潜砂制限がアサリの成長に及ぼす影響

抄録

In Hakodate Bay, Hokkaido, the short-necked clam Ruditapes philippinarum shows highly depressed growth rate above 30 mm in shell length. In this area, clams often burrow into the sediment at an angle, which seems to be caused by boulders in the sediment. For benthic bivalves, burrowing into sediment is an important behavior to escape severe environmental conditions and predation. To examine possible effects of boulders on burrowing behavior and growth of the clam, we compared burrowing depth of the clams in contact with boulders and those clear of boulders. The ratio of burrowing depth to shell length was higher in those clear of boulders than those touching boulders. To understand this depressed growth mechanism, we carried out a field growth experiment and a laboratory feeding experiment. In the field experiment, we compared the shell growth increments of clams with restricted burrowing into sediment and those of unrestricted burrowing for three months. Shell growth increments of the clams with restricted burrowing were smaller regardless of their initial shell sizes. In the laboratory experiment, we compared the clearance rates of clams under six different situations: predatory crabs were present or not and burrowing treatments were unrestricted (deep sediment), restricted (shallow sediment) or impossible (i.e., no sediment). Both the presence of predators and restriction of burrowing decreased the clearance rate of clams; however their interaction was not significant. These results suggest boulders in the sediment restrict burrowing, and then depress shell growth of the clams. Sediment features must be taken into account for fishery analysis and stock recovery potential in clams with burrowing behavior.