Silversides in South Brazil: morphological and ecological aspects.

The coastal plain of the Rio Grande do Sul state (RS) in southern Brazil, has an enormous lagoon complex in its southern portion formed by Patos Lagoon, Mirim Lagoon, Mangueira Lagoon and several other smaller lakes. This system was originated from successive transgression and regression cycles, which took place since the upper Pleistocene (Vilwoock, 1984). Patos Lagoon is a huge chocked lagoon (10.360 area km), with an estuarine area comprising 10% of the total area in its southern portion, where one can find small, oligohaline shallow water embayment less than 3m maximum depth (Delaney, 1965). Mirim Lagoon has an area of 3.749 km2, but only 2.382 km is located on Brazilian’s territory because the remaining belongs to Uruguay. Mirim Lagoon communicates with Patos Lagoon through the São Gonçalo channel (Delaney, 1965). Other small coastal lagoons are located along the coastal line, and are locally known as “lakes in rosary”. The larger one is the Mangueira Lake, a closed lake with an area of approximately 800 km2. Its northern portion is connected with the Taim wetland, which is formed by small lakes (Jacaré, Nicola, Flores). The other lakes are shallow (from 2 to 3 m) and are situated in the north portion of the Rio Grande do Sul state (Fig. 1). Ten species of silversides, comprising the genus Odontesthes, occur along the RS coastal plain (Bemvenuti, 2002; Malabarba and Dyer, 2002). In Patos Lagoon estuary and its adjacent marine coastal area occurs O. argentinensis (Valenciennes) and O. incisa (Jenyns), whereas in the freshwater habitats of PatosMirim lagoon system can be found O. bonariensis (Valenciennes), O. humensis De Buen, O. retropinnis (De Buen), O. aff. perugiae Evermann and Kendall and O. mirinensis Bemvenuti. In contrast, O. bicudo Malabarba and Dyer, O. piquava Malabarba and Dyer and O. ledae Malabarba and Dyer are endemic of a chain of small shallow lakes spread along the RS north coastline. Most of Odontesthes species co-occur in the same habitats and they are characterized by morphological similarities, which hinder their taxonomic identification.