Seasonal occurrence and vertical distribution of appendicularians in Toyama Bay, southern Japan Sea

Abstract

To investigate seasonal variation in the community structure of appendicularians, vertical hauls (0–500 m) with a Norpac net were made at an offshore station in Toyama Bay at intervals of 2–4 weeks from February 1990 to January 1991. Additional samples were collected with MTD nets at 12–17 different depth layers between the surface and a depth of 600–700 m at the same position in June, September and December 1986, and March 1992, to examine the vertical distribution of appendicularians. Twenty-one species (including two unidentified species) belonging to five genera were found, and the dominant species were divided into three groups by their occurrence period. Oikopleura longicauda, Fritillaria borealis f. typica and F. borealis f. sargassi occurred throughout most of the year. Fritillaria pellucida, O. fusiformis and O. rufescens were found in summer and autumn. Oikopleura dioica was found in spring and winter. Oikopleura longicauda was overwhelmingly the most abundant species throughout the year. This species was always distributed in the upper 100 m depth, with a peak at a depth of 0–50 m that corresponded to the peak of chlorophyll a concentration during the day and night in all seasons. The day–night vertical distribution patterns of F. borealis f. typica, F. pellucida, O. fusiformis and O. rufescens were similar to that of O. longicauda. Seasonal variations in abundance of appendicularians are considered to be the result of biological factors rather than physical factors such as temperature and salinity. In particular, O. dioica seems to be affected by food availability.