Mermaid’s Purse of the Elasmobranchs
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).
The Mermaid’s Purse is an egg case that can be from a variety of species of sharks, skates and rays (elasmobanchs). For many elasmobranchs reproduction is oviparous (egg-bearing) where 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. Some species can produce egg cases that contain up to 7 embryos.
Egg cases come in a variety of shapes and small in size with most being 8 cm (3.0 inches) to 10 cm (4.0 inches) in length, quadrangular with hollow tendrils or sticky filaments projecting from each corner. The tendrils are used to attach the egg case to structure, such as seaweed, or substrate (often rocks) where the embryo will develop. When deposited the egg cases are soft and pliable, but after a few hours they become hardened and leathery. They also change color with time being amber colored when laid, and becoming dark green or dark brown to black with maturity. 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 to hatch. The hatchlings are miniature replicas of their parents and normally receiving no help from their parents.
The shape of the Mermaid’s Purse vary by species. Dogfish Purses, for example, tend to be an elongated rectangle, with fine, curly tendrils at one end and a flattened “tail” at the other. Horn Shark Mermaid Purses are cone-shaped with thin, spiral, flanges that give the case the appearance of a fat screw. The 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 are frequently found washed up on the beach and are often mistaken for another marine organism. Most are found as spent cases but some still retain partially developed embryos. Some of the more translucent cases can be help up to a bright light, to reveal a silhouette of the embryo(s).