Zoologger: Death by world's longest animal

Species: Cyanea capillata

Habitat: cold waters of the Arctic, north Atlantic and north Pacific Oceans

A venomous medusa-like beast as long as a blue whale has emerged as an unlikely defender of the world’s oceans.

The lion’s mane jellyfish is the largest jellyfish known and a contender for the longest animal of all time. Its bell can be 2.5 metres across, and its tentacles can stretch over 30 metres – about the same length as a blue whale. This is 10 metres longer than the tentacles of the famous Portuguese man-of-war – which in any case is not a true jellyfish but a hydrozoan.

Now Aino Hosia of the Institute of Marine Research in Bergen, Norway, and Josefin Titelman of the University of Gothenburg in Sweden have found that captive lion’s manes will readily prey on sea walnuts – transparent animals of the comb jelly type and a voracious invaders of the world’s oceans – and may help to control their numbers in the wild.

The sea walnut is native to the western Atlantic, but has now spread to the North Sea and even to the chill waters of the Baltic. It feeds on tiny plankton, devastating their populations – and in turn it brings about crashes in the numbers of fish that depend on the plankton for food. Like many species invading new territories, it had been thought to have avoided significant predation – but no longer.

Four ages

The lion’s mane jellyfish passes through four stages over the course of its life: larva, polyp, ephyra and the familiar medusa. Each medusa is either male or female, and the female carries her fertilised eggs with her until they develop into larvae.

Each tiny, free-swimming larva – also known as a planula – heads off and installs itself on a rock, which is carefully selected to allow it to hang head-down in the shade, and develops into a polyp. This looks like a miniature sea anemone, and is also known as the scyphistoma because the terminology wasn’t confusing enough already.

After feeding for several months the polyp begins reproducing asexually, making many identical ephyrae. This generally happens in early spring. Each ephyra breaks away and sets off on its own, and may – if it gets enough gelatinous prey – eventually become a full-size medusa.

Sunscreen and stings

The name “lion’s mane” comes from the jellyfish’s tentacles, which come in shades between yellow and red. They are covered with tiny cells bearing dangerous toxins that can paralyse prey animals and cause fatal heart attacks in lab rats.

Humans who get caught in the tentacles generally suffer only mild reactions, unless they are allergic or receive many stings. Slathering yourself with sunscreen apparently protects against the worst of it.

Nevertheless, and despite giving only a limited ability to move around, the tentacles make the lion’s mane a formidable predator. It readily catches small fish, and also targets many other jellies, including the large moon jellyfish.

So it should come as no surprise that the lion’s mane targets the invasive sea walnut – though it is not as successful as it is against more familiar prey. Hosia and Titelman found that the sea walnuts escaped 90 per cent of the time, though they generally suffered damage in the process and were more likely to succumb after repeated assaults.

Still, with enemies like the sea walnut, we need friends like this.

Journal reference: Journal of Plankton Research, DOI: 10.1093/plankt/fbq106

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