Multi-leveled locomotion system of the fan worm facilitates underwater crawling inside a slippery tube

The fan worm (Annelida: Sabellidae) inhabits a mineral tube and obtains its nourishment by filtering surrounding water through its corolla-like tentacles. This creature features ultrafast retraction to its narrow tube for escaping external dangers, a process greatly aided by its self-secreted mucus lubricating the tube wall. However, the mucus may render the tube wall too slippery for the worm feet (parapodia) to crawl on. How the fan worm can steadily crawl in the tube against the mucous surface remains highly elusive. Here, we show that the fan worm exhibits a triple-leveled locomotion system and that each level can cooperate to achieve stable motion when the worm is extending out from the slippery tube underwater. In addition, we designed a fan-worm-inspired robot equipped with bionic foot pads that can stably crawl on mucous surfaces. This biological system can spark new design ideas for intelligent robots that explore the slippery environment underwater.