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ABSTRACT The aim of this study was to characterize a nursery ground (Ohama Beach, Hiuchi-nada) for the Japanese flounder Paralichthys olivaceus in the central Seto Inland Sea having insufficient availability of mysids, Nipponomysis ornata and Iiella oshimai . We examined daily growth rates of wild juveniles; this measure can be used as an index of quality of a nursery. Juveniles consumed not only mysids, but also Crangon uritai and fishes. Mysid availability was lower at the study beach than in the nursery in the Sea of Japan; however, mean growth rates (MGRs) from May to June varied from 0.75 to 1.84 mm day −1 and equaled that in the nursery with sufficient availability of mysids in the Sea of Japan. Prey availability was high from May to June and decreased after July. Additionally, water temperatures after late July were higher than the optimal temperature. Multiple regression analysis suggested that the low availability of prey and high water temperatures reduced the MGR after July. The results indicate that the optimal season for releasing hatchery-reared juveniles is from May to June, a period characterized by high availability of prey.

Distribution patterns of the three species of sand-burrowing mysids, Archaeomysis vulgaris, Archaeomysis japonica and Iiella ohshimai were investigated, and also the diel and tidal distribution patterns of the dominant mysid A. vulgaris were investigated in the surf zone of an exposed and mesotidal sandy beach at Fukiagehama, Kagoshima Prefecture, southern Japan. The morphodynamic state of the beach was an intermediate type with a low-tide terrace or ridge/runnel. Habitat zonation was recognized among the three species during the entire study period (May and October of 2002 and 2003). A. vulgaris occurred from the high-water shoreline to the low-water shoreline during the daytime, whereas A. japonica was observed off the low-water shoreline and I. ohshimai was found far from the longshore bar. Habitats of A. vulgaris shifted along the beach slope, depending on the tidal changes at the shoreline. However, at low water in May of 2002 and 2003, most individuals of A. vulgaris were distributed in the intertidal runnel and on the lowtide terrace, rather than off the low-water shoreline. A. vulgaris had the highest population density of the three species, and fluctuations in its total mean density in the surf zone were estimated at each tide in May of 2003. These ranged from 2,956 indiv./square m at low water at night to 172 indiv./square m at middle water during the daytime. Distribution patterns characteristic of A. vulgaris are ecologically interpreted in relation to beach morphology, water flow regimes in the surf zone, and avoidance of fish-predation pressure.

Diel and tidal changes in distributions of the 3 species of sand-burrowing mysids, Archaeomysis kokuboi Ii, A. japonica Hanamura, Jo & Murano and Iiella ohshimai (Ii), were studied during summer on a sandy beach in Otsuchi Bay, on the Pacific coast of northeastern Honshu, mainland Japan. During the day A. kokuboi, the closest dweller to the shoreline, showed remarkable tidal migrations that differed among the developmental stages and sexes. During the daytime, intraspecific zonation of A. kokuboi relative to the shoreline was maintained during the habitat shift with tide. However at night A. kokuboi emerged from the sand and swam in the water column. The swimming activities became progressively less with growth through juveniles, immatures, mature males, non-brooding females to brooding females. A. japonica, which occurred just below the intertidal zone, showed no tidal migration, but also emerged into the water column at night. Its swimming activities differed among developmental stages, i.e. adults were inactive, while immatures and juveniles swam into the water column to expand their horizontal distribution offshore. Likewise, I. ohshimai, mostly juveniles or immatures <3.0 mm standard length, showed no tidal migration, but emerged into the water column at night. The spatiotemporal distribution patterns of the 3 mysid species of various developmental stages showed very little overlap. The daytime behaviors of sand-burrowing mysids, such as tidal migration or burrowing, are unique among mysidaceans, and may be adaptations to avoid visual predators. On the other hand distribution patterns of the different species and stages at night seem well adapted to ensuring feeding or reproduction.

Seasonal abundance and distribution patterns of the 3 species of sand-burrowing mysids, Archaeomysis kokuboi Ii, A. grebnitzkii Czerniavsky and Hella ohshimai (Ii), were studied on a sandy beach at Otsuchi Bay, northeastern Honshu, Japan. Clear interspecific bathymetrie zonation was recognized among the 3 species. Their habitats shifted seasonally and according to development in species-specific ways. A. kokuboi was the shallowest dweller and was distributed from the swash zone to the step area, where it showed clear intraspecific zonation, i.e. juveniles were mainly distributed just below the step, the most mature mysids occurred above the step and most of the brooding females were restricted to the area close to the shoreline. A. grebnitzkii was found in more offshore areas than A. kokuboi and for the most part their habitats did not overlap. I. ohshimai was distributed in deeper water than the other 2 species and migrated into shallower sites during May to October. Ontogenetic migration to deeper water was evident in A. grebnitzkii and I. ohshimai. Inter- and intraspecific distribution patterns of sand-burrowing mysids may be explained by differences in their responses to wave action or flow regimes near the bottom and also by the distributional patterns and quality of diet in their habitats and their feeding behaviors.