Sibling Rivalry

Sharks are endlessly fascinating creatures. Sleek, beautiful and perfectly adapted to their environment, they have diversified over millions of years and have evolved some very different approaches to reproduction.

For example, of the approximately 400 species of sharks in the world, only about 40% lay eggs. Some hatch in the ocean while others hatch inside the mother, resulting in babies (known as pups) that are born live. Those eggs that leave the mother's body before they hatch are covered in protective sacs or hard casings that can attach to coral or plants so that unhatched pups can develop in safety. In some species, such as the Horn shark, the egg cases are even buried in sand or sediment or pushed into crevices between rocks.

Once empty, the egg cases often wash ashore and can be found among the strandline on beaches and are commonly known as "mermaid's purses". But not every type of shark pup is lucky enough to hatch from a safe, protective casing - some are forced to fight for their very survival before they’ve even been born.

Some of the types of sharks that bear live young, such as Great Whites, Makos or Sandtigers, exhibit a completely unique trait called “embryophagy”, which means that their pups must survive within the womb without nourishment from the mother and are forced to feed on their unborn siblings to survive. Thus while many embryos can hatch within the mother’s womb, only one or two actually make it to birth. To date, sharks are the only creatures in the world where this behaviour has been observed, bringing a completely new meaning to the concept of “survival of the fittest.”