The class of animals known as Chondrichthyes contains over 1,000 species of cartilaginous fishes, including sharks, rays, skates, and chimaeras. Within this group, some of the animals give birth to live young, called viviparity, and some lay eggs, called oviparity. Those that are oviparous develop a rough and leathery casing made of protein around the developing embryo before laying the egg, which is deposited on the seafloor to gestate. After the egg hatches and the animal inside swims off, that leather case washes up on shore, just waiting for a beachcomber to find it. And find it they do: by the 1900s, these egg cases, with their rough exteriors, rectangular shape, and long “horns” on each corner, were collected often enough that they were given the nickname “mermaid’s purse.”
Egg cases are particularly vulnerable to predation because the mother does not watch over the egg after it has been laid, although she does choose spots on the seafloor that are well-hidden from any potential predator. As each egg case species comes in various shapes, sizes, and colors, marine biologists can count the number and types of egg cases that wash up on shore to help give an estimate of the Chondrichthyes population in a certain area.
A vast number of shark and ray species are endangered due to human activities, so the knowledge that marine biologists can gain from egg case abundance in the wild is incredibly important. Plus, they are very useful for a mermaid looking to carry her belongings!
This article appeared in the Beachcombing Magazine May/June 2020 issue.