Shelter preference for the Japanese mitten crab Eriocheir japonica: A field experiment in a river, southern central Japan

The Japanese mitten crab Eriocheir japonica is an edible species commonly inhabiting rivers throughout the Japanese Archipelago. The present study investigated daytime shelter preference of the crab by a field experiment. The riverine experiment was done in an enclosure that was screened with an agricultural plastic netting. The enclosure was composed of sixteen 1-m² sections, each section with one of the eight different riverbed substrate types( loose boulders with interstices, embedded boulders, loose cobbles with interstices, embedded cobbles, pebbles, sand, leaf litter, or woody debris; each with two replicates). Sixty-nine adult crabs were released into the enclosure and their hiding substrate types during the daytime were observed on a daily basis starting on the following day for 12 consecutive days. Of the total of 334 observations, 61.0% of the crabs were found in loose boulders with interstices, followed by woody debris (21.2%) and loose cobbles with interstices (12.8%). Significant positive rank correlations were observed between the carapace width of the crabs and the dimensions of the interstices used by these individuals (width: τ = 0.232, height: τ = 0.155, depth: τ = 0.091), indicating that the preferred shelter size by E. japonica changes according to the body size.