Mortality causes of the bivalve, Ruditapes philippinarum, in the intertidal zone of the Seto Inland Sea, eastern Yamaguchi Prefecture : A field study by caging experiments

To evaluate the mortality of the bivalve Ruditapes philippinarum caused by predatory animals, two experiments were conducted in the coastal area of the western Seto Inland Sea around the Tana Marine Biological Laboratory, Yamaguchi Prefecture, Japan. On the tidal flat, four cages of different designs were set on the bottom, and 200 individuals of R. philippinarum (14.8 mm in average shell length [SL]) were put into each cage (Experiment 1). From the number of survivors after four months, the mortalities due to environmental factors, predators entering from the upper side and those from the bottom were estimated to be 0.615, 0.225 and 0.170, respectively. On the other hand, in the raft experiment, containers filled with sand and R. philippinarum juveniles (< 5 mm SL) were set into the cages hanging from the raft at depths of 1, 3, 5 and 7 m (Experiment 2 ). Five individuals of the sand dollar Scaphechinus mirabilis were also introduced into the container at each depth. Four months later, large-sized individuals (20-30 mm SL) of R. philippinarum were observed in the containers, indicating that the juvenile bivalves survived and grew even in the presence of S. mirabilis. Therefore, this study shows that S. mirabilis is not the major cause of mortality of R. philippinarum.