Population genetics of the silverside Odontesthes argentinensis (Teleostei, Atherinopsidae): Evidence for speciation in an estuary of southern Brazil

Allozyme electrophoresis was used to investigate genetic population structu re among 245 individuals of the silverside Odontesthes argentinensis sample d from localities in southern Brazil and Uruguay. Striking differences in a llozyme frequencies and genetic variability were detected among localities. Analysis of these differences indicated two distinct populations: one dist ributed in a wide marine coastal zone; and one in the peripheral environmen t of the Patos Lagoon estuary in southern Brazil. The population in Patos L agoon appears to have originated from an ancestral marine population and be en established in the estuary by a founder effect. The extent and pattern o f genetic divergence found in our study, the existence of diverse reproduct ive strategies in the estuarine silversides, and differences in morphology of eggs and larvae, indicate that speciation is taking place in Patos Lagoo n. It is suggested that the two populations should be considered separately when formulating management and conservation strategies.