The Littorina ziczac (Gmelin, 1791) species complex was studied in Rio de Janeiro State, southeastern Brazil. The occurrence of three species was confirmed, through the analysis of penis morphology: L. ziczac (Gmelin, 1791), L. lineata d'Orbigny, 1841 and L. lineolata d'Orbigny, 1840. There is a correlation between the morphology of the penis and shell shape, thus it was possible to make a discrimination model based in conchologic data, with an efficiency of 75%. However, this model cannot be applied to others samples that weren't included in its formulation, due to phenotypic plasticity of the shell caused by biotic and abiotic factors. In the 9 areas studied L. lineolata was always the most abundam species. No vertical stratification in the distribution of the species was verified. From the nine enzymatic systems examined, only Pgi, Pgm e Mpi, presented identifiable patterns and no diagnostic locus was identified in any species. Nevertheless, S and I suggest the individualization of L. ziczac and L. lineata,respectively.
Littorina; species complex; vertical distribution; biochemistiy; Rio de Janeiro; Brazil