Annual population dynamics of the opossum shrimp Neomysis americana Smith, 1873 (Crustacea, Mysidacea) from an estuarine sector of the Argentine Sea

Abstract

The opossum shrimp Neomysis americana Smith, 1873 (Crustacea: Mysidacea) is abundant in coastal and estuarine sectors of the southwest Atlantic ((30°S-40°S) where it plays a key role as food for fish. In the present work the population dynamics of the species in Samborombón Bay (Río de la Plata estuary, Argentina) during an annual cycle is analyzed. Samples were obtained from March 1987 through to March 1988 with a small Bongo net (20 cm diameter, 200 µm mesh size). N. americana occurred all year round with maximum densities in austral spring and summer. Three annual generations were identified. The overwintering generation had a growth rate of about 0.037 mm day^-1 and duration of 4.9 months, the spring generation grew at a rate of 0.146 mm day^-1 during 1.5 months, the summer generation had a growth rate of 0.076 mm day^-1 and matured in 1.5 months. The sex ratio was favourable to females, which had a higher growth rate and a larger size than males. The species reproduced constantly throughout the year. The proportion of gravid females was higher during spring and summer but the brood size was larger in winter (20-25 embryos female^-1) than in spring and summer (10-15 embryos female^-1). The results obtained were related to the environmental conditions. The implications of the spatial and temporal distribution patterns of Neomysis americana for the local food web were discussed.