Accumulation of dietary and aqueous cadmium into the epidermal mucus of the discus fish Symphysodon sp.

The discus fish Symphysodon sp. is an Amazonian cichlid with a unusual form of parental care where fry obligately feed from parental mucus for the first few weeks of life. Here, we investigated the possible impact of environmental cadmium on this species, particularly with respect to mucus contamination. We exposed groups of fish to cadmium either through their food (400 mg kg−1) or through the water (3 μg l−1) for 4 weeks, and measured tissue concentrations and ATPase activities at weekly intervals. Cadmium significantly accumulated in all tissues (except for muscle) after 7 days, and tissue concentrations increased until the end of the experiment. Significant alterations in ATPase activities of intestine and kidney were observed at day 7 and 14, but no alterations in gill ATPase activities occurred. The epidermal mucus showed a high accumulation of cadmium from both exposures, but particularly from the diet, indicating that dietary cadmium can be transferred from gut to mucus. Combining this data with approximations of fry bite volumes and bite frequencies, we constructed daily estimates of the cadmium that could potentially be consumed by newly hatched fry feeding on this mucus. These calculations suggest that feeding fry might consume up to 11 μg g−1 day−1, and hence indicate that this species' dependency on parental mucus feeding of fry could make them particularly susceptible to cadmium contamination of their native habitat.