Deep-sea adaptation of the epipelagic chaetognath Sagitta elegans in the Japan Sea

Abstract

The Japan Sea is a semi-isolated marginal sea connected to the Pacific by 4 shallow straits. The only important current flowing Into this sea today is the warm Tsushima Current entering through the southern strait. Between 20000 and 10000 yr ago, the cold Oyashio Current flowed into the Japan Sea through its northern straits. The epipelagic chaetognath Sagitta elegans was introduced into this sea during this period, but shallow straits prevented the invasion of deep-sea chaetognaths. All other chaetognath species were introduced into this sea by the warm current within the last 10000 yr and inhabit the epipelagic layer. S. elegans is distributed as deep as 1000 m or more, but feeding activity is low in the deep water. The size and number of grasping splnes and teeth of the Japan Sea specimens are larger than those from the Pacific. They have also developed intestinal tissue containing oil droplets, like meso-and bathypelagic specles. These features are adaptations to the deep-water environment. The vertical dispersal of S. elegans is facilitated by the absence of competitors such as Eukrohnia hamata, E. fowleri and S. macrocephala in this marginal sea.

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

“…The hook length was represented by the longest hook on each specimen (Terazaki 1993b). The head width, hook length and total length (Ͻ10 mm) were measured to the nearest 0.01 mm under a dissecting microscope with an eyepiece micrometer.…”

“…As a possible cause, Terazaki (2001) noted that the depths of all straits connecting the Japan Sea to the North Pacific are too shallow (Ͻ130 m) to allow invasion of meso-and bathypelagic species from the adjacent North Pacific Ocean to the Japan Sea. While Parasagitta elegans is an epipelagic species in the North Pacific, they are known to extend their vertical distribution down to the mesopelagic zone in the Japan Sea, possibly because of the absence of competitors such as Eukrohnia hamata in the Japan Sea (Terazaki 1993b, Ozawa et al 2004. Chaetognath species, other than P. elegans, recorded in the Japan Sea are all warm-water species, and are thought to be transported from the Pacific Ocean with the Tsushima warm-water current, a branch of the Kuroshio current (Kitou 1974).…”

Spatial and temporal changes in zooplankton abundance, biovolume, and size spectra in the neighboring waters of Japan: analyses using an optical plankton counter

“…1 to 2 mm 18.6 ± 7.1 37.0 ± 17.3 39.0 ± 9.6 66.9 ± 16.9 88.7 ± 46.2 3.3 ± 2.2 *** D A B1 B2 B3 C 2 to 3 mm 7.7 ± 4.1 14.5 ± 12.8 16.5 ± 6.1 41.5 ± 12.1 181.4 ± 160.9 1.0 ± 0.9 *** D A B1 B2 B3 C 3 to 4 mm 3.9 ± 4.1 16.5 ± 10.9 8.5 ± 7.5 13.9 ± 9.1 30.5 ± 42.7 0 ** D A B2 B3 B1 C 4 to 5 mm 1.1 ± 1.6 23.3 ± 24.1 3.5 ± 4.3 7.7 ± 9.9 6.7 ± 9.0 0 *** D A B2 C B3 B1 temperature is low, the large-sized cold-water species, i.e., chaetognath Parasagitta elegans, amphipod Themisto japonica and euphausiid Euphausia pacifica are known to perform diel vertical migration and distribute on the surface at night (Ikeda et al 1992;Iguchi et al 1993;Terazaki 1993). The proportion of 4 to 5 mm ESD in group B1 was much higher than that in the other groups ( Figure 5C).…”

Chaetognath species-specific responses to climate regime shifts in the Tsushima Warm Current of the Japan Sea

“…), and S. nagae Alvariño (mixed-water sp., Nagai et al 2006). The latter three dominant species in TWC water were selected for further analysis in the present study, while S. elegans, a cold-water species, is distributed in the water column containing the cold Proper Water according to Terazaki (1993, Fig. 2).…”

“…Water temperature (WT) and salinity (SAL) data represent the integrated-average values of the 0-100 m layer, as warmwater and mixed-water species of chaetognaths are mostly distributed in the upper 100 m of the TWC water (Terazaki 1993, Nishihama & Hirakawa 1998. The eastward (positive) volume transport (VT) of TWC water was calculated from the geostrophic current assuming 500 dbar (10 4 Pa) as an indicative level of no motion along the PM transect (from Station PM 1 to 10, Fig.…”