Mass mortality of the yellow clam, Mesodesma mactroides (Bivalvia: Mactracea) in Monte Hermoso beach, Argentina Original Research Article

Yellow clam was the most abundant species in the sandy mesolittoral community of Monte Hermoso (Buenos Aires Province, Argentina). In November 1995, a sudden massive mortality almost decimated its population. The number of dead individuals was estimated in near 63 million, with a total biomass of 428.03 ton (fresh weight with shells). Simultaneous samples of sediments, seawater and clams did not allow detecting the probable cause of this phenomenon. No abnormality was found in the physicochemical parameters that could justify the mortality. Several specific mortality factors were discarded, as the analysis for heavy metal contamination, phytotoxins, abnormal phytoplankton composition, and protozoan tissue-parasites gave negative results. The meteorological conditions were rather stringent during a period immediately previous to the mortality, this accounting for a probable switch condition. This sole factor can not however account for the progressive decimation of the species observed from 1993 on: in the term of 3 years mass mortalities of Mesodesma mactroides occurred in Brazil, Uruguay and Argentina, covering the entire area of distribution of the species. The yellow clam is now in a risk condition as for its conservation status.