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Unlike many fishes, sharks reproduce internally. Males have modified pelvic fins, called claspers. Usually females are larger than males, but rarely the males are larger than their counterparts. Males do have larger teeth than the females in certain species. Since the female is held by these teeth during copulation, these species' females have thickened skin behind the head.
Sociable species spend much of their lives in single sex groups. These two groups only meet during a mating phase. Little else is known about this aspect of reproduction because only a few have been seen in their native habitat and sharks for the most part don't thrive well in captivity.
Sharks are not nurturing parents. There are three reproductive strategies that they use to give birth which give varying levels of protection to the offspring. Oviparous sharks lay eggs in a leathery shell. The shells are species specific. Ovoviviparous sharks have a weak shell and the young hatch inside the mother after their yolk is used up. Viviparous has hardly any egg shell and the yolk sac is at the end of an umbilical ord. As yolk is used up the mother becomes a nourisher through food and blood; analagous to mammals, but not quite the same. Some species are nourished by a uteral lining called uterine milk rather than mother's blood. After the young leave the mother's body there is no parental care, and the young often have to hide from the parents or be eaten as prey. These three systems are useful but not completely reliable measns of species identification.
In Hammerhaed sharks and a few others, while still in the uterus, the larger embryos begin eating the less developed embryos and eggs in a survival of the fittest frenzy. Also, unlike many other fishes, sharks are not hermaphroditic but are a single sex their whole life.
The reproductive organs of sharks are uniform and are linked with their urinary excretory system. Eggs are made in the ovaries. Although there are always two, often only one is functional. They are long pale organs
