HOW SEA STARS OPEN BIVALVES

¹ Syracuse University, Syracuse, N. Y.

1. An investigation was made into the possibility that sea stars secrete a substance which is toxic or anesthetic for bivalves. Extracts prepared from the organs of feeding and non-feeding Asterias forbesi were introduced into the adductor muscle and the mantle cavity, or perfused over the beating heart, of Mytilus edulis. The effects of such solutions were, generally, identical to those produced by sea water or distilled water.

2. Sea stars were induced to feed upon specially prepared mussels, so that the forces which their tube feet exerted on the shells could be measured manometrically. The adductors of the mussels used in such experiments had been severed and replaced by steel springs or plastic cylinders which could not be affected by any alleged toxin. It was found that the tube feet did pull the valves apart and forces of over 3000 grams were recorded. It was observed also that a very minute opening between the valves (0.1 mm.) was sufficient to permit the insinuation of the asteroid stomach.

3. The common interpretation of the mechanical theory, which asserts that the sea star "fatigues" the mollusc, appears inaccurate in view of the findings of this research. There is evidence that the opening of the valves is a rapid process involving overwhelming, discontinuous forces, so that the predator may be considered to relax its pull upon the valves at intervals and to allow its stomach to be compressed between the valves until it pulls them apart again.