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Reproduction Reproduction in all Chondrichthyes is internal and the male uses modified pelvic fins called claspers to insert sperm. The presence or absence of claspers makes it easy to distinguish male from females. Great white shark claspers Reproduction During copulation a clasper is inserted into the female’s cloaca and hooked in place by spines at the tip. Sperm is ejaculated into a groove in the clasper and a muscular siphon sac filled with seawater is squeezed which washes the sperm down the groove into the cloaca from where the sperm swim up the female’s reproductive tract. Reproduction The sharks use of internal fertilization is coupled with their use of a reproductive strategy in which a few young are invested in heavily. The energy investment is provided by the female who retains and nourishes a small number of offspring within her body. Reproduction Energy is provided either in the form of egg yolk or is delivered to the developing babies via the mother’s reproductive tract. The mode of nutrition depends on whether reproduction is oviparous or viviparous. Reproduction All skates and some sharks are oviparous and lay eggs soon after fertilization. The eggs hatch later. Most oviparous sharks produce large eggs with big yolks and a proteinaceous case is secreted around the fertilized egg. Reproduction Protuberances on the case entangle in vegetation or the substrate and hold it in place. Development takes 6-10 months within the case. Movements of the embryo bring in oxygen and flush out wastes. Developing skate in its egg case http://www.youtube.com/watch?v=ZEpOmj Lb2e8 Reproduction Other sharks are ovoviviparous. The eggs develop within the mothers body and hatch either in her or just after being released from her. Egg case of cat shark Embryo of deep sea cat shark. There is a very large yolk sac to support the embryo’s growth. Reproduction The remaining species of shark are viviparous and the offspring are nourished by a placenta, unfertilized eggs or smaller siblings. These forms of food supply are collectively referred to as matrotrophy. Placental feeding of young Some sharks develop long stringy extensions of the oviduct. These secrete a milky substance into the mouths and gill openings of the young. The commonest form of viviparity in sharks uses a yolk sac placenta which allows the developing baby to obtain nutrition from its mothers blood stream. In great white sharks and sand tiger sharks the young feed on extra eggs ovulated by the mother and also on their siblings. Sand tiger shark eats siblings in utero http://www.youtube.com/watch?v=ZrqgPjZ 07Ts&feature=related Life history strategy of sharks Sharks because they invest heavily in individual offspring produce relatively few young. This reproductive strategy is similar to that of humans and elephants. Life history strategy of sharks Humans, elephants and sharks all have a high expectation of survival and they have what is called a type I survivorship curve. Life history strategy of sharks Survivorship curves can be classified into three general types Type I, Type II, and Type III Figure 52.5 Number of survivors (log scale) 1,000 I 100 II 10 III 1 0 100 50 Percentage of maximum life span Type I curve Type I curve typical of animals that produce few young but care for them well (e.g. humans, elephants). Death rate low until late in life where rate increases sharply as a result of old age (wear and tear, accumulation of cellular damage, cancer). Type II curve Type II curve has fairly steady death rate throughout life (e.g. rodents). Death is usually a result of chance processes over which the organism has little control (e.g. predation) Type III curve Type III curve typical of species that produce large numbers of young which receive little or no care (e.g. Oyster). Survival of young is dependent on luck. Larvae released into sea have only a small chance of settling on a suitable substrate. Once settled however, prospects of survival are much better and a long life is possible. Life history strategy of sharks Because sharks are slow breeders their populations are very vulnerable to an increase in adult mortality and/or a reduction in survival of offspring. In recent years fishing has drastically increased adult mortality and caused many shark populations to decline sharply. Fishing and sharks Historical records by early explorers, merchants and others often mention the number and large size of the sharks that trailed their ships. Sharks were competitors for the schools of herring, mackerel, capelin and other commercial that humans hunted, but sharks themselves were not fished for. In the early 20th century the seas of the world still teemed with sharks, but that has changed dramatically. Fishing and sharks In the 1950’s long-line fisheries for tuna, swordfish, marlin and other prized species treated sharks as a nuisance by-catch and many were cut free. Today the growing wealth of Asian countries, where shark fins are a delicacy, has made them a