It is well known that aluminium is the principle toxicant killing fish in acidified freshwater systems, and it has been shown that crucian carp (Carassius carassius) can survive exposures to aqueous aluminium levels toxic to most other freshwater fish species. The crucian carp has a remarkable ability to survive anoxic conditions, and the aim of the present study was to reveal if the tolerance to aluminium can be associated with the ability to survive prolonged anoxia. Crucian carps were exposed to either acidic Al-rich water (pH 5.8; 960 μg Al/l), acidic Al-poor water (pH 5.8; 50 μg Al/l) or untreated control water (pH 6.5; 50 μg Al/l). Blood, muscle and gill samples were collected from exposed fish, and closed respirometry was performed to measure critical O2-tension an normoxic O2-consumption. The results show an increased gill surface area in Al-exposed fish, while the critical O2-tension did not change. The normoxic O2-consumption was lower in Al-exposed fish and might be due to a reduced metabolic rate. The results suggest that crucian carp exposed to aluminium do not become hypoxic, since haematocrit, plasma lactate and blood ethanol did not differ from that of control fish after 14 days of exposure. We also observed an initial loss of plasma chloride and sodium, followed by a stabilisation of these ions at a lower level than in control fish. The decrease in plasma ions caused a transient increase in haematocrit and water content in muscle tissue, returning to control levels when the ion concentrations stabilised, suggesting that the water balance was restored. We conclude that the high tolerance to aluminium in crucian carp is associated with its ability to avoid hypoxia as well as an ability to counteract a continuous loss of plasma ions.
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