Effects of salinity and hypoxia on cadmium bioaccumulation in the shrimp Palaemon longirostris

The aim of the present study was to investigate the role of two key environmental factors of estuarine ecosystems, salinity and hypoxia, on the Cd bioaccumulation by direct exposure in the white shrimp Palaemon longirostris. Two types of experiments were performed in the laboratory. First, we studied Cd accumulation by shrimp after metal exposure at two salinities (0.2 and 10 per thousand) and/or water oxygen levels (21 and 6 kPa). We also investigated the role of hypoxia in more detail and, in particular, its interaction with water Cd concentration by subjecting shrimp at low salinity (0.2 per thousand) to two oxygen levels (21 and 6 kPa) and four concentrations of dissolved Cd metal (0.2, 0.5, 2, and 10 microg/L). Second, we studied the ventilatory and circulatory responses of P. longirostris to changes in oxygen and Cd concentrations to understand some basic aspects of the underlying mechanisms involved in the accumulation process. Our findings allow us to confirm that salinity is the main factor acting on dissolved Cd bioaccumulation processes. However, we demonstrate that hypoxia also must be classified as being of primary importance. Through its physiological effect on the prawn, hypoxia strongly enhances the rate of Cd accumulation in gills and hepatopancreas. Its magnitude is inversely related to the metal concentration, because its influence increases when the metal concentration is low. Our results show that chemical speciation is, indeed, the primary factor that influences metal contamination. On the other hand, at low salinity, under conditions in which metal bioavailability is increased, we show that ventilation modifies metal accumulation in the gills by a factor of two.