The largest recorded specimen was measured off the coast of Massachusetts in 1865 and had a bell with a diameter of 210 centimetres (7 feet) and tentacles around 36.6 m (120 ft) long.

[2] 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.

Juveniles are lighter orange or tan, very young lion's manes are occasionally colorless and adults are red and start to darken as they age.

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.

[citation needed] Normally, there is no real danger to humans (with the exception of people suffering from special allergies), but in cases when someone has been stung over large parts of their body by not just the longest tentacles but the entire jellyfish (including the inner tentacles, of which there are around 1,200[11]), medical attention is recommended as systemic effects can be present.

[20][11] In July 2010, around 150 beachgoers were stung by the remains of a broken-up lion's mane jellyfish along Wallis Sands State Beach in Rye, New Hampshire, US.

[21] A photograph widely distributed on the internet appears to show an anomalously large lion's mane dwarfing a nearby diver.

[22] On the television program QI, the show claimed that the longest animal in the world was the lion's mane jellyfish.

This was later corrected – in 1864, a bootlace worm (Lineus longissimus) was found washed up on the coast of Fife, Great Britain, that was 55 m (180 feet) long.

[23] The creature is referred to as the culprit in the 1926 short story "The Adventure of the Lion's Mane" in The Case-Book of Sherlock Holmes by Sir Arthur Conan Doyle.

[24] Seabirds, larger fish such as ocean sunfish, other jellyfish species, and most sea turtles will only attack juveniles or smaller specimens while a fully grown adult is incapable of being eaten, due to their massive size and the abundance of stinging tentacles they possess, although both adults and juveniles have been documented eaten by anemones.