First record of a brackish radiolarian (Polycystina):: Lophophaena rioplatensis n. sp in the Rio de la Plata estuary

Vertically stratified bottle plankton samples collected in the Rio de La Plata estuary (Atlantic coast of South America at similar to35degreesS) and in coastal waters off Mar del Plata (similar to38degreesS) in December 1999 and November 2001 yielded up to 394 live cells l(-1) of a single new nassellarian species: Lophophaena rioplatensis n. sp. (family Plagoniidae). In estuarine waters, the species was recorded at salinities as low as 15.4 p.s.u.; densities in excess of 100 cells l(-1) were found at salinities ranging from 16.9 p.s.u. These extremely high concentrations (the highest ever reported in the literature), as well as the fact that >90% of the individuals recorded contain cytoplasm, indicate that these are self-sustaining populations which thrive in the estuary (and in nearshore coastal waters), probably due to plentiful dissolved silica and an abundant food supply. Lophophaena rioplatensis is the first polycystine brackish-water species described. This finding shows that radiolarian fossils are not unequivocally associated with open-ocean conditions, but may also be useful indicators of coastal and brackish estuarine paleoenvironments.