Selenium is an important essential nutritional trace element which has been shown to provide protection against certain other metal poisoning. However, it is a suspected carcinogen and teratogen. The uptake, depuration, and toxicity of selenium in Daphnia pulex have been examined. The LC50 at 48 and 96 hr for juvenile animals is 0.6 mg/L and 0.1 mg/L respectively, and for adults it is 1.3 mg/L and 0.5 mg/L respectively. Uptake in adult unfed animals is rapid, reaching a maximum at about 12 hr, but depuration is slow. In fed animals, uptake is slower, reaching a maximum at 96 hr, but initial depuration is followed by a slower prolonged loss. Localization in cells is primarily in the cytoplasmic compartment although evidence is presented which suggests nucleolar localization. Ultrastructural damage is detected by 16 hr after exposure and is initially confined to the mitochondria. Dense deposits accumulate in the mitochondrial matrices. The nature of these deposits is unknown; they may represent a calcium- or phosphate-selenium complex. With time, the mitochondria degenerate. It is clear that relatively low concentrations of selenium are toxic to these aquatic organisms and render them incapable of survival in the natural environment. Concentrations higher than those lethal to Daphnia can be expected, at least in local areas, from the burning or conversion of fossil fuels.
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