Ramnogaster arcuata (Jenyns, 1842) and Platanichthys platana (Regan, 1917), found at the estuary of Lagoa dos Patos and coastal lagoon (Lagoa Mirim) at the South of Brazil, are two sympatric clupeid fishes with very similar features. In order to clarify their taxonomic and geographical situation, delimiting its occurrence in fresh waters, a comparative study of the bony structures of the head, anal and dorsal fins and ventral scutes of these two sardines was developed. The osteological study with clear and stain technique included more information about the differentiation of the groups from the bones of the supramaxilla, dentary, pterygiophore and epipleural ribs. Variations on the body shape of those species were analyzed through the multivariate morphometry, in order to obtain differentiation patterns between the groups. Thirty specimens were selected from each species. The measurements were obtained by truss network, by using 10 homologous anatomical landmarks. The analysis of the principal components, adjusted by Burnaby's orthogonal projection, indicated two main groups, separated along the PC 1. which explained 63.2% of the variation between groups. The diagnostic characters that differentiate R. arcuata from P. platana, were related to segments of distance of the head, longitudinal growth, body height and caudal peduncle. Furthermore, two groups of P. platana were found. In spite of their morphological similarities, they presented a differentiation gradient between the estuary population of the Patos Lagoon and the Mirim Lagoon, possibly resulting from two ecologically different environments.
Clupeidae; multivariate morphometry; morphological variation; osteology