valuable catch. http://advocacy.britannica.com/blog/advocacy/wp-content/uploads/shark-fin.jpg Shark Fins http://www.lessfeelsbetter.net/upload/1215781554SHARKFI8crop.jpg Fishing and sharks Dried sharkfin can cost more than $500 a kilo and sharkfin soup up to $90 a bowl. Longliners can set lines as much as 100km long that contain 30,000 baited hooks. In 1997 Hawaiian longliners caught more than 100,000 sharks and tossed almost 99% of the body mass back. Why? They just kept the fins. http://mythix.com/images/ projects/ Shark_Finning_ -_dead_shark_in_ocean.jpg http://www.scubadiving.com/upload/images/Travel/20070326_sharkfinning_head.jpg Fishing and sharks As worldwide fish stocks have declined, and often collapsed, less desirable species including sharks have been targeted by commercial fishing fleets. As cod stocks collapsed, species such as spiny dogfish (marketed as “rock cod”) began to be served as a replacement in fish and chips. Fishing and sharks The Norwegian fishing fleet targeted sharks of the genus Lamna (porbeagles or salmon sharks) for intensive fishing to sell as steaks as a substitute for swordfish. Initial harvests were as much as 8060 tons in a year from the northeast Atlantic. Within seven years the catch collapsed to 207 tons and hasn’t been over 100 tons since the 1970’s. Porbeagle http://dsc.discovery.com/sharks/shark-types/porbeagle-shark.jpg Fishing and sharks Data from analyses of catch records worldwide show similar massive declines worldwide (see Callum Roberts’ “The Unnatural History of the Sea” for sources). More than 90% of sharks have been taken from massive areas of the world’s oceans. Sharks caught on longlines. http://onfinite.com/libraries/1353690/324.jpg Fishing and sharks Some species populations have been devastated. Once the oceanic whitetip was probably the commonest large animal in the world. Today it’s numbers have declined 150-fold in the Gulf of Mexico and probably by the same amount elsewhere. Oceanic whitetip (Red Sea) http://www.flmnh.ufl.edu/fish/gallery/ Fishing and sharks Along with declines in numbers another pattern common to other fisheries has emerged, the sizes of the animals caught has fallen. Between the 1950’s and 1990’s the size of individuals caught fell in a variety of species. Fishing and sharks Percentage decline in size of individuals caught Oceanic whitetips (-33%) Mako (-50%) Blue (-50%) Dusky (-60%) Silky (-83%) Fishing and sharks This decline is because fishing often preferentially removes older animals, and even if it doesn’t, fishing pressure is so intense that animals don’t live long enough to grow large. Fishing and sharks The simple truth is that shark populations cannot be intensively harvested sustainably. They are long-lived, slow maturing and slow reproducing. Fishing and sharks For example, female spiny dogfish do not mature until about 10-12 years of age and produce only 2-14 pups biennially. They can live 40-50 years, but not with fishing pressure. Other sharks have similar reproductive profiles. Fishing and sharks Even with a total ban on fishing, overfished shark populations will take many, many years to recover. Skates and rays More than half of all elasmobranchs are skates and rays. More species (about 534-631 depending on who’s classifying them) than there are sharks. As is the case for sharks, skates and rays have a cartilaginous skeleton and an enlarged oilfilled liver that reduces their buoyancy. They also possess the same electro sensors sharks do (Ampullae of Lorenzini) Skates and rays Skates and rays have characteristically dorsoventrally flattened bodies greatly enlarged pectoral fins, which attach to the side of the head. gill slits placed ventrally and eyes dorsally placed. http://www.flmnh.ufl.edu/fish/education/questions/rayskatesawfish.jpg Skates and rays There are also dorsoventrally flattened sharks e.g. the wobbegong and angel sharks. They have large wing-like pectoral fins but these are not attached to the head. Spotted wobbegong http://www.biolib.cz/IMG/GAL/122294.jpg Skates and rays Most skates and rays depend on undulating their pectoral fins to swim but some such as the torpedo rays use their caudal (tail) fin for movement. Atlantic torpedo ray http://www.junglewalk.com/shop/Products/Atlantic-Torpedo-Ray-Magnet-4623.htm Skates and rays Skates and rays should not be confused with flatfishes (e.g. sole and halibut), which are bony fishes. In flatfish the body is twisted during development to bring both eyes and gills to the dorsal surface, but not symmetrically. Flatfish (flounder a bony fish note the asymmetrical head) http://biochemicalsoul.com/2009/02/adaptation-of-the-week-flatfish-recapitulation/ Skates and rays The group is specialized for bottom dwelling and feeding on hard foods (e.g. molluscs and crustaceans) that have to be ground up. Teeth are flat crowned plates that form an arrangement like paving stones. Stingray teeth http://www.flmnh.ufl.edu/fish/Gallery/ Descript/atlanticstingray/atlanticstingray.html http://misc.thefullwiki.org/Ray_and_Skate The mouth