PREDATION ON BIVALVES ON A SANDY BEACH IN SOMA, JAPAN

Predation may be an important cause of early postsettlement mortality of bivalves, but little is known of in situ predation processes on bivalves by multiple predators on sandy beaches. To address this issue, bivalve shells in the guts of demersal fishes and benthic invertebrates were counted and measured monthly from April to November 2009 in Soma, Japan. For the Sakhalin surf clams Pseudocardium sachalinense and Raeta pulchella, the frequency of occurrence and number of shells in guts of predators per transect peaked around May-June, when the length of shells in guts tended to be smaller than in later months. These two indices decreased to 0 in July for P. sachalinense and to a low level in August for R. pulchella, which was associated with increasing shell length (ShL) in guts. For the bivalve Mactra crossei, both occurrence and number of shells in guts of predators per transect increased around November in some of the predators. The largest ShL of P. sachalinense in guts tended to be similar among the predators and was 1.1-1.5 mm. The largest ShL of R. pulchella and M. crossei tended to vary among the predators, and were 5.6-17.8 mm and 1.0-7.2 mm, respectively. The spiny sand sea star Luidia quinaria and red tongue sole Cynoglossus joyneri had a high frequency of occurrence and number of shells per gut per transect. These results indicate that predation on P. sachalinense and R. pulchella was most intense in the early postsettlement period of May-June in Soma, Japan. The size of the bivalve shell may have provided protection, but was dependent upon on the predator.