The mysterious objects often found washed ashore by beachcombers are commonly known as “devil’s purses.” These curious, leathery capsules are not marine debris but rather the protective egg casings produced by certain cartilaginous fishes, including sharks, skates, and chimaeras. Encountering one of these empty, dark structures often sparks curiosity about the creature that created it and the secret life that developed within. This natural packaging represents a specialized reproductive strategy in the ocean.
Anatomy of the Egg Case
The physical structure of a devil’s purse is designed for maximum security in the turbulent marine environment. The case itself is constructed primarily from collagen, a tough, fibrous protein that gives the exterior its characteristic dark brown to black color and leathery, almost plastic-like texture. This robust composition provides substantial defense against abrasion from sand and rocks, as well as protection from most predators during the extended gestation period.
The architecture of the case includes specialized features for anchoring it securely to the seabed. Many species produce long, coiled tendrils or rigid, hook-like horns that extend from the corners of the main body. These appendages wrap around seaweed, sponges, or other fixed debris on the ocean floor, preventing the capsule from being swept away prematurely by strong currents. This tethering mechanism ensures the developing embryo remains stable throughout the months it takes to mature.
Identifying the Parent Species
The specific morphology of a washed-up egg case can often reveal the identity of the cartilaginous fish that produced it. A primary distinction exists between the cases of skates and those of most egg-laying sharks, such as the catsharks. Skate egg cases are typically rectangular or trapezoidal in shape, featuring four distinct, stiff horns extending from each corner of the capsule.
Shark egg cases, by contrast, are generally more purse-like or spiral in their construction, sometimes resembling a flattened cylinder. These shark purses often possess long, coiled, or sticky tendrils that emerge from the ends, designed to become entangled in surrounding ocean growth. Chimaeras, sometimes called ratfishes, represent a third group, producing uniquely tapered, spindle-shaped cases that are less frequently encountered by beachcombers.
The size, shape, and specific texture are species-specific identifiers, allowing scientists to catalog local populations based on the empty shells they find. The number and length of the anchoring appendages are highly specific to the species responsible for the deposition.
Development Inside the Purse
The internal environment of the devil’s purse is a highly controlled incubator that supports the complete embryonic development of the fish. Within the protective walls, the embryo is suspended in a fluid and draws all necessary sustenance from a large yolk sac, which is analogous to the yolk found in a bird’s egg. This rich food supply fuels the growth of the fish for the duration of the lengthy gestation period, which can range from several months to well over a year depending on the species and water temperature.
A sophisticated mechanism is built into the casing to ensure the embryo receives a constant supply of oxygenated water. The egg case features small slits or fine tendrils at the ends, which act as openings for water circulation. The developing fish often exhibits rhythmic movements, subtly flexing its tail to actively pump fresh seawater through these vents and across its developing gills.
This passive and active water exchange prevents the buildup of waste and ensures adequate respiration for the growing juvenile. Once the embryo has absorbed the yolk sac and reached a size capable of independent survival, the juvenile fish emerges by pushing its way out of one of the case’s slits or openings. This releases a miniature version of the adult into the marine ecosystem.
Where to Find Them
Empty devil’s purses are most commonly discovered on beaches, typically deposited along the high tide mark, known as the wrack line. Strong currents or storms can dislodge the anchored cases, and once the juvenile fish hatches, the empty capsule drifts until it is washed ashore. They are frequently found near areas with extensive seaweed beds or other debris where the parent fish prefers to lay its eggs.
If a beachcomber encounters a case that appears soft, heavy, or translucent enough to show a developing form inside, ethical practice suggests leaving it undisturbed in the water or secured to its anchoring point. Reporting the location of these finds to local marine organizations or citizen science projects provides valuable data, helping researchers track the distribution and reproductive success of local skate and shark populations.