Movement and aggregation in the fluted giant clam (Tridacna squamosa L.)

Aquaculture has been the traditional focus of tridacnid giant clam research whereas their ecology and behaviour have received much less attention. This study was based on the observation that juvenile fluted giant clams (Tridacna squamosa), when evenly distributed in a tank, will move and aggregate over time. We observed movement in clams ranging from 10 to 313 mm in shell length and ‘climbing’ up the sides of tanks was noted for clams with lengths between 10 and 22 mm. Locomotion also occurred after byssal attachment to the substrate; there was a highly significant association between type of movement (i.e. translation, rotation and no movement) and presence of attachment. Tests for phototaxis were negative. Aggregation was examined by placing clams in regular patterns on grids. After three days in the aquarium and 24 h in the field, their positions were analysed to obtain a statistical parameter for ‘clumpiness’. This was found to be greater in the live clam runs than both random walk and random distribution simulations, suggesting that clams were attracted to conspecifics. The latter was tested by recording clam movement with respect to five types of fixed ‘targets’ (i.e. live clam, fouled clam shell, foul-free clam shell, random inanimate object and none). The test clams moved, non-significantly, towards live clam targets and displayed higher mobility compared to tests with other target objects; a negative correlation between mobility and clam length was also observed. A choice experiment using bidirectional water inflow with clam effluent as one source resulted in clams moving toward the effluent, offering the first direct support for positive chemotaxis among conspecifics in Bivalvia. Together, our results indicate the presence of chemical signalling among clams, leading to movement toward one another and clumping. Aggregation could serve several ecological functions, such as defence against predation, physical stabilisation and facilitation of reproduction. With worldwide decline in natural giant clam densities, the opportunity for conspecific clumping is reduced, and local stocks could be facing increased vulnerability to Allee affects.