is located underneath the body and can be rapidly protruded to suck up prey. Differences between skates and rays Skates usually have an elongated but thick tail stalk, which has two dorsal fins and a caudal fin at the end. Generally skates also have a rostrum a pointed nose-like extension of the braincase. http://img.21food.com/20110609/product/1 306507703542.jpg Skates are oviparous. Skate egg case http://people.whitman.edu/~yancey/skateEggCase.JPG Differences between skates and rays Rays typically have a whip-like tail and the tail fins are replaced by serrated venomcontaining barbs. Rays are viviparous and most lack a rostrum http://www.marinebiodiversity.ca/skatesandrays/Classification%20Overall.htm Skates and rays The spiracles (openings behind the eye) are much larger in rays than in sharks because water for the gills enters exclusively through them because the mouth is usually buried in the sand. http://media.photobucket.com/image/spirac les%20in%20rays/nairboarding/sp.jpg Skates and rays Skates and rays are usually well camouflaged and sit on the bottom to hide from predators. A few species are dangerous because of their sharp and barbed tail (stingrays) or because they can generate severe electric shocks (electric rays). Most species are bottom feeders that eat invertebrates. However, the largest species (e.g. manta rays), like whale sharks and basking sharks, are planktivores. Blue spotted ray Manta Ray Stingray A stingray’s tail possesses a sharp pointed, barbed spine halfway along the length of the tail that it uses for defense. Many species of stingray have a venom gland associated with the spine and the venom can be fatal to humans. Stingray spines have been used as spear points by many cultures. Stingray barb. http://elasmodiver.com/Sharkive%20images/Stingray-Barb-004.jpg http://www.flmnh.ufl.edu/fish/Gallery/Descript /atlanticstingray/atlanticstingray.html Electric rays The up to 40 species of electric rays belong to the order Torpediniformes. They can produce an electrical discharge that is used for hunting and protection. The discharge can range from 8 to 220 volts and be up to 30 amperes (it’s the amperage that’s dangerous. Amperage is a measure of the amount of electricity flowing). Marbled torpedo ray http://elasmodiver.com/MarbledTorpedoRay.htm Electric rays The electric organs are modified from striated muscle fibers and consist of stacks of flattened cells supplied with nerves on one side. Because the cells are stacked in columnar series (called electroplaques) the small electrical charges generated by each cell sum together to produce a large charge that can stun or kill other fish. Electric rays The electric organs are up to one sixth of an electric rays body weight and are located on the side of the ray just ahead of the pectoral fins. In some species the organs are wired to direct a shock upwards. In others they are pointed downwards. Electric rays appear to hunt mostly at night and may slowly move over a fish and then stun it or lurk buried under sand until a suitable prey fish swims overhead. Sawfishes The seven species of sawfishes inhabit tropical and subtropical coastal bays and estuaries and prefer shallow silty water. They are big (1.4 to 7 meters in length). They possess a distinctive “tooth” edged rostrum or saw (the teeth are modified tooth-like structures called denticles). The rostrum is covered with motion and electro detectors that allow the sawfish to detect hidden prey. http://www.elasmoworld.org/sawfish.html http://www.elasmoworld.org/sawfish.html Sawfishes Usually sawfish forage on the bottom and eat crabs, crustaceans and small fish. They often stir up silt and mud with their saws to startle prey. Sawfish also slash with the saw to kill or disable prey and use the saw to defend themselves also. Subclass Holocephali: Chimaeras Chimaeras are a small group (about 33 species) of deep sea (>80m and usually considerably deeper) cartilaginous fishes known commonly as ratfish or ghostfish. Because they live mainly in deep water they are not a well known group. Their lineage diverged from the sharks about 400 mya. Formerly they were more diverse. Male spotted ratfish Subclass Holocephali: Chimaeras Have a large head often with an elongated snout filled with sensory structures Have a cover over the gills (operculum) as in bony fishes) Have a venomous spine located in front of the dorsal fin. Lack both a spiracle and stomach. The tail is thin and tapers to a point (hence the name ratfish) and not much use in swimming. Instead, chimaeras depend on flapping their pectoral fins for much of their movement. Chimaeras Like other Chondricthyes males use claspers for mating and eggs are laid in leathery cases. Males also possess retractable sexual appendages on the head and in front of the pelvic fins that apparently are used to grasp the female during mating. Chimaeras They appear to mostly feed on sea urchins, shrimp, and mollusks that they grind using plate-like grinding teeth. Unlike in sharks the upper jaws are fused to the skull.