By Scott Rowan
Sharks have two ways to birth that vary from species to species: live birth (vivaparous) or laying eggs (oviparous).
Oviparous sharks give birth to egg cases that contain a single shark pup and a yolk sac to nourish it. The protective shell is constructed of a compound that combines collagen and keratin, which is similar to human fingernails. Resilient, yet flexible, the keratin/collagen shell anchors itself to seaweed and other objects with either curly tendrils or horns that hook onto submerged debris in order to keep the growing pup safe from predators.
After the shark pup is born, the empty egg case remains and is often found washed ashore on beaches throughout the world.
Mermaid purses are not exclusive to just oviparous sharks, other members of the elasmobranch family including skates and rays also use eggcases to give birth. The size, color and shape of the egg case varies from species to species.
In the United Kingdom, the Shark Trust has an ongoing project to identify and preserve egg cases of all species called The Great Eggcase Hunt Project.
It is relatively easy to see how the unusual egg cases became mythologized by sailors hundreds and thousands of years ago when stories of mermaids patrolling the mysterious waters were believed to be true. After breeding season, when countless thousands of egg cases began washing ashore, there’s no wonder that creative stories were told to explain the confusing little pockets of material that covered the sandy shoreline.