Zoology Notes 008: the shark that eats its siblings, in utero

Embryonic sand tiger sharks will attack and eat each other inside their mother Photograph: Rafa Rivas/AFP/Getty Images

It makes for gruesome but fascinating reading.

In 1983, marine biologists published an in-depth study of the embryonic development of the sand tiger shark, (Carcharias taurus), also known as the grey nurse shark. The paper is noteworthy for one particular detail, summed up by the following sentence:

“At about 100mm TL [total length], the embryo begins to hunt and consume other intrauterine embryos.”

At the early stages of pregnancy, a female sand tiger shark may have a litter of as many as ten embryos, five in each of two discrete uterine horns. By the end of gestation, only two survive, one on each side.

The mother turns into a battleground soon after the first and largest embryo hatches inside her when it reaches around 10cm long. The feeding frenzy appears to be far from indiscriminate. In one case, researchers dissected a uterus to find a 33cm embryo with four of its much smaller siblings in its throat. There were a further 70 capsules in the uterus, of which two contained embryonic sharks. Both of these had been “punctured numerous times”, whilst every single one of the 68 embryo-free capsules had been left untouched.

Female sand tiger sharks start a pregnancy with a lot of embryos. Only two ever get born. Photograph: Alamy

This intrauterine cannibalism or embryophagy does not appear to occur in other closely related sharks, which can give birth to ten or so pups at a time. It seems strange that selection should favour a system in which the maximum litter size ends up as two when it could be more. But on this matter, the authors of the 1983 study had some ideas:

“Although only two young are produced at the end of a lengthy gestation period, they have several selective advantages as top predators in marine food webs,” they wrote. The newborn sand tiger sharks are so large they maybe better able to see off predators. It is also possible, they suggest, that the experience of hunting in utero may give them an edge.

Interestingly, a more recent study has revealed that female sand tiger sharks often have litters sired by multiple males but, as a result of the cannibalism, they can end up with offspring from just one.

Gilmore, R.G. et al. (1983) Reproduction and embryonic development of the sand tiger shark. Fishery Bulletin 81: 201-225 [PDF, with some cool pictures]