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How do gametes get into coelom, and, later out of it? Excretion can be under quite heavy pressure, see https://www.youtube.com/watch?v=1OsRbuEINXI
As question has not received answer, will answer to best of my ability. Eggs are plopped off from ovary, into coelom. Duct out of coelom is the oviduct that opens into it. The fish must apply pressure to the whole of the coelom, I guess, to push it out. That part is a bit of a mystery to me, an expert answer would be better.
Dispersal of fish eggs by water birds &ndash just a myth?
How do fish end up in isolated bodies of water when they can&rsquot swim there themselves? For centuries, researchers have assumed that water birds transfer fish eggs into these waters &ndash however, a systematic literature review by researchers at the University of Basel has shown that there is no evidence of this to date.
Small lakes with a surface area of less than 100 m 2 represent the majority of global freshwater ecosystems. Many of these lakes are found in remote, often mountainous areas with no inflow and outflow. Yet in most of these lakes, there are fish. So how do fish reach lakes and ponds that are not connected to other bodies of water?
This question was already addressed by some of the leading natural scientists of the 19th century such as Charles Darwin, Alfred Russel Wallace and Charles Lyell, who all came to the same conclusion &ndash water birds must be responsible for fish dispersal.
And they had a plausible explanation for this: fish eggs of some species are sticky and can survive for some time out of water. The theory is thus that the fish eggs stick to water birds&rsquo feathers or feet the birds then fly from one body of water to the next, where the fish hatch from their eggs.
Conclusive studies are lacking
A study carried out by environmental scientists from the University of Basel has now shown that although the research community considers this to be a proven theory, no studies have been published to confirm it.
To objectively measure the lack of evidence, the Basel research team conducted a systematic literature review. The result shows that no in-depth scientific studies exist to prove that water birds disperse fish eggs.
To rule out the possibility that the unsuccessful search was due to their method, the researchers also used the same approach to look for evidence of the dispersal of aquatic invertebrates. In this case, they found numerous scientific publications supported by experiments and field studies.
Still widespread today
For their study, the Basel researchers also reviewed online forums and surveyed around 40 experts from research, private institutions, and enviromental NGOs. Their aim was to determine the prevalence of the theory of fish dispersal by water birds both inside and outside the research community. The majority of experts that took part in the survey found the theory so plausible that they deemed the mystery to have been solved. However, none of them could draw on any empirical evidence.
&ldquoThe lack of evidence does not mean that water birds are not responsible for the dispersal,&rdquo says Dr. Philipp E. Hirsch from the University of Basel. &ldquoBut we simply do not yet know what roles are played by birds, humans and other processes.&rdquo
Understanding the way that fish are dispersed in remote bodies of water is important for the maintenance of biodiversity. The knowledge of how species colonize new habitats forms the basis for the preservation of refuges and targeted reintroduction and also helps prevent the spread of invasive species.
Most male fish have two testes of similar size. In the case of sharks, the testes on the right side is usually larger [ citation needed ] . The primitive jawless fish have only a single testis, located in the midline of the body, although even this forms from the fusion of paired structures in the embryo. 
Under a tough membranous shell, the tunica albuginea, the testis of some teleost fish, contains very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that from puberty into old age, develop into sperm cells (also known as spermatozoa or male gametes). The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly created sperm cells mature (see spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.
However, most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures called sperm ampullae. These are seasonal structures, releasing their contents during the breeding season, and then being reabsorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebrates, including the same range of cell types. 
In terms of spermatogonia distribution, the structure of teleosts testes has two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in Atherinomorph fish they are confined to the distal portion of these structures. Fish can present cystic or semi-cystic spermatogenesis in relation to the release phase of germ cells in cysts to the seminiferous tubules lumen. 
Many of the features found in ovaries are common to all vertebrates, including the presence of follicular cells and tunica albuginea There may be hundreds or even millions of fertile eggs present in the ovary of a fish at any given time. Fresh eggs may be developing from the germinal epithelium throughout life. Corpora lutea are found only in mammals, and in some elasmobranch fish in other species, the remnants of the follicle are quickly resorbed by the ovary.  The ovary of teleosts is often contains a hollow, lymph-filled space which opens into the oviduct, and into which the eggs are shed.  Most normal female fish have two ovaries. In some elasmobranchs, only the right ovary develops fully. In the primitive jawless fish, and some teleosts, there is only one ovary, formed by the fusion of the paired organs in the embryo. 
Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct.  Gymnovaries are the primitive condition found in lungfish, sturgeon, and bowfin. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.  Secondary gymnovaries are found in salmonids and a few other teleosts.
The eggs of fish and amphibians are jellylike. Cartilagenous fish (sharks, skates, rays, chimaeras) eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development. Most fish species spawn eggs that are fertilized externally, typically with the male inseminating the eggs after the female lays them. These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog, Chiromantis xerampelina.
Intromittent organs Edit
Male cartilaginous fishes (sharks and rays), as well as the males of some live-bearing ray finned fishes, have fins that have been modified to function as intromittent organs, reproductive appendages which allow internal fertilization. In ray finned fish they are called gonopodiums or andropodiums, and in cartilaginous fish they are called claspers.
Gonopodia are found on the males of some species in the Anablepidae and Poeciliidae families. They are anal fins that have been modified to function as movable intromittent organs and are used to impregnate females with milt during mating. The third, fourth and fifth rays of the male's anal fin are formed into a tube-like structure in which the sperm of the fish is ejected.  When ready for mating, the gonopodium becomes erect and points forward towards the female. The male shortly inserts the organ into the sex opening of the female, with hook-like adaptations that allow the fish to grip onto the female to ensure impregnation. If a female remains stationary and her partner contacts her vent with his gonopodium, she is fertilized. The sperm is preserved in the female's oviduct. This allows females to fertilize themselves at any time without further assistance from males. In some species, the gonopodium may be half the total body length. Occasionally the fin is too long to be used, as in the "lyretail" breeds of Xiphophorus helleri. Hormone treated females may develop gonopodia. These are useless for breeding.
Similar organs with similar characteristics are found in other fishes, for example the andropodium in the Hemirhamphodon or in the Goodeidae. 
Claspers are found on the males of cartilaginous fishes. They are the posterior part of the pelvic fins that have also been modified to function as intromittent organs, and are used to channel semen into the female's cloaca during copulation. The act of mating in sharks usually includes raising one of the claspers to allow water into a siphon through a specific orifice. The clasper is then inserted into the cloaca, where it opens like an umbrella to anchor its position. The siphon then begins to contract expelling water and sperm.  
Oogonia development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilisation processes. Changes in the nucleus, ooplasm, and the surrounding layers characterize the oocyte maturation process. 
Postovulatory follicles are structures formed after oocyte release they do not have endocrine function, present a wide irregular lumen, and are rapidly reabsorbed in a process involving the apoptosis of follicular cells. A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned. This process can also occur, but less frequently, in oocytes in other development stages. 
Some fish are hermaphrodites, having both testes and ovaries either at different phases in their life cycle or, as in hamlets, have them simultaneously.
In fish, fertilisation of eggs can be either external or internal. In many species of fish, fins have been modified to allow Internal fertilisation. Similarly, development of the embryo can be external or internal, although some species show a change between the two at various stages of embryo development. Thierry Lodé described reproductive strategies in terms of the development of the zygote and the interrelationship with the parents there are five classifications - ovuliparity, oviparity, ovo-viviparity, histotrophic viviparity and hemotrophic viviparity. 
Ovuliparity means the female lays unfertilised eggs (ova), which must then be externally fertilised.  Examples of ovuliparous fish include salmon, goldfish, cichlids, tuna and eels. In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their gametes into the surrounding water.
Oviparity is where fertilisation occurs internally and so the female sheds zygotes (or newly developing embryos) into the water,  often with important outer tissues added. Over 97% of all known fish are oviparous (needs confirmation, since the ovuliparity is a new term which may be confused with oviparity. If ovuliparity is used, most of the fishes have ovulipaprity breeding strategy).  In oviparous fish, internal fertilisation requires the male to use some sort of intromittent organ to deliver sperm into the genital opening of the female. Examples include the oviparous sharks, such as the horn shark, and oviparous rays, such as skates. In these cases, the male is equipped with a pair of modified pelvic fins known as claspers.
Marine fish can produce high numbers of eggs which are often released into the open water column. The eggs have an average diameter of 1 millimetre (0.039 in). The eggs are generally surrounded by the extraembryonic membranes but do not develop a shell, hard or soft, around these membranes. Some fish have thick, leathery coats, especially if they must withstand physical force or desiccation. These type of eggs can also be very small and fragile.
The newly hatched young of oviparous fish are called larvae. They are usually poorly formed, carry a large yolk sac (for nourishment) and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed metamorphosis) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on zooplankton prey, a process which depends on typically inadequate zooplankton density, starving many larvae.
In ovoviviparous fish the eggs develop inside the mother's body after internal fertilisation but receive little or no nourishment directly from the mother, depending instead on a food reserve inside the egg, the yolk.  Each embryo develops in its own egg. Familiar examples of ovoviviparous fish include guppies, angel sharks, and coelacanths.
There are two types of viviparity, differentiated by how the offspring gain their nutrients.
- Histotrophic (tissue eating) viviparity means embryos develop in the female's oviducts but obtain nutrients by consuming other tissues, such as ova (oophagy) or zygotes.  This has been observed primarily among sharks such as the shortfin mako and porbeagle, but is known for a few bony fish as well such as the halfbeakNomorhamphus ebrardtii.  An unusual mode of vivipary is adelphophagy or intrauterine cannibalism, in which the largest embryos eat weaker, smaller unborn siblings. This is most commonly found among sharks such as the grey nurse shark, but has also been reported for Nomorhamphus ebrardtii. 
- Hemotrophic (blood eating) viviparity means embryos develop in the female's (or male's) oviduct and nutrients are provided directly by the parent, typically via a structure similar to, or analogous to the placenta seen in mammals.  Examples of hemotrophic fish include the surfperches, splitfins, lemon shark, seahorses and pipefish.
Aquarists commonly refer to ovoviviparous and viviparous fish as livebearers.
