Importance of waterborne cadmium and zinc accumulation in the suspension-feeding amphioxus Branchiostoma belcheri

The amphioxus is often used as a model marine animal in biological research because of its special position in evolution. These animals have transparent surfaces and nonrespiratory gills, and whether such specialization plays a role in the accumulation of metals remains unknown. In the present study, we determined the biokinetics of Cd and Zn in the amphioxus Branchiostoma belcheri, including metal assimilation efficiency (AE), the dissolved Cd and Zn uptake rate constants, and the efflux rate constant. Metal AEs were 7.6 to 34.3% for Cd and 8.4 to 42.9% for Zn in amphioxus feeding on 5 different algae, and the AEs of Zn were significantly influenced by the algal con centration. The dissolved uptake rate constants were 0.107 l g−1 d−1 for Cd and 0.061 l g−1 d−1 for Zn. The efflux rate constants were 0.012 to 0.033 d−1 for Cd and 0.031 to 0.040 d−1 for Zn following dietary or waterborne metal exposure. Overall, these determined bio kinetics were comparable to those found in other suspension feeders, such as marine bivalves. However, amphioxus had a higher absorption efficiency from the dissolved phase than those found for marine bivalves, largely caused by the unique surface sorption of metals. Such unique surface uptake contributed to the dominance of waterborne Cd accumulation in amphioxus under most environmental conditions. For Zn, dietary exposure appeared to be more important than aqueous uptake due to its relatively high particle reactivity. The predicted trophic transfer factors of Cd and Zn were <1 under most conditions.