Comparison of metal uptake rate and absorption efficiency in marine bivalves

Recent studies have quantified extensively metal assimilation efficiency from ingested food sources in aquatic invertebrates. Metal absorption efficiency (alpha) from the dissolved phase is analogous to metal assimilation efficiency, but it remains poorly defined and quantified. In this study, the alpha of four trace metals [Cd, Cr(VI), Se(IV), and Zn] was determined in three species of marine bivalves (green mussel [Perna viridis], black mussel [Septifer virgatus], and clam [Ruditapes philippinarum]). Individual bivalves were first measured for their clearance rates, followed by measurements of the metal influx rate, after which the metal alpha and the uptake rate constant (Ku) were then computed. Among the four metals considered, the highest Ku and alpha were found for Zn, followed by Cd > Cr(VI) > Se(IV). The Ku values were comparable between the two mussels but were 1.8- to 3.3-fold lower in the clams. Interspecific difference in metal Ku was strongly related to, but intraspecific difference in Ku was not affected by, the bivalve's clearance rate. Interspecific difference in metal alpha was smaller than the metal Ku and was independent of the clearance rate, whereas the intraspecific difference in metal alpha correlated with the individual variations of the clearance rate. Within each bivalve species, a significant negative correlation was found between the metal alpha and the clearance rate, implying that an individual pumping a greater amount of water was coupled to a lower alpha. Significant correlation between the alpha of four metals was also documented in all three species of bivalves. Thus, metal bioavailability from the aqueous phase was directly related to the physiological conditions of the animals. Both the aqueous chemistry and the physiology of the animals can be important in affecting metal bioavailability from the dissolved phase.