Hermaphroditism occurs when a given individual in a species possesses both male and female reproductive organs, or can alternate between possessing first one, and then the other. Hermaphroditism is common in invertebrates but rare in vertebrates. It can be contrasted with gonochorism, where each individual in a species is either male or female, and remains that way throughout their lives. Most fish are gonochorists, but hermaphroditism is known to occur in 14 families of teleost fishes. 
Usually hermaphrodites are sequential, meaning they can switch sex, usually from female to male (protogyny). This can happen if a dominant male is removed from a group of females. The largest female in the harem can switch sex over a few days and replace the dominant male.  This is found amongst coral reef fishes such as groupers, parrotfishes and wrasses. It is less common for a male to switch to a female (protandry).  : 162 As an example, most wrasses are protogynous hermaphrodites within a haremic mating system.   Hermaphroditism allows for complex mating systems. Wrasses exhibit three different mating systems: polygynous, lek-like, and promiscuous mating systems.  Group spawning and pair spawning occur within mating systems. The type of spawning that occurs depends on male body size.  Labroids typically exhibit broadcast spawning, releasing high amounts of planktonic eggs, which are broadcast by tidal currents adult wrasses have no interaction with offspring.  Wrasse of a particular subgroup of the family Labridae, Labrini, do not exhibit broadcast spawning.
Less commonly hermaphrodites can be synchronous, meaning they simultaneously possess both ovaries and testicles and can function as either sex at any one time. Black hamlets "take turns releasing sperm and eggs during spawning. Because such egg trading is advantageous to both individuals, hamlets are typically monogamous for short periods of time–an unusual situation in fishes."  The sex of many fishes is not fixed, but can change with physical and social changes to the environment where the fish lives. 
Particularly among fishes, hermaphroditism can pay off in situations where one sex is more likely to survive and reproduce, perhaps because it is larger.  Anemone fishes are sequential hermaphrodites which are born as males, and become females only when they are mature. Anemone fishes live together monogamously in an anemone, protected by the anemone stings. The males do not have to compete with other males, and female anemone fish are typically larger. When a female dies a juvenile (male) anemone fish moves in, and "the resident male then turns into a female and reproductive advantages of the large female–small male combination continue".  In other fishes sex changes are reversible. For example, if some gobies are grouped by sex (male or female), some will switch sex.  : 164 
The mangrove rivulus Kryptolebias marmoratus produces both eggs and sperm by meiosis and routinely reproduces by self-fertilization. Each individual hermaphrodite normally fertilizes itself when an egg and sperm that it has produced by an internal organ unite inside the fish's body.  In nature, this mode of reproduction can yield highly homozygous lines composed of individuals so genetically uniform as to be, in effect, identical to one another.   The capacity for selfing in these fishes has apparently persisted for at least several hundred thousand years. 
Although inbreeding, especially in the extreme form of self-fertilization, is ordinarily regarded as detrimental because it leads to expression of deleterious recessive alleles, self-fertilization does provide the benefit of fertilization assurance (reproductive assurance) at each generation. 
Sexual parasitism Edit
Sexual parasitism is a mode of sexual reproduction, unique to anglerfish, in which the males of a species are much smaller than the females, and rely on the females for food and protection from predators. The males give nothing back except the sperm which the females need in order to produce the next generation.
Some anglerfish, like those of the deep sea ceratioid group, employ this unusual mating method. Because individuals are very thinly distributed, encounters are also very rare. Therefore, finding a mate is problematic. When scientists first started capturing ceratioid anglerfish, they noticed that all the specimens were female. These individuals were a few centimetres in size and almost all of them had what appeared to be parasites attached to them. It turned out that these "parasites" were highly reduced male ceratioid anglerfish. This indicates the anglerfish use a polyandrous mating system.
The methods by which the anglerfish locate mates are variable. Some species have minute eyes unfit for identifying females, while others have underdeveloped nostrils, making it unlikely that they effectively find females using olfaction.  When a male finds a female, he bites into her skin, and releases an enzyme that digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level.  The male becomes dependent on the female host for survival by receiving nutrients via their now-shared circulatory system, and provides sperm to the female in return. After fusing, males increase in volume and become much larger relative to free-living males of the species. They live and remain reproductively functional as long as the female stays alive, and can take part in multiple spawnings.  This extreme sexual dimorphism ensures that when the female is ready to spawn she has a mate immediately available.  Multiple males can be incorporated into a single individual female with up to eight males in some species, though some taxa appear to have a one male per female rule.  In addition to the physiological adaptations, the immune system is altered to allow the conjoining. 
One explanation for the evolution of sexual parasitism is that the relative low density of females in deep-sea environments leaves little opportunity for mate choice among anglerfish. Females remain large to accommodate fecundity, as is evidenced by their large ovaries and eggs. Males would be expected to shrink to reduce metabolic costs in resource-poor environments and would develop highly specialized female-finding abilities. If a male manages to find a female parasitic attachment, then it is ultimately more likely to improve lifetime fitness relative to free living, particularly when the prospect of finding future mates is poor. An additional advantage to parasitism is that the male's sperm can be used in multiple fertilizations, as he stays always available to the female for mating. Higher densities of male-female encounters might correlate with species that demonstrate facultative parasitism or simply use a more traditional temporary contact mating. 
Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilization. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell. The first all-female (unisexual) reproduction in vertebrates was described in the Amazon molly in 1932.  Since then at least 50 species of unisexual vertebrate have been described, including at least 20 fish, 25 lizards, a single snake species, frogs, and salamanders.  As with all types of asexual reproduction, there are both costs (low genetic diversity and therefore susceptibility to adverse mutations that might occur) and benefits (reproduction without the need for a male) associated with parthenogenesis.
Parthenogenesis in sharks has been confirmed in the bonnethead  and zebra shark.  Other, usually sexual species, may occasionally reproduce parthenogenetically, and the hammerhead and blacktip sharks  are recent additions to the known list of facultative parthenogenetic vertebrates.
A special case of parthenogenesis is gynogenesis. In this type of reproduction, offspring are produced by the same mechanism as in parthenogenesis, however, the egg is stimulated to develop simply by the presence of sperm - the sperm cells do not contribute any genetic material to the offspring. Because gynogenetic species are all female, activation of their eggs requires mating with males of a closely related species for the needed stimulus. The Amazon molly, (pictured), reproduces by gynogenesis.
The elkhorn sculpin (Alcichthys elongatus) is a marine teleost with a unique reproductive mode called “internal gametic association”. Sperm are introduced into the ovary by copulation and then enter the micropylar canal of ovulated eggs in the ovarian cavity. However, actual sperm-egg fusion does not occur until the eggs have been released into sea water. 
Inbreeding depression Edit
The effect of inbreeding on reproductive behavior was studied in the poeciliid fish Heterandria formosa.  One generation of full-sib mating was found to decrease reproductive performance and likely reproductive success of male progeny. Other traits that displayed inbreeding depression were offspring viability and maturation time of both males and females.
Exposure of zebra fish to a chemical environmental agent, analogous to that caused by anthropogenic pollution, amplified the effects of inbreeding on key reproductive traits.  Embryo viability was significantly reduced in inbred exposed fish and there was a tendency for inbred males to sire fewer offspring.
The behaviors of juvenile Coho salmon with either low or medium inbreeding were compared in paired contests.  Fish with low inbreeding showed almost twice the aggressive pursuit in defending territory than fish with medium inbreeding, and furthermore had a higher specific growth rate. A significant effect of inbreeding depression on juvenile survival was also found, but only in high-density competitive environments, suggesting that intra-specific competition can magnify the deleterious effects of inbreeding.
Inbreeding avoidance Edit
Inbreeding ordinarily has negative fitness consequences (inbreeding depression), and as a result species have evolved mechanisms to avoid inbreeding. Numerous inbreeding avoidance mechanisms operating prior to mating have been described. However, inbreeding avoidance mechanisms that operate subsequent to copulation are less well known. In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between sperm of rival males for achieving fertilisation.  In competitions between sperm from an unrelated male and from a full sibling male, a significant bias in paternity towards the unrelated male was observed. 
Inbreeding depression is considered to be due largely to the expression of homozygous deleterious recessive mutations.  Outcrossing between unrelated individuals results in the beneficial masking of deleterious recessive mutations in progeny. 
Classification Features of Animals
Animals are classified according to morphological and developmental characteristics, such as a body plan. With the exception of sponges, the animal body plan is symmetrical. This means that their distribution of body parts is balanced along an axis. Additional characteristics that contribute to animal classification include the number of tissue layers formed during development, the presence or absence of an internal body cavity, and other features of embryological development.
Figure 15.1.2: The phylogenetic tree of animals is based on morphological, fossil, and genetic evidence.
Which of the following statements is false?
- Eumetazoa have specialized tissues and Parazoa do not.
- Both acoelomates and pseudocoelomates have a body cavity.
- Chordates are more closely related to echinoderms than to rotifers according to the figure.
- Some animals have radial symmetry, and some animals have bilateral symmetry.
Animals may be asymmetrical, radial, or bilateral in form (Figure 15.1.3). Asymmetrical animals are animals with no pattern or symmetry an example of an asymmetrical animal is a sponge (Figure 15.1.3a). An organism with radial symmetry (Figure 15.1.3b) has a longitudinal (up-and-down) orientation: Any plane cut along this up&ndashdown axis produces roughly mirror-image halves. An example of an organism with radial symmetry is a sea anemone.
Figure 15.1.3: Animals exhibit different types of body symmetry. The (a) sponge is asymmetrical and has no planes of symmetry, the (b) sea anemone has radial symmetry with multiple planes of symmetry, and the (c) goat has bilateral symmetry with one plane of symmetry.
Bilateral symmetry is illustrated in Figure 15.1.3c using a goat. The goat also has upper and lower sides to it, but they are not symmetrical. A vertical plane cut from front to back separates the animal into roughly mirror-image right and left sides. Animals with bilateral symmetry also have a &ldquohead&rdquo and &ldquotail&rdquo (anterior versus posterior) and a back and underside (dorsal versus ventral).
Watch this video to see a quick sketch of the different types of body symmetry.
Most animal species undergo a layering of early tissues during embryonic development. These layers are called germ layers. Each layer develops into a specific set of tissues and organs. Animals develop either two or three embryonic germs layers (Figure 15.1.4). The animals that display radial symmetry develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). These animals are called diploblasts. Animals with bilateral symmetry develop three germ layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three germ layers are called triploblasts.
