In this great educational segment, Jonathan explores the reproductive strategies of sharks, explaining how some sharks lay eggs (oviparity), produce live young (viviparity) or produce eggs that develop internally (ovoviviparity).
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Are they born from eggs or live born? Or both? How do sharks reproduce? I’m Jonathan Bird and this is shark academy!
Unlike fish, which produce large amounts of eggs, most of which never reach maturity, sharks produce far fewer, but larger, offspring, and they have a much better chance of reaching adulthood.
Sharks can reproduce in several ways. Many small sharks, like the horn shark, the swell shark and cat sharks actually lay eggs on the bottom. The baby shark develops in there, living off a yolk sac filled with nutrients. The baby, a miniature version of its parents, is born after a few months to a year of gestation, depending on water temperature and the species. Sharks using this reproductive strategy are called oviparous. Meaning egg-laying.
Other sharks are viviparous. They give birth to live young which develop inside the mother shark, receiving nutrients and oxygen from mom through an umbilical cord, just like humans. Larger sharks like Blue sharks, Bull sharks and Hammerheads use this technique.
The last group of sharks are ovoviparous, (also known as Aplacental Yolk Sac Viviparous). In this system, the mother produces eggs like the oviparous system, but instead of laying her eggs on the bottom, she carries them inside her body until they hatch. When the babies pop out, you might think the shark was viviparous, but the babies had no umbilical cord inside, they lived off a yolk sac.
In the weird category, the Sand Tiger shark has a strange variation on its ovoviviparous reproduction known as intrauterine cannibalism. The mother produces up to 50 pups in each of two uteruses, but the first baby in each uterus that reaches about 4" long eats all its siblings! After 12 months of gestation, the mother then gives birth to a pair of 3 foot long pups! Pups that are well fed!
Because sharks put so much more time, effort and energy into producing a viable offspring than bony fish do, they give birth to far fewer pups. A whale shark produces the most pups of any shark—around 300 at a time—but then again, mom is the size of a bus. The Thresher and Sand Tiger only produce two pups at a time. The Blue shark can produce 135 at a time, which is a lot for a shark not the size of a bus.
Gestation is long too, averaging 9-12 months. Baby Spiny Dogfish take 22 months—almost two years—for their young to develop.
Most sharks take quite a while to become mature enough to reproduce as well. Great Hammerheads take 9 years. Lemon and Bull sharks take 15 years. Spiny Dogfish? 20 years.
So, between the small litters, a long gestation period and long time to reach maturity, sharks just don’t reproduce very quickly. This is the reason they are so vulnerable to overfishing.
If you’re interested in sharks, there are more than 30 Shark Academy episodes to watch! You can also join my underwater adventures on Jonathan Bird’s Blue World!
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