Arctic sponges leave trails as they wander

Adult marine sponges are usually thought to be stationary, picking a spot on the seafloor while still in their larval stage and sticking to it. Lacking muscles to move around, they’re referred to as ‘sessile’ – fixed in one place, as opposed to ‘motile’ marine creatures.

But a paper published in Current Biology has described trails across the Arctic seafloor, made of brown spicules: spikes that belong to the sponges and provide structural support. This indicates that the mature sponges were on the move.

Typical sponge spicule trails. Credit: AWI OFOBS team, PS101; Morganti et al./Current Biology

The researchers examined videos taken by the icebreaker Polarstern on a 2016 mission around the ice-covered Langseth Ridge in the Arctic Ocean. They found a densely populated community of sponges (mostly from three species: Geodia parva, G. hentscheli and Stelletta rhaphidiophora), 10° further north than any previously reported sponge grounds.

“We observed trails of densely interwoven spicules connected directly to the underside or lower flanks of sponge individuals, suggesting these trails are traces of motility of the sponges,” write the researchers, who are based at the Max Planck Institute of Marine Microbiology and the Alfred Wegener Helmholtz Centre for Polar and Marine Research, both in Germany.

Over 70% of the images containing sponges also featured trails.

“This is the first time abundant sponge trails have been observed in situ and attributed to sponge mobility,” add the authors.

Sponge spicule trails on the seafloor. Credit: AWI OFOBS team, PS101

3D modelling of the trails suggested that the sponges changed direction as they moved, and that they moved uphill. This implies the sponges were moving themselves, rather than being shifted by gravity and ocean currents.

The authors suggest a few reasons for the wandering sponges: they may need to move to better access food in the sparse Arctic environment, or movement could assist in reproduction by providing ‘asexual budding’ or leaving substrates for larval sponges to settle on.

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