2021 Volume 76 Pages 26-30
To investigate the population dynamics of Donax semigranosus, its shell length was measured each month (March 2006 to March 2007) at Kujukuri Beach, Chiba Prefecture. To examine the effects of predation of the foot part by the sanderling Calidris alba, I conducted an experiment to test whether individuals whose foot parts were preyed upon could still burrow into the sand. The results showed that D. semigranosus began growing from March onwards, reproduced in the summer, and died in the winter. Therefore, the lifespan of the bivalves at the study site was estimated to be 1–1.5 years, similar to previous reports. Additionally, successive recruitments occur between August and October and overwintering occurs from December to March with a shell length of 6.9±1.5 mm (mean±SD, n=2,308). Unlike reports from the Sea of Japan, bivalves were common in the swash zone during winter (average of 10,701 ind./m2 from December to March). The experiment showed that most of the individuals (21 of 25 ind.) whose foot parts had been preyed upon were unable to burrow into the sand. Therefore, bivalves might be stranded due to wave action and eventually die on the beach. If the foot parts were less damaged, they could burrow into the sand and be able to regenerate the foot. Thus, foraging of foot parts by sanderlings could be a form of sublethal predation.
