Cold, warm, temperate and brackish: Bivalvia diversity of a complex area of the Southwestern Atlantic (Uruguay)

SCARABINO F., J.C. ZAFFARONI, D. G. ZELAYA, JM ORENSANZ, A. CARRANZA, L. ORTEGA Y F. GARCÍA RODRÍGUEZ

México DF

Congreso; Mollusca 2014: el Encuentro de las Americas. XIII RENAMAC, IX CAMA, LXXX AMS y XLVII WSM; 2014

The Uruguayan marine and estuarine coastlines (ca. 500 km) include sandy beaches interrupted by coastal lagoons (without mangroves) and rocky outcrops, often associated with islands. The extended inner shelf (ca.60 nautic miles wide) is sedimentologically heterogeneous. Uruguayan waters concentrate critical biogeographical and ecological boundaries created by the interaction between the Río de la Plata estuary and subtropical and subantarctic waters. In fact, 24 % of the total (110 spp.) bivalve species reported from the shelf have their northernmost or southernmost records there. This is well exemplified with the distribution patterns of lucinid clams, until now very poorly documented in the region: outer shelf and slope influenced either by subtropical (Dulcina cf. lens) or by subantarctic waters (Epicodakia? falklandica and Lucinoma sp.). Additionally, coastal waters appear to have been colonized intermittently by Ctena cf. pectinella.The cold temperate mytilid Aulacomya atra exemplifies northwards dispersion via rafting on kelp holdfasts adrift, and unusual recruitments associated with cold oceanographic events. The venerids Tawera and Anomalocardia, which no longer live in Uruguayan waters, are respectively indicative of colder and warmer Quaternary thermal regimes. These cases illustrate the need of well dated records to establish current distribution ranges.Other noticeable features of the Uruguayan bivalve biodiversity include: (a) poor diversity but high biomasses of estuarine species including the phylogenetically singular genus Erodona, (b) a marine soft bottom warm-temperate inner shelf fauna represented by large biomasses of tellinoids (Donax, Psammacoma), mactroids (Mactra, Mesodesma), veneroids (Pitar, Amiantis), corbulids (Corbula) and even protobranchs (Ennucula), (c) high degree of deep sea endemism, still to be tested, and (d) methane/sulfide rich environments mostly represented by the first records of Acharax, Lucinoma and Vesicomya?.Future challenges include the detailed analysis of the whole fauna using integrative taxonomy in order to test purportedly broad latitudinal ranges of distribution in the western Atlantic.