Ovoviviparous shark pups depend on yolk for nutrition, and when an embryo has expended its own yolk sac, it turns to the eggs around it. Some species practice intrauterine cannibalism, or eating the other fertilized or unfertilized eggs in the womb.
The best-known intrauterine cannibal is the sand tiger shark. Although the sand tiger shark has two uteri and produces many eggs, each litter yields just two pups -- one from each uterus. Sand Tiger Shark "intrauterine cannibalism"