ENVIRONMENTAL-STRESS ON CORBICULA-FLUMINEA (BIVALVIA) IN THE PARANA RIVER DELTA (ARGENTINA) - COMPLEX POLLUTION-RELATED DISRUPTION OF POPULATION STRUCTURES

Corbicula fluminea was collected at monthly intervals in November 1991 -June 1993, and in March, April 1995 at a site located in the vicinity of strong sources of industrial and sewage effluents (confluence of t he Rivers San Antonio and Vinculacion, SaV); and in November 1994 and March, April 1995 at a presumably less polluted one (Parana de las Pal mas River, PP), in the lower delta of the Parana River (Argentina). Ab undances at SaV ranged between 430 and 10,300 ind. m(-2), being notice ably higher than those at PP, as well as those at 19 stations between and around these two locales. Shell lengths at SaV were monotonous thr oughout the 19-months period, over 90% of the clams varying between 16 and 22.5 mm in length; no discernible cohorts were recorded. At PP, o n the other hand, juveniles below 5 mm were largely dominant, and shel ls above 25-30 mm in length were very common. Larvae retrieved from th e gills of gravid clams showed much greater mortality rates in waters from SaV (up to 75% after 120 hr), than in those from PP and in the co ntrol (approximately 10% after 192 hr). The first internal growth-rela ted mark in clams from SaV was closer to the umbo (mean: 14.2 mm), tha n that in clams from PP (mean: 24 mm). It is concluded that inadequate water quality at SaV is responsible for 100% mortalities of newborns, for dwarfed adult clams, and for the lack of discernible cohorts.