Shark eggs are some of the most unusual reproductive structures in the animal kingdom. They come in two main forms: the tough, leathery egg cases produced by species like catsharks and horn sharks — often called “mermaid’s purses” — and the soft, yolk-rich eggs retained inside live-bearing sharks. Not all sharks lay eggs at all. Understanding what shark eggs look like, which species produce them, and how shark reproduction works reveals how diverse these animals truly are.
What Do Shark Eggs Look Like? The Mermaid’s Purse
The most recognizable shark egg is the egg case of oviparous (egg-laying) sharks — a tough, leathery, rectangular or pouch-shaped capsule commonly called a “mermaid’s purse.” These are frequently found washed up on beaches worldwide.
Key features of shark egg cases:
- Material: Tough collagen fibers — similar to fingernail material. Flexible but extremely durable.
- Shape: Varies significantly by species. Most are rectangular with curling tendrils at each corner that anchor the egg to seaweed or rocky substrate. Horn shark eggs are distinctive spiral-shaped corkscrews, completely unlike the standard mermaid’s purse.
- Color: Typically dark brown, olive, or black when fresh. They bleach to tan or white after washing ashore and drying out.
- Size: Ranges from 5 cm (small catshark species) to 30+ cm (larger species like the nurse shark). The egg case of the swell shark (Cephaloscyllium ventriosum) is about 10–12 cm.
- Tendrils: The curling filaments at the corners anchor the case to vegetation or structure on the seafloor. Some species have long, thread-like tendrils; others are short or absent.
Which Sharks Lay Eggs?
Approximately 40% of shark species are oviparous — they lay eggs. The rest are viviparous (live-bearing) or ovoviviparous (eggs hatch inside the mother). Common egg-laying species include:
- Catsharks (family Scyliorhinidae) — the most common source of mermaid’s purses found on beaches. Dozens of species worldwide, including the small-spotted catshark common in European waters.
- Horn sharks (Heterodontus spp.) — produce unique spiral-flanged egg cases the female wedges into rock crevices. The flanges screw the egg firmly in place.
- Bamboo sharks (Chiloscyllium spp.) — popular in home aquariums due to their egg-laying; the eggs hatch in 3–4 months.
- Nurse sharks — actually ovoviviparous, despite common misconception
- Swell sharks (Cephaloscyllium spp.)
- Dogfish sharks (some species) — though many dogfish are live-bearing
What Is Inside a Shark Egg?
Inside each egg case is a single embryo surrounded by a large yolk sac — its only food source throughout development. If you hold a fresh, intact shark egg case up to a bright light, you can see the embryo and yolk sac through the translucent walls. The embryo begins as a small disc and develops over weeks to months into a fully formed juvenile shark.
As the embryo grows, it absorbs the yolk. By the time it hatches, the yolk sac is fully absorbed and the pup is ready to feed independently. There is no parental care — once the egg is laid and anchored, the female moves on.
How Long Do Shark Eggs Take to Hatch?
Incubation time varies significantly by species and water temperature:
- Small-spotted catshark: 6–9 months in typical British waters
- Horn shark: 6–9 months
- Bamboo shark: 3–4 months (warmer water speeds development)
- Swell shark: 7–10 months
Warmer water generally accelerates development. Cold-water species can take over a year to hatch.
Mermaid’s Purses on Beaches
Empty shark egg cases wash ashore on beaches worldwide after the pup has hatched and the case has broken free from its anchor. Fresh cases are dark and pliable; older beached cases bleach and become brittle. The Shark Trust in the UK runs a “Great Eggcase Hunt” citizen science program that has logged over a million egg case sightings — a valuable dataset for shark population monitoring.
If you find an egg case on the beach, check whether it is empty (the pup has hatched) or still contains an embryo. Intact, unhatched cases can sometimes be returned to the sea — though survival chances are low once the case has been disturbed.
Shark Egg Facts
- Common name: Mermaid’s purse
- Material: Tough collagen — similar to keratin
- Species that lay eggs: ~40% of all shark species
- Most distinctive egg: Horn shark — spiral corkscrew shape
- Incubation: 3 months to 1+ year depending on species and temperature
- Contents: Single embryo + yolk sac (sole food source)
Frequently Asked Questions
Are mermaid’s purses from sharks or rays?
Both. Rays and skates also produce egg cases, and their cases wash up on beaches alongside shark cases. Skate egg cases tend to be larger and more rectangular with stiff horns rather than tendrils. The Shark Trust provides identification guides to distinguish shark from ray and skate cases.
Do great white sharks lay eggs?
No. Great white sharks are ovoviviparous — the embryos develop inside the mother and are born live. The young hatch from eggs inside the uterus and may consume unfertilized eggs (oophagy) before birth. Great whites give birth to 2–14 live pups.
Can shark eggs be kept in an aquarium?
Yes — bamboo and catshark eggs are commonly kept in home marine aquariums. They require clean, well-oxygenated saltwater and appropriate temperature. Watching the embryo develop through the translucent case is a popular educational experience.
What happens if a shark egg washes ashore?
An intact, unhatched egg case that washes ashore has a low chance of survival — it needs seawater and the embryo may already be compromised. Some wildlife rescue organizations attempt to care for stranded intact cases in aquarium conditions. Empty cases (where the pup has already hatched) are harmless and make interesting natural history specimens.