Mysids are used routinely by regulatory authorities for conducting 96 h LC50 toxicity tests to evaluate the potential hazards of pollutants to aquatic ecosystems. Data from these acute tests suggest that the European estuarine mysid Neomysis integer (Peracarida: Mysidacea) is comparatively sensitive to trace metals and organophosphate pesticides, and may be a suitable alternative to the frequently-used sub-tropical American mysid Americamysis (≡Mysidopsis) bahia for testing the toxicity of chemical contaminants to European estuarine biota. There is, however, growing demand for the development of toxicity tests which are more representative of the effects of toxic contaminants on natural populations, and which provide results that are more readily extrapolated to natural ecosystems, than acute tests. Behavioural disruption, particularly of swimming ability, is used increasingly in laboratory toxicity studies as a sensitive endpoint for assessing the effects of contaminants on aquatic biota. This paper describes a sensitive laboratory technique, using an annular flume, to determine the effects of an organophosphate pesticide on the swimming behaviour of N. integer. Following 7-day exposure to 0.038 μg chlorpyrifos l−1, mysids became hyperactive and more swam forward into a slow current (3 cm s−1) than control mysids. Despite this hyperactivity, pesticide-exposed mysids were unable to swim faster than 15 cm s−1, whereas control mysids were able to swim faster than 18 cm s−1. Other changes in swimming behaviour following pesticide exposure included fewer mysids maintaining position, and more mysids swimming with the current at high current velocities (18 cm s−1), than control individuals. These responses of chlorpyrifos-exposed N. integer are predicted to cause reduced ability of N. integer to maintain position in the natural estuarine habitat. Furthermore, the swimming behaviour of N. integer was affected at pesticide concentrations below the 7 day LC50 (0.084 μg chlorpyrifos l−1), highlighting the importance of using sub-lethal toxicity studies for predicting environmental consequences of pollutant discharge.
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