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.

This study determined the effect of cadmium (Cd) toxicity comparatively on two copepods, Eurytemora affinis (Poppe 1880) from a temperate region (Seine Estuary, France) and Pseudodiaptomus annandalei (Sewell 1919) from a subtropical region (Danshuei Estuary, Taiwan), according to their sex and reproductive stages. In addition, the effect of Cd to their life cycle traits was quantified. In the first experiment, both copepod sexes were exposed to 40, 80, 150, 220, and 360 µg/L of Cd and a control cultured in salinity 15, except that the temperature was 18 °C for E. affinis and 26 °C for P. annandalei. This allowed calculating median lethal concentration (LC50) of Cd after 96 h. This was 120.6 µg/L Cd for P. annandalei males which were almost twice as sensitive as P. annandalei females (LC50 = 239.5 µg/L Cd). For E. affinis females, the LC50 was 90.04 µg/L Cd, reflecting a 1.4 times higher sensitivity of females than of males (LC50 = 127.75 µg/L Cd). The males of both species were similarly sensitive; however, the E. affinis females were 2.7 times more sensitive than the P. annandalei females. We also compared the sensitivity of ovigerous females (OVF) and non-ovigerous females (NOF) of both species to Cd. Mortality was higher in NOF than in OVF of both copepod species in both the control and the 40 µg/L Cd treatment. Finally, the total population, fecundity and female morphology of both copepod species were estimated after exposing one generation cycle (nauplius to adult) to 40 μg/L Cd (for E. affinis) and 160 μg/L Cd (for P. annandalei). A significant decrease in cohort production, survival and clutch size but no significant difference in the prosome length of both copepod species exposed to Cd were detected. The ratio of OVF:NOF was high in both copepod species exposed to Cd. Cd toxicity did not significantly affect the M:F sex ratio and % OVF of E. affinis. However, the effect of Cd toxicity in P. annandalei was significant in the M:F sex ratio and was in favor of females and their reproductive activities due to an increase in % OVF. Moreover, there was a significant decrease in total production of P. annandalei due to high mortality in their nauplii and copepodid developmental stages. Toxicity to Cd appears to be affected by multiple factors including sex, reproductive life stage and species. The ecological implication of Cd toxicity on E. affinis and P. annandalei copepod ecology is more related to a skewed sex ratio, low egg production, reduced hatchability and reduced survival that affects the recruitment potential of the copepod nauplii resulting in a decreasing copepod population. Mortality, reproduction and population growth of model species may provide important bio-indicators for environmental risk assessment.