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 (<0.8 degree slope) beaches. These results indicate that mildly-sloped and well-oxygenated beach areas are distinctive habitat characteristics of C. muromiensis.

Contents of supplementary materials: Appendix 1. Density of macrobenthic invertebrates found during this survey at the Tama River Estuary in 2007 and 2009.

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On April 14, 1997, the inner part of Isahaya Bay was isolated from the Ariake Sea by a dike for reclamation. Thereafter, red tides and episodes of hypoxia began to occur frequently in the Ariake Sea. The government introduced seawater inside of the dike from April 24 to May 20, 2002, but the reclamation project was continued and finally completed on November 20, 2007. We periodically monitored the grain-size of bottom sediments and the benthic macrofauna using sediment samples collected from (1) 32–50 fixed stations around the inner part of the Ariake Sea in June or November every year from June, 1997, to June, 2007, and (2) 88 fixed stations throughout the entire Ariake Sea in June of 1997, 2002, and 2007. During the decade between 1997 and 2007, the median particle diameter of the bottom sediment decreased at many stations around Isahaya Bay and also in some stations around the mouth of the Ariake Sea. As for the benthic macrofauna, a total of 38,221 individuals were collected from the 88 fixed stations in June, 1997, and the most dominant taxa were polychaetes, gammaridean amphipods, and bivalves. In June of 2002, 74,273 individuals of benthic animals were collected from the same stations, but in June of 2007, just 12,386 individuals. Some opportunistic species such as Modiolus (M.) comptus and Corophium spp., those inhabit fine sediment around the mouth of Isahaya Bay, became abundant in June, 2002, but most of them disappeared by June, 2007 because of hypoxia and a change in the composition of the bottom sediments.

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Growth pattern of the wedge clam Latona cuneata (Linnaeus) (Donacidae, Veneroida, Bivalvia) was surveyed on a sandy beach in Mitoma, Fukuoka City, Japan by monthly sampling during 16 months. Juveniles recruited to the intertidal area from July to August with around 3.2 mm shell length (SL), which exhibited rapid growth till the next November except during winter (December–February) and summer (July–August). In the second winter, their mean SL was 20 mm. In March, clams with 18–27 mm SL were divided into three cohorts that had overwintered 2–4 times. Large clams with SL>28 mm that had overwintered five times were rarely found, suggesting that in L. cuneata mortality before the fifth winter is high. Clam distribution on the beach was surveyed by quadrat sampling along a 50-m line toward the seashore in May, July and November. L. cuneata changed its distribution pattern with topographic changes of the beach; however, all clams concentrated in the foreshore area (retention zone). Clams aggregated at a high density in May, but showed dispersed polymodal patterns with a low density in July and November. Two types of growth lines were categorized on L. cuneata shells, one deeply notched (Line A) and one with a hyaline margin (Line B). SL of three generations divided by the number of lines on L. cuneata collected in June roughly agreed with that of cohorts from the June population. SL at growth lines also agreed with cohort analysis results from December (with Line A) and August populations (with Line B). Thus, the growth lines were indicative of overwintering and reproduction in summer.

Contents of supplementary materials: Photographs of study animals and study site.

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