The larval distribution and feeding habits of Cynoglossus abbreviatus and Cynoglossus lighti were investigated around the Rokkaku estuary in Ariake Bay during October 2004 and March and April 2005. Cynoglossus abbreviatus started and completed metamorphosis at larger sizes than C. lighti. Developmental phases consisted primarily of individuals at metamorphosis and postmetamorphosis, which were mainly distributed inside and just outside the Rokkaku River in March-April for C. abbreviatus and October for C. lighti. Tidal changes in vertical distribution just outside the river mouth differed between the closely related species; C. abbreviatus was distributed in the surface and middle layers at flood tide, and aggregated near the bottom (where the current speed was lowest) at ebb tide. Cynoglossus lighti stayed mainly near the bottom during all tidal phases. Larvae of both Cynoglossus species selectively fed on copepods (Pseudobradya sp.), which were scarce in the waters. These facts suggest that the nursery grounds of the two Cynoglossus species were different in terms of their tidal vertical distribution patterns.

To examine the importance of the upper estuarine areas of Ariake Bay as a nursery ground for fish, assemblages of larvae and juveniles were compared among various aquatic habitats. The upper estuaries of the bay (the Rokkaku and Hayatsue estuaries) are brackish, highly turbid waters with high tidal velocities, and differ substantially from the Isahaya area, which has been separated from the bay by a man-made dike, to the middle estuary (the Kikuchi estuary). Abundances of larvae and juveniles were higher in the estuaries than in the open bay and Isahaya areas. Abundant species in the upper estuaries were similar to each other, but differed from those of the middle estuary. This was primarily due to larvae and juveniles of fishes that occurred almost entirely in the upper estuaries, such as Acanthogobius hasta, Boleophthalmus pectinirostris, Coilia nasus, Cynoglossus abbreviatus, Nibea albiflora, Odontamblyopus lacepedii, Trachidermus fasciatus and Tridentiger barbatus. These results suggest that the upper estuaries play an important role as nursery grounds for fishes.

Zonation has been a major focus of intertidal ecology. In general, sandy beaches are thought to have three zones (supralittoral, littoral, and sublittoral zones). In the present study, 41 areas of sandy beaches were surveyed along the Japan Sea coast of Honshu (from Aomori to Yamaguchi Prefectures) characterized by a small tidal range, large seasonal fluctuations in the mean tide level, and harsh wind-driven waves in winter, and the applicability of the three-zone scheme was examined. Overall, a total of 316 beach stations were surveyed, 28 taxonomic units were identified, and the most abundant species was the amphipod Haustorioides japonicus. The taxonomic units were grouped using multivariate techniques into a total of 7 assemblages, and the relationship of these assemblages to zonation was examined. Investigation of the indicator species of the 7 assemblages and their vertical position on the beaches showed that one assemblage corresponded to the supralittoral zone (indicators were talitrid amphipods and the isopod Tylos granuliferus). Another assemblage corresponded to the sublittoral zone, and the other five assemblages to the littoral zone. The assemblage of the sublittoral zone tended to disappear on the northern beaches, while one of the littoral assemblages whose indicators included a mysid species, Archaeomysis kokuboi, tended to occur on the northern beaches. Thus, the three-zone scheme can be applied to sandy beaches of the Japan Sea, but there are large variations among the beaches.

Ethics approval: All the procedures performed in the studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. This study was approved by the ethics committee of Kyoto University Graduate School and the Faculty of Medicine (R0546-2) Consent: Informed consent was obtained from all individual participants involved in the study.Data and/or Code availability: All data generated or analysed during this study are included in this published article.