Comparative accumulation of 109Cd and 75Se from water and food by an estuarine fish (Tetractenos glaber)

Few data are available on the comparative accumulation of metal(loid)s from water and food in estuarine/marine fish. Smooth toadfish (Tetractenos glaber), commonly found in estuaries in south-eastern Australia, were separately exposed to radio-labelled seawater (14 kBq L-1 of 109Cd and 24 kBq g-1 75Se) and food (ghost shrimps; Trypaea australiensis: 875 Bq g-1 109Cd and 1130 Bq g-1 75Se) for 25 days (uptake phase), followed by exposure to radionuclide-free water or food for 30 days (loss phase). Toadfish accumulated 109Cd predominantly from water (85%) and 75Se predominantly from food (62%), although the latter was lower than expected. For both the water and food exposures, 109Cd was predominantly located in the gut lining (60e75%) at the end of the uptake phase, suggesting that the gut may be the primary pathway of 109Cd uptake. This may be attributed to toadfish drinking large volumes of water to maintain osmoregulation. By the end of the loss phase, 109Cd had predominantly shifted to the excretory organs e the liver (81%) in toadfish exposed to radio-labelled food, and in the liver, gills and kidney (82%) of toadfish exposed to radio-labelled water. In contrast, 75Se was predominantly located in the excretory organs (gills, kidneys and liver; 66e76%) at the end of the uptake phase, irrespective of the exposure pathway, with minimal change in percentage distribution (76-83%) after the loss phase. This study emphasises the importance of differentiating accumulation pathways to better understand metal(loid) transfer dynamics and subsequent toxicity, in aquatic biota.