Figure 15.1.4: During embryogenesis, diploblasts develop two embryonic germ layers: an ectoderm and an endoderm. Triploblasts develop a third layer&mdashthe mesoderm&mdashbetween the endoderm and ectoderm.
Presence or Absence of a Coelom
Triploblasts may develop an internal body cavity derived from mesoderm, called a coelom (pr. see-LŌM). This epithelial-lined cavity is a space, usually filled with fluid, which lies between the digestive system and the body wall. It houses organs such as the kidneys and spleen, and contains the circulatory system. Triploblasts that do not develop a coelom are called acoelomates, and their mesoderm region is completely filled with tissue, although they have a gut cavity. Examples of acoelomates include the flatworms. Animals with a true coelom are called eucoelomates (or coelomates) (Figure 15.1.5). A true coelom arises entirely within the mesoderm germ layer. Animals such as earthworms, snails, insects, starfish, and vertebrates are all eucoelomates. A third group of triploblasts has a body cavity that is derived partly from mesoderm and partly from endoderm tissue. These animals are called pseudocoelomates. Roundworms are examples of pseudocoelomates. New data on the relationships of pseudocoelomates suggest that these phyla are not closely related and so the evolution of the pseudocoelom must have occurred more than once (Figure 15.1.2). True coelomates can be further characterized based on features of their early embryological development.
Figure 15.1.5: Triploblasts may be acoelomates, eucoelomates, or pseudocoelomates. Eucoelomates have a body cavity within the mesoderm, called a coelom, which is lined with mesoderm tissue. Pseudocoelomates have a similar body cavity, but it is lined with mesoderm and endoderm tissue. (credit a: modification of work by Jan Derk credit b: modification of work by NOAA credit c: modification of work by USDA, ARS)
Protostomes and Deuterostomes
Bilaterally symmetrical, triploblastic eucoelomates can be divided into two groups based on differences in their early embryonic development. Protostomes include phyla such as arthropods, mollusks, and annelids. Deuterostomes include the chordates and echinoderms. These two groups are named from which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from Greek words meaning &ldquomouth first,&rdquo and deuterostome originates from words meaning &ldquomouth second&rdquo (in this case, the anus develops first). This difference reflects the fate of a structure called the blastopore (Figure 15.1.6), which becomes the mouth in protostomes and the anus in deuterostomes. Other developmental characteristics differ between protostomes and deuterostomes, including the mode of formation of the coelom and the early cell division of the embryo.
Figure 15.1.6: Eucoelomates can be divided into two groups, protostomes and deuterostomes, based on their early embryonic development. Two of these differences include the origin of the mouth opening and the way in which the coelom is formed.
Circulatory System Variation in Animals
The circulatory system varies from simple systems in invertebrates to more complex systems in vertebrates. The simplest animals, such as the sponges (Porifera) and rotifers (Rotifera), do not need a circulatory system because diffusion allows adequate exchange of water, nutrients, and waste, as well as dissolved gases, as shown in Figure 21.3 a . Organisms that are more complex but still only have two layers of cells in their body plan, such as jellies (Cnidaria) and comb jellies (Ctenophora) also use diffusion through their epidermis and internally through the gastrovascular compartment. Both their internal and external tissues are bathed in an aqueous environment and exchange fluids by diffusion on both sides, as illustrated in Figure 21.3 b . Exchange of fluids is assisted by the pulsing of the jellyfish body.
Figure 21.3. Simple animals consisting of a single cell layer such as the (a) sponge or only a few cell layers such as the (b) jellyfish do not have a circulatory system. Instead, gases, nutrients, and wastes are exchanged by diffusion.
For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body therefore, more complex circulatory systems evolved. Most arthropods and many mollusks have open circulatory systems. In an open system, an elongated beating heart pushes the hemolymph through the body and muscle contractions help to move fluids. The larger more complex crustaceans, including lobsters, have developed arterial-like vessels to push blood through their bodies, and the most active mollusks, such as squids, have evolved a closed circulatory system and are able to move rapidly to catch prey. Closed circulatory systems are a characteristic of vertebrates however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptation during evolution and associated differences in anatomy. Figure 21.4 illustrates the basic circulatory systems of some vertebrates: fish, amphibians, reptiles, and mammals.
Figure 21.4. (a) Fish have the simplest circulatory systems of the vertebrates: blood flows unidirectionally from the two-chambered heart through the gills and then the rest of the body. (b) Amphibians have two circulatory routes: one for oxygenation of the blood through the lungs and skin, and the other to take oxygen to the rest of the body. The blood is pumped from a three-chambered heart with two atria and a single ventricle. (c) Reptiles also have two circulatory routes however, blood is only oxygenated through the lungs. The heart is three chambered, but the ventricles are partially separated so some mixing of oxygenated and deoxygenated blood occurs except in crocodilians and birds. (d) Mammals and birds have the most efficient heart with four chambers that completely separate the oxygenated and deoxygenated blood it pumps only oxygenated blood through the body and deoxygenated blood to the lungs.
As illustrated in Figure 21.4 a Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. The atrium collects blood that has returned from the body and the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated this is called gill circulation. The blood then continues through the rest of the body before arriving back at the atrium this is called systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish’s systemic circuit. The result is a limit in the amount of oxygen that can reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish.
In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, which is called pulmonary circulation, and the other throughout the rest of the body and its organs including the brain (systemic circulation). In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation.
As shown in Figure 21.4 b , amphibians have a three-chambered heart that has two atria and one ventricle rather than the two-chambered heart of fish. The two atria (superior heart chambers) receive blood from the two different circuits (the lungs and the systems), and then there is some mixing of the blood in the heart’s ventricle (inferior heart chamber), which reduces the efficiency of oxygenation. The advantage to this arrangement is that high pressure in the vessels pushes blood to the lungs and body. The mixing is mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit. For this reason, amphibians are often described as having double circulation.
Most reptiles also have a three-chambered heart similar to the amphibian heart that directs blood to the pulmonary and systemic circuits, as shown in Figure 21.4 c . The ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood. Some reptiles (alligators and crocodiles) are the most primitive animals to exhibit a four-chambered heart. Crocodilians have a unique circulatory mechanism where the heart shunts blood from the lungs toward the stomach and other organs during long periods of submergence, for instance, while the animal waits for prey or stays underwater waiting for prey to rot. One adaptation includes two main arteries that leave the same part of the heart: one takes blood to the lungs and the other provides an alternate route to the stomach and other parts of the body. Two other adaptations include a hole in the heart between the two ventricles, called the foramen of Panizza, which allows blood to move from one side of the heart to the other, and specialized connective tissue that slows the blood flow to the lungs. Together these adaptations have made crocodiles and alligators one of the most evolutionarily successful animal groups on earth.
In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles, as illustrated in Figure 21.4 d . The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. The four-chambered heart of birds and mammals evolved independently from a three-chambered heart. The independent evolution of the same or a similar biological trait is referred to as convergent evolution.
Rohu: Systematic Position, Distribution and Structure | Bony Fish
In this article we will discuss about Rohu:- 1. Systematic Position of Rohu 2. Habit and Habitat of Rohu 3. Geographical Distribution of Rohu 4. External Structures of Rohu 5. Skeletal Structures of Rohu 6. Coelom of Rohu 7. Digestive System of Rohu 8. Hydrostatic Organ of Rohu 9. Respiratory System of Rohu 10. Circulatory System of Rohu 11. Venous System of Rohu 12. Nervous System of Rohu 13. Urinogenital System of Rohu.
- Systematic Position of Rohu
- Habit and Habitat of Rohu
- Geographical Distribution of Rohu
- External Structures of Rohu
- Skeletal Structures of Rohu
- Coelom of Rohu
- Digestive System of Rohu
- Hydrostatic Organ of Rohu
- Respiratory System of Rohu
- Circulatory System of Rohu
- Venous System of Rohu
- Nervous System of Rohu
- Urinogenital System of Rohu
1. Systematic Position of Rohu:
Subphylum Vertebrata (= Craniata)
Scientific Name Labeo rohita
2. Habit and Habitat of Rohu:
It is abundantly found in fresh water in ponds, lakes, rivers and reservoirs. It is chiefly vegetarian and bottom feeders but young fry feed on zooplankton. It breeds in June and July in running water.
3. Geographical Distribution of Rohu:
Labeo rohita is found in tropical and tem­perate regions. It is the commonest cap in the plains of India, except in the south. It is also common in Bangladesh and Myanmar (Burma). Labeo rohita, commonly called the Rohu fish, is one of the typical fresh-water bony fish­es of India. This particular fish is studied as the type specimen of bony fishes in many Indian Universities.
4. External Structures of Rohu:
The Rohu fish has a spindle-shaped body measuring up to 1 m in length and weighing about 20-25. kg. The dorsal side of the body is blackish in colour and the ventro-lateral sides are silvery. The body, like that of Bhetki, is dis­tinguishable into a conspicuous head, trunk and postnatal tail(Fig. 6.19).
The head extends from the snout up to the posterior margin of the operculum. The snout is depressed and projects beyond the jaws. Labeo rohita is characterised by having no lateral lobes in the snout. Two nostrils are present on the dorsal side of the snout.
The mouth is a crescentic transverse opening bounded by thick fringed lips. Teeth are absent in the jaws. The eyes are prominent and are lidless. One or two pairs of barbels are present on the dorsolateral sides of the mouth. The maxillary barbels are relative­ly short and delicate that the rostal barbels.
The trunk is elongated and oval in cross-sec­tion. It is covered over by thin overlapping cycloid scales. The lateral line runs along the lateral sides of the body. The scales along the lateral line contain pores which are connected with a tubular canal. The vent is situated ventrally and just in front of the anal fin.
Both paired and unpaired fins are well- developed. The pectoral and pelvic fins are borne by the respective girdles. The pectorals are located at the anterolateral side of the trunk behind the operculum. Each pectoral fin is sup­ported by 19 fin rays. The pelvic are situated on the ventral side behind the pectorals.
Each pelvic fin contains 9 fin-rays. There is only one dorsal fin in Rohu which arises from the mid- dorsal line of the trunk half-way between the snout and the base of the tail. The anal fin lies posterior to the anus. The dorsal fin consists of 13 fin-rays and anal fin has 4-6 fin-rays. The tail fin is homocercal with two symmetrical lobes. Several fin-rays support the tail fin.
