http://www.sciencemag.org/news/2021/04/sponges-can-crawl-it-costs-them-bits-their-bodies

Sea sponges don’t move, or so many scientists believed. But researchers report today in Current Biology that deep in the Arctic Sea sponges do creep, and they sacrifice pieces of their own bodies to do so.

During an Arctic expedition, scientists aboard the icebreaker Polarstern surveyed an underwater mountain ridge, using a boat-towed camera and a remote-controlled aquatic vehicle. At depths between 1000 and 580 meters, beyond the reach of sunlight, the researchers observed a thriving community of sponges. They also found snaking trails of spicules—fragments of the sponge skeleton—connected to many of the creatures.