Born Tough

Part of why sharks have lasted so long is how carefully they reproduce.

They do not flood the ocean with eggs and hope. Instead they make a few, well-protected young:

• Some lay leathery egg cases, the spiral or pouch-shaped "mermaid's purses" you find washed up on beaches.
• Many give birth to live, fully formed pups ready to hunt on day one.
• A few grow their young inside the mother for over a year before birth.

This slow, high-investment strategy produces strong survivors. It is also exactly what makes sharks so hard to replace once their numbers fall.