If you’ve been swimming or surfing in the sea in Cornwall during the summertime, there’s a good chance you’ve laid eyes on, or brushed past, a jellyfish or two! The good news is that most are completely harmless and just minding their own business, floating around like the chill dudes they are. However, there are a couple of species that do sting, so it’s good to know what’s what. Let’s dive into all the details!
Moon Jellyfish (Aurelia aurita)
Moon jellies are the most common type of jellyfish in Cornwall. You can identify them by the four pink or purple rings (gonads) in the centre of their translucent ‘blob’, also known as the bell, which can reach up to 40cm across, though in most cases are a lot smaller. Their short tentacles are hard to see and only about half a centimetre long.
Moon Jellyfish Sting
These jellyfish can be found in Cornish waters from April to September. While they do sting, it’s so mild that you’re unlikely to feel it at all, making them perfectly safe to swim with.
Did you know?
Moon jellyfish can emit a faint glow, this bioluminescence is due to their ability to produce light!
A Guide to Jellyfish in Cornwall & Their Stings 6
Barrel Jellyfish (Rhizostoma pulmo)
Also known as dustbin-lid jellyfish, the barrel jellyfish is the largest species found in UK waters and can reach up to 90 cm in diameter – now that’s a big jelly! They have a large, dome-shaped bell that appears quite mushroom-like and come in pink, pale violet, or bluish-white hues, with a frilly edge and ruffled oral arms underneath. Unlike other jellyfish, they don’t have true tentacles and instead use these arms to capture plankton and tiny marine organisms.
Barrel Jellyfish Sting
Their sting is very mild, similar to a nettle sting, and usually harmless to humans. Barrel jellyfish are also known to actively swim away from movement in the water, making them even less of a threat. They’re most commonly spotted between July and September.
Did you know?
Barrel jellyfish are an important food source for leatherback turtles!
Barrel Jellyfish with dome dome-shaped bell, frilly edge and ruffled oral arms.
Compass Jellyfish (Chrysaora hysoscella)
Sadly, compass jellies won’t help you navigate the seas, but they’re named for the brown V-shaped markings on their creamy white or yellow bell, which resemble a compass rose. They can grow up to 30 cm in diameter, with tentacles reaching up to a metre long in addition to shorter, frilly oral arms.
Compass Jellyfish Sting
These almost-invisible tentacles can deliver a nasty sting, so it’s best to give them space! They can even detach their tentacles, so if you’re stung, check for lingering ones and remove them with tweezers, then rinse with warm water.
Compass jellyfish are less common than other UK species but are most likely to be spotted between July and September. Some believe they allow small fish to shelter among their tentacles, not to protect them, but to lure in more prey. A little mischievous villain of the sea!
Did you know?
Compass jellyfish are active swimmers and often pulse rhythmically through the water.
Compass Jellyfish with a radial pattern which resembles a compass rose
Lion’s Mane Jellyfish (Cyanea capillata)
Known for their thick, flowing tentacles resembling a lion’s mane, this jelly is the largest known jellyfish species in the world – king of the jellies, if you will. Young Lion’s Mane jellies are pink or light orange, while mature ones turn a deep reddish-brown or dark purple.
They’re typically found in the Arctic, North Pacific, and North Atlantic. It’s extremely rare to spot one in Cornwall, as they prefer colder temperatures. While many are smaller, their bell can reach 2.4 metres, and their hair-like tentacles can stretch up to a jaw-dropping 36.5 metres.
Lion’s Mane Jellyfish Sting
The sting of a Lion’s Mane Jellyfish is very painful, so avoid them even when they wash up on the beach.
Did you know?
The longest known animal on Earth was a Lion’s Mane jellyfish measuring 36.6 metres long, which is longer than a blue whale!
Blue Jellyfish (Cyanea lamarckii)
Surprisingly, these striking jellies are relatives of the Lion’s Mane, as they’re in the same genus. They start life yellow or translucent and turn vivid blue or violet as they mature. Their bell reaches around 25 cm in diameter, with a scalloped edge and four blue oral arms underneath.
Blue Jellyfish Sting
Their tentacles can grow up to a metre long, forming a blue, hair-like cluster under the bell. While less common, they can be seen along the Cornish coast between April and July. Their sting is mild, much like a nettle, so no need to panic if you meet one of these violet-hued friends.
