Apical uptake of Cd in the gills and Cd influx to the circulatory system in zebrafish exposed to different Cd and Zn concentrations in the water (mean±S.D., N=6). The lines were fitted by non-linear regression (for results see Table 3), except for the uptake at 16 μM Zn where the line was fitted by linear regression. *The influx in the 16 μM group significantly lower than the influx in the 2 and 4 μM Zn groups (Mann–Whitney U test, P