Invertebrates
Lion's Mane Jellyfish
Cyanea capillata · also called lion's mane jellyfish, giant jellyfish, hair jelly
The lion’s mane is the biggest jellyfish there is — a rippling orange-brown bell that can span two metres, trailing a mane of hundreds of fine tentacles that in the coldest northern seas stretch for tens of metres behind it. By tentacle length it rivals the longest animals on Earth.
Overview
Lion’s mane jellyfish drift through the cold and cool waters of the northern hemisphere — the Arctic, the North Atlantic, the North Pacific. They are largest and most abundant in the far north; further south, in the English Channel or off New England, they are smaller and less common. Each jelly grows from a polyp in spring, feeds and swells through summer on plankton, small fish, and other jellyfish caught on its tentacles, and dies in autumn — a life of about one year.
The largest jellyfish
The bell is typically 30–50 centimetres but reaches around 2 metres in the Arctic. Beneath it hang hundreds of tentacles in eight clusters, trailing 10–30 metres and, in one 1870 Massachusetts specimen, a recorded 37 metres behind a 2.3-metre bell. Tentacle length is stretchy and hard to pin down, but even cautiously the lion’s mane is the biggest jellyfish and among the longest animals.
The sting
For a healthy adult a lion’s mane sting is painful but rarely dangerous — burning, welts, cramps, fading over hours. The risks are a massive sting from a very large jelly or an allergic reaction; a few deaths are attributed to those cases. Detached tentacles still sting, so washed-up jellies must not be touched.
A drifting nursery
Juvenile fish — whiting, haddock, and especially the prowfish — shelter among the tentacles, apparently immune to the stings, gaining protection and stealing the jelly’s catch.
Status
The lion’s mane jellyfish has not been assessed for conservation. It is abundant across the cold northern oceans, with populations that swing widely from year to year.
Lion's Mane Jellyfish: common questions
Is the lion's mane jellyfish really the largest jellyfish in the world?
Yes, by both bell diameter and tentacle length. In the cold seas of the far north, bells can reach around two metres across, and the fine tentacles — hundreds of them, in eight clusters — trail for tens of metres. The famous 1870 Massachusetts specimen was recorded with a 2.3-metre bell and 37-metre tentacles. Tentacle length is elastic and hard to measure precisely, so the "longer than a blue whale" claim should be taken with some caution, but even conservatively the lion's mane is the biggest jellyfish and one of the longest animals by any reasonable reckoning.
How bad is a lion's mane sting?
For most healthy people, unpleasant but not serious. Contact with the tentacles causes an immediate burning pain, red welts, and sometimes muscle cramps, nausea, and sweating, usually easing over a few hours. It is not in the league of a box jellyfish. However, a very large jelly can deliver a huge number of stings at once, which can cause a severe systemic reaction, and people with allergies are at higher risk; a small number of deaths have been attributed to lion's mane stings, generally in those circumstances. Detached tentacle fragments in the water or on the beach can still fire their stinging cells, so washed-up jellies should not be touched.
What should you do if you are stung?
Get out of the water. Carefully remove any tentacle pieces without rubbing (use a gloved hand, tweezers, or the edge of a card). Rinse the area with seawater, not fresh water, which can trigger unfired stinging cells; vinegar is recommended for some jellyfish but its effect on lion's mane is uncertain, and hot water (as hot as can be tolerated) or a hot pack often helps with the pain. Do not urinate on the sting — that is a myth and can make it worse. Seek medical help for a large sting, a sting near the eyes or mouth, or any sign of a whole-body reaction such as difficulty breathing.
Why do fish swim among the stinging tentacles?
A lion's mane jellyfish drifting through open water is a mobile refuge. Juveniles of several fish — including whiting, haddock, and particularly the prowfish — shelter within the curtain of tentacles, where larger predators will not follow. The young fish appear to be immune to or tolerant of the stings, or careful enough to avoid the most potent tentacles, and they also pilfer prey the jellyfish has caught. It is a loose, one-sided association — the fish benefit, and the jellyfish is largely indifferent, though it may occasionally eat one.