Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

All legs and no body (Image: Norbert Wu/Minden Pictures/FLPA)

Species: The Southern Ocean giant sea spider (Colossendeis megalonyx)

Habitat: The ocean floor around Antarctica, South America, Africa and Madagascar

The Southern Ocean giant sea spider is the stuff of nightmares – its leg span of 25 centimetres more or less equals that of the world’s largest land spiders like the Goliath Bird-Eating tarantula of South America.


But there are two reasons why arachnophobes shouldn’t fear this beast: it lives at the bottom of the ocean around Antarctica and it’s not actually a spider.

Sea spiders belong to a group of arthropods called the pycnogonids, which are found scuttling along the bottom of many of the world’s oceans and seas. Most are relatively small – it’s only around the poles that sea spiders grow large, which is a trait they share with many marine species. Exactly why this happens remains a mystery.

Many sea spiders are carnivorous, dining on worms, jellyfish and sponges. “They have a giant proboscis to suck up their food,” says Florian Leese at Ruhr University Bochum in Germany.

Like true spiders, some sea spiders have eight legs. But not all do. “Some have 10 and even 12 legs,” says Leese.

Bizarre anatomy

Curiously, though, their bodies don’t appear to have much else apart from their long legs and proboscis. “They don’t really have a body,” says Leese. “They have their organs in their legs.”

These creatures are sometimes called the pantopoda – meaning “all legs” – because of their bizarre anatomy.

The lack of an obvious body means sea spiders don’t need to bother with a respiratory system. Simple diffusion can deliver gases to all of the tissues.

The Southern Ocean giant sea spider is one of the most common sea spiders in the waters around Antarctica. It also lives in coastal waters off South America, South Africa and Madagascar, down to a depth of 4.9 kilometres.

It is so widespread that some have wondered whether it really is a single species. To find out, Leese and his colleagues examined DNA taken from 300 specimens. Animal cells usually carry two forms of DNA: most is in the form of nuclear DNA in the cell’s nucleus, but there is a second form of DNA in the mitochondria – often called the “powerhouse of the cell”. Mitochondrial DNA is usually only inherited down the female line.

How many species?

The mitochondrial genes fell into about 20 distinct groups, apparently suggesting the Southern Ocean giant sea spider should really be broken up into 20 distinct species.

But the nuclear DNA showed that many of these apparently distinct species can and have interbred in the recent past. In fact, the team says, if the Southern Ocean giant sea spider is divided into several distinct species, we should probably recognise only five – not 20.

Why is this? The mitochondrial DNA sequences are so distinct that the sea spiders probably began to diverge about a million years ago – perhaps during glacial periods when a deterioration in conditions left small populations of sea spiders isolated from one another in ice-free “refugia”, where they could each develop their own genetic mutations.

But when environmental conditions improved and the spider lineages began expanding out of those refugia, they began to interbreed and hybridise. That’s not unlike the way different human lineages like the Neanderthals, Denisovans and our species interbred when they came into contact after thousands of years of isolation.

The results are important for conservation. Mitochondrial and nuclear DNA often show the same general pattern, says Leese, so when easier-to-analyse mitochondrial DNA indicates one species actually breaks down into several “cryptic” species, conservationists want to protect all of the lineages. But nuclear DNA sequences might show that many of those cryptic species don’t really exist.

“The study advises caution in calling distinct mitochondrial lineages species,” says Leese.

Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.140424