Mermaid’s Purses
Have you ever found a mermaid’s purse among seaweed on the beach? These small leathery pouches are the cast-off egg capsules of some sharks and their kin. Most fish release their eggs and sperm into the ocean, leaving survival of their offspring to chance. But sharks, skates, rays and chimeras give their young a far better odds at life because they practice internal fertilization which means their young develop in eggs within the mother's body. In many species the young are born as miniature adults but in other species the females produce egg cases that are simply left on the seabed, attached to something underwater or tucked inside a crevice. The pup develops inside its egg case, hatches, and then swims away. Most egg cases look like little pillows with hooks and tendrils that at one time attached them to seaweed or underwater debris, but egg cases of some others have a corkscrew shape. One spring, SRI interns painted egg cases they had found on the beach with bright colors and gave them to local science classes in presentations on sharks and mermaids cases.