The diploid chromosome number is 52 in Labeo rohita.
The integument is covered by an outer soft epidermis and an inner dermis. The epidermis is composed of epithelial cells with numerous unicellular mucous glands. The musculature of the body wall lies below the integument. The disposition of the body wall musculature is similar to that of other fishes.
5. Skeletal Structures of Rohu:
The endoskeleton of Rohu described by D. S. Sarbahi in 1932. The scales and fin-rays constitute the exoskeleton of the Rohu fish.
The endoskele­ton is completely ossified and consists of:
(a) An axial skeletal portion and
(b) An appendicular skeletal part.
The axial skeleton is com­posed of the skull, vertebral column with the ribs and the skeletal elements supporting the median fins.
The skull of Rohu has a very com­plicated structural organisation. Many inves­ting and replacing bones participate in the for­mation of the skull (Fig. 6.20). It is composed of the cranium, the sense capsules and the vis­ceral arches.
The cranium and the sense cap­sules are immovably united together while the visceral arches are loosely attached with the skull. Basically the skull is composed of a pos­terior basai plate and anterior trabecular region. The auditory capsules are united with the basal plate and the nasal capsules are attached with the trabecular region.
The lateral walls are mostly incomplete and are derived from the orbital cartilage. This carti­lage joins posteriorly with the auditory capsule and the nasal capsule on the anterior end. The palate quadrate (visceral arch) articulates ante­riorly with the trabecular region of the skull by a basal process and posteriorly with the audi­tory capsule by an otic process.
The skull in adult fish assumes an elonga­ted shape.
It is broadly divided into:
(b) A posterior occipital region,
(c) The otic region consisting of the bones of the auditory capsules,
(d) An orbitotemporal region and
(e) an anterior nasal (ethmoidal) region.
The dorsal side is more or less convex. A shallow supra-temporal groove is present on its posterodorsal side. This groove extends posterola­teral towards the main occipital spine. The posterior wall or the occipital region of the skull bears three apertures, a median foramen mag­num and two large oval fenestrae.
Each fenestra pierces the exoccipital bone and forms a char­acteristic feature of the cyprinoid skull. The occipital region is composed of a supraoccipi­tal, a basioccipital and a pair of exoccipital bones. The main posterior part of the skull is formed by the supraoccipital bone which does not form the dorsal boundary of the foramen magnum in Rohu.
It is divided into dorsal and posteroinferior portions. The wider anterior end of the dorsal portion is overlapped by the pari­etal bones. A median vertical occipital spine is present on the dorsal portion of the supraoccip­ital. The posteroinferior portion of the supraoc­cipital is composed of the occipital spine (or keel).
The exoccipitals are large bones, each consisting of:
(b) A paroccipital process and
The basal plate forms part of the floor of the cranial cavity and the paroccipital process forms the posterior boundary of the auditory capsule and the side wall of the cranial cavity. The dorsal process encloses the foramen magnum.
The basioccipital is a large bony piece and is roofed over by the occipital condyle. A deep depression is present on the posterior surface of the occipital condyle. A deep depression is pre­sent on the posterior surface of the occipital condyle. A large oval masticatory process is borne by the ventral surface of the basioccipital.
The otic region is represented by the paired auditory capsules each situated on the posterior side of the skull and lies between the seventh and the ninth cranial nerves. Each auditory capsule is derived from an otic carti­lage growing round the internal ear.
The otic cartilage is transformed into the prootic, epiotic, sphenotic, pterotic and opisthotic bones in other teleosts. But in Rohu, the opisthotic bone is lacking and the other four bones form a compact inverted cup-like structure. The orbit temporal region of the skull is composed of the temporal (or sphenoidal) region and the orbit.
The temporal region is subdivided into:
(a) The parietal region and
The parietal region is made up of the parietals, alisphenoids and Para sphenoid. The frontal region includes the frontals, orbitosphenoids and Para sphenoid. Besides these bones, the supra-temporal is located at the posterolateral angle of the skull.
The nasal (or ethmoidal) region of the skull comprises of the bones which develop in relation to the nostrils and the snout. The par­ticipating bones of this region are the paired nasals, ectoethmoids and lacrymals, a median mesethmoid, a vomer and a rostral. Of these bones, the mesethmoid ectoethmoid and ros­tral are replacing bones, while the nasals, lacrymals and the vomer are all investing bones.
The visceral skeleton consists of a series of seven half-hoops encir­cling the pharyngeal wall. The half-hoops of two sides unite with each other along the mid- ventral line forming seven visceral arches. All the visceral arches are united with one anoth­er mid-ventrally to form a basket-like visceral skeleton. The first visceral arch is called mandibular arch, the second is the hyoid and the rest five are the branchial arches.
Of the five branchial arches, the four support the gills while the fifth one forms the inferior pharyn­geal bones. The inferior pharyngeal bones develop into the masticating plates armed with large teeth. The mandibular arch is divided into a dorsal palatopterygoquadrate bar and a ventral Meckel’s cartilage forming the primary lower jaw.
The palatopterygoquadrate beco­mes closely associated with the cranium and forms the primary upper jaw. The primary upper jaw becomes ossified by the following replacing bones, palatine, meta-pterygoid and quadrate.
Two investing bones premaxilla and maxilla support the anterior margin of the mouth and together form the secondary upper jaw. Each half of the lower jaw is composed of a small articular, a large dentary and a small angular.
Two dentaries unite in the middle line. The hyoid arch is also divided into two parts: the upper hyomandibular and lower hyoid cornu. The hyomandibular forms the suspensorium by which the jaws remain sus­pended to the cranium.
Many investing bones are connected with the hyoid arch and support the operculum (Fig. 6.21 A). The bones of the operculum are opercular, pre-opercular, sub-opercular and inter-opercular. Each branchial arch is ossified by four replacing bones, pharyngobranchial, epibranchial, ceralobranchial and hypobranchial.
The vertebral column is a completely ossified structure and com­posed of 37-38 vertebrae (Fig. 6.21 B). The vertebrae are of amphicoelous (i.e., both the ends of the centrum bear concavity) type.
The vertebral column is distinguishable into the following parts:
(a) An anterior trunk region consisting of 21 trunk vertebrae (Fig. 6.21 C) and bearing movable ribs and
(b) A posterior caudal region. The vertebrae of the caudal region lack ribs and possess haemal arches (Fig. 6.21 D).
The first four trunk verte­brae are greatly altered since these vertebrae connect the swim-bladder with the internal ear. The last 3 or 4 trunk vertebrae bear posteroventral processes. The last three caudal vertebrae are modified for the support of the caudal fin.
The posterior most caudal vertebra is transformed into an upturned rod-like urostyle. It is a solid structure with a ventral groove wherein the proximal ends of the hypourals fit.
Typical trunk vertebra:
A typical trunk vertebra is composed of a deeply biconcave centrum (Fig. 6.21 C). In the embryonic stage the concavities are communicated by a nar­row notochordal canal perforating the body of the centrum. The notochordal canal becomes closed in an adult. The edges of the centra are united by connective-tissue liga­ments and the spaces enclosed by the verte­brae are filled with the remains of noto­chordal elements.
A pair of backwardly directed processes arising from the antero­lateral borders of centrum, enclose the spinal cord and unite above to form the neural arch. The neural arch gives a long dorsal back­wardly directed neural spine. A pair of small blunt processes, the prezygapophyses, is present anteriorly at the base of the neural arch.
Another pair of postzygapophyses arise from the posterolateral edges of the vertebra. The postzygapophyses are pointed upwards and backwards. A pair of short parapophyses originating from the ventrolateral surfaces of the centrum is directed downwards. The ribs are attached with the parapophyses by ligaments.
Typical caudal vertebra:
The caudal part of the vertebral column is composed of 16 or 1 7 caudal vertebrae.
Like that of a trunk verte­bra (Fig. 6.21 D), a typical caudal vertebra has:
(a) An amphicoelous centrum with a median dorsal, a medium ventral and two lateral depressions,
(b) A neural arch with a long backwardly directed neural spine and
(c) Artic­ulating processes like pre- and post­zygapophyses, are present, in the same posi­tion.
From the anterolateral margins of the centrum, a pair of backwardly directed pro­cesses meets in the mid-ventral line to form a canal for placement of the caudal blood ves­sels. This process is known as the haemal arch. This arch produces a backwardly direc­ted haemal spine.
A pair of small blunt anteroventral processes is present at the bases of each haemal arch. Similar processes on the posterolateral side of the centrum are present. These posteroventral processes are directed downwards and backwards as seen in posterior trunk vertebrae.
The skeleton suppor­ting the median fin consists of a series of:
(a) Somactidia or endo-skeletal radials and
(b) The dermotrichia or dermal fin-rays (Fig. 6.21 E).
The somactids are parallel bony rods lying embedded within the body muscles. Each somactid is divided into a proximal, a mesial and a distal segment. The dermotrichia support the fold of the fin. In Rohu, the dermo­trichia are branched and jointed and are usu­ally called lepidotrichia. Besides these fin- rays, delicate horny-rays (actinotrichia) are present at the free edges of the fins.
The dorsal fin is supported by lepi­dotrichia. There are fifteen or sixteen lepi­dotrichia seated on fourteen radials. The prox­imal segment of each radial is enlarged and dagger-shaped. These are called interspinous bone or axonost. The median segment is short and the distal sector is greatly reduced. The anal fin bears a series of eight fin-rays support­ed by seven radials.
The first six are well- formed. The caudal fin is supported by a num­ber of flattened bony rods. Two epiurals and a radial are present on the dorsal side of the urostyle while nine hypourals are present on the ventral side. The fin-rays are attached with the hypourals and epiurals in two symmetrical halves.
The pleural ribs are segmentally disposed paired slender bony rods. The ribs are attached with the distal ends of the para­pophyses. There are seventeen pairs of ribs and the first pair are attached with the para­pophyses of the fifth trunk vertebrae. The ribs are present between the muscles and the peri­toneum and encircle the abdominal cavity.
Besides the ribs, there are series of rib-like Y- shaped inter-muscular bones which support the connective-tissue septa or myocommas. The inter-muscular bones originate from the neural arch of all the vertebrae composing the verte­bral column.
