The lion's mane jellyfish (Cyanea capillata)

Human encounters with the jellyfish can cause temporary pain and localized redness.[14] In normal circumstances, however, and in healthy individuals, the stings of the jellyfish are not known to be fatal; vinegar can be used to deactivate the nematocysts. If there is contact with a large number of tentacles, however, medical attention is recommended after exposure.

There may be a significant difference between touching a few tentacles with fingertips at a beach and accidentally swimming into the jellyfish. The initial sensation is more strange than painful and feels like swimming into warmer and somewhat effervescent water. Some minor pain will soon follow. 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[8]), medical attention is recommended as systemic effects can be present.[15] Although rarely, severe stings in deep water can also cause panic followed by drowning.[16][8]

On a July day in 2010, around 150 beachgoers were stung by broken-up Lion's mane jellyfish remains in Wallis Sands State Beach, Rye, New Hampshire, USA. Considering the size of the species, it is possible that this incident was caused by a single specimen.[17
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