The term “mermaid’s purse” is the common, evocative name for what scientists call a shark egg case. These peculiar objects are highly durable, protective capsules created by certain shark species to house their developing offspring outside the mother’s body. Found washed ashore on beaches, often tangled in seaweed or nestled in the sand, they capture the imagination of beachcombers. These leathery, dark-colored structures offer a glimpse into the reproductive strategy of a minority of cartilaginous fish, representing a secure, self-contained nursery for the embryo.
Anatomy and Function of the Shark Egg Case
The tough, leathery texture of the shark egg case is due to its composition, which is primarily a highly cross-linked protein called collagen. This material is secreted by the female shark’s oviducal gland and forms a structurally complex network that provides both mechanical protection and necessary permeability. While the case is robust enough to shield the embryo from physical damage, it must also permit the exchange of gases and waste with the surrounding seawater.
Shark egg cases exhibit variations in shape, acting as specialized anchoring mechanisms for different marine environments. Many species, such as catsharks, produce rectangular cases with long, curly tendrils extending from the corners. These tendrils function to securely fasten the egg case to substrate like kelp fronds or rocky crevices, preventing the developing embryo from being swept away by currents.
Some sharks, like the Horn Shark, have evolved a distinctive auger- or corkscrew-shaped case. This spiral design allows the female to wedge the egg deep into rock crevices on the seafloor. The case’s architecture, whether rectangular or spiral, includes small slits or pores that facilitate the constant flow of oxygenated water required for the embryo’s survival.
The Sharks That Lay Eggs
The majority of shark species give birth to live young, but a smaller group reproduces through oviparity, or egg-laying. This reproductive method, which involves depositing an egg case into the environment, is characteristic of several bottom-dwelling shark families. Notable oviparous species include the Swell Sharks and the Catsharks, which are among the most common egg-layers. Horn Sharks are also well-known for their unique, spiraled egg cases, which are typically wedged into rocks for protection. This method contrasts with viviparous sharks, which retain the eggs within the mother’s body until they hatch and are born as fully formed pups.
Skates, which are cartilaginous relatives of sharks, also produce egg cases often mistaken for shark purses. Skate egg cases are generally flatter and more rectangular than those of most sharks. They feature four distinct, stiff horns instead of the curly tendrils. These horns are used to anchor the case into soft sediment on the seafloor. The specific shape and external features of a recovered egg case can often identify the exact species that laid it.
Embryonic Development Inside the Purse
Development inside the protective purse is a long process, often lasting for several months and sometimes exceeding a year, depending on the species and water temperature. The single embryo within the case is entirely reliant on a large, internalized yolk sac, which provides all the necessary nutrition until it is ready to hatch. Initially, the case may be sealed, with the embryo relying on oxygen stored within the egg jelly.
As the embryo grows and its respiratory needs increase, the case opens to the surrounding water through small slits. At this stage, the developing shark must actively move its tail in a rhythmic, undulating motion to pump oxygen-rich water through the capsule. This continuous flushing action ensures that the embryo receives enough dissolved oxygen to sustain its growth and prevents suffocation.
The developing shark undergoes significant morphological changes, absorbing the external yolk sac into its body to form an internal store of nutrients. Although protected by the thick, leathery casing, the embryo is not completely isolated from its environment. Research shows that some embryos can sense the minute electrical fields generated by potential predators and will cease movement to avoid detection. Once the yolk is fully consumed, the miniature shark is prepared for its emergence into the ocean.
The Journey of the Empty Case
The process of hatching involves the fully developed young shark tearing or chewing its way out of the tough, collagenous case. This action leaves a small slit or hole at one end of the purse, marking the exit point of the newborn shark. The newly emerged shark is a miniature version of the adult and immediately swims away to begin its independent life.
Once empty and detached from its anchor point, the lightweight and durable egg case becomes buoyant. Ocean currents then carry the discarded capsule, which is why these objects are frequently found washed up on the strandline by beachcombers. These empty “mermaid’s purses” serve as a valuable indicator for marine scientists, providing clues about the breeding grounds and nursery areas of oviparous shark species.