Abundance and community structure of chaetognaths from the epipelagic through abyssopelagic zones in the western North Pacific and its adjacent seas

Abundance and community structure of chaetognaths were studied based on the vertical stratified zooplankton samples from the epipelagic through abyssopelagic zones (maximum: 5,000-5,800 m) at four stations in the western North Pacific (44°N, 39°N, 30°N, and 25°N) and one station each in the Japan Sea, Okhotsk Sea, and Bering Sea. Chaetognath standing stocks (indiv. m-2) were greatest at subarctic stations (44°N, Okhotsk Sea, and Bering Sea). Vertically, chaetognath density (indiv. m-3) was the greatest in the shallowest depths, and decreased with increasing depth. Chaetognaths occurred down to 4,000-5,000 m at subarctic stations, while chaetognaths were not found below 3,000 m at subtropical stations (30°N and 25°N). The number of chaetognath species was the greatest (22 representing 14 genera) at the transition station (39°N), and the least (1 species) at the station in the Japan Sea. Species diversity indices (H′) were low at subarctic stations, but high at subtropical stations. Vertical profiles of H′ varied also between these stations; it peaked at the mesopelagic zone at subarctic stations, and at the epipelagic zone at subtropical stations. Cluster analysis separated chaetognath communities of the study region into 5 groups (A-E) characterized by discrete spatial distribution: group A; the mesopelagic zone at subtropical and transition areas, group B; the epipelagic zone at subtropical and transition areas, group C; the bathy- and abyssopelagic zone (except the Japan Sea), group D; all depths in the Japan Sea, and group E; the epi- and mesopelagic zones in the subarctic area. For the four most abundant species at the subarctic stations, allometric data showed greater head width to total length ratios, and greater hook length to total length ratios for deeper-dwelling species. Relatively larger head width (i.e. large mouth) and longer hooks of deep-sea chaetognaths are considered to be an adaptation significant to the successful capture of prey zooplankton in resource limited deep-sea environments