These ‘Immobile’ Sea Sponges Apparently Had Places to Go

With their rigid structures and lack of appendages, sponges can seem more like plants or fungi than the animals that they are. Long assumed to be basically immobile, sponges have been spotted leaving trails on the Arctic ocean floor, evidence they have more going on than scientists previously thought.

The trails are made of the sponges’ spicules—the hard bits of the sponge that protect its softer tissues and provide the creatures’ structure. Spicules are generally spiky and can look just like jacks in some sponge species. Like cheese grating along the seafloor, the sponges left bits of themselves as they moved. The researchers published their findings today in Current Biology.

“You couldn’t tell what the seafloor was because there was such a thick layer of sponge spicules and sponges all over the ground,” said study co-author Autun Purser, a deep sea ecologist at the Alfred Wegener Helmholtz Centre for Polar and Marine Research, in an email. “The sponges reminded me of an endless living plain of life, like something from a Stanislaw Len story.”

To a casual observer, sponges don’t seem so much languid as completely stationary. (Can you blame them for not moving much? Sponge in deep polar regions, especially, have to deal with dreadfully cold water as well as pitch blackness). In their larval stage, the animals can move, but adults have generally been considered sessile. Sponges have no muscles specifically for locomotion, either, so when they’ve moved in laboratory settings, it’s been by expanding and retracting parts of their bodies. In the wild, the researchers suggest, the sponges could be moving around to find food, more hospitable locations, or to give their progeny a fighting chance.

“We thought sponges, when adult, had to make the best of whichever area they end up settling on after the larval stage,” Purser said. “Now it seems if some species do not like their current situation, they can slowly move off in search of better conditions, or alternatively, to give their offspring some space.”

Purser suspects the latter. The team observed the sponges in 2016, using mounted cameras on a titanium sled rig on the research vessel Polarstern. They recorded sponges of varying sizes and species, from over 3 feet wide to less than half an inch across. Over 400 images included spicule trails, and the juvenile sponges tended to be found in the wake of an adult, mobile sponge. That lends credence to the idea that the adult sponges are moving to other areas to give their young some breathing room.

“I think, in a way, this is perfectly linked to the other findings on Antarctic waters,” said César Cárdenas Alarcón, a biologist with the Chilean Antarctic Institute who was not affiliated with the new paper, wrote in an email. “This impressive research about movement in polar sponges is just another piece to the puzzle that helps us understand the mechanisms that sponges, which are dominant members in polar benthic communities, use to survive in such extreme environments.”

Sponges are an extremely ancient group of animals, originating before the explosion of biodiversity that came in the Cambrian Period some 550 million years ago. Despite their long tenure, secrets of their life still elude us, perhaps because they live in environments just as alien to us as Mars.

More: Scientists found unexpected life half a mile under an Antarctic ice shelf