ABSTRACT
Stalked crinoids in the family Isocrinidae are generally passive, but have sometimes been observed to actively wave their arms up and down in the water column. Using video cameras on a manned submersible, we investigated the causes of these active arm movements. Incidence of arm waving increased with the amount of time crinoids were illuminated by the submersible lights, as did the abundance of small crustaceans attracted to the lights. Close-up video indicated that many arm movements were in direct response to crustaceans contacting the arms. The observations suggest that arm flexure is used to rid the arms of crustaceans which may rob food from the food grooves and also sand or detritus falling on the crown. The effective stroke of an arm flexure is caused by contraction of dorsal longitudinal muscles. The return stroke appears to result from elastic recoil of large ligaments near the aboral sides of the arms.
