Bioaccumulation of metals in calanoid copepods by oral intake

We demonstrated that oral intake of water by two calanoid copepods, Pseudodiaptomus annandalei and Eurytemora affinis takes place and has implications for their ecotoxicology. In the first experiment, copepods were exposed to a dyed medium, which allowed us to visually examine the possibility of water uptake by the copepod. We observed that both copepod species were taking in water orally and evacuated dye at different speeds. This exposure left concentrated dye in the guts of the copepods indicating adsorption into the gut epithelium. This was further demonstrated by exposing both copepod species independently to dissolved metals (Cd,17 µg/L; Cu,13.8 µg/L; Ni, 29.3 µg/L) and to dietary metals (Cd,18.8 µg/g; Cu, 35.3 µg/g; Ni, 32.5 µg/g). The results showed that although the concentration of dissolved metals they were exposed to were lower than those of the dietary metals, nevertheless, uptake of metals by both copepod species from the dissolved phase alone was substantially higher than from dietary exposure. This provides clear evidence to support our hypothesis that higher metal body burden observed in copepods exposed to dissolved metals than in those exposed to dietary metals is an implication of oral intake of water. P. annandalei showed higher excretion rate of metals when exposed to dissolved metals than E. affinis. However, the excretion rate of metals from both copepod species exposed to dietary metal was similar. We conclude here that both copepod species take in water orally. Our study further showed that metal uptake depends on the exposure routes and the uptake and excretion rates are dependent on the type of metals, amounts and the species.