The biogeography and ecological preferences of Neotropical freshwater ostracods are poorly known, and more so the dynamics of populations and habitat selection of species living in pleustonic environments of temporary ponds. In the present survey we analyze the population changes of ostracods living in pleustonic environments of small freshwater bodies on Martín García Island (Río de la Plata, Argentina). Between June 2005 and June 2007, monthly samples of floating vegetation from eight different ponds on the island were collected, and limnological parameters were measured in situ. The results of multivariate logistic regression showed that the presence of ostracods was significantly related to high dissolved oxygen content and high water temperature. In addition, multivariate regression analysis indicated that, when ostracods were present, their total abundance was negatively related to floating vegetation dry weight. Four ostracod species were found: Strandesia bicuspis, Chlamydotheca incisa, Cypridopsis vidua, and Bradleytriebella trispinosa. The seasonal variation in abundances indicated that populations of the most common species, S. bicuspis and C. incisa, were denser during the summer and autumn months. The results of canonical correspondence analysis showed that individuals of S. bicuspis were more abundant at higher temperatures and lower conductivity than C. incisa. Further research is needed to clarify the observed negative correlation between floating vegetation dry weight and ostracod density and the possible differential thermal preference of the two species studied.

Deroceras reticulatum is a misanthropic European species spread widely throughout South America. At the moment this species is considered a 'pest' in direct sowing such as maize, soybean, sunflower, wheat, alfalfa and clovers, among others. The aim of this paper is to report the first record of D. reticulatum in the Reserva de Usos Multiples Isla Martín García, Buenos Aires province and to provide information about this species distribution in five Argentina provinces.Keywords: Deroceras reticulatum, distribution, pest, exotic species, Argentina. Distribução de Deroceras reticulatum (Müller, 1774) (Pulmonata, Stylommatophora) na Argentina com primeiro registro na Reserva de Usos Múltiplos Ilha Martín García, Río de la Plata superior ResumoDeroceras reticulatum é uma espécie sinantrópica oriunda da Europa e com grande dispersão na América do Sul. Esta espécie é considerada atualmente praga dos cultivos de semeadura direta de milho, soja, trigo, girassol, alfafa e trevos, entre outros. O presente trabalho tem por objetivo comunicar o primeiro achado do agriolimácido D. reticulatum na Reserva de usos múltiplos Ilha Martín García, província de Buenos Aires e proporcionar informação sobre a distribuição desta espécie em cinco províncias argentinas.Palavras-chave: Deroceras reticulatum, distribuição, praga, espécie exótica, Argentina.

The Island of Martin Garcia lies at the confluence of the Uruguay and Paraná Rivers (upper Río de la Plata). This island is an outcrop of the crystalline basement. Due to basalt exploitation the island exhibits several ponds covered by carpets of free-floating macrophytes. Seven major environmental variables were measured: water and air temperature, percentage of oxygen saturation, dissolved oxygen, electrical conductivity, total dissolved solids and pH. Eleven leech species were found, Helobdella striata, H. diploides, H. adiastola and H. hyalina were new records. UPGMA clustering of species based on their occurrence in different ecological conditions revealed three main species groups. Canonical Correspondence Analysis (CCA) explained 97.2% of the correlation between species and environmental variables. H. triserialis shows the widest range of tolerance, H. hyalina shows positive relationships to conductivity; H. diploides shows a high correlation with dissolved oxygen, H. striata, H. lineata, and S. similis are negatively correlated with water temperature, and H. simplex is positively correlated with pH. Relationships between the species richness (S) and the sampling sites were negatively correlated with water temperature and positively correlated with dissolved oxygen. Leech biodiversity from the water bodies of Martín García Island, shows a great diversity of species and a wide plasticity regarding the characteristics of the environmental factors considered.Keywords: biodiversity, CCA, cosmopolitan, ecology, endemics, leeches. A Biodiversidade e a Ecologia de

No abstract