The appendicular skeleton includes the supporting structures of the paired fins and the corresponding girdles (Fig. 6.22).
The pectoral girdle is situ­ated immediately behind the last branchial arch. It consists of a reduced ‘primary’ endo-skeletal and well-formed secondary der­mal girdle. The primary girdle is composed of two lateral halves, each is transformed into three replacing bones, scapula, coracoid and mesocoracoid (Fig. 6.22A).
The two halves of the girdle do not meet in the mid-ventral line. The secondary dermal girdle consists of inves­ting bones. Each side of this girdle is made up of a cleithrum (or clavicle) a supracleithrum, a post-temporal and a post-cleithrum. The dermal girdle is attached with the pterotic process of the skull by the post-temporal.
The scapula is a ring-lime bony piece with a large scapular foramen for the impass of the branchial artery and the nerve. The coracoid is an irregular triangular bone lying internal to the scapula but ventral to the mesocoracoid. The mesocoracoid is an inverted Y-shaped bone.
The coracoid and the scapula parti­cipate in the formation of the glenoid articula­tion to which three of the four radials of the pectoral fins are movably attached. The cleithrum of the secondary pectoral girdle is the largest bone and it completely covers the primary pectoral girdle.
The posterior inner surface of the cleithrum is connected with a stout curved bone, the post-cleithrum. The supracleithrum is an elongated dagger-shaped bone which covers the dorsal end of the cleithrum. The supracleithrum is articulated at its dorsal end with a minute conical post- temporal bone which in turn remains attached with the supra-temporal bone.
The pectoral fin is supported by nineteen lepidotrichia attached with four radials. The radials are articulated directly with the scapula.
The pelvic girdle is located anterior to the anal fin. It consists of two simi­larly constructed halves. Each half is mostly composed of a large osseous pelvic bone with a small cartilaginous rod attached to the pos­terior end of the pelvic bone (Fig. 6.22B). The pelvic bone is distinguishable into an anterior elongated part and a posterior stout rod-like part.
The anterior part bears a ventral deep groove and its frontal end is forked. The forked end is connected with the ribs of the twelfth trunk vertebra by ligaments. The posterior rod like parts of the two halves of the pelvic girdle is united with the middle line.
Each pelvic fin is supported by nine fin-rays and three radials. The fin-rays are attached to the radials proximally and the radi­als are articulated with posterior border of the pelvic bone. The first two radials bear two fin-rays while the rest are borne by the third radial.
The coelom is lined with peritoneum and divisible into an anterior pericardial cavity containing the heart and a posterior perivis­ceral cavity accommodating the main viscera.
7. Digestive System of Rohu:
The digestive system is composed of an extremely long alimentary canal and associa­ted digestive glands (Fig. 6.23). The descrip­tion of this system was presented by D. S. Sarbahi in 1939. This particular fish is herbi­vorous in habit. The alimentary canal is divid­ed into mouth, buccal cavity, pharynx, oesophagus, intestinal bulb, intestine and rec­tum with its external opening, the anus.
The mouth is bounded by soft upper and lower lips. The free edges of the lips are broad and are beset with four or five rows of black­ish conical papillae. The buccal cavity is a short dorsoventrally compressed cavity with a flat floor and an arched roof. The mucous membrane lining the buccal cavity contains minute papillae.
There is no distinct tongue in Rohu, but the mucous membrane lining the floor of the buccal cavity is provided with highly developed thick muscles. The buccal cavity leads into a dorsoventrally flattened pharynx. The pharynx is bounded by gill- arches and is well-demarcated into an anteri­or respiratory part and a posterior narrow masticatory part.
The anterior portion is nar­rower and is perforated laterally by gill-slits. The posterior portion of the pharynx bears closely set pharyngeal teeth on its ventrolat­eral walls and the ventral wall is highly fold­ed transversely. The pharyngeal teeth help to crush solid foods.
These teeth are all alike (homodont) and are arranged in three rows, one alternating the other. Each tooth has a narrow basal root and a cylindrical projecting crown. The root remains embedded in the mucous membrane and the crown is laterally compressed.
The posterior portion of pharynx leads into a very short tube, the oesophagus. The mucous membrane of the oesophagus is thrown into a number of longitudinal folds. The ductus pneumaticus of the swim-bladder opens into the oesophagus.
True stomach is absent in Rohu. The anterior part of the intes­tine becomes swollen into a sac just behind the oesophagus. This sac is designated as intestinal swelling or intestinal bulb which stores food.
The gastric glands are absent in the intestinal bulb and it resembles histologi­cally the intestine. The mucous lining of the intestinal bulb contains absorptive and mucous cells. Absence of stomach in Rohu and related forms like Labeo gonius, Cirrhina mrigala, Catla catla and many other cyprinids is difficult to explain.
Lack of stomach is not actually related to feeding habit but may be a case of neoteny as suggested by Barrington (1957). The intestine is extremely elongated (measuring about 7.5 metres) and thin-walled. The intestine is more or less of uniform diam­eter and forms a number of coils.
Elongation of intestine and its extensive coiling are related to its herbivorous food habit. The intestine is lined by absorptive and mucus-secreting cells. The absorptive cells bear a free striated mar­gin. The muscular layer of the intestine is thin.
The mucous membrane of the intestine pre­sents different types of foldings. The anterior portion of intestine shows oblique transverse folds, while the posterior part of the intestine is characterised by having distinct longitudinal folds. The terminal part of the intestine is slightly dilated and forms a thin-walled sac called rectum.
Like that of the anterior part of the intestine the mucous lining of the rectum exhibits indistinct oblique transverse folds. The rectum opens to the exterior through the anus located just in front of the urinogenital opening. The pyloric caeca are lacking in Rohu.
The digestive glands comprise of liver and pancreas. The liver is a massive gland of dark- brown colour. The liver is divided into a narrow right lobe and a broader left lobe. The two lobes of the liver are connected at three regions, ante­riorly by a median lobe, medially by a median connective lobe and posteriorly by a median mass.
The gall-bladder is an elongated sac of about 8 cm in length and 2.5 cm in diameter. The gall-bladder is located between the right and left lobes of the liver. A cystic duct, after originating from the anteroventral end of the gall-bladder, receives three hepatic ducts. The pancreas remains diffused along the coils of the intestine. It extends into the liver.
The exocrine part of the pancreas may form acini. The cells of the acini are large in size. Each cell is colum­nar in nature with a prominent nucleus and divided into two portions—basal and apical. The basal part contains homogeneous cyto­plasm while the apical end possesses large zymogen granules. Presence of exocrine pan­creatic cells within spleen tissue in Rohu, Catla and Mrigal fishes is a peculiar feature.
Mechanism of digestion:
The mechanism of digestion is not clear. The absence of stomach is compensated by the production of pancreatic trypsin and erepsin as well as enterokinase from the intestinal mucosa. Amylase is produced from the pancreatic cells. Lipase and maltase are also reported to be present in the intestinal extracts while their place of secretion has not been recorded.
In this genus of fish which lacks stomach, pepsin and hydrochloric acid are absent. As this fish is herbivorous, the con­centration of carbohydrate-splitting enzymes is highest and the protein-splitting enzymes are lowest in concentration.
8. Hydrostatic Organ of Rohu:
The swim-bladder is a thin-walled perivis­ceral gas-filled sac. It lies in the cavity of the coelom and situated dorsal to the gut. The swim-bladder is divided into two, an anterior and a posterior chamber. The anterior cham­ber is connected to the oesophagus by a slen­der ductus pneumaticus. The swim-bladder acts as hydrostatic organ.
9. Respiratory System of Rohu:
There are four pairs of gills contained in the branchial chambers. Each branchial cham­ber is covered by operculum and the branchiostegal membrane which is attached to the posterior margin of the operculum. The wall of the pharynx is perforated by five gill slits on each side and is separated by four gill-arches or inter-branchial septa.
There are four pairs of gills and the gills are holobrach type. Each gill has a double row of gill-filaments (holobranch) and is supported by gill-arch with gill-rakers. The two rows of gill lamellae are separated by the inter-branchial septum which is short and compact.
Each gill arch bears one afferent and two efferent branchial vessels (Fig. 6.24). The pseudo branch of the hyoid arch consists of a comb-like body. Each pseudo branch is com­posed of a single row of giIl-gilaments on the inner surface of operculum.
Physiology of respiration:
Rohu utilizes the oxygen dissolved in water. The physical mecha­nism of respiration can be described under two sequences (Fig. 6.25).
During inspiration, the outer opening of the gill-chamber remains tightly closed to the body wall by the branchiostegal membrane and the two opercula bulge out to increase the accommodating capacity of the pharyngeal and buccal cavities. As a conse­quence, water from exterior rushes inside through the opened mouth and fill in the buc­copharyngeal cavity.
Immediately with the entry of water, the pharyngeal and the buccal cavities contract and exert pressure to the contained water. As the mouth, by this time, becomes closed by oral valves, the contained water finds the way out through the gill-slits.
The operculum as well as the branchiostegal mem­brane is lifted by this time and the water from the gill-chambers goes out through the open­ing of the gill-chamber. The dilatation and the contraction of the pharyngeal cavity are caused by the alternate retraction and protrac­tion of the hyoid arch supporting the buc­copharyngeal cavity.
Physiology of gaseous exchange:
The gills are highly vascular structures and are supplied by afferent and efferent branchial arteries. The afferent branchial artery carrying the deoxygenated blood is situated very superficially on the outer edge of the gill. The afferent brachial artery breaks up into capillaries in the sub­stance of the gill.
During the transit of water through the gill-slits, the deoxygenated blood in the capillaries of the gill-filaments takes oxy­gen dissolved in water and gives out carbon- dioxide by diffusion. The blood thus aerated, is collected by efferent branchial arteries and is conveyed to the different parts of the body.
10. Circulatory System of Rohu:
The circulatory system of Rohu is basical­ly built on teleostean plan.
The heart is composed of a sinus venosus, an auricle and a ventricle. The conus arteriosus is absent as such and is represented by a pair of valves. A non-contractile bulbus arteriosus is present. The sinus venosus is pro­portionately larger and bears a pair of lateral appendages (Fig. 6.26). It is spongy in nature.
