Distribution and Habitat Characteristics of Cyathura muromiensis (Crustacea: Isopoda) in the Tidal Flats of the Tama River Estuary, Tokyo Bay, Japan

To clarify the habitat characteristics of the brackish-water isopod Cyathura muromiensis, we conducted a field survey of the macrobenthic invertebrates, including C. muromiensis, and several environmental variables at 27 sites (3 stations×9 transect lines) on the tidal flats of the Tama River Estuary (Tokyo Bay, Japan) in summer in 2007 and 2009. Cyathura muromiensis was concentrated along certain transect lines between 0.5 km below and 2 km above the river mouth and was often limited to the shoreward stations. Canonical correspondence analysis (CCA) indicated that the major macrobenthic species may be distributed in response to two integrated environmental gradients: one is longitudinal and correlated with distance from the river mouth (as an indicator of salinity), median sediment grain diameter, and elevation; the other is not longitudinal but is correlated with the beach slope and sediment redox potential (ORP). The response pattern of C. muromiensis to the latter gradient distinguished this isopod from the other major macrobenthic species. The distribution of C. muromiensis along the gradient of each environmental variable selected by CCA was also checked. This isopod occurred abundantly at sites of mesohaline to polyhaline interstitial water in oxidized sandy sediment containing 2–18% mud and showing 60–140 mV ORP at 5 cm depth, in the middle to lower intertidal zone of mildly sloping (