CONICET | Buscador de Institutos y Recursos Humanos

The temperate zone of the Southwestern Atlantic Ocean (23-42ºS), which includes the Large Patagonian Marine Ecosystem and the Subtropical Convergence Zone, is one of the most productive areas of the Southern Hemisphere. Key features of this region are a wide shelf, the convergence of cold and warm currents, and the continental freshwater input of the La Plata River. The Uruguayan marine and estuarine waters are at the core of this zone. The marine and estuarine bivalve fauna of Uruguay received relatively good attention since the publication of the Voyage of Alcide d´Orbigny (1834-1846). Here we provide an overview of taxonomic, faunistic and biogeographic issues, identifying knowledge gaps and highlighting priorities for future research. The main threats for that fauna are discussed, with emphasis on species of current or potential socioeconomic interest. Of the 212 species reported from the area, only four species are strictly estuarine there: Erodona mactroides Bosc, 1801, Tagelus plebeius (Lightfoot, 1786), Brachidontes darwinianus (d´Orbigny, 1842) and Mytella charruana (d´Orbigny, 1842), all of these having large biomasses, as is also the case for the marine eurihaline Mactra isabelleana d´Orbigny, 1846. A total of 113 deep-sea species are recorded for the area of interestconsidered / studied, including almost every known group occurring elsewhere in deep-sea basins, with the exception of sunken wood associated species. Of these, 37 have been recorded only from the Argentine Basin. Some new records are preliminarily reported and discussed, including Acharax (Solemyidae), Lucinoma, Graecina (Lucinidae), and Callogonia (Vesicomyidae), all from the continental slope. A total of 19 warm and warm/temperate bivalve species have their southern distribution boundary in Uruguayan waters associated to warm waters of/or derived from the Brazil Current, including species distributed from the USA to Uruguay or from southeast Brazil to Uruguay. On the other hand, at least 8 exclusively cold-water bivalves exhibit their northernmost distribution boundary off La Plata River; their occurrence there is associated with offshore sub-Antarctic waters. We encountered three major problems with published information: (a) ?Abbott´s effect? (i.e., the tendency to identify species from the South Hemisphere based on North Hemisphere criteria), which affects many identifications and biases complied distributions; and (b) inferred ranges of geographic or bathymetric distribution, as in other shelly marine invertebrates, are distorted by the inclusion of regionally extinct species in check-lists of extant fauna; and (c) uncritical inclusion in regional checklists of extra-limital records of species that show up only occasionally along the coasts and shelves of Uruguay and southernmost Brazil. Knowledge gaps include (i) small-sized species (notably microbivalves) such as the Galeommatoidea and Cyamioidea, but also other less cryptic but morphologically variable species, such as members of Hiatella, Abra and the Corbulidae; (ii) an extensive, deep-water sector of the area that remains virtually unexplored; (iii) hard bottoms located on the outer shelf and slope, that remain undersampled; and [(iv)ii] deep-sea wood-associated bivalves (either epibenthic or borers). We also emphasize the need to utilize dredges specially designed for deep penetration into the sediments. Threats include the Rapa whelk (Rapana venosa (Valenciennes, 1846)), the extraction of several species, one of these affected by natural mass mortalities, and habitat degradation. The Uruguayan waters represent a critical biogeographical and ecological crossroads because of the complex interaction of currents and water masses. This region is thus particularly well suited as a system for the study of processes underlying biodiversity patterns. Pending challenges in taxonomic and biogeographic research will be successfully addressed only if multinational collaborative initiatives are undertaken in a framework of integrative taxonomy.

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