This sea spider can be found along Norwegian shores

Researchers have identified several thousand species of sea spiders. Most of them live deep in the sea, at depths of over a thousand metres.

But one of these species can be found in the Norwegian tidal zone.

"Many legs that are jointed together"

For some, the sight of sea spiders might evoke cold fear. In Norwegian waters, they range from just a few millimetres wide up to about 25 centimetres

The largest sea spiders in the world can span about 70 centimetres.

"They barely have a real body. They're basically many legs that are jointed together," Gro I. van der Meeren tells Science Norway.

She is a researcher and ecosystem specialist at the Institute of Marine Research (IMR), which has mapped many different species of sea spider on the Norwegian seabed.

They are so slender that some of their internal organs are actually located in their legs.

The Latin name, Pycnogonida, means many knees. 

They are predators

"People working in oil, gas, and underwater filming often come across them," says van der Meeren. 

For most of us, encounters with these creatures are rare. However, in colder regions, some sea spiders live much closer to the surface – between 12 and 18 metres deep.

"They're predators. They feed on molluscs by piercing them with a proboscis and sucking out the insides," he says.

Can be found along Norwegian shores

Van der Meeren points to a small variant that lives among seaweed in the tidal zones along the Norwegian coast: 

"It's called the sea rose louse, though it's not a louse at all," she says.

It is a small and compact sea spider. Van der Meeren points out that it is especially adapted to the very harsh conditions in the tidal zone, where both predators and crashing waves are a constant threat. 

The sea rose louse has adapted with a compact body and short legs. It is also only a few millimetres in size. 

This is just one of the many ecological niches sea spiders have adapted to.

In a recent study published in PNAS, researchers describe a species of sea spider that lives exclusively along cracks on the seabed in the Pacific Ocean. There, it lives off methane. 

Members of the same family have been found along the Mid-Atlantic Ridge in the southern Atlantic.

So far, this specific species has not been found in our parts of the world's oceans, but there are similar methane emissions from cracks on, for example, the seabed in the Barents Sea.

Gro van der Meeren believes they might be here too, even though no one has discovered them yet. 

"No one's really searched for these kinds of sea spiders," she says.

They are only a few centimetres in size and live only near cracks where methane seeps out.

Are they spiders?

Sea spiders have existed for hundreds of millions of years.

"Sea spiders are a very ancient animal group. It shows how incredibly long they've been crawling around on this planet," the researcher says.

But are they really spiders? 

Sea spiders are not the same as land spiders, known as arachnids, according to the American NOAA. 

Both sea spiders and land spiders are arthropods, but even that classification is debated.

"They are considered part of the arthropods, but some question that too, because they're so unusual," says van der Meeren.

This remains an ongoing debate. 

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Translated by Alette Bjordal Gjellesvik

Read the Norwegian version of this article on forskning.no

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