Vertical distribution of Large-size chaetognaths in Suruga Bay, central Japan

The vertical distribution of large-size chaetognaths was investigated on the basis of 44 samples collected from Suruga Bay, central Japan, during four cruises carried out in August 1980, November 1981, February 1982 and May 1983. The samples were collected by simultaneous tows with a 160 cm ring net (mesh aperture: 2 mm) at five or seven layers between surface and 1,000 meters. A total of 19 species representing 12 genera of 3 families were identified. The number of species was relatively high in August and November and low in February and May. Species composition was distinguishable between the epipelagic zone (0-200 m depth) and the mesopelagic zone (under 200 m-depth). In the epipelagic zone, Zonosagitta nagae dominated through all seasons, associated with Flaccisagitta enflata in August and November and Serratosagitta pacifica in February. In the mesopelagic zone, the chaetognath assemblage was characterized by the occurrence of mesopelagic species such as Pseudosagitta lyra, Solidosagitta zetesios and Eukrohnia fowleri with relatively high species diversity. The centers of vertical distribution of each species estimated by weighted mean depth involved the following three zones of the water column: l) Surface zone (above 200 m-Depth): Ferosagitta ferox, F. robusta, Flaccisagitta enflata, Serratosagitta pacifica, Zonosagitta nagae. 2) Upper mesopelagic zone (200-500 m): Flaccisaqgitta hexaptera, Mesosagitta decipiens, Pseudosagitta lyra. 3) Lower mesopelagic zone (500-1,000 m): Eukrohnia hamata, E. fowleri, E. bathypelagica, Solidosqgitta zetesios, Caecosagitta macrocephala. Among these species, Z. nagae and F, enflata living in the epipelagic zone represented remarkable diel vertical migration, but P. lyra and S. zetesios living in the mesopelagic zone had no difference in the vertical distribution between day and night. For S. zetesios, maximum abundance was found in shallower sampling layers in February and May compared with August and November. This seasonal change of vertical distribution seems to be related with ontogenetic vertical migration of this species.