6 species of jellyfish to look out for when swimming in the sea

There’s nothing like a swarm of jellyfish to cause a bit of a stir amongst sea swimmers. And, typically, just as people feel they might want to take a dip in the sea, jellies also tend to appear as the sun starts to shine and the sea begins to warm in spring and summer!

Fortunately, not all sting when they accidentally drift against your skin. Jellyfish are really quite beautiful. Some kinds come in a range of striking colours, like the purple jellyfish (which can be orange or brown!) and striped patterns (like the compass jellyfish). Others are plainer, and can all look very similar, especially those such as the moon jellyfish which bud off as identical clones from a “parent” polyp on the seabed.

Whether you’d like to see lots of jellies, or you would prefer to avoid them, you might like to know that their numbers tend to build up in sheltered coves, when onshore winds and currents bring them in. They are a food source for some surprising animals. Leatherback turtles grow slowly but steadily on a diet of jellies. Ocean sunfish, too, grow big on a jelly diet. People eat them, though they haven’t caught on as a UK staple – at least not yet! Should you touch them? Best not. I wouldn’t normally be put of from a dip in the sea by a handful of spineless, brainless jellies in the vicinity – but it’s good to know which ones pack a serious sting!

Lion’s Mane Jellyfish (Cyanea capillata)

A large, striking animal, beautiful but dangerous, with a potentially severe sting. Has large, reddish brown, umbrella-shaped bell with a mass of long, thin hair-like tentacles.Usually found in northern waters, at any time of year.
The species featured as a surprise killer in a Sherlock Holmes story, “The Adventure of the Lion’s Mane”.

Above: A Lion’s mane jelly. Photo by Dan Hershman (https://www.flickr.com/photos/hershman/ (https://www.flickr.com/photos/hershman/253773774/) [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons)

Moon Jellyfish (Aurelia aurita)
Mostly harmless, though may sting sensitive skin. Very common, can bloom in large numbers when our chilly seas begin to warm up, or cool down. It has a transparent, umbrella-shaped bell edged with short hair-like tentacles, and four rings towards centre. It can be up to 40cm in diameter, but is usually less.
Found all around the North Atlantic region, especially common in sheltered waters in the west of Scotland.
Forms mating aggregations in late summer, seemingly using the sun as a compass.

Image credit: André Karwath

Compass Jellyfish (Chrysaora hysoscella)
Possibly the most typical-looking jellyfish, with round bell-shaped body and long tentacles. Also called sea nettle, this jellyfish stings. Colour is variable, but usually has pale umbrella-shaped bell with brown V-shaped markings, a little like the divisions on a compass. Typically up to 30cm.
Found mainly to the west of Britain, but can appear anywhere. Sometimes found with a juvenile fish sheltering amongst its tentacles, apparently unharmed by its stings.

Image credit: Anna Fiolek, NOAA Central Library.

Barrel Jellyfish (Rhizostoma octopus)
A surprisingly substantial jelly, robust and spherical, with no tentacles but eight thick frilled arms. Bulky and white with pretty purplish fringe. Up to a metre in diameter.
Found to the south and west.
This is a favourite food of the leatherback turtle and the ocean sunfish – two of the very biggest creatures in our seas.

Image credit: Ales Kladnik from Ljubljana, Slovenia

Blue Jellyfish (Cyanea lamarckii)
Similar in shape to lion’s mane, but much smaller, and a relatively mild sting – though this can still be painful. Purplish blue lines radiate visibly through the bell-shaped body. Up to 30cm.: Found all around the UK
While it appears quite similar to the lion’s mane, it is a distinctly separate species.

Image credit: Denis Barthel

Mauve stinger (Pelagia noctiluca)
A small, attractively marked jelly, but possesses a surprisingly powerful sting. Has a deep bell with pink or mauve warts, up to only 10cm.All too common in the Med. Eruptions of huge numbers, in spring or autumn, can cause real problems for fish farms – collectively the small jellies can kill many hundreds of salmon.

Image credit: Hans Hillewaert

If you are wondering why the Portuguese man of war isn’t listed, it is because it isn’t actually a jellyfish but rather a colony of thousands of different organisms. Find out all about the Portuguese man of war here

8 things you didn’t know about jellyfish

A reader tip: These “jellies” are an important but little studied group, and the Marine Conservation Society wants to hear about your sightings of them, HOWEVER SOME JELLYFISH STING, SO DON’T TOUCH!

The NHS has some advice here on what to do if you do get stung by a jellyfish