The Calanoid Copepod Euchaeta Antarctica from Southern Ocean Atlantic Sector Midwater Trawls, with Observations on Spermatophore Dimorphism

Abstract

Stage V amd VI copepodids of the predatory calanoid copepod Euchaeta antarctica were counted from Southern Ocean Atlantic Sector trawl samples collected during 9 months over a 5-year period. The proportion of CVI males to all CVI animals reached a maximum in June; the proportion of CVI males to total males reached a maximum in July. Two morphologically distinct kinds of spermatophores were attached to 2 different areas on the genital segment of females, and were correlated with 2 kinds of spermatophores held in leg 5 and contained within the bursa of different males. Possible adaptive significance ofspermatophore dimorphism and placement for Euchaetidae is discussed and further studies are outlined. From 1962 to 1972, the National Science Foundation's United States Antarctic Research Program (USARP) supported surveys of Southern Ocean marine organisms. Collections included over 1,000 3-m Isaacs-Kidd midwater trawl samples of the pelagic midwater fauna. In the Atlantic Sector, samples were taken during most months of the year. Samples were collected initially by the research staff of the University of Southern California and later by scientists from the Smithsonian Oceanographic Sorting Center, Woods Hole Oceanographic Institution, and Texas AM Wolfenden, 1908, 1911; Farran, 1929; Vervoort, 1951, 1957, 1965; Yamanaka, 1976; Park, 1978). Records north of the Antarctic Convergence by Yamanaka (1976) and Park (1978) off Argentina, and Vervoort (1957) in the Indian Ocean generally are from greater depths. Recently specimens have been collected from as shallow as 10 m off Isla Madre de Dios in the Chilean fjord system (Marin and Antezana, 1985). The vertical distribution limits of E. antarctica have yet to be studied systematically through 1 year, although Hopkins'(1985a) collected it to 1,000 m (his deepest samples) in March and April; Yamanaka (1976) reported specimens to 4,000 m in July, August, October, and December. Its presence in plankton samples taken under ice has been noted in the Ross Sea by Farran (1929) at Cape Evans, Ross Island, and by Bradford (1981) at McMurdo Sound. Littlepage (1964) studied seasonal changes in lipid content of adults from McMurdo Sound. Reports of biological interactions include an outline of its trophic position in the pelagic community during March and April (Hopkins, 1985b) (it fed on pelagic copepods, e.g., Oncaea and Metridia, and was eaten by a fish, Electrona antarctica), and its predation by a benthic brittle star, Astrotoma agassizii (Dearborn et al., 1986).