A contribution to the knowledge of the behaviour of Anodontites trapesialis (Bivalvia : Mycetopodidae). The effect of sediment type on burrowing

Large specimens of Anodontites trapesialis (n = 17) were observed in the laboratory and three locomotory patterns are described : burrowing, horizontal locomotion and rotation. Burrowing which consists of successive digging cycles (events that occur in association with each downward movement) was observed and recorded in large specimens added on two types of substrate : mixed sand (0.28\%< 62 mu m; n = 6) and native sediment (40.65\%< 65 mu m; n = 6) at 25 degrees C. The burrowing process was similar in both sediments and the rate of digging cycles, expressed as number of digging cycles per 10 min interval, decreased gradually as the shell intruded into the substrate, probably due to a decrease in penetrability with depth. In both types of sediment, time and number of digging cycles associated with the deepening into the substrate (burrowing) were significantly greater than those associated with penetration of the foot into the substrate and lifting of the shell which occurred on the surface of the substrate. Compared to other bivalves, A. trapesialis is a very slow burrower. Although the Burrowing Rate Index (BRI = [specimen mass (g)(0.33)/burrowing time(s)]x10(4)) in mixed sand (6.0 +/- 1.5)(mean +/- SE) did not differ significantly from that in native sediment (8.3 +/- 1.1), the total number of digging cycles performed during the burrowing was significantly greater in mixed sand (69.7 +/- 7.1) than in native sediment (43.7 +/- 6.8) which, while penetrable, is probably more cohesive than mixed sand, affording a better purchase of the foot and thus increasing its power to drag the shell into the sediment on each digging cycle. Thus, each digging cycle in native sediment probably produces a greater deepening than in mixed sand and so a smaller number of digging cycles is needed to finish burrowing.