The largest jellyfish specimen was found in 1870 off the coast of Massachusetts. Its bell averaged 2.3 meters in diameter and its tentacles reached 36.5 meters in length.
The lion's mane jellyfish, also known as the giant jellyfish or the hair jelly, is the largest known species of jellyfish. Its range is confined to cold, boreal waters of the Arctic, northern Atlantic, and northern Pacific Oceans. It is common in the English Channel, Irish Sea, North Sea, and in western Scandinavian waters south to Kattegat and Øresund. It may also drift into the southwestern part of the Baltic Sea (where it cannot breed due to the low salinity). Similar jellyfish – which may be the same species – are known to inhabit seas near Australia and New Zealand.
The largest recorded specimen was measured by Alexander Agassiz off the coast of Massachusetts in 1865 and had a bell with a diameter of 210 centimeters (7 feet) and tentacles around 36.6 m (120 ft) long. Lion's mane jellyfish have been observed below 42°N latitude for some time in the larger bays of the east coast of the United States. The lion's mane jellyfish uses its stinging tentacles to capture, pull in, and eat prey such as fish, sea creatures, and smaller jellyfish.
Lion's mane jellyfish remain mostly very near the surface, at no more than 20 m (66 ft) depth. Their slow pulsations weakly drive them forward, so they depend on ocean currents to travel great distances. The jellyfish are most often spotted during the late summer and autumn when they have grown to a large size and the currents begin to sweep them to shore.
Most encounters cause temporary pain and localized redness. In normal circumstances, and in healthy individuals, their stings are not known to be fatal. Vinegar can be used to deactivate the nematocysts, but due to a large number of tentacles, medical attention is recommended after exposure.