Mermaid’s Pruse, Rajidae Sp. Egg Case

Mermaid’s Purse. A Representative Elasmobranch Mermaid’s Purse most likely from a the Rajidae Family of Skates. Pictured case: 7.25 cm (2.9 inches) x 3.4 cm (1.3 inches).

Mermaid’s purses are often found washed up on the beach and may be mistaken for a marine organism. They are in fact egg cases from a variety of species of sharks, skates and rays (elasmobranchs). Some species of elasmobranchs are oviparous (egg-bearing). For these species, fertilization of the egg occurs inside the female’s body, and the fertilized embryo is then enclosed in an egg case which is deposited in the environment. The case may contain up to seven embryos. Egg cases come in a variety of shapes and sizes. Most are quadrangular and have hollow tendrils or sticky filaments projecting from each corner. The tendrils are used to attach the egg case to a structure (regularly seaweed) or substrate (often rocks) where the embryo will develop. Initially the egg cases are soft and pliable so they can be laid, but after a few hours they become harder and more leathery. The egg cases also change color with time. Most egg cases are amber colored when they are laid, becoming dark green or dark brown to black. The egg cases are primarily made of collagen. Embryos can develop within the egg case in as little as 8 weeks, or they may take over a year before hatching. When they do hatch, the little animals are miniature replicas of their parents. They are on their own once they hatch, receiving no help from their parents.

The shape of the Mermaid’s Purse can give some indication of the type of animal that it contains. Dogfish purses tend to be a elongate rectangle, with fine, curly tendrils at one end and a flattened “tail” at the other. Horn Shark egg cases are cone-shaped. They have thin, spiral, flanges that give the case the appearance of a fat screw. These flanges help hold the case in crevasses, protected from most predators and water motion. The cases of Skates, in the Family Rajidae, are rectangular, with thick, slightly curved, horn-like, tendrils at each corner.  The Mermaid’s Purses found on the beach may either be spent cases, having already hatched, or they may still contain an embryo. Some, more translucent, cases can be help up to a bright light, revealing a silhouette of the embryo.