Afferent branchial arteries:
There are four pairs of afferent branchial arteries, supplying deoxygenated blood to the corresponding gills. The ventral aorta divides anteriorly into the first two afferent branchial arteries. The second, third and fourth pairs of the afferent branchial arteries have separate and independent origin from the ventral aorta.
Efferent branchial arteries:
After oxygenation, the blood from the gills is collected by efferent branchial arteries. The venous system, exactly like that of Bhetki, includes the paired anterior and posterior car­dinals, unpaired hepatic and renal portal veins and paired subclavian veins. The renal portal vein is well-represented.
11. Venous System of Rohu:
The venous system of Rohu consists of the systemic veins and the portal veins. These veins directly or indirectly convey the deoxy­genated blood from the different parts of the body to the heart.
Systemic venous system:
The blood is carried to the sinus venosus by right and left ductus Cuvieri. Each ductus Cuvieri is formed by three principal veins: an anterior cardinal sinus, a jugular sinus and a posterior cardinal sinus.
The anterior cardinal sinus brings blood from the anterior part of the body (see Fig. 6.27) and the posterior cardinal sinus brings blood from the posterior part of the body. Both the posterior cardinal veins receive segmental veins, renal veins, genital veins, etc.
In addition to the above mentioned three principal veins, the pectoral and pelvic veins form the pectoral and pelvic fins respectively and the slender hepatic vein opens into the ductus Cuvieri.
The blood from the tail region is conveyed by a caudal vein which just entering into the trunk bifurcates into two branches. The right posterior cardinal sinus passes through the substance of the right kidney and opens into the right ductus Cuvieri. The left posterior cardinal vein originates from the capillaries of the renal portal vein.
The portal venous system is composed of a special vein which originates in capillaries and end in capillaries and secondly the blood from these veins before going to the heart passes through some inter­mediate organs. When the intermediate organ is the kidney, such a system constitutes the renal portal system and when the organ is liver, the system is called the hepatic portal system.
The left branch of the caudal vein after entering into the left kidney breaks up into capillaries and forms the renal portal vein. These capillaries reunite and form the left posterior cardinal vein.
Hepatic portal system:
The capillaries from the alimentary canal and its associated structures unite to form a hepatic portal vein which enters into the substance of liver and breaks up into the capillaries. The capillaries reunite to form the hepatic vein which opens to the ductus Cuvieri.
12. Nervous System of Rohu:
The brain of Rohu is typically built on the piscine plan. The cerebral hemispheres are small and undivided. The corpora striata are prominent, but the pallium is thin and non-nervous. The olfactory lobes are mode­rately developed.
The reduced diencephalon is provided with a dorsal pineal body and a ventral pituitary body. The optic lobes are large with two large lobi inferiores on the ventral side. The optic chiasma is absent and the optic nerves simply cross one another. The cerebellum is conspicuous and prolongs anteriorly to form the valvula cerebelli.
There are ten pairs of cranial nerves in Rohu. The origin and branching of the cranial nerves are, as in Bhetki, similarly disposed. The organs of special sense are well-developed and are built on typical teleostean plan. The sense of taste is highly developed. Numerous taste-buds are present in the lips, in the epithelium lining the first three gill-slits and on the barbels.
The tactile receptors are abundant all over the body specially on the lips and barbels. The organisation of lateral line system, eyes and ears is strikingly similar to those of other bony fishes already described. The ear is composed of utriculus, sacculus, lagena and three semicircular canals.
The otoliths are remarkably large in size and are three in number:
(a) Sagitta is present inside the sacculus,
(b) Asterisus fills the lagena and
(c) Lapillus is present in the utriculus.
13. Urinogenital System of Rohu:
The kidneys are extremely elongated bodies extending along the whole length of the visceral cavity. They are situated on the dorsal side of the body wall above the swim-bladder and are distinct anteriorly but become partly fused in the middle region.
The kidneys are of mesonephric (designated to be of opisthonephros type by Greham Kerr) type. The ureters, one from each kidney, open into a thin-walled urinary bladder situated ventral to the cloaca. The urinary bladder opens into the urinogenital sinus (Fig. 6.28).
During the breeding season, the pectoral fins become greater than or equal to anal fins in males, but in females, the pectoral fins are smaller than the anal fins . Choudhury (1959) has reported that the pectoral fins have a rough dorsal surface in males during breeding season but in females the surface of the pectoral fins is smooth.
Rohu attains maturity at the end of second year and is polygamous. The sex play lasts about 5-10 seconds.
The gonads become greatly enlarged dur­ing breeding season. In the males, the testes extend the whole length of the abdominal cav­ity. From the posterior end of each testis, a vas deferens arises which finally opens into the urinogenital sinus.
In the female, the ovaries are also paired structures which attain larger size than the testes. The oviducts are lacking. The eggs are released in the body cavity from where the eggs emerge out through a pair of genital pores formed temporarily from the anterior wall of the urinogenital sinus.
In Rohu, enormous number of eggs is laid at a time and the eggs sink to the bottom. Immediately after discharge, the eggs come in contact with the spermatic fluid (milt) and fer­tilization takes place externally. The mecha­nism of cleavage and subsequent develop­mental sequences are not known in Rohu.
It is expected that the development should be like that of other bony fishes. Labeo rohita is a fast- growing carp. The eggs hatch within 2-15 hours.
Freshly hatched youngs (hatchlings) have prominent yolk sac attached to the ven­tral side of the body. Absorption of the yolk sac requires 5-7 days and the young ones (called fry) begin to feed.
The fry attains a size from about 2 mm to 3 mm and is characterised by having fringed lips and a prominent vertical dark spot at the base of the tail which dis­appears in course of growth. When the fry becomes 5 mm long, it is designated as the fingerling which also varies from 5 mm – 15 mm in length. Attainment of sexual maturity requires about two years.
9.5 How to plan a small hatchery
- a sheltered area , small shed or building, with sufficient space to accommodate your hatching and early rearing units
- sufficient space , usually in outside facilities such as tanks, ponds and/or cages or enclosures for broodstock and potential broodstock
- if required, suitable outside facilities , such as tanks, cloth incubators, small ponds, etc. for rearing fry and fingerlings
- a good water supply , sufficient for the routine requirements of the hatchery and the outside facilities, plus any additional amounts for cleaning, major water replacement, etc.
- layout, site access, and equipment suitable for quick and efficient handling and transfer of broodstock, eggs, fry, fingerlings, etc.
- good security and sufficient storage for equipment, materials, etc.
2. Before deciding on the full details of the design and layout of your hatchery, there are several points you should consider further.
(a) Examine the site or possible sites available. Check on the area available and its overall topography (see Topography ), the potential water supplies (see Water 4 ) and consider the possible construction features you might need (see Construction, 20 ).
- hatchery production is for one or several species
- the species are single or multiple-spawning
- production is at a specific time of the year, or throughout a particular season, or throughout the year
- production is dependent on outside broodstock, or on stock grown, held and/or conditioned at the hatchery
- you need to produce early fry, advanced fry, or fingerlings
- you are supplying for your own needs or for other farmers, or for both, and what quantities are involved, i.e. define your target productions
- you will do the work yourself or be able to get assistance during important work periods.
- the various quantities of stock
- the type, size and number of equipment items
- the type and size of water supply needed.
- Stock and production plans are given in Table 25b for three different species and selected target productions of early fry.
(d) Consider this stock and production plan further, using information on broodstock availability, spawning time, hatching time, and first feeding and early rearing period, to make an outline schedule of operation (see two examples below). Each sequence of spawning, hatching, fry rearing, etc. can be defined as a batch or cycle.
(e) Look at the timing of such a batch or cycle, and consider whether you will operate with a single batch each season or year, or with several batches (possibly of a different species). Thus once a tank or incubator has finished one batch, it may be cleaned out and used for the next one. In this way, you will get more production from the facilities you have set up. Prepare your final schedule of operation .
(f) Check approximately to make sure you will have enough space and water supply for the intended production. The chart below can be used for guidance. Modify your plans if necessary, either by changing the target production or by changing the number of cycles. Thus you may get the intended production with smaller facilites but a greater number of cycles.
Planning the hatchery layout
3. Once you have estimated the number and size of hatchery units involved, you can plan the position and layout of the hatchery. To do so you should consider the following.
(a) You can also use the chart on this page to estimate the overall internal areas required for holding and spawning tanks, hatching equipment, water supplies, access space, storage and work areas and possibly office space. This will be the main hatchery unit . In most cases it is housed in a single building, although in larger and more complex systems several buildings may be used, such as a broodstock unit, a hatching unit and a service/storage unit.
Note: space required includes piping, stands, etc. remember to allow for access - for hatchery staff, nets, bins, etc., as well as storage space and access to pipe valves for maintenance.
(b) Estimate the external areas required for broodstock ponds, conditioning or spawning facilities, early rearing facilities and access roads. Identify any areas that should be near the main hatchery unit, and which may possibly be partially sheltered and/or enclosed.
(c) Review the site(s) available, and identify a suitable and convenient position , sufficient to accommodate the areas required but reasonably compact in layout, and permitting the easy arrangement of water supplies, access and security. Identify the location of the hatchery building itself.
- water supply and drainage should be simple and easily managed
- there should be good access and maintenance space (particularly for water controls, stock handling and movement)
- layout should be neat and uncluttered, easy to keep clean and in good condition, and should allow stock to be observed without undue disturbance
- there should be suitable and convenient storage space
- there should be suitable space for hatchery operations , for example for examining eggs or larvae or dissecting broodstock
- construction should be simple, and should allow reasonable opportunities for modification and/or expansion later.
Note: see below for a plan and a cross-section of a simple carp hatchery of 30 m 2 .
Typical hatchery dimensions for the production of common carp early fry (see previous example in table 25b) or tilapia fingerlings are summarized in the following chart.
The hatchery water supply
4. The arrangement of the hatchery water supply is particularly important for its successful operation. Some points to consider are listed here.
(a) You may use gravity-fed river or stream supplies, pumped water from rivers, ponds, lakes, etc. or groundwater sources.
(b) Make sure there is sufficient water of the quality you require during the periods of hatchery use. Check water quality (Chapter 2).
