Morphology of the naupliar stages of some Oithona species (Copepoda: Cyclopoida) occurring in Toyama Bay, southern Japan Sea

Abstract

Adults of Oithona atlantica, Oithona plumifera and Oithona similis occurred throughout the year in Toyama Bay (0-500 m), but in the surface layers (0-50 m), the adults of O. atlantica and O. plumifera were observed in Toyama Bay during different seasons. Based on the differences in adult distribution, O. atlantica nauplii were found in samples collected during spring (April to May) while O. plumifera were found in samples collected during autumn (September to October). Identification of O. atlantica and O. plumifera nauplii and comparisons of their morphological differences were based on this temporal separation. Nauplii of O. atlantica, O. plumifera and O. similis differed in their ratio of body height (maximum length of dorsal-ventral axis) to length (H/L). This characteristic ratio was common to all naupliar developmental stages and is therefore useful as a basis for distinguishing between naupliar stages of the three species. The population and community structure and densities of these species of Oithona were investigated in the surface water layer (0-50 m depth) of Toyama Bay from 1997 to 1999. Major population structural changes occurred in different sampling seasons, and may have been influenced by changes in hydrographic conditions, including temperature and water current. In winter and spring (from February to June), many O. similis and O. atlantica nauplii appeared, and the former nauplii were more dominant. Oithona similis nauplii are suggested to may be the most important species as prey for fish larvae.