Sand Tiger Sharks: The Epitome of Sibling Rivalry

Flickr Photo by Maritime Aquarium at Norwalk

Anyone who has siblings knows that brothers and sisters do not always get along.  Sisters don't always play with their Barbie dolls happily and brothers don't always cooperate in building their fort together.  Often siblings compete with each other to be the tallest or strongest, to be the best soccer player, or to receive the most attention from their parents and friends.  Sometimes this sibling rivalry intensifies and can end in clenched fists, bared teeth, or pulled hair.  However, this rivalry is trivial compared to the sand tiger shark, where shark embryos are forced to compete with their siblings for survival before they are even born.  

Female sand tiger sharks have two uteri, and although they produce many eggs, each litter only yields two pups:  one from each uterus.  In the uterus, many eggs exist, some fertilized and others unfertilized.  The eggs contain yolk in which the embryos feed on until they deplete this source.  Then the eggs hatch and the embryos can search for other food sources inside the uterus.  Since the eggs hatch at different times, some embryos are more developed than others.  The oldest and most developed embryos form teeth first, in which they can feed on fertilized or unfertilized eggs or other embryos in the uterus.  This phenomenon provides an example of survival of the fittest through cannibalism inside the uterus.

This embryonic cannibalism may seem like a grotesque discovery, but in fact can be quite useful in protecting the sand tiger shark population.  Currently tiger sharks are considered a near threatened species.  If scientists can discover a way to recreate the conditions in the shark womb, then scientists can extract embryos from female sharks and nourish the embryos through a normal gestation period.  The scientists can then release these sharks back into the wild, replenishing the sand tiger shark population.