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Paralichthys species have high commercial value and constitute a resource for demersal fisheries in the Southwestern Atlantic Ocean. Although six species have been described based on morphological characters for the Uruguayan and Argentinean waters, only the presence of three species is accepted, being their distribution still uncertain. Genetic tools are useful for species identification as a complement of morphological characters. This paper determines the genetic and morphometric variation between three species of Paralichthys that inhabiting Uruguayan waters. The interorbital width (IN) and the length from the mouth to the dorsal fin base (SD) of P. patagonicus differed significantly from the other two species (P. orbignyanus and P. brasiliensis). From a morphological perspective, individuals identified as P. orbignyanus formed two different groups. Clade Po_A included those belonging to oceanic coastal waters of Uruguay and deeper waters of the Rio de la Plata together with individuals of P. brasiliensis from ItajaÃ-, Santa Catarina (Brazil). Individuals collected in the Rio de la Plata coastal and beach zone formed the clade Po_B group. Genetic analysis confirmed for the first time the presence of P. brasiliensis in Uruguayan and Rio de la Plata shelf waters and extends its known distribution area further south. Discriminant scores for P. orbygnianus separate individuals in two groups corresponding to the presence of two species. Molecular and morphological analysis showed individuals in the clade Po_A includes those collected in Uruguayan waters together with those collected in northern Brazilian waters belonging to the species P. brasiliensis. [ABSTRACT FROM AUTHOR]

The presence of hybrids in the wild represents a threat to the integrity of species. In nature, sympatric species with the capacity to hybridize have mechanisms to prevent introgression, but hybrids originating from aquaculture may pose a threat to natural populations by inevitable escapes from captivity and their interaction with pure individuals in the wild. Given the increased vigour of these hybrids, they may compete better than native individuals.The species of the genus Pseudoplatystoma, commonly named sorubims, are some of the most emblematic species of the La Plata River basin, and are an important part of fisheries and aquaculture in the region.In this work, nuclear and mitochondrial genetic markers were used to evaluate the presence of hybrids in the lower La Plata River basin. Although hybrids of Pseudoplatystoma corruscans and Pseudoplatystoma reticulatum were previously detected in the upper Paraná and Paraguay rivers in Brazil, the presence of hybrids is here extended to the middle section of the Paraná River in Argentina. Hybrids were not found in the lower Paraná River or in the lower and middle sectors of the Uruguay River.This result has implications for conservation, as it shows the impact of the aquaculture of sorubims in the Paraná River, although apparently they have not yet affected the lower and middle Uruguay River. [ABSTRACT FROM AUTHOR]

Micropogonias furnieri is widely distributed in the southwestern Atlantic Ocean. In Uruguay, Laguna de Rocha and Río de La Plata estuary have been reported as reproductive and nursery areas. In Laguna de Rocha, individuals reach maturity at smaller total length than their oceanic counterparts. It has been difficult to establish whether Laguna de Rocha represents a biologically distinct population or simply ecophenotypic variation. More generally, the possible presence of several distinct populations of white croakers in Uruguayan waters has been hypothesized, but limited data exist to substantiate them. A recent mitochondrial DNA analysis suggested divergence between the Río de La Plata and the Oceanic front populations. Using seven microsatellites loci, we studied the population structure of M. furnieri in the nursery areas suggested by the literature, as well as in three additional localities to test these hypotheses. The individuals of Laguna de Rocha showed a moderate genetic differentiation with respect to some of the other populations surveyed. Specimens of Montevideo showed the higher genetic distinctiveness. Given the apparent absence of geographical barriers, other factors may be responsible for the observed differentiation. The complex pattern of forces interacting in this system makes it difficult to disentangle the causes of the population structure found. The adaptation to local environmental conditions could be playing an important role in population differentiation, as well as the possible selective pressures imposed by fisheries. The results obtained in this work offer clues about the processes responsible for differentiation of fishes in estuarine and marine environments. [ABSTRACT FROM AUTHOR]