Age- and sex-related variation in sensitivity to the pyrethroid cypermethrin in the marine copepod Acartia tonsa Dana

Acute effects of cypermethrin, a pesticide used to treat ectoparasite infes tations of salmon, were assessed using the calanoid copepod Acartia tonsa. A. tonsa has been widely used for hazard assessment of chemicals in the mar ine environment using acute toxicity tests, but only with randomly selected adults, assuming a sex ratio of 1:1. The present study assesses the enviro nmental hazard of cypermethrin by exposing nauplii and adult males and fema les, separately. Our results showed that the naupliar stages were 28 times more sensitive to cypermethrin than adults after 96 h of exposure, with LC( 50)s of 0.005 mug L-1 and 0.142 mug L-1, respectively. Significant differen ces in sensitivity between sexes were only found during the first 24 h of e xposure, with males being approximately twice as sensitive as females. The results of age-related variation in sensitivity are supported by studies wi th other species of copepods and toxicants, where the increased capacity of the adults for detoxification, the allometric differences in weight and si ze, and the molting process are given as explanations. Variation in sensiti vity between sexes is discussed in terms of faster depuration rates in fema les through egg production and implications of feeding rate changes after 2 4 h of exposure. Our results suggested that standard toxicity test methods using A. tonsa are unsatisfactory because the most sensitive life stage is not included and sex-related differences in tolerance are not taken into ac count. We also found that cypermethrin caused significant mortality in Acar tia at exposures concentrations from one to three orders of magnitudes lowe r than the recommended field treatment concentration for copepodic infestat ions.