Beachcombers sometimes discover mysterious, leathery pouches washed ashore, often called “mermaid’s purses.” These are not mythical accessories but protective egg cases produced by certain marine animals. Their structures safeguard developing young before they emerge into their marine habitat. These cases provide valuable insights into the reproductive strategies of some fascinating aquatic species.
Biological Identity
Mermaid’s purses are specialized egg capsules of oviparous chondrichthyans, a group of cartilaginous fish including some sharks, all skates, and chimaeras. These tough, flexible casings, primarily composed of collagen protein strands, provide a resilient barrier for the developing embryo. Their primary function is to shield the embryo from predators and physical damage.
A single embryo typically develops inside, nourished by a yolk sac. Some species, like the big skate and mottled skate, can produce egg cases containing multiple embryos, sometimes up to seven.
After internal fertilization, the female deposits these cases onto the seafloor, often in sheltered locations such as kelp forests, seagrass meadows, or rocky crevices. This structure acts as a detached uterus, allowing offspring to develop independently for several months, or even over a year, depending on the species and environmental conditions.
Appearance and Identification
Mermaid’s purses exhibit a variety of shapes, sizes, and colors, reflecting species diversity. Most commonly, they are dark brown or black, though some can appear green or orange-yellow. Their texture is often described as leathery or fibrous, feeling rough or smooth. Sizes vary significantly from a few centimeters to 28 centimeters, as seen in the flapper skate.
Distinctive features help differentiate between egg cases. Skate egg cases are typically rectangular or square-shaped, with four stiff, horn-like projections at each corner. These horns help anchor the case securely into soft seafloor sediments, preventing drift.
In contrast, many shark egg cases, particularly from catsharks, are more rounded or purse-shaped with long, curly tendrils at their corners. These tendrils wrap around seaweed or coral, suspending the egg case in the water column. Chimaera egg cases are uniquely spindle- or bottle-shaped, sometimes with lateral fins or ridges, and are laid directly on the seafloor. When found dried out, soaking a mermaid’s purse in water can rehydrate it, making features more defined for identification.
Finding and Understanding Them
Mermaid’s purses are often found washed ashore on beaches, nestled among seaweed in the strandline. They can also be found in tide pools or occasionally inland in sand dunes. Most cases found are empty, meaning the embryo has hatched and the young animal has swum away, leaving the discarded casing to be carried by ocean currents. Rough weather and storms can dislodge cases from their anchoring points, leading to more washing ashore.
If an empty mermaid’s purse is found, it can contribute to marine conservation efforts. Organizations encourage beachgoers to report findings, as these cases provide data on species presence, distribution, and potential nursery areas.
However, if a case appears translucent with a visible embryo when held to light, it indicates the case may still be alive. In such instances, gently return the egg case to the sea, ideally in a sheltered area like a tide pool or amongst seaweed, allowing nature to take its course. The tough, leathery material of the egg case, made of keratin, allows empty cases to persist for a long time in dry conditions, making them durable finds.