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.

The systematic relationships in the species of the genus Odontesthes Evermann & Kendall, 1906 were analysed by multivariate morphometry, using data adjusted for alometric size effects by orthogonal projections. The results are discussed in terms of morphological differentiation to each geographic units. It was concluded that seven species were identified in the southern Brazilian coast. Five of them inhabit freshwater environments, O. bonariensis (Valenciennes, 1835), O. humensis De Buen, 1953, O. retropinnis (De Buen, 1953), O. aff. perugiae Evermann & Kendall, 1906 and O. mirinensis Bemvenuti, 1995. The freshwater atherinids with the largest body size, O. retropinnis, O. bonariensis and O. humensis, can be distinguished by snout size, mouth position and number of gill rakers on the first branchial arch. The small size silversides, Odontesthes aff. perugiae and O. mirinensis, showed features of similar morphologic and sympatric distribution. The remaining species were represented by O. incisa (Jenyns, 1842), that occurred in coastal areas and O. argentinensis (Valenciennes, 1835) in brackish waters. In the last species, intraespecific morphological variation is attributed to phenotypic plasticity, resulted from environmental conditions found in that ecosystem.

Gut contents of 304 individuals of the silverside Odontesthes humensis de Buen, 1953 were analyzed using frequency of occurrence (FO%) and gravimetric (P%) methods. A total of 207 individuals (68.1%) had some food itens in the gut, while 97 individuals (31.9%) had empty guts. The silverside Odontesthes humensis has presented a benthic carnivorous diet, preying mainly on molluscs and arthropods. The molluscs Heleobia sp. (FO = 61.35%) and Corbicula fluminea (Müller, 1774) (FO = 57.97%) were the most frequent itens, followed by Neocorbicula limosa (Maton, 1811) (FO = 17.39%). Among the arthropods, the coleoptera insects (FO= 18.84%) were dominant followed by, insect larvae (FO = 6.76%), the crustacean Palaemonetes argentinus Nobili, 1901 (FO = 1.93%) and isopods (FO = 1.45%). Vegetal remains, organic matter and digested fish were grouped due to low frequency (FO = 9.13%) being considered occasional. Juvenile fed mainly on insect larvae and moluscs, while the adults preferred molluscs and coleoptera. On the description of its feeding apparatus the importance of a protrusible upper jaw was observed, being important on the capture of prey in inaccessible places. A protrusible mouth and the format of the pharingean plates, are important morphological characters that assist on the capture and handling of prey. The molariform shaped pharingean teeth help break hard food items, as shells and carapaces.

Odontesthes mirinensis, a new species of silverside, is described in coastal lagoons system of Rio Grande do Sul, Brazil. The new species is distinguished from other Odontesthes species by short snout and having 24-29 gill rakers on the lower branch of the first branchial arch. Osteological features (bones of the skull, axial skeleton and girdles) are included and discussed. Meristic and morphometric variables were analyzed separately, through multivariate procedures. Principal Component Analysis show that Odontesthes mirinensis, sp.n. does not exhibit significant geographic variation on body shape.