Community structure and spatial distribution of chaetognaths in Tosa Bay on the temperate Kuroshio coast of Japan

Studies on seasonal and spatial variations in the community structure of chaetognaths are limited despite their importance in marine plankton ecosystems. We examined community structures of chaetognaths in one-year monthly samples collected at three stations from inshore (Stn 1) to the shelf edge (Stn 3) in Tosa Bay on the Kuroshio coast of Japan. The yearly mean density of chaetognaths was 22.2 ind. m-3, consisting of Zonosagitta nagae (50.0%), Flaccisagitta enflata (17.4%), Serratosagitta pacifica (9.4%), and Aidanosagitta regularis (9.3%). The monthly dominant species were Z. nagae from March to July, F. enflata and A. reguralis from August to October, and F. enflata and S. pacifica from November to February. The densities of Z. nagae and F. enflata in the top 50 m were generally higher at Stn 1. In the 200-m water column at Stn 3, the densities of the dominant species were generally highest in the 0–25 or 25–50 m layer, and A. regularis was generally distributed in the shallowest layer, followed by F. enflata, Z. nagae and S. pacifica according to their population mean depths. The present results on the community structure and vertical distribution agree well with those in bays located further downstream the Kuroshio Current, indicating that the results are general features in temperate Kuroshio coastal waters. Comparison of chaetognath community structures with those in adjacent regions indicates that the dominance of Z. nagae in regard to the yearly mean abundance is characteristic to temperate Kuroshio Current coasts.