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Giant sea spiders can only move at glacial speeds, but they are still deadly predators that can catch prey much faster than them.

Sea spiders are found throughout the ocean. Despite their name, they are not true spiders but belong to .

Many sea spiders are 鈥渟tealth vampires鈥�, says at the University of Hawai鈥榠 at M膩noa. They creep around the seabed in search of immobile prey, like anemones or corals. Once a sea spider stumbles on a victim, it punches small holes in it with its proboscis: a long tube with a mouth on the end. Then it sucks up body fluids, often without killing the victim.

鈥淚t鈥檚 more like grazing or parasitism,鈥� Moran says.

But she and her colleagues have found that some sea spiders can hunt more actively.

They studied Southern Ocean giant sea spiders (), which are bright orange and have a legspan of 25 centimetres, comparable to the largest true spiders.

In the waters around Antarctica, Moran saw them feeding on jellyfish and small molluscs called sea butterflies (), which look like soggy pink popcorn and often live nearer the surface.

They wondered how the sluggish sea spiders could catch the relatively speedy sea butterflies. To find out, they captured some sea spiders and put them in tanks with sea butterflies. They recorded what happened with time-lapse photography.

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While the sea butterflies mostly float around, they occasionally hit the seabed, which stuns them temporarily. At this point a sea spider would stab the sea聽butterfly with its proboscis and feed.

The attacks were still 鈥渧ery leisurely鈥�, Moran says. They lack the whip-like speed of, say, a chameleon鈥檚 tongue. 鈥淚 guess it鈥檚 spectacular for an Antarctic sea spider, but it鈥檚 not super fast,鈥� she says.

It鈥檚 not clear how the sea spiders can detect their prey. They have eyes, but these would be little use at lower depths or during the long dark of an Antarctic winter. Instead, they may detect movement in the water, or use chemical sensors to 鈥渟mell鈥� prey.