Abstract
The pea crab Pinnotheres bidentatus Sakai, 1939 is a parasite that lives inside bivalve mollusks in brackish waters. In Hitotsuba Inlet on the east coast of Kyushu, Japan, we conducted two sampling surveys of its host bivalves from May, 2004 to September, 2005 and from May, 2011 to July, 2012 to elucidate the life history of the crabs based on their size distribution. The main host bivalves were two psammobiid species, Nuttallia japonica and Psammotaea virescens. While the densities of these bivalve populations in the inlet decreased 25–50% from 2004 to 2011, the density of pea crabs did not. The proportion of infested Nuttallia japonica increased from 5.5% in 2004–2005 to 25.0% in 2011–2012. In the inlet, the crab Pinnotheres bidentatus spawns from late May to late September, the next generation infests bivalves in fall to winter, and the infested crabs grow until the next spawning season.
