What Is a Mermaid's Purse? Shark Egg Cases on the Sefton Coast

4 March 2026

What it actually is

A mermaid's purse is the egg case of a cartilaginous fish — usually a small-spotted catshark (Scyliorhinus canicula), which most people in the UK know by its older name, dogfish. They're common in the Irish Sea. They're not dangerous. At full size they're about 60–80cm long and live on the seabed.

The leathery case — made from collagen fibres, structurally similar to fingernail — is produced by the female to protect a single fertilised egg. She deposits it on rocky or kelpy seabed, where the four curly tendrils at the corners anchor it in place. The embryo develops inside for nine to twelve months, nourished by a yolk sac. Once the pup hatches, the empty case detaches and drifts until it washes ashore.

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A freshly washed-in mermaid's purse on Ainsdale Beach. The curly tendrils anchored it to kelp on the seabed. The embryo chamber is visible when held to the light.

Why they wash up on the Sefton Coast

The Irish Sea is one of the UK's more productive fishing grounds for small-spotted catshark. The species is not endangered — it's one of the most abundant elasmobranchs in UK waters. The Sefton Coast, with its exposed westerly aspect and significant tidal range, collects what the Irish Sea delivers, which is why beachcombers here find mermaid's purses year-round.

Numbers peak after storms and high tides, when wave action scours the seabed. Early morning after an overnight high tide is the best time to look — the tide deposits them while you sleep and they haven't been disturbed yet. Winter is more productive than summer for exactly this reason.

How to identify it

Small-spotted catshark cases are roughly rectangular, dark brown to black, leathery but flexible. Size varies — typically 4–7cm across the body, with tendrils adding several centimetres. Fresh cases are flexible and slightly slippery. Older ones dry out, become brittle, and lighten in colour.

Hold a fresh case up to bright light and you can often see the hollow embryo chamber, sometimes with the remains of the yolk sac. This is easiest with a torch or strong sunlight behind the case.

You may also find skate and ray egg cases — these are larger, harder, and have pointed horns rather than curly tendrils. The starry ray and thornback ray both occur in the Irish Sea, and their cases turn up occasionally on Sefton beaches.

The Great Eggcase Hunt

The Shark Trust runs the Great Eggcase Hunt — a citizen science project that uses public finds to map shark and ray distribution in UK waters. Recording a mermaid's purse takes about two minutes: photograph it, note the location and date, and submit via the Shark Trust website or app.

The Sefton Coast data feeds into a national picture of where small-spotted catshark is breeding and where other species may be establishing themselves. It's one of those rare cases where picking something up off the beach and taking a photo has a direct conservation use.

What else to look for

The tideline along the Sefton Coast regularly produces razor clams, whelk egg masses (also called sea wash balls or mermaid's necklaces — a white, spongy cluster of tiny capsules), and various bivalve shells. In winter and early spring, jellyfish strandings are common — compass jellyfish and barrel jellyfish both wash up regularly after storms. Full guide at the link below.

The best beachcombing stretch is at low tide on the broad flat sands at Ainsdale or along the Formby Point tideline. Go early, go after a high tide, and bring a bag.