Effect of salinity and substrate grain size on larval settlement of the asari clam (Manila clam, Ruditapes philippinarum)

Abstract

Spatial and temporal variation in the amount of larval settlement is a fundamental factor in determining adult abundance for bivalves in soft sediments, along with post-settlement factors. Salinity and substrate grain size are potential factors which influence larval settlement rates of bivalves, but the effects have been poorly examined for asari clam (Manila clam, Ruditapes philippinarum). In this study, we examined the effect of salinity, substrate depth, and substrate grain size on larval settlement of the asari clam. Larval settlement rates (percent settlement after 3days) of the clam differed significantly by salinity. The larval settlement rate showed a higher value as the salinity decreased until it reached 13, which is almost the low salinity limit for the clam. This implies that spatial and temporal variation in freshwater inflow strongly influences larval settlement of the clam. Larvae would not settle on the substrate unless they encountered a water area with an appropriate low-salinity range. The larval settlement rate did not significantly differ by substrate grain size. However, fine sand may be unsuitable, because under observation of the microscope, the larvae often failed to dig into the sediment grain size of 0.125 and 0.063mm. The larval settlement rate was very low without substrate. These results are valuable for understanding the variation in larval settlement in the field in relation to freshwater inflow and sediment grain size. The results are also valuable to improve hatchery rearing techniques of the clam spat.