Many scientists believed that sea sponges did not move. However, researchers report today in Current Biology that sponges do creep deep in the Arctic Sea, and they sacrifice pieces of their own bodies to do so.
During an Arctic expedition, scientists aboard the icebreaker Polarstern used a boat-towed camera and a remote-controlled aquatic vehicle to survey an underwater mountain ridge. The researchers discovered a thriving community of sponges at depths ranging from 1000 to 580 metres, beyond the reach of sunlight. They also discovered spicule trails—fragments of the sponge skeleton—that were linked to many of the creatures.
Because many of the animals were found on the uphill ends of these trails, and the site lacked evidence of strong flows, the researchers ruled out gravity and currents as possible sponge-moving forces. Instead, the sponges are moving on their own, according to the team.
The sponges, according to the scientists, sink their spicules into the ground and pull on them to propel their bodies forward. The embedded spicules rip off the animals’ bodies as they move forward, leaving a trail of skeletal fragments and fleshy bits behind. (The image above shows a zig-zagging spicule trail.) Although laboratory experiments had shown that some sponges were capable of this behaviour, no evidence had been found in the wild.
The reason the animals are crawling around in the first place, according to the researchers, is to scavenge for food in the nutrient-depleted polar depths. Another possibility is that the sponges move to disperse their offspring, or that they create spicule trails to provide surfaces for sponge larvae to settle on.