Many people assume all sharks give birth to live young, but the few species that lay eggs produce some of the ocean’s most unusual and fascinating structures. These external egg casings are a marvel of biological engineering, designed to protect the developing embryo from predators and harsh ocean conditions. Their appearance varies dramatically, ranging from rectangular, leathery pouches to strange, corkscrew-shaped objects. The distinct visual features of these capsules offer a unique window into the reproductive lives of the species that create them.
Reproductive Diversity in Sharks
The diverse appearances of shark eggs are only produced by a small fraction of the species, those with an oviparous reproductive strategy. This method involves the female depositing a fertilized egg, enclosed in a protective case, outside her body to develop independently. The eggs laid by oviparous sharks, such as certain catsharks and bullhead sharks, are the only ones a person might ever encounter washed up on a beach.
The majority of sharks utilize other reproductive methods where the young develop internally. Ovoviviparity is the most common strategy, where eggs hatch inside the mother’s oviduct. The embryos are nourished solely by a yolk sac until they are born alive, and there is no direct placental connection.
Viviparity is the third strategy, similar to the reproductive process in mammals. The embryo develops in the uterus and receives nourishment through a placental connection, leading to the live birth of fully formed young. Since these eggs never leave the mother’s body, the unusual external cases only belong to the oviparous minority.
The Anatomy of the Egg Case
The external egg case is often called a “mermaid’s purse” when found on the shore. This casing is not a hard shell like a bird’s egg, but a tough, leathery material composed primarily of collagen protein. The female shark’s oviducal gland secretes this material, which surrounds the fertilized egg.
The primary function of the case is to provide a durable barrier for the developing embryo against physical damage and predation. Inside the case, the embryo is suspended and nourished by a yolk sac for its development, which can take several months to over a year depending on the species and water temperature. The capsule must allow for gas exchange and waste removal.
This is achieved through microscopic pores or narrow slits located at the corners or edges of the case, which facilitate the circulation of water and the uptake of oxygen. The entire structure represents a self-contained life-support system designed for long-term incubation in the harsh marine environment. Once the embryo is fully developed, it escapes through an opening, typically a pre-formed slit at one end of the capsule.
Variations in Shape and Appearance
The most striking feature of oviparous shark eggs is their variety in shape, which is directly linked to where the female deposits them for incubation. Catsharks produce cases that are typically oblong or rectangular, often featuring long, coiled tendrils at each of the four corners. These tendrils function as anchors, allowing the female to secure the egg case to seaweed, kelp, or coral to prevent it from drifting away.
The Port Jackson shark produces an egg case with a distinctive spiral or auger shape. The female will wedge this spiral case into a crevice or between rocks, where the ridges help secure it in place against the currents. This protects the egg in rocky habitats where there is little seaweed for anchoring.
Skate egg cases are often mistaken for shark eggs, but they are generally more rectangular and flatter, featuring pointed, horn-like projections at the corners instead of tendrils. Shark egg cases vary in texture, ranging from smooth and rubbery to dark brown or black with a rough, striated surface. These color and texture variations provide natural camouflage against the seabed where they are laid.