Acorn-foraging activity and feeding behaviour by two species of freshwater crabs (Brachyura: Potamidae) from Okinawa Island, Ryukyu Islands, Japan

Abstract

Acorn foraging activity and feeding behaviour by two species of potamid freshwater crabs (Geothelphusa grandiovata and Candidiopotamon okinawense) were studied at Yanbaru forest, northern Okinawa Island, Ryukyu Islands, Japan. Adult females of the two species were observed to inhabit burrows that were filled with acorns of Castanopsis sieboldii and Quercus miyagii. The burrows with the crabs as well as gathered acorns were distributed relatively close to a stream (0.8-5 m) at slopes of the forest. Acorns were either eaten with a part of the lower part or seed coat remaining, or vertically broken up and only the inside consumed. Feeding experiments under laboratory conditions indicated that the acorns eaten by G. grandiovata had the same feeding scars as the ones found around crabs burrows, suggesting that the crabs were indeed feeding on acorns. There were 1-24 uneaten acorn(s) observed from all burrows with gathered acorns. Some of the uneaten acorns were even germinated at both around opening of and in the burrow. The accumulation of acorns by the crabs thus inadvertently helps the dispersal of C. sieboldii and Q. miyagii along the slopes of valleys, where the seeds cannot easily lodge and germinate.