Lion's mane jellyfish

Cyanea capillata, expanding (top), contracting (bottom)

The taxonomy of the Cyanea species is not fully agreed upon; some zoologists have suggested that all species within the genus should be treated as one. Two distinct taxa, however, occur together in at least the eastern North Atlantic, with the blue jellyfish (Cyanea lamarckii Péron & Lesueur, 1810) differing in color (blue, not red) and smaller size (10–20 cm [3+7⁄8–7+7⁄8 inches] diameter, rarely 35 cm [14 inches]). Populations in the western Pacific around Japan are sometimes distinguished as Cyanea nozakii, or as a subspecies, C. c. nozakii. In 2015, Russian researchers announced a possible sister species, Cyanea tzetlinii found in the White Sea.[5]

Lion's mane jellyfish (Cyanea capillata) are named for their showy, trailing tentacles reminiscent of a lion's mane. They can vary greatly in size: although capable of attaining a bell diameter of over 2 m (6 ft 7 in), those found in lower latitudes are much smaller than their far northern counterparts, with a bell about 50 cm (20 in) in diameter. Larger specimens are typically further offshore than smaller ones. 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.

While most jellyfish such as the moon jelly have a circular bell, the bell of the Lion's Mane is divided into eight lobes, resembling an eight-pointed star. Each lobe contains about 70 to 150 tentacles,[6][7] arranged in four fairly distinct rows. Along the bell margin is a balance organ at each of the eight indentations between the lobes – the rhopalium – which helps the jellyfish orient itself. From the central mouth extend broad frilly oral arms with many stinging cells called Cnidocytes.[8] Closer to its mouth, its total number of tentacles is around 1,200.[9]

The long, thin tentacles which emanate from the bell's subumbrella have been characterised as “extremely sticky”; they also have stinging cells. The tentacles of larger specimens may trail as long as 30 m (100 ft) or more, with the tentacles of the longest known specimen measured at 36.6 m (120 ft) in length, although it has been suggested that this specimen may actually have belonged to a different Cyanea species.[10] This unusual length – longer than a blue whale – has earned it the status of one of the longest known animals in the world.[11]

Lion's mane jellyfish ephyrae form between two medusas.

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. Unlike most pelagic jellyfish, they are completely solitary and rarely travel in groups.

Like other jellyfish, lion's manes are capable of both sexual reproduction in the medusa stage and asexual reproduction in the polyp stage.[12] Lion's mane jellyfish have four different stages in their year-long lifespan: a larval stage, a polyp stage, an ephyrae stage, and the medusa stage.[12] The female jellyfish carries its fertilized eggs in its tentacle, where the eggs grow into larvae. When the larvae are old enough, the female deposits them on a hard surface, where the larvae soon grow into polyps. The polyps begin to reproduce asexually, creating stacks of small, immature medusae called ephyrae.[13] The individual ephyrae break off from the stacks, where they eventually grow into the mature medusa stage to become full-grown jellyfish.[14]

Lion's mane jellyfish swimming, side view

Human encounters with the jellyfish can cause temporary pain and localized redness.[15] 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[9]), medical attention is recommended as systemic effects can be present.[16] Although rarely, severe stings in deep water can also cause panic followed by drowning.[17][9]

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. Considering the size of the species, it is believed that this incident was caused by a single specimen.[18]

The lion's mane jellyfish appears in the Sherlock Holmes short story "The Adventure of the Lion's Mane" published in The Case-Book of Sherlock Holmes. Holmes discovers at the end of the story that the true killer of a professor who died shortly after going swimming (shouting "the lion's mane" before he succumbed) was actually this jellyfish. Suspicion was originally laid upon the professor's rival in love, until the latter was similarly attacked (he survived, although badly stung). In the context of the story, it is only because the school professor has a weak heart that he succumbs, as is confirmed by the survival of the second victim.

A photograph widely distributed on the internet appears to show an anomalously large lion's mane dwarfing a nearby diver. The photo was subsequently shown to be a hoax.[19]

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. This claim is, however, disputed because bootlace worms can easily stretch to several times their natural length, so it is possible the worm did not actually grow to be that length. If that is the case, the lion's mane jellyfish would indeed be the longest animal in the world. [20]

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.[12] The leatherback sea turtle feeds almost exclusively on them in large quantities during the summer season around Eastern Canada.[21]

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  Lion's mane jellyfish, expanded
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  Lion's mane jellyfish capturing three moon jellyfishes
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  Lion's mane jellyfish with full threads visible
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  Lion's mane jellyfish contracted
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  Lion's mane jellyfish in star formation
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  Lion's mane jellyfish open, showing underside
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  Life-sized model in the Smithsonian Museum of Natural History, Washington DC, US
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  Small, dead lion's mane jelly washing up on the beach

Wikimedia Commons has media related to Cyanea capillata.

• Video footage of stranded Lion's Mane Jellyfish
• Photos of Lion's mane jellyfish on Sealife Collection