PARK, D.; MINOR, M.D.; PROPPER, C.R.*: The Effect of a Common Insecticide on Mosquitofish Morphology is Seasonally Dependent

Reproduction in many species follows a seasonal pattern governed by appropriate changes in endocrine physiology. Sensitivity to endocrine disrupting compounds (EDCs) may therefore vary seasonally as the endocrine system cycles. In order to determine if there is seasonal variation in morphological responses to EDC exposure, we treated breeding and non-breeding female mosquitofish, Gambusia affinis to increasing, environmentally appropriate doses of the common insecticide, endosulfan for 30 days. Following exposure, we measured the condition factor (body weight in g/body length mm), relative anal fin length (anal fin length mm/body length mm), and gonadosomatic (GSI; ovary weight g/body length mm) index for all animals and compared values across treatment groups. We also histologically determined the size (diameter and surface area) of thyroid follicles in non-breeding, endosulfan-exposed females. Between breeding and non-breeding controls, breeding fish were longer, heavier and had a higher condition factor than non-breeders. Neither GSI nor relative anal fin length differed between control groups. Non-breeding fish exposed to 0.5 ppb endosulfan had longer relative anal fin lengths compared to controls whereas breeders did not, suggesting that masculinizing effects of endosulfan are seasonally dependent. Furthermore, non-breeding fish exposed to endosulfan had larger thyroid follicles, suggesting that endosulfan disrupts thyroid gland function. No other parameter was affected in either seasonal group. Our results suggest that evaluation of potential EDC effects may need to include seasonal studies to fully understand exposure risks. Supported by Grant # 4661R1 Council for Tobacco Research, U.S.A., Inc.