Footage from a recent Arctic expedition shows spiny trails left by sponges as they rove across the seafloor, contradicting earlier assumptions that the animals stay put once they reach adulthood.
Scientists discovered trails left by the slow-motion journeys in 69% of the images they captured that contained living sponges. The animals are likely spurred into motion by the need to search for food or disperse their offspring, the researchers reported April 26 in Current Biology.
"Maybe other sponges do move very slowly, too, but we just haven't seen it anywhere else," said Autun Purser, a senior researcher at the Alfred Wegener Helmholtz Centre for Polar and Marine Research in Germany and co-author of the findings.
He said it's possible that in other parts of the world, there are more particles, falling sand and detritus to cover up such trails over time, "whereas here in the Arctic, where there's very little stuff raining down, the sponge trails remain visible for a long time."
Purser and his colleagues noticed the intriguing trails on a 2016 expedition of an area about 700 kilometers from the North Pole. The permanently ice-covered habitat of the submerged peaks of the Langseth Ridge offers few nutrients.
However, scientists have known for more than a decade that sea sponges inhabit the area, since a previous team recovered a massive sponge that filled most of a metal 50-centimeter-by-50-centimeter metal box that was sent to the seafloor.
When Purser and his team towed a marine camera behind their ship, they found a thriving community of sponges dwelling on the seamounts, hundreds of meters below the surface.
"This whole chain was completely covered in sponges," Purser said. "Sometimes 100% of the seafloor was just sponge."
The researchers also noticed a dense mat of abandoned spicules — spines that normally protrude from the animals' soft bodies — covering much of the rocky substrate. Criss-crossing this mat were fresh trails made from densely interwoven spicules, many of which led right to the underside or flanks of sponges.
"It looks," Purser said, "as if you've grabbed the chalk and pushed it across the seafloor; it's a really solid line all the way through."
The researchers sent a robot known as a hybrid remotely operated vehicle down for a closer look, and found that the trails were about 10 centimeters thick and could stretch on for meters, forming a sort of three-dimensional ridge behind the sponges.
"You can see in some of the photographs that other animals, like shrimp and fish, are hiding behind this trail," Purser said. "It's actually a feature of the seafloor; as the sponges move, they are providing ecosystem niches for other animals to use, as well."
He and his colleagues used the images and video captured by the robot to create three-dimensional models of the seafloor. They saw that the sponge trails were frequently interwoven and overlapped each other, suggesting that the animals sometimes change direction. The team also realized that many of the sponges were on the uphill ends of their trails, which means the animals are actively moving and not just sliding downhill due to gravity.
The sponges may leave the trails behind them while looking for scraps of food or a place to filter-feed. The trails might even trap bits of floating detritus within their spines to be discovered and gobbled up later. This could explain why the seemingly barren environment of the seamounts can support so many sponges, which could reach more than a meter in diameter.
"We were surprised to see so many animals there, and one possible reason is that this mat that has been made by the sponges as they move around is really good for accumulating particles, which ... are transported in the ocean currents under the ice into this area," Purser said.
Purser also suspects that the trails play a role in the sponges' reproduction. In many of the photos, whitish juvenile sponges were visible amid the spines. The trails may protect the vulnerable sponges and also leave them plenty of room to feed without competing with their parents.
"If you leave the offspring and move away, you're giving that offspring the chance to filter the water surrounding it," Purser said.
Although a few smaller sponge species have been observed to move in adulthood, the Arctic-dwelling sponges were thought to be among the many species that become stationary after their larval stage. The sponges don't have any muscles, but Purser and his team have an idea of how they might be crawling along the seafloor.
"They expand across their spines and then sort of suck themselves across one side of themselves and leave the other spines behind," he said. "It's a strange, pulsing sort of movement."
The scientists did not witness the sponges hustling along the seafloor; Purser suspects the animals only move a few millimeters per day, if that. In the future, he plans to return to the area and drop a camera to monitor the area over a period of several years in hopes of catching the sponges in motion.
The secluded habitat favored by the sponges may not be permanently covered by ice in future years, perhaps opening the region to other kinds of animals.
"It's important to try and understand and discover these strange ecosystems before they vanish, and also to show how we may be losing biodiversity that we don't even know about by ice retreat if we don't explore it all while we still have the chance to do so," Purser said.
The study, "In situ observation of sponge trails suggests common sponge locomotion in the deep central Arctic," published April 26 in Current Biology, was authored by Teresa M. Morganti, Max Planck Institute of Marine Microbiology; Antje Boetius, Max Planck Institute of Marine Microbiology and Alfred Wegener Helmholtz Centre for Polar and Marine Research; Autun Purser and Laura Hehemann, Alfred Wegener Helmholtz Centre for Polar and Marine Research; Jonas Blendl, Ludwig Maximilians University and Alfred Wegener Helmholtz Centre for Polar and Marine Research; Hans Tore Rapp, University of Bergen; Christopher R. German and Michael V. Jakuba, Woods Hole Oceanographic Institution; and Beate M. Slaby, GEOMAR Helmholtz Centre for Ocean Research Kiel.