Winter ontogenetic migrations and the onset of gonad development in large dominant calanoid copepods in the Weddell Gyre (Antarctica)

Abstract

Data on abundance, stage composition and vertical distribution, sex ratio, maturity states, and the inferred seasonal/ontogenetic migrations of Calanoides acutus, Calanus propinquus, and Rhincalanus gigas obtained during the Winter Weddell Gyre Study 1992 in early to mid-winter are presented. During this period copepodite Stages CIV and CV and adult females of C. acutus underwent an ontogenetic downward migration, which in CIV seemed to be delayed until early winter if moderate biomass of phytoplankton was sustained in the upper water layers. The descent of CV and CVI was close to completion already by June while CIV continued to sink down slowly during the winter. By mid-winter, sexual differentiation in CV, maturation of males, and the onset of female maturation were observed for C. acutus. Males of C. acutus maintained rather restricted vertical distribution centered in the core of the Warm Deep Water. C. propinquus showed no indication of regular ontogenetic migration, but did show some seasonal changes in the vertical distribution of particular stages with a strong geographical variability. The bulk of the overwintering C. propinquus population in the Weddell Gyre consisted of CIII and did not leave the Winter Water. The maturation of males and females in C. propinquus began in winter as well. R. gigas underwent a downward migration, which was only detected in those parts of the Weddell Gyre where some recruitment of that species took place, i.e. at the Weddell Front and in the Maud Rise area. Although sexual differentiation in CV was in progress in winter, the onset of maturation of both males and females in R. gigas appeared to be delayed until at least the end of winter.