In coastal waters, mysterious pouches sometimes wash ashore, tangled in seaweed or nestled in rocky tide pools. They’re not trash, nor seaweed pods, but the egg cases of sharks and rays – nature’s ingenious design for protecting their young in a variety of marine habitats

As Easter approaches and egg motifs appear in shops and celebrations across many parts of the world, it’s a fitting time to highlight a lesser-known kind of egg – one that doesn’t come from a hen or a chocolate factory, but from the ocean depths. While most people associate eggs with land animals, some species of sharks, rays, and skates also produce eggs, and these egg cases –  often referred to as “mermaid’s purses” – are some of the most fascinating structures in the marine world. To the untrained eye, they may look like sea debris. But for scientists – and increasingly, for curious beachgoers – they’re a precious clue to where certain species live, breed, and grow.

Close-up of a shark egg. The yolk of the egg provides essential nutrients to the developing embryo, supporting its growth and development until it is ready to hatch. CC: John M. Chase 

Do all sharks and rays lay eggs? No, some sharks and rays give birth to live young, viviparous, a trait that sets them apart from most other fish. Other species are ovoviviparous, where embryos develop inside the mother’s body, nourished by a yolk sac, and are born alive without direct nutritional support from the mother. Lastly, some species are oviparous, laying eggs that develop and hatch outside the mother’s body. These include several species of catsharks, skates (which are rays), and a few deep-sea sharks and chimaeras. In tropical and subtropical regions –  including throughout Latin America and the Caribbean – oviparous species can be found inhabiting coral reefs, rocky bottoms, mangrove channels, and even deep-sea habitats.

The egg cases of sharks and rays are remarkable examples of nature’s engineering. Made primarily of tough, leathery collagen, these capsules are highly resistant to both predators and physical damage. Most are rectangular or oblong, measuring between 4 to 12 centimeters in length, though some species produce much larger cases. Depending on the species, the egg cases may feature long, tendril-like structures or stiff, horn-shaped extensions at each corner, which help anchor them securely to the substrate and prevent them from drifting. 

How to Tell the Difference Between a Shark Egg and a Ray Egg?

A question you may have is how to differentiate between a shark egg and a ray egg, often referred to as a mermaid’s purse. One of the most common ways to tell them apart is by examining the shape and external features of the eggcase, specifically the presence of horns or tendrils.

Ray and skate egg cases typically have two pointed horns at each end of the central capsule, which houses the developing embryo. These spikes help anchor the egg securely into soft seafloor sediment, where it can remain protected throughout development.

In contrast, shark egg cases – especially those from species like catsharks, swell sharks, and horn sharks – often feature long, curly tendrils. These tendrils are used to wrap the eggcase around seaweed or coral, keeping it safely suspended in the water column.

Comparison Chart: Key Features for Differentiating Ray/Skate Eggcases from Shark Eggcases.

Inside the egg case, a single embryo develops, attached to a yolk sac that provides all the nutrients it needs. Development time varies depending on the species and environmental conditions, particularly temperature. In warm, shallow waters, embryos may hatch in a few months, while in colder or deeper environments, development can take more than a year. As the embryo grows, it uses up the yolk and gradually fills the egg capsule. Once fully developed, it emerges as a fully formed miniature shark or ray.

A shark embryo inside its egg case, nourished by a yolk sac that provides all the nutrients it needs to develop. CC: Letoakin

For those walking along the beach, egg cases are sometimes found washed ashore, especially after storms. Recognizing them requires a bit of practice. Most egg cases are dark brown or black, slightly flattened, and may feel rubbery or fibrous. They are empty by the time they reach shore, often showing a small slit or hole at one end where the young animal emerged. 

Although they may look strange or even unappealing at first glance, egg cases are a valuable source of scientific information. By identifying species and documenting locations where egg cases are found, researchers can better understand breeding areas, seasonal patterns, and habitat use – all critical data for elasmobranch conservation. For example, if a particular reef or bay regularly yields egg cases, it could indicate the presence of a local nursery area. Protecting these areas can have a significant impact on the survival of vulnerable shark and ray populations.

If you’re interested in identifying mermaid’s purses (shark and ray eggcases), there are excellent resources available for citizen scientists and marine enthusiasts. However, it’s essential to use a species identification guide tailored to your region. With so many egg-laying shark and ray species found worldwide, using a localized guide ensures accurate identification and contributes valuable data to marine conservation efforts.