Is the lion's mane jellyfish truly the longest animal on our planet?

Nature · Published 29 September 2026 Swipe more like this →

In short: With tentacles stretching over 36 meters, the lion's mane jellyfish is a contender for the title of the world's longest animal. Yet, this giant of the cold, boreal seas remains a mystery, as its true dimensions are often debated and its stinging reach can persist long after the creature has vanished.

The lion's mane jellyfish (Cyanea capillata) is a master of the cold, boreal waters of the Arctic, Atlantic, and Pacific Oceans. Named for its trailing, hair-like tentacles, this species is one of the largest jellyfish known. While their bells can reach diameters of over two meters, size varies significantly by latitude; those in northern regions grow much larger than their counterparts in warmer, southern waters. The bell itself is uniquely divided into eight lobes, creating an eight-point star appearance, with each lobe housing a balance organ called a rhopalium to aid orientation.

The creature's reputation for extreme length stems from a specimen measured off the coast of Massachusetts in 1865, which reportedly featured tentacles 36.6 meters long. This measurement has sparked skepticism among researchers due to a lack of standardized documentation. The only potential rival for the title of longest animal is the bootlace worm (Lineus longissimus), which was described in 1864 as exceeding 55 meters. However, because bootlace worms are highly elastic and can stretch well beyond their natural size, the lion's mane jellyfish remains a strong candidate for the record.

Beyond its size, the jellyfish is a solitary drifter, relying on ocean currents to navigate the upper 20 meters of the water column. Its life cycle is complex, involving four distinct stages: larva, polyp, ephyra, and the mature medusa. While their stings are generally not fatal to healthy humans, they are notoriously painful. The danger is compounded by the fact that detached tentacles can retain their stinging ability for up to 25 days, posing a persistent threat to marine life and swimmers alike.

Taxonomy within the genus Cyanea remains a subject of active debate, with some zoologists suggesting that various species might actually be one. Despite this, the lion's mane occupies a distinct place in both marine biology and culture, famously appearing as the antagonist in Sir Arthur Conan Doyle's 1926 Sherlock Holmes story, 'The Adventure of the Lion's Mane'.

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