Egg case (Chondrichthyes)

An egg case or egg capsule, colloquially known as a mermaid's purse or devil's purse, is a casing that surrounds the fertilized eggs of some sharks, skates, and chimaeras. They are made of collagen protein strands.

Because they are lightweight, they are commonly washed up by the sea, often found at the strandline, the farthest point of the high tide. The egg cases that wash up on beaches are usually empty, the young fish having already hatched out. Furthermore, egg cases are often found in commercial fishing gears like nets and pots, and on the sea floor during surveys using an ROV.

Among the 9 extant orders of sharks, egg-laying is found only in three:

The egg cases of bullhead sharks (Heterodontidae) have spiral flanges that allow them to be wedged inside crevices or into sand.

The bamboo sharks (Hemiscylliidae) and the zebra shark (Stegostomatidae) lay eggs on the bottom, while the other carpet sharks give live birth. The egg cases are oval and covered with adhesive fibers that serve to secure them to the bottom.

Some catsharks (Scyliorhinidae) and the finback catsharks in the genus Proscyllium are the only members of their order that lay eggs. The egg cases of catsharks are purse-shaped with long tendrils at the corners that serve to anchor them to structures on the sea floor.

The size of egg cases vary; those of the small-spotted catshark or lesser spotted dogfish, Scyliorhinus canicula, are around 5 centimetres (2 in) long, while those of the greater spotted dogfish, S. stellaris, are around 10 centimetres (4 in). That excludes the four long tendrils found in each corner, which assist in anchorage. Egg cases from rays vary in that they have points rather than tendrils. The colours and shapes of egg cases also vary greatly from species to species.

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