Diel and tidal migrations of the sand-burrowing mysids, Archaeomysis kokuboi, A. japonica and Iiella ohshimai, in Otsuchi Bay, northeastern Japan

Abstract

Diel and tidal changes in distributions of the 3 species of sand-burrowing mysids, Archaeomysis kokuboi Ii, A. japonica Hanamura, Jo and 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.