Seasonal abundance and distribution of pelagic tunicates (Chordata: Thaliacea) in the central South China Sea

A total of 168 macro-zooplankton samples from 42 stations in the central South China Sea (12°~20° N, 111°~118° E, an area of about 64×10⁴ km²) were collected in September 1983 (autumn), April 1984 (spring), August 1984 (summer) and December 1984 (winter). Twenty-three species and subspecies of tunicates were found, of which Thalia democratica complex (including T. d. orientalis and T. d. echinata) and Doliolum denticulatum were the dominant species, and accounted for 95.7%,90.0%, 91.8% and 90.5% of the total tunicates found in autumn, winter, spring and summer, respectively. The highest abundance (with a mean of 2.37 ind./m³) occurred in autumn. There are strong correlations between the abundances of the tunicates and those of phytoplankton and chlorophyll a concentration. However, tunicates also aggregate in areas with low primary production in the autumn survey, probably due to the water circulation pattern.