(c) It may be necessary to clean the water supply with screens and/or fifters (see Section 2.9). For ground/well water supplies, you may need to aerate the water to make sure it has sufficient oxygen (see Section 2.7).
(d) If water supplies are only available at certain times, you may need to provide storage . Check the daily use of water by the hatchery and allow for the number of hours or days of storage required. Concrete tanks or earthen ponds can be used (see Chapter 4, Water, 4 ). If it is found on land higher than the hatchery, the water could be supplied by gravity otherwise it could be pumped.
A hatchery uses 10 I/min of water for hatching, 10 m 3 /day for exchanging water in broodstock tanks and 5 m 3 /day for washing, cleaning, etc. The total daily water use is: (10 I/min x 60 min x 24 h/1 000) + 10 m 3 /day + 5 m 3 /day = 14.4 m 3 /day + 10 m 3 /day + 5 m 3 /day = 29.4 m 3 /day. If storage for 10 days' operation is required, this will be equivalent to 29.4 x 10 = 294 m 3 this amount could for example be provided with a pond of approximately 300 m 2 x 1 m average water depth.
(e) Water quality requirements may be different for broodstock, their final ripening and fry rearing. You should normally supply the best quality water to the ripening tanks and egg hatchery areas. However, you may be able to save on water treatment, or use a different water supply for the other parts of the system. If necessary, you may also be able to reuse hatchery water in fry or broodstock ponds.
(f) You may need another small domestic water supply for washing, cleaning, etc. Wastes from this should not drain into ponds, as they may contain detergents, chemicals, etc.
5. Water supplies and drainage for the external ponds can be arranged as for normal farm ponds, using canals, pipes, and sluices (see Chapter 8, Construction, 20 ).For external tanks and internal areas of the hatchery, a piped water system is usually used. Its main features are as follows.
(a) Water is fed by gravity or pumped either directly to the main supply pipe, or more commonly, to a header tank , which provides short term storage and regulates flow to the other tanks. It is usually large enough for at least 10 minutes of flow . You need a 1 m 3 tank for a flow of 100 I/min. In some cases a storage tank can be used as a header tank (see hatchery design above).
(b) The header tank is usually set with its base at least 1 m above the hatchery tanks . It is typically 0.5 to 1 m deep it may be set up on a wall, on a timber stand, or on the roof of the hatchery. The tank can be made of concrete, timber (with a polythene or butyl liner), fibreglass or plastic. In some cases, domestic water supply tanks can conveniently be used.
(c) The main supply pipe normally runs from the header tank to the hatchery tanks. From this are run the secondary supply pipes , serving small groups of tanks, and individual supply pipes to the tanks themselves. These pipes are usually made of PVC or ABS. They are sized according to the flow rate required and the head available from the header tank (see Section 3.8, Construction, 20 ). Table 26 shows some typical dimensions which you may use for guidance.
Setting up the hatchery
6. Once you have reviewed the layout and water supply details, you can proceed to the construction and development of the hatchery. Some points to be considered are listed here.
(a) Select a suitable type of building for the hatchery itself. This could be a simple shade structure, a prefabricated building, a conventional local structure or a heavier brick/concrete building. Make sure foundations are suitable.
(b) Check on the overall costs of the site development (for example see Section 12.8, Construction, 20 ), external areas, hatchery building, hatching facilities and water supplies. Amend quantities and/or specifications if costs do not match your budget. Check that costs per fry or fingerling produced are reasonable by local standards.
(c) When you decide to go ahead, prepare for construction . Proceed according to the guidance given in Chapter 12, Construction, 20 .
(d) Remember to plan the timing of construction to allow for factors such as availability of local labour, wet or dry seasons, the supply of broodstock and the timing of the breeding season.
(e) Finish off the hatchery area with suitable access roads, security fences, drainage, and other services if required.
(f) Drainage from the hatching units and tanks is normally either by channel, usually brick or concrete, or by drainage pipe, set in or on the hatchery floor see Section 3.8 and Section 8.2, Construction, 20 , for details on sizing. Remember to allow for tanks or incubators being emptied. Make sure the drainage areas can be easily cleaned and disinfected.
Let’s look a little closer at sharks parts, habits, and biology:
How do sharks maintain neutral buoyancy?
Neutral buoyancy means being as heavy or dense as the fluid around you so that you don’t sink down or float up.
Sharks have several adaptations that can help them be neutrally buoyant. Sharks lack true bone but instead have cartilaginous skeletons that are much lighter. Sharks also have large livers full of low-density oils, which provide some buoyancy.
While sharks lack a swim bladder that many bony fish have, some species of shark, like the sand tiger (Carcharias taurus), can actually gulp air into their stomach, which can provide additional buoyancy.
How can you tell a male from a female shark?
Male sharks have paired intromittent organs called claspers. Claspers are modifications of the pelvic fins and are located on the inner margin of the pelvic fins. Females do not have claspers.
How many kinds of fins do sharks have?
Sharks have five different types of fins: pectoral, pelvic, dorsal, anal, and caudal. These fins are rigid and supported by cartilaginous rods.
The paired pectoral fins are located ventrally near the anterior (front) end of the shark. They are used primarily for lift as the shark swims.
The paired pelvic fins located behind the pectoral fins are used for stabilization while the shark swims.
The dorsal fin is the one that commonly appears skimming along the water’s surface. Sharks may have one or two dorsal fins that act to stabilize the shark during swimming. The second dorsal fin is usually smaller than the first dorsal fin and is located posteriorly (toward the tail) to the first larger dorsal.
Stability is the main function of the anal fin for sharks that have one, other sharks may lack this fin. It is located on the ventral (bottom) side between the pelvic and caudal fins.
The caudal fin is also called the tail fin. The upper half and the lower half of the shark’ tail are not equal in size with the upper portion usually the larger. This is especially pronounced in the thresher shark that has an upper tail lobe longer than the shark’s body. This fin is responsible for propelling the shark through the water as it swims.
Do sharks lay eggs or give live birth?
Sharks exhibit a great diversity in reproductive modes.
Egg cases from nurse shark. Photo © George Burgess
There are oviparous (egg-laying) species and viviparous (live-bearing) species. Oviparous species lay eggs that develop and hatch outside the mother’s body with no parental care after the eggs are laid. The embryos are nourished by a yolk-sac inside the egg capsule.
Viviparous species can be separated into two categories: placental (having a placenta, or true connection between maternal and embryonic tissue), or aplacental (lacking a placenta). Among the aplacental species, there are those whose embryos rely primarily on a yolk-sac for nutrition during gestation and those that consume yolk-filled, unfertilized egg capsules (oophagy).
There is even one species, the sandtiger (Carcharias taurus), in which the two largest embryos that were fertilized first, consume the other embryos of the litter (adelphophagy). It has also been suggested that the young of some viviparous species may be nourished by uterine secretions during some part of gestation, but this has not been conclusively documented. There is no parental care after birth among viviparous sharks.
*Aplacental viviparity can also be described as ovoviviparity, although that term is rarely used today.
What is a mermaid's purse?
This name is given to the egg cases of many sharks and skates. This tough, protective purse-shaped egg case contains one fertilized egg. A young shark or skate later emerges from the mermaid’s purse. Shark species that utilize this mode of reproduction include the swell shark, dogfish, and angel sharks.
How do sharks reproduce?
All sharks have internal fertilization. Mating has been observed in relatively few species of sharks, but both hormonal and behavioral cues are likely involved. The female is typically passive as the male bites and grasps her with his teeth to hold on during copulation. Then, the male inserts a clasper into the female’s cloaca and releases sperm. Depending on species, sperm may or may not be stored in the female prior to fertilization of the oocyte, or ovum.
During ovulation, the female releases oocytes from the ovary. Then, these oocytes are fertilized by sperm, and the fertilized ova are encapsulated in an egg case in a specialized organ called the nidamental or shell gland. In all oviparous species and most viviparous species, a yolk sac is packaged in the egg case along with the ovum.
Development of the embryo then proceeds according to the mode of reproduction and embryonic nutrition of the particular species (see question above). In oviparous species, eggs are laid. In viviparous species, gestation takes place in the uterus. Sharks are hatched or born as juveniles, or smaller versions of the adult. There is no larval stage.
What is the correct term for a baby shark?
A baby shark is referred to as a pup.
Can sharks live in freshwater?
Most sharks live only in the marine environment in full-strength saltwater. Some coastal shark species can survive in brackish estuaries with mixed fresh- and saltwater. Many juvenile sharks use these brackish areas as nursery grounds.
There are only a couple shark species that are capable of surviving in freshwater for any length of time, and these have special physiological adaptations that allow this. These species are the bull shark (Carcharhinus leucas) and the speartooth shark (Glyphis sp.).
Bull sharks have been captured
2,100 miles (3,480 km) up the Amazon River, and
1,700 miles (2,800 km) up the Mississippi River. Bull sharks have also been documented to traverse
108 miles (175 km) of rapids in the Rio San Juan leading up to Lake Nicaragua from the Caribbean Sea.
The speartooth shark has been captured over
60 miles (100 km) up the Adelaide River in Australia. Though these sharks are capable of surviving in freshwater, there are no populations living in completely landlocked freshwater lakes. They always have a route that will connect them to the ocean.
Do sharks have tongues?
Sharks have a tongue referred to as a basihyal. The basihyal is a small, thick piece of cartilage located on the floor of the mouth of sharks and other fishes.
It appears to be useless for most sharks with the exception of the cookiecutter shark. The cookiecutter shark uses the basihyal to rip chunks of flesh out of their prey.
Taste is sensed by taste buds located on the papillae lining the mouth and throat of the shark. The taste receptors help the shark decide if the prey item is suitable or not prior to ingesting the item.
Are sharks warm or cold blooded?
Most sharks, like most fishes, are cold blooded, or ectothermic. Their body temperatures match the temperature of the water around them.
There are however 5 species of sharks that have some warm blooded, or endothermic capabilities. The family lamnidae, known as mackerel sharks, includes the white (Carcharodon carcharias), shortfin mako (Isurus oxyrinchus), longfin mako (Isurus paucus), porbeagle (Lamna nasus), and salmon (Lamna ditropis). This family has the unique ability to elevate their internal body temperatures above that of their surrounding environment by the use of a highly-developed network of blood vessels that retain the heat produced by their muscles.
