Mesopelagic crustacea in and around a warm-core eddy in the Tasman Sea off eastern Australia

Abstract

Decapod Crustacea were collected during five cruises (in August, September, and October 1979, February and May 1980) at sites inside, at the edge, and outside of warm-core eddy J. These sampling sites were considered to have come from different domains of the Tasman Sea water mass. All 146 samples in this time series were taken in the upper 500 m at night using horizontal tows with an RMT 8. A total of 21 494 individuals belonging to 41 species and five larval categories was found. Nine of the 18 abundant species were cosmopolitan species: typical of samples from outside, at the edge of, and inside eddy J. Oplophorus spinosus was typical only at the eddy edge. Four species were typical of the outside and edge domains, and another four were typical of the inside and edge domains. Species abundances outside the eddy were dominated by Sergia prehensilis, Gennadas gilchristi, and Acanthephyra quadrispinosa. The first two species, plus Systellaspis debilis, were dominant at the eddy edge. Six species (Systellaspis debilis, Sergia prehensilis, Sergia scintillans, Sergia splendens, Sergestes atlanticus, and Parapandalas cf. richardi) were dominant at various times inside the eddy. There were no significant differences in the abundance of Sergia prehensilis in the three domains. Systellaspis debilis was significantly more abundant inside and at the eddy edge than outside the eddy, and Gennadas gilchristi was significantly more abundant outside and at the eddy edge than inside the eddy. An analysis of the community changes with time showed that the outside communities remained very similar in spite of the 5°30' change in latitude of eddy J between August 1979 and May 1980. In contrast, the inside communities were quite dissimilar between months, and there was no trend in this pattern. The edge communities were very similar except in May, when a large increase in the abundance of Gennadas gilchristi was found. Comparisons of communities between domains within each month showed the outside and edge communities were very similar over the entire period. In contrast, the outside and inside communities became increasingly different in time, mainly caused by changes in the abundances of species inside the eddy. The inside and edge communities were also quite different, but no consistent pattern in their dissimilarity with time was seen. Differences in the size-frequency distributions of Sergla prehensilis and Systellaspis debilis between domains through time suggest that breeding and recruitment were occumng at different times inside and outside the eddy. There was little evidence for colonization of the eddy by Tasman Sea species. We conclude that a secondary succession has been initiated, probably in response to the different physical and biological environments present inside eddy J as compared with the surrounding Tasman Sea.