Species composition and depth distribution of chaetognaths in a Kuroshio warm-core ring and Oyashio water

Abstract

Chaetognath species composition, diversity and depth distribution were investigated inside and outside a warm-core ring oV the north-east coast of Japan. Time series samples were collected at a station inside a Kuroshio warm-core ring (KWCR) and at a station in the surrounding Oyashio water. Greater mean abundance of 2.51± 0.116 (mean ± SE) chaetognaths m –3 was found outside the ring compared with 1.75± 0.321 chaetognaths m –3 collected within the ring. However, species diversity values were higher within the KWCR (Shannon–Weaver index). Eukrohnia hamata was the dominant species outside the ring, comprising 79.8–87.9% of the total chaetognaths per haul. Inside, Sagitta minima was dominant, comprising 35.1–44.3%. Most E. hamata were collected in deeper layers within the KWCR and their abundance was on average only 9% of that found in the Oyashio region. Only E. hamata and Sagitta scrippsae had diVerent depth distributions in the KWCR. The vertical distribution of E. hamata by body size appeared altered by the KWCR. Although the mean length of E. hamata was not signiWcantly diVerent between regions, sexual development appeared inhibited in the ring. Sagitta elegans collected in the KWCR were mostly small in size (<10 mm), signiWcantly smaller than in the Oyashio water.