Jellyfish on the Sefton Coast — What Are They and Do They Sting?

4 March 2026

Barrel jellyfish — the big flat one

If you've found a large, flat, disc-shaped jellyfish — dark brown or grey-purple, iridescent in the light, the size of a dinner plate or bigger — that's a barrel jellyfish (Rhizostoma pulmo). It's the UK's largest jellyfish species, capable of reaching 90cm across the bell and 35kg in weight. The ones that wash up on the Sefton coast are typically 30–60cm.

The sting of a barrel jellyfish is considered negligible for most people. The tentacles are fused and the nematocysts are too small to penetrate adult human skin effectively. You can handle one without concern. That said, don't rub your eyes afterwards, and don't let the dog eat it — digested nematocysts can still cause a reaction.

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A barrel jellyfish on Ainsdale Beach. The dog has correctly identified it as interesting. The iridescent purple colouring is typical of freshly stranded specimens.

Compass jellyfish — the one with the markings

Compass jellyfish (Chrysaora hysoscella) are identifiable by the brown V-shaped markings radiating outward from the centre of the bell, like the points on a compass. They're typically smaller than barrel jellyfish — 15–30cm bell diameter — and more translucent, with long trailing tentacles.

These do sting. The sting is mild compared to lion's mane jellyfish, but it's real and it's uncomfortable — similar to a nettle sting that lingers. If you or a child makes contact, rinse with seawater (not fresh water, which causes nematocysts to discharge further), don't rub, and remove any remaining tentacle fragments with something flat. Most people have no reaction beyond temporary irritation.

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Compass jellyfish (Chrysaora hysoscella) — identifiable by the brown V-shaped patterning. This species has a real sting. Rinse with seawater if contact occurs.

Why they strand here

Jellyfish strandings are a normal feature of the Sefton coast in winter and early spring. Storms and strong westerly winds push them toward shore; the broad flat beaches and significant tidal range strand them as the tide drops. It's not a sign that something is wrong — in fact, jellyfish blooms follow plankton blooms, which signal a productive, healthy sea.

Numbers vary year to year. Some winters produce dozens of strandings per day over a stretch of beach; some years the coast is mostly clear. A notable stranding is worth reporting to the Marine Conservation Society's jellyfish survey, which tracks distribution and abundance around UK coasts.

Dogs and jellyfish

Dogs are attracted to jellyfish on beaches and will try to bite or roll on them. A stranded jellyfish retains functional nematocysts for hours after death — the sting reflex is automatic and does not require the animal to be alive. Compass jellyfish in particular can sting a dog's mouth and tongue, causing drooling, pawing at the mouth, and occasional vomiting.

The standard advice: keep dogs away from stranded jellyfish, particularly anything with visible trailing tentacles. If contact occurs, rinse the dog's mouth with water and monitor. Severe reactions are uncommon but seek veterinary advice if symptoms persist.

Other species to be aware of

Moon jellyfish (Aurelia aurita) also wash up occasionally — these are the classic translucent disc with four pink rings visible through the bell. They have no significant sting.

Portuguese man o'war (Physalia physalis) is a different organism entirely — a siphonophore, not a true jellyfish — with a potentially serious sting. It has been recorded on UK Atlantic coasts but is genuinely rare on the Sefton coast. It looks like a blue/purple translucent bladder with long trailing tentacles and is unmistakable. If you see one, don't touch it, move children away, and report to the Marine Conservation Society.