2015 Volume 44 Pages 1-9
The spatial distribution, population structure and feeding behavior of the mole crab Hippa marmorata (Hombron & Jacquinot, 1846) was investigated on Iriomote-jima Island, Ryukyu Islands, in the Western Pacific region. Changes in the geographic distribution, body-size distribution, abundance, and vertical migration of mole crabs in response to the circatidal rhythms were examined by sampling sediments using a modified rake. Diel changes in the number of crabs exhibiting a pre-feeding posture to capture zooplankton were examined by visual counts. Mole crabs were more abundant at stations near the mouth of an inlet where terrestrial hermit crabs (Coenobita brevimanus) were previously observed to have released their larvae. The body-size distribution, measured as carapace length (CL), ranged from 5.3 to 20.3 mm in females and 4.1 to 12.1 mm in males. The pre-feeding posture was only observed at night, and increased and decreased in crab numbers observed with the flood (incoming) and ebb (receding) tides, respectively. Regardless of the tidal cycle, mole crabs were only collected in the swash zone, and the number of crabs collected was higher at night. The population structure and tidal migration characteristics of the mole crabs observed in this study closely correspond to the observations of previous studies. The temporal patterns observed in the adoption of pre-feeding posture among mole crabs may be an adaptation to feeding on the zooplankton that typically emerge at night.
