A skate egg is a protective, rectangular capsule laid by a female skate, a cartilaginous fish related to sharks and rays. This structure is commonly known as a “mermaid’s purse” due to its distinctive, pouch-like shape. Skates are oviparous, meaning they lay eggs that develop outside the mother’s body on the ocean floor. The durable egg case sustains the developing embryo for many months in the marine environment.
Identifying the Skate Egg Case
The skate egg case is frequently found washed ashore, appearing as a dark, leathery, or horny object among debris. These empty capsules are typically rectangular, measuring between two and six inches in length, depending on the skate species. A defining feature is the presence of four rigid projections, described as horns or tendrils, extending from each corner of the case. The color usually ranges from olive-green when freshly laid to dark brown or black after exposure to the sun and sand.
The term “mermaid’s purse” is also applied to the egg cases of some sharks, which look different from those of skates. Shark egg cases, such as those from catsharks, often have long, curly, fibrous tendrils used to secure the case around kelp or fixed objects. In contrast, the skate’s case features straighter, stiffer horns designed for anchoring into soft seafloor sediment. Finding a skate egg case on the beach usually means the young skate successfully hatched, or the capsule was empty and washed away from the nursery site.
Function and Structure of the Casing
The durability of the skate egg case comes from its composition: a tough, keratinous material similar to human fingernails. This shell protects the single embryo contained within from predators and abrasive ocean currents. Upon being laid, the female skate attaches the capsule to the seabed. The four horns function as anchoring points, securing the case to sand, mud, or seaweed, preventing it from being swept away.
The developing skate depends on a constant supply of oxygenated seawater, facilitated by the case’s structural design. Small openings or slits are present at the ends of the horns, allowing water to flow into the central cavity. The embryo performs rhythmic movements, often described as tail-lashing, to help pump water through these slits and circulate fresh, oxygen-rich water across its gills. This constant ventilation is necessary for respiration and waste removal throughout the developmental period.
Development and Hatching
Development inside the egg case is a long process, occurring on the seafloor without parental care. The embryo receives nourishment from a large, internal yolk sac, which is gradually absorbed during gestation. The length of time a skate spends inside its case depends on the species and the surrounding water temperature. For many species, development typically takes several months, ranging from three months to over a year.
In extremely cold waters, such as those found in deep-sea habitats, the development of some skate species can extend for over three years before the young skate is ready to emerge. As the embryo grows, it eventually fills the entire capsule and utilizes the remaining yolk reserves. Hatching is triggered when the young skate is fully formed and strong enough to tear through the tough, leathery casing, often exiting through one of the end slits. A fully developed skate emerges as a miniature version of the adult, capable of swimming and feeding immediately after hatching. The empty, post-hatch case often splits or tears where the juvenile skate pushed its way out, eventually washing ashore.