Did you know?
Blue jellyfish can glow in the dark thanks to bioluminescence, emitting a soft blue or green light to confuse predators or attract prey!
Crystal Jellyfish (Aequorea victoria)
These near-invisible jellies are named for their almost completely transparent bodies, which glow bright green when disturbed, thanks to their bioluminescence. Their bell is small (around 12 cm) with delicate white lines radiating from the centre. They can have up to 80 tentacles, reaching lengths of 1.8 metres – impressive for such a little jelly!
Crystal Jellyfish Sting
Crystal jellyfish are rarely seen outside Cornwall in the UK, so you could consider them locals. Though, they also appear along the West Coast of North America. Their sting is very weak and you are unlikely to feel much if anything at all.
Did you know?
Scientists discovered Green Fluorescent Protein in crystal jellies, now used in medical and genetic research to study cancer, brain function, and more. The discovery even won a Nobel Prize in 2008!
Stalked Jellyfish (Calvadosia campanulata)
These little guys are quite different from the others. Rather than drifting, stalked jellies attach themselves to rocks, shells, or seaweed via a stalk. Their trumpet-shaped bodies have eight arms, each ending in a cluster of tentacles, more like a mix between a sea anemone and a jellyfish.
Stalked Jellyfish Sting
They’re tiny, just 0.5 to 4 cm tall, and very hard to spot. Look closely along the seafloor — their bright red, green, orange, or brown colours help them blend with seaweed. You’ll also find them in the North Pacific and Antarctica.
Did you know?
Scientists believe stalked jellyfish can cartwheel short distances (aww!) – though they usually stay put once they’ve found the perfect perch.
The Imposters
Comb Jellyfish (Mnemiopsis)
Not true jellyfish, comb jellies are often mistaken for them. They have transparent, oval, or ribbon-like bodies with eight rows of tiny cilia (hair-like projections) that shimmer with rainbow colours in the light. It’s like a jelly light show!
Most have no tentacles, but some (like the sea gooseberry species) have two sticky ones for catching prey.
Did you know?
Comb jellies have a one-way digestive system, their opening serves as both mouth and… the other end.
A Comb Jellyfish with rows of tiny cilia that shimmer different colours.
By-the-Wind Sailors (Velella velella)
You might spot these By the Wind Sailors on Cornish beaches in autumn, when they wash ashore. They consist of a floating blue disc and a triangular sail (up to 7 cm), which catches the wind and determines their drift direction.
By The Wind Sailors Sting
Like the Portuguese man o’ war, they are colonies of polyps working together. They use short tentacles to catch plankton and larvae. They’re generally harmless, but can cause mild irritation to sensitive skin. If you’re unsure of which species it is, it’s best to steer clear.
Did you know?
Velella reproduces both asexually (within the colony) and sexually (by releasing medusa-like offspring into the ocean)!
Portuguese Man o’ War (Physalia physalis)
These venomous stingers are usually found in warmer oceans, but can appear in Cornwall in autumn, blown in by south-westerly storms. Though they look like jellyfish, they’re actually colonial hydrozoans, made up of different specialised polyps.
Portuguese Man o’ War Sting & What to do
They have a gas-filled, iridescent float (up to 30 cm) and tentacles that can reach 30 metres! Their powerful nematocysts deliver a highly venomous sting to humans. It’s rarely fatal, but serious reactions can occur, especially in children or allergic individuals. A lot of beaches in Cornwall are manned by RNLI lifeguards who are able assist you if you suspect you’ve been stung.
Did you know?
Their predators include loggerhead turtles, ocean sunfish, and the blue dragon nudibranch (Glaucus atlanticus), which eats their tentacles and steals their stinging cells!
Portuguese Man-o-War Jellyfish with a gas-filled, iridescent float.
Whether you find them fascinating or freaky, jellyfish are an incredible part of Cornwall’s coastal wildlife. From the gentle Moon Jelly to the dramatic Portuguese Man o’ War, each species plays a unique role in the Cornish marine ecosystem. While a few can sting you, most are harmless, and armed with your new knowledge, you’ll be able to identify and appreciate these squishy sea-dwellers.
Some beaches in Cornwall have more common sightings of jellyfish, if you’re jelly-curious – here are some of the best spots to go looking: Gwithian, Polzeath, Harlyn, Porthtowan, Perranporth and Portreath.
