The Feeding Behavior of the Seastar Evasterias troschelii Stimpson

Kymograph recordings of shell movements of 4.0-5.5 cm long clams (Protothaca staminea) during attacks by the seastar (Evasterias troschelii) showed: (1) in 21 experiments the valves were pulled apart at a mean rate of about 1.0 mm per minute, as measured at the mid-ventral edge of the clams; (2) clams constantly tried to close during the initial opening of them by the seastars and then at longer intervals thereafter for a period usually not exceeding 4 hours. A specially constructed, false lucite "clam" containing Mytilus meat as bait was opened by Evasterias against known resistances applied to the appoximate center of the upper, free-moving valve of the "clam." Kymograph recordings of shell movements of this "clam" during seaster attacks showed: (1) the curves showing the openings agree closely with those showing openings of live clams; (2) an average sized Evasterias may exert forces in excess of 5000 grams; (3) having lifted such weights, the seastar may hold them lifted for several hours; (4) in 14 experiments, weights varying from 700 to 5500 grams were lifted at an average speed of about 0.8 mm per minute, equal to an increase in gape of about 1.2 mm per minute during the lift. Maximum lifts varied from 2.8 to 13.5 mm with a mean of 6.6 mm. Experiments and observations showed that only the longitudinal musculature of the tube feet is responsible for the forces exerted by the seastar in opening bivalves. However, there are strong indications that the seastar is capable of "locking" itself in a rigid state before commencing the pull. If prevented from applying force, Evasterias weill still succeed in feeding on bivalves, but the digestion period is considerable prolonged due to difficulties in inserting all lobes of the stomach into the prey.