- Published on Wednesday, 20 February 2013 21:29
- Written by Super User
(Photo by Sue Mobley courtesy of N.C. Aquariums)
ANSWER - Some do and others bear live young. Statistics indicate 60 to 70 percent of shark species give live birth, and 30 to 40 percent lay eggs. Science has much to learn about the reproductive methods of these top predators, making it difficult to generalize about their populations as a whole.
What we do know is that the most common type of birth involves eggs remaining inside the mother until they have absorbed the yolk, developed, and hatched internally. The mother then gives birth to live, self-sufficient pups. This occurs in whale, basking, thresher, tiger, spiny dogfish and many other shark species. Sharks that lay eggs are notably fewer and include such species as bamboo, cat and horn sharks.
Interestingly, some live bearers have a biological twist in the internal development of young. The pups inside the mother do not receive primary nutrients from a yolk sac. Instead, they are nourished by eating surrounding infertile eggs. Other live bearers produce many fertilized eggs, but few young survive internally because the developing young feed on their less developed or weaker siblings. Developing young of the shortfin mako, sand tiger and great white sharks are examples that use this cannibalistic method. A third reproduction process is used by about 10 percent of known shark species. This involves the egg’s yolk sac becoming a placenta attached to the mother’s uterine wall to transfer nutrients from mother to pup, similar to humans and other mammals. This occurs primarily in larger sharks, such as bull, blue, lemon and hammerheads. Depending on the species, live-birth pups measure between 17 and 23 inches long. Egg layers produce egg capsules ranging from about 4 to 10 inches long. The protective capsules have a leathery, horn-like texture, and are often unusual shapes with cord-like tendrils that anchor the eggs to stationary objects like coral or plants to keep them from being swept away. Some sharks, such as the horn shark, push egg cases into the sea bottom, under rocks or into crevices.
All sharks reproduce using internal fertilization. Some species mate year-round, others prefer certain seasons and regions. Some bear young each year, while others stop reproducing at irregular intervals that can last several years. Live bearers have a gestation period of 6 to 22 months, depending on the species. As apex predators, these ancient fish play a vital role in the balance of the world’s oceans. Today, some 200 species are on the Endangered Species List. Some states are imposing fishing restrictions on selected species.