Sand tiger sharks (also known as grey nurse sharks) are relatively placid sharks, but their offspring are scarier than anything seen in the movies. Unlike most sharks, sand tiger sharks give birth instead of lay eggs. Although dozens of baby sharks are fertilized, only one or two survives to be born.
Say Hello to Intrauterine Cannibalism
The male and female sand tiger shark breeds sexually. His sperm fertilizes some of her eggs, although she may release both fertile and infertile eggs into both of her uteri. Each of the fertilized shark eggs subsist for a while on a packet of yolk attached to their growing bodies. Similar yolk sacks can be seen in the fry of many fish species, such as the betta or Siamese fighting fish.
But these yolk sacs are soon exhausted. In many other fish species, the fry have already hatched from the eggs or have been expelled from the mother's body. But not with sand tiger sharks. The baby sharks remain in the two uteri and then eat the unfertilized eggs. When the eggs run out, they turn on each other. The baby sharks eat each other until there is only one left in each womb. It is those two shark pups that get born. Sometimes one of those pups died during birth or for some other reason, so a sand tiger shark may only give birth to one pup.
Why would a shark evolve such an unusual reproductive strategy? Most sharks just dump the fertilized eggs and are done with them. But this is the problem. These tiny eggs and newly hatched fry are dangerously vulnerable to predators – which can include small fish. The shark fry needs to dodge predators for many months or even years until it has grown big enough to defend itself. Theoretically, an entire shark egg clutch could be wiped out before any of the offspring are large enough to reproduce.
When sand tiger sharks are born, they are a couple of feet long, have formidable teeth and know how to hunt. They are well prepared to take on many fish. These one or two pups have excellent chances of surviving to reproduce. Although it takes more energy on the mother's part (not to mention the surviving shark's siblings) it is more logical to spend a lot of energy to ensure that one offspring lives rather than take chances with several hundred that will most likely die anyway.
Unfortunately, sand tigers are heavily poached and caught in commercial fishery bycatch even though they are a protected species. Currently, they cannot reproduce fast enough in order to maintain the heavy predation on their species. There are some adventurous plans to take fertilized eggs out of female sharks and raise them separately and then release them when they grow large enough. But so far they are only in the planning stages.