The white shark is able to maintain its stomach temperature as much as 57ºF (14ºC) warmer than the ambient water temperature.
The salmon shark is possibly the most warm blooded shark, and maintains its body temperature at around 77ºF (25ºC). This is 70ºF (21ºC) warmer than the sub-arctic waters where it lives.
What is a nictitating membrane?
The nictitating membrane is a thin, tough membrane or inner eyelid in the eye of many species of sharks. This membrane covers the eye to protect it from damage, especially just prior to a feeding event where the prey may inflict damage while trying to protect itself.
How do sharks detect prey?
Sharks have many keen senses that are mostly geared towards helping them locate prey. Depending on the species or the environment certain senses are more or less important to them for locating their targeted prey, which is most often fish. Sharks use the senses of smell (chemoreception), vision, hearing, the lateral line system, and electroreception (ampullae of Lorenzini) for capturing prey.
The lateral line system, which all fishes possess, allows them to detect waves of pressure or mechanical disturbances in the water.
The ampullae of Lorenzini are receptors that can detect weak electric fields. This sense is unique to sharks and their relatives. Sharks primarily use this sense to locate cryptic prey which can not be detected by their other senses, such as stingrays buried in sand. The stingray, like all living animals, emit weak electric fields produced by muscular contractions in the body. Sharks have the extra predatory advantage of being able to detect those fields at close range. More information on electric organs of elasmobranchs and other fish adaptations.
Hunting habits of three species are summarized below: the white (Carcharodon carcharias), tiger (Galeocerdo cuvier), and bull (Carcharhinus leucas) sharks. These species are also responsible for the majority of attacks on humans.
The white shark, when targeting seals in coastal areas, is thought to be a primarily visual ambush predator. It cruises on or near the ocean bottom looking up to the surface for basking or swimming seals. When a seal is spotted it will make a high speed rush and attempt to mortally wound it in the first strike. It is believed that most white shark attacks on surfers are the result of the shark mistaking the surfboard for a seal. In most cases, once the shark gets a mouthful of fiberglass or neoprene instead of a fatty seal, it will tend to leave the scene. This initial strike can however leave a victim with serious wounds.
Tiger sharks are usually nomadic in their movements, and therefore use their “long-range” senses, like smell and hearing, for helping to key in on prey. They can detect the scent of dead fish, birds, or turtles from very long distances, and follow the odor corridor to its source. Once close enough, vision becomes the more dominant sense leading up to the consumption of the prey. Of course, chance visual encounters with live prey often occur as well.
Bull sharks tend to occur in shallow coastal waters where visibility is often poor. They have smaller eyes than other closely-related sharks, and it is therefore believed that bull sharks do not rely on vision as much as some of their other senses. When relying more on the sense of hearing, smell, or their lateral line, they can more easily mistake human activity in the water as that of their prey which is mostly comprised of schooling fishes.
Why are sharks called apex predators?
Apex predators are at the top of the food chain and have few or no natural predators. Sharks fit this category, feeding on fish, seals, and large invertebrates and having few predators.
Apex predators keep populations of prey animals in check, and are thereby important in maintaining the ecological balance of its environment.
What is the lateral line and its purpose?
The lateral line is comprised of a series of tubes located just below the surface of the skin, running lengthwise on both sides of the shark’s body, from the head all the way to the tail. The pores are open to the outside where water flows through, into the tubes below the skin. These tubes are lined with hair-like projections that are connected to sensory cells.
When moving water, sound, vibration, and pressure changes stimulates these sensory cells, the shark is alerted to potential prey in nearby waters.
Why do basking sharks swim with their mouths open?
The basking shark is usually seen swimming with its mouth wide open, taking in a continuous flow of water. Food is strained from the water by gill rakers located in the gill slits. The 1000-1300 gill rakers in the basking shark’s mouth can strain up to 2000 tons of water per hour.
Occasionally the basking shark closes its mouth to swallow its prey. These sharks feed along areas that contain high densities of large zooplankton (i.e., small crustaceans, invertebrate larvae, and fish eggs and larvae).
There is a theory that the basking shark feeds on the surface when plankton is abundant, then sheds its gill rakers and hibernates in deeper water during winter. Alternatively, it has been suggested that the basking shark turns to benthic (near bottom) feeding when it loses its gill rakers. It is not known how often it sheds these gill rakers or how rapidly they are replaced.
Why do hammerhead sharks have broad heads?
Hammerheads get their common names from the large hammer-shaped head. This compressed head, also referred to as a cephalophoil, allows for easy distinction from other types of sharks.
The cephalophoil is broad and flattened, with eyes located on the outer edges of the cephalophoil, and nostrils also spread far apart. It is thought that the head structure may give the shark some sensory advantages. The broad head may be adapted to maximize lateral search area. With an increased distance between the nostrils, hammerheads may be able to better track scent trails.
Along with the pectoral fins, the cephalophoil may provide additional lift and maneuverability as the shark moves through the water. Hammerheads have larger musculature in the head region than other Carcharhiniform sharks and have a wider range of head movement. This allows them increased hydrodynamics and to maneuver quickly at high speeds.
What is a cookiecutter shark?
The cookiecutter shark (Isistius brasiliensis) lives in tropical and warm temperate seas throughout the world. It is a small shark, reaching sizes of about 20 inches (50 cm).
This shark’s name comes from the cookiecutter-shaped bites it takes out its victims which includes large fish and whales. A vacuum is created when the cookiecutter shark’s lips attach to the victim. It then takes out an oval-shaped bite of flesh by using its saw-like teeth, leaving behind a cookiecutter-shaped wound.
Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization. In oviparity , fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. Chicken eggs are an example of this second type.
In ovoviparity , fertilized eggs are retained in the female, but the embryo obtains its nourishment from the egg’s yolk and the young are fully developed when they are hatched. This occurs in some bony fish (like the guppy Lebistes reticulatus), some sharks, some lizards, some snakes (such as the garter snake Thamnophis sirtalis), some vipers, and some invertebrate animals (like the Madagascar hissing cockroach Gromphadorhina portentosa).
In viviparity the young develop within the female, receiving nourishment from the mother’s blood through a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some cartilaginous fish, and a few reptiles.
Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is isolated within the female, which limits predation on the young. Internal fertilization enhances the fertilization of eggs by a specific male. Fewer offspring are produced through this method, but their survival rate is higher than that for external fertilization.
Hox Genes, Differential Gene Expression, and Segment Identity
Organogenesis (and development in general!) is characterized by changes in which specific genes are expressed in different cells. This differential gene expression, or turning on and turning off different genes, is what determines a specific cell’s form and function. It is also the and is the process underlying differentiation (for more on this topic, see the Gene Regulation page on the Bio1510 website). For example, during differentiation, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal cells. Other ectoderm cells will move into the interior of the embryo to form the central nervous system, and will express genes specific to the nervous system. The process of differentiation is largely regulated by induction, or cell-cell communication during development. How do induction and differential gene regulation work together to induce development of specific organs and body structure?
The information below was adapted from OpenStax Biology 27.1 and Khan Academy “Homeotic Genes.” All Khan Academy content is available for free at www.khanacademy.org
Changes in gene regulation during development are carefully regulated in both time and space. As we previously discussed, the eggs of protostomes and some deuterostomes contain cytoplasmic determinants, which cause cells of the developing embryo to have different identities as early as the first cell division (during cleavage). Cytoplasmic determinants are often regulatory genes that direct the expression of other genes, thus initiating a developmental “cascade” of changes in gene expression that ultimately lead to proper development of the animal. Each regulatory gene activates a new set of regulatory genes in the next set of cell divisions as the embryo progresses through development, as shown below.
In Drosophila, a developmental regulatory cascade beginning with maternal effects genes ultimately results in activation of Hox genes which dictate a segment’s ultimate identity. Image credit: Adapted from Khan Academy https://www.khanacademy.org/science/biology/ap-biology/developmental-biology/signaling-and-transcription-factors-in-development/a/homeotic-genes originally modified from Figure 6. Module Predictions within the Segmentation Gene Network by Mark D. Schroeder et al. CC BY 4.0
A key set of genes involved in differential gene expression and morphogenesis in animals are the homeobox or Hox genes. This family of genes is responsible for determining the general body plan, such as the number of body segments of an animal, the number and placement of appendages, and animal head-tail directionality. All animal phyla except sponges have a set of Hox genes. Each body segment is “specified” by a specific combination of Hox genes. In other words, Hox genes determine “segment identity,” or where along the body different body parts develop:
Expression of different Hox genes results in different “segment identity.” The break mark (//) in the chromosome indicates that these two clusters of genes are separated by a long intervening region that’s not shown. Image credit: Khan Academy https://www.khanacademy.org/science/biology/ap-biology/developmental-biology/signaling-and-transcription-factors-in-development/a/homeotic-genes originally modified from Hox genes of fruit fly, by PhiLiP, public domain
A single Hox mutation in the fruit fly can result in an extra pair of wings or even appendages growing from the wrong body part, as shown below.
A single mutation in a Hox gene results in development of legs instead of antennae on the fly’s head. Image credit: Khan Academy https://www.khanacademy.org/science/biology/ap-biology/developmental-biology/signaling-and-transcription-factors-in-development/a/homeotic-genes originally modified from Antennapedia mutation by toony, CC BY-SA 3.0. The modified image is licensed under a CC BY-SA 3.0 license
While there are a great many genes that play roles in the morphological development of an animal, what makes Hox genes so powerful is that they serve as master control genes that can turn on or off large numbers of other genes. Hox genes do this by coding transcription factors that control the expression of numerous other genes. Hox genes are homologous in the animal kingdom, that is, the genetic sequences of Hox genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor, from worms to flies, mice, and humans (Figure). Hox genes have undergone at least two duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve.
Hox genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters: Hox-A, Hox-B, Hox-C, and Hox-D. Genes within these clusters are expressed in certain body segments at certain stages of development. Shown here is the homology between Hox genes in mice and humans. Note how Hox gene expression, as indicated with orange, pink, blue and green shading, occurs in the same body segments in both the mouse and the human. Image credit: modified from Features of the animal kingdom: Figure 4 by OpenStax College, Biology, CC BY 4.0, with edits based on Lappin et al.
If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?
This video describes developmental regulatory genes in general and focuses on the importance of Hox genes in particular: