Shark

Hexanchiformes

Reproduction

The sex of a shark can be easily determined. The males all have their pelvic fins modified into a pair of claspers. The name is somewhat misleading as they are not used to hold on to the female, but are the shark's version of the mammalian penis. (As a side note, Class Chondrichthyes has the distinction of having the animal with the largest intromittent organ ? an organ used for transmitting sperm ? in relation to body length. This animal is the clearnose skate (Raja eglanteria) which has claspers of 15 cm (6 in) in size on a fish that reaches 1 m (3 feet) in length.)

Related Topics:
Sex - Pelvic - Clasper - Penis - Intromittent organ - Sperm - Clearnose skate

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Mating has rarely been observed in sharks. The smaller catsharks often mate with the male curling around the female. In the less flexible species the two sharks swim parallel to each other while the male inserts the clasper into the female's oviduct. Many females in the larger species have bite marks that appear to be a result of a male grasping her to maintain position. The bite marks can also come from the courtship of the sharks. The male may come and bite the edges of the female to show his interest. In some species, females have evolved thicker skin to withstand the sharks bite marks during mating.

Related Topics:
Mating - Oviduct

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Sharks have a much different reproductive strategy than most fishes. Instead of producing huge numbers of eggs and larvae (99.9% of which never reach sexual maturity in fishes that use this strategy) sharks normally produce around a dozen pups, some species up to 70-80 and some as few as 2-3. These pups are either protected by egg cases or born live. No known sharks provide parental protection for their young, but females have a hormone that is released into their blood during the pupping season that apparently keeps them from feeding.

Related Topics:
Fishes - Hormone

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There are three ways in which shark pups are born:

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• Oviparity - Some sharks lay eggs. In most of these species, the developing embryo is protected by an egg case with the consistency of leather. Some of these cases are corkscrewed into crevices for protection. When they wash up empty on beaches, the egg cases are sometimes called mermaid's purses. Oviparous sharks include the horn shark, catsharks, Port Jackson Sharks, and the swell shark.
• Viviparity - These sharks actually maintain a placental link to the developing young, more analogous to mammals than other fishes. The young are born alive and fully functional. Hammerheads, the requiem sharks (like the bull and tiger sharks), the basking shark and the smooth dogfishes fall into this category. Dogfishes also have the longest known gestation period of any shark, 22 months. The blue shark produces the most young of sharks that have had the number of pups recorded, the maximum reported being 82.
• Ovoviviparity - Most sharks utilize this method. The young are nourished by the yolk of their egg and by fluids secreted by glands in the walls of the oviduct. The eggs hatch within the oviduct, and the young continue to be nourished by the remnants of the yolk and the oviduct's fluids. As in viviparity, the young are born alive and fully functional. Sometimes they are functional even before being born, as some species practice oophagy, where the first to hatch eat the remaining eggs in the oviduct. Sand tigers, makos, threshers, porbeagles and possibly great whites have oophagous young. The survival strategy for the species that do this is that the young are able to grow to an even larger size before being born. The whale shark is now considered to be in this category after having been classified as oviparous for a long time. Whale shark eggs found are now thought to have been aborted. Most ovoviviparous sharks generally give birth in sheltered areas, including bays, river mouths, and shallow reefs. They choose such areas mainly because of the protection from predators (mainly other sharks) and the abundance of food. A slightly modified form of ovoviviparity, oophagy is where the young hatch inside of the uterus within the first three months of gestation and consume the eggs, which the mother makes during the gestational period. Utilized by the lamnoid, great whites, crocodile sharks, makos, sand tigers, threshers, and false catsharks. Even more complex is embryophagy, where the first of the embryo?s to hatch kills its brothers and sisters before consuming the eggs provided for nourishment from its mother.