Rapid Arm Movements in Stalked Crinoids

Stalked crinoids in the family Isocrinidae have been observed to wave individual arms actively. Using video cameras mounted on a manned submersible, we studied these movements and investigated the factors that elicit them. Crinoids wave their arms in response to sand or detritus dropped on their crowns, to entanglement in tentacles of adjacent sea anemones, and to contact by small crustaceans that might steal from the food grooves. There was no evidence that arm waving functions in food collection. In most cases, the movements could be attributed directly to mechanical stimulation by some natural stimulus. The rapid effective stroke of an arm flexure is caused by contraction of dorsal longitudinal arm muscles. The slower return stroke results from the elastic recoil of large ligaments near the aboral sides of the arms.