From the deck of their magnificent sailing boat, in a breathtaking natural scenery, researchers are studying an opportunistic sea lily that hitchhikes on the back of a crab.
Between 2015 and 2020, from the deck of Oceania, a Polish polar expedition sailing vessel, oceanographers set photo traps 35 meters down in the western fjords of Svalbard. “Oceania is incredibly seaworthy and you can do really good research from her deck,” describes Kajetan Deja, an oceanographer at the Institute of Oceanology of the Polish Academy of Sciences. “Sailing on this vessel is kind of romantic and offers good access to the water and the surrounding nature.” Indeed, his team of researchers has just been going through the recordings, and made a great discovery. A bit like oceanographers on their sailboats, sea lilies attach to the backs of moving crabs, whereas this filter-feeding animal usually hangs on the bottom.
This curious and unique pairing was described for the first time in Polar Biology on July 8. According to the authors, the sea lily in question, Heliometra glacialis, moves in this way towards new spaces opened up by retreating glacial tongues. Despite its name and appearance, a sea lily is not a plant. Rather, it belongs to the Crinoides group, closely related to sea urchins, sea cucumbers and starfish.
It’s a species that lives in the northern hemisphere, in the cold waters of the Arctic. “It has often been recorded on the outer shelf of the Barents Sea, Greenland and the North Atlantic,” describes Kajetan Deja in detail. Usually, this featherstar prefers open water to bays or small fjords, because “it needs strong currents and water movement, conditions usually found along the edges of the shelf”, he adds. Yet the crab-riding sea lilies have been detected on underwater photo traps at the bottom of Mijenfjorden and Eklanfjorden, on Svalbard, in the sediment plumes of melting glaciers. The question is, why do they travel so far?
These freed-up spaces are attracting them to a new phenomenon: underwater plankton cascades. Tiny saltwater organisms such as copepods and diatoms perish on contact with the incoming freshwater in estuaries. “Freshwater currents between land and sea are accelerated by global warming,” says Agnès Baltzer, a sedimentologist at the University of Nantes. As a result, plankton are sinking down through the water column. “Heliometra glacialis feeds on fresh organic suspended matter, large diatoms and zooplankton, mainly copepods”, points out Kajetan Deja.
The lily filters the water fixed on the substrate. “The most typical bottom for this species is gravel mixed with mud and sand”, Deja notes.”To find food, it can move around on its own, walking with its cirri or is carried by the current. But the question is why settle on the back of a crab?
Near glacier tongues, unstable bottom surfaces and turbid water are not ideal for sea lilies, unless they find a solid substrate such as a crab. This prompts a new question: is it symbiosis? Apparently not, since the lily can unhook itself, and unlike Oceania‘s Arctic oceanologists, it can leave the vessel at any time to further move around in Svalbard’s fjords.
Camille Lin, PolarJournal
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