Uptake and loss of dissolved 109Cd and 75Se in estuarine macroinvertebrates Original Research Article

Semaphore crabs (Heloecius cordiformis), soldier crabs (Mictyris platycheles), ghost shrimps (Trypaea australiensis), pygmy mussels (Xenostrobus securis), and polychaetes (Eunice sp.), key benthic prey items of predatory fish commonly found in estuaries throughout southeastern Australia, were exposed to dissolved 109Cd and 75Se for 385 h at 30 kBq/l (uptake phase), followed by exposure to radionuclide-free water for 189 h (loss phase). The whole body uptake rates of 75Se by pygmy mussels, semaphore crabs and soldier crabs were 1.9, 2.4 and 4.1 times higher than 109Cd, respectively. There were no significant (P > 0.05) differences between the uptake rates of 75Se and 109Cd for ghost shrimps and polychaetes. The uptake rates of 109Cd and 75Se were highest in pygmy mussels; about six times higher than in soldier crabs for 109Cd and in polychaetes for 75Se – the organisms with the lowest uptake rates. The loss rates of 109Cd and 75Se were highest in semaphore crabs; about four times higher than in polychaetes for 109Cd and nine times higher than in ghost shrimps for 75Se – the organisms with the lowest loss rates. The loss of 109Cd and 75Se in all organisms was best described by a two (i.e. short and a longer-lived) compartment model. In the short-lived, or rapidly exchanging, compartment, the biological half-lives of 75Se (16–39 h) were about three times greater than those of 109Cd (5–12 h). In contrast, the biological half-lives of 109Cd in the longer-lived, or slowly exchanging compartment(s), were typically greater (1370–5950 h) than those of 75Se (161–1500 h). Semaphore crabs had the shortest biological half-lives of both radionuclides in the long-lived compartment, whereas polychaetes had the greatest biological half-life for 109Cd (5950 h), and ghost shrimps had the greatest biological half-life for 75Se (1500 h). This study provides the first reported data for the biological half-lives of Se in estuarine decapod crustaceans. Moreover, it emphasises the importance of determining metal(loid) accumulation and loss kinetics in keystone prey items, which consequently influences their trophic transfer potential to higher-order predators.