The effects of cadmium administered via ambient water or food on plasma ions of the African freshwater cichlidOreochromis mossambicus were studied for 2, 4, 14, and 35 days, in low calcium (0.2 mM) and high calcium (0.8 mM) water. In low calcium water, an environmentally relevant concentration of 10 μg/L water-borne cadmium induced a significant and dramatic hypocalcemia on days 2 and 4. Recovery of plasma calcium was observed on days 14 and 35. Hypermagnesemia was observed on day 2, but normal levels were already found on day 4. In high calcium water adapted fish, the extent of hypocalcemia and hypermagnesemia was less pronounced than in fish from low calcium water. Water-borne cadmium caused no significant changes in plasma phosphate, sodium, potassium, or osmolality. On days 2 and 4, dietary cadmium (averaging 10 μg Cd/fish/day) caused hypermagnesemia and hypocalcemia in low calcium wateradapted fish. Recovery was observed on days 4 and 14, respectively. In fish from high calcium water, dietary cadmium caused a significant reduction in plasma calcium on day 4 only; plasma magnesium was unaffected. Hyperphosphatemia was apparent on day 14, irrespective of the water calcium concentration. No changes in plasma sodium, potassium, or osmolality were found.
The results show that sublethal concentrations of cadmium, administered via the water as well as via the food, affect calcium and magnesium metabolism in tilapia. High water calcium ameliorates the effects of both water and dietary cadmium on plasma calcium and magnesium levels.
Among the various heavy metal pollutants, cadmium is frequently present in natural water bodies as a result of discharges from industrial processes or other anthropogenic contamination. The harmful effects of cadmium on mammals and other terrestrial animals have been widely studied and reviewed (Flicket al. 1971; Vallee and Ulmer 1972; Webb 1979; Korte 1983; Foulkes 1986). Aquatic vertebrates such as fish, live in very intimate contact with the environment through their gills. This makes them very susceptible to aquatic pollutants.
Since it is well established that freshwater fish take up most of the ions necessary for homeostasis from the water via the gills (Eddy 1982), cadmiuminduced plasma ionic disturbances are apparently caused by impaired uptake and diffusional losses of ions via these organs (Larssonet al. 1981; Giles 1984). Ionic disturbances have also been reported after exposure of fish to sublethal concentrations of heavy metals. For example, changes in the plasma ionic composition have been observed in fish exposed to copper and zinc (Lewis and Lewis 1971; Spry and Wood 1985), mercury (Locket al. 1981), and chromium (Van der Putteet al. 1983). With respect to cadmium, exposure of rainbow trout to sublethal levels induced hypocalcemia, with reduced plasma sodium, potassium, chloride and increased plasma magnesium (Giles 1984). In European flounder, cadmium-induced hypocalcemia and elevated levels of plasma phosphate, magnesium and potassium were observed (Larssonet al. 1981).
In addition to water, food could also be a source of cadmium for fish, since it accumulates in aquatic organisms through trophic transfers (Anonymous 1971; Williams and Giesy 1978; Coombs 1979). Indeed, Bryan (1976) concluded that food as a source of Zn, Mn, Co, and Fe for molluscs, crustaceans and fish was more important than water. From various studies on both water-borne and food-containing metals, reviewed by Dallingeret al. (1987), there is evidence that uptake of heavy metals such as Cd, Cu, Co, Pb, Hg, and Zn from food is also the predominant pathway in freshwater fish. Koyama and Itazawa (1977) reported significant hypocalcemia and elevated plasma phosphate levels in cadmium-fed carps. Similarly, plaice and thornback ray both accumulated more cadmium from food than from seawater (Pentreath 1977). In general, cadmium concentrations in natural waters are extremely low and a more important route of cadmium uptake by fish may be represented via the gut. Experiments with dietary cadmium may therefore yield more representative information for field situations.
In this investigation, we have compared the effects of a sublethal concentration of cadmium administered via the water or via the food in the African cichlid fishOreochromis mossambicus (tilapia). Plasma ions and osmolality were determined. Cadmium was administered at sublethal concentrations, in the order of magnitude that may occur in natural waters (⩽10 μg Cd/L). In many studies aimed at evaluating the effects of cadmium on fishes, high concentrations (>1 mg Cd/L) of cadmium have been used. Hence severe physiological, behavioral and detrimental effects have been reported. Such high concentrations are rarely found in nature, except in cases of spillage or heavily polluted waters. The Working Group on Cadmium Toxicity (EIFAC 1977) has suggested that chronic exposure to low cadmium concentrations is more relevant to understanding the mechanisms involved in the intoxication process in teleost fish.
We further studied the influence of relatively low and high calcium concentration of the water on the toxic effects of cadmium. The effects of water hardness (mainly Ca2+ and Mg2+ ions) on heavy metal toxicity have been demonstrated in various species of teleosts (Pärtet al. 1985). Increased toxicity of cadmium to fish in soft water as compared to hard water has been demonstrated in catfish and guppies (Kinkade and Erdman 1975), goldfish (McCartyet al. 1978), striped bass (Palawskiet al. 1985), brook trout (Carrollet al. 1979) and rainbow trout (Calamariet al. 1980; Pascoet al. 1986). Similar observations on teleosts exposed to zinc, copper and lead (Sinleyet al. 1974; Zitko and Carson 1976; Judy and Davies 1979; Laurén and McDonald 1986) indicate a protective role of calcium against the toxic effects of heavy metals. It was also investigated whether the protective effect of the water-calcium concentration is limited to water-borne cadmium only, or also applies to dietary cadmium.