Predation intensity of the portunid crabs <b><i>Charybdis japonica</i></b> and <b><i>Portunus pelagicus</i></b> on the hermit crab <b><i>Diogenes nitidimanus</i></b> according to season, predator size, and prey shell type and size

Abstract

Portunid crabs are strong predators in shore ecosystems, and often prey on hermit crabs. In a previous study, we observed the loss of larger individuals of the hermit crab Diogenes nitidimanus during the mid-reproductive season. Therefore, we experimentally investigated predation intensity of the portunid crabs Charybdis japonica and Portunus pelagicus on D. nitidimanus collected from the tidal flat of the Waka River to elucidate the effects of predator body size, prey shell type and shell size, and season. Each predator was given six prey items with a combination of two shell types Umbonium and Batillaria and three shell size classes. We replaced eaten prey items daily with identical ones in terms of shell size and type, and recorded the number of prey items eaten over 10 days per predator. However, we were unable to analyze the effect of season for P. pelagicus because of the species limited periods of collection. C. japonica preferred prey with small, Umbonium-type shells. The total number of prey items eaten increased in summer, but not with body size. By contrast, P. pelagicus preferred prey with Batillaria-type shells, and predator body size tended to be a more important factor in the context of medium-and small-sized shells. Batillaria-type shells are likely more resistant to crushing pressure than Umbonium-type shells. More smaller prey were eaten during the experiment, which does not explain the loss of larger individuals at the study site.