Mechanisms generating and maintaining the admixture of zooplanktonic molluscs (Euthecosomata: Opistobranchiata: Gastropoda) in the Subtropical Front of the South Atlantic

The aim of this study was to test the hypotheses of faunistic mixing. Various different mechanisms of the mixture of zooplanktonic organisms may exist in the transition zones. The distribution of planktonic gastropods of the order Thecosomata, which show a high fidelity to the water masses, was analysed. The study area comprises the southern part of the Brazil Current, the Brazil-Malvinas Confluence Zone, the Brazil Current Front, the Subtropical Front, the South Atlantic Current and the southern Benguela Current. The Subtropical Front presents sectors with contrasting dynamic characteristics which allow us to test hypotheses about the mechanisms generating faunistic mixture on a mesoscale. The evidence presented in the present study suggests that at least two mechanisms of faunistic mixture exist in the Subtropical Front. One is passive, resulting from environmental mixture in very dynamic front areas, where eddies are formed. The other mechanism occurs when the water column is vertically stratified. This mechanism depends on the migratory behaviour of the zooplankters and requires that there is stable stratification, that the depth of layers does not exceed the migratory capacity of the larger individuals and that the species can survive the physicochemical gradient. Received: 30 September 1998 / Accepted: 9 May 1999