Distribution and abundance of gymnosomata (Gastropoda: Opisthobranchia) in the Southwest Atlantic

The distribution and abundance of gymnosome gastropods in the Argentine Sea and Brazil--Malvinas Confluence during 1978-1979 and 1988 were studied. The collections analyzed included 768 quantitative samples obtained between 48°W and the coast, and from 35°S to 55°S. Two species were found. Spongiobranchaea australis was the most frequent and abundant (up to 730 per 1000 m3); its presence in the area was associated with the core of the Malvinas Current. Clione antarctica was less abundant (maximum abundance: 230 per 1000 m3) and was also associated with the Malvinas Current. The geographic ranges of both species in the area are wider than previously described. Since the range of S. australis in the area extends far from the range of its prey Clio, it is not clear whether S. australis can feed on the thecosomatous pteropod Limacina (and not only on Clio, as described in the bibliography) or it starves in that area. During the 1978-1979 annual cycle, the abundance of both species followed neither the abundance patterns of their prey nor of the total zooplankton, and differed from each other. The residence time of swarms of both gymnosomes were shorter than one month. As a general pattern, the aggregates are rapidly transported northward by the Malvinas Current and also penetrate the outer shelf water, but they remain there only during a short period and cannot preclude the final expatriation. So, the abundance of gymnosomes in the area depends on passive migration more than intrinsic population factors.Fil:Dadon, J.R. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Pteropoda (Gastropoda, Thecosomata e Gymnosomata) coligidos ao largo dos arquipélagos de São Pedro e São Paulo, costa nordeste, Brasil Pteropoda (Gastropoda, Thecosomata and Gymnosomata) collected along the northeast coast of the archipelago of São Pedro and São Paulo, Brazil

<abstract language="eng">Pteropoda studies were carried out offshore São Pedro and São Paulo Archipelago (0º56'2"N e 29º20'6"W) to assess biodiversity and spatial distribution. A Bongo net (mesh size 300 micrometer) was hauled obliquely from a depth between 0 and 50 m and Oand 100 m by the REVIZEE-SCORE/NE I oceanographic expedition on board of the Brazilian Navy Oceanographic Ship Antares. Samples were collected at 16 stations from September to October 1995. A total of 880 organisms distributed in 20 species, 14 genera and 9 families were identified. At the layer 0-50 m, Creseis virgula (Rang (828) was the most frequent species (71.43%), followed by Limacina inflata (Orbigny, (836) and Cavolinia inflexa (Lesueur, 18(3) (64.28%, each). ln terms of relative abundance outranked L. inflara (varying between 57.5 and 92.24%) and C. virgula (varying between 60.0 and 72.73%). At the layer 0-100 m, C. virgula was very frequent (80%), L. inflata (70%) and Cuvierina columnella (Rang (827) (40%) were frequent, other species had less than 30% of frequency. L. inflata varied from 37.5 to 100% and C. virgula from 42.42 to 100% of relative abundance. Species diversity varied from medium to low ( < 2.0 bits.ind-1) in both layers. At the layer 0-50 m C. virgula was directly correlated to temperature and salinity, while at the layer 0-100 m L. inflata was correlated to these parameters, showing that these species are Tropical Water indicators