2020 年 15 巻 2 号 p. 146-155
The community structure and life cycle of dominant chaetognath species were studied in a 0–1000–m water column in the Oyashio region, western subarctic Pacific, six times during the year from March 2003 to February 2004. The abundance of chaetognaths ranged from 3235 to 6691 inds. m−2. Throughout the study period, ten chaetognath species belonging to nine genera were identified. Among the chaetognaths, Eukrohnia hamata accounted for 39–62% of the abundance, followed by E. hamata/Eukrohnia bathypelagica juveniles, Parasagitta elegans, and E. bathypelagica. The species diversity (H') of chaetognaths varied between 1.12 and 1.50. For E. hamata, individuals had a body length ranging from 2.3–23.8 mm. The abundance of juveniles fluctuated from 22.0–48.3% and was higher in June 2003. Based on cohort analysis, recruitment of E. hamata juveniles occurred from spring to summer, and they reached a body length of 8 mm in one year. The overwintered population showed rapid growth from June to October, when they reached 15 mm in body length. Eukrohnia hamata showed little growth in winter and reproduced the subsequent spring. Thus, a two–year life cycle of E. hamata is proposed for the population in the Oyashio region. For comparable information about the life cycle of E. hamata, an eight to ten–month generation length was reported for the population in the eastern subarctic Pacific. These regional differences in the generation length of E. hamata are attributed to differences in habitat temperature, with much higher temperatures in the eastern (3.8–6.0°C) than the western (2.3–2.9°C) subarctic Pacific.
