Sublethal effects of cadmium and diazinon on reproduction and larval behavior in zebrafish (Brachydanio rerio)

Abstract

For a fish population to survive, it must successfully reproduce. Thus, environmental perturbation of reproductive processes could have significant ecological impacts. Fish are often exposed to lethal and sublethal contaminant concentrations. At sublethal concentrations, there is still a concern that contaminant exposure may influence life history parameters. Sublethal contaminant exposure can impair many reproductive processes, and I was interested in examining the effects of cadmium and diazinon on reproductive success, larval swimming behavior, and a pheromonally regulated steroid. Adult zebrafish (Brachydanio rerio) were sublethally exposed to either cadmium or diazinon and median spawning time, fecundity, fertilization success, gonadal 17a,20pdihydroxy- 4-pregenen-3-one (DHP) concentrations (diazinon only), gonadosomatic index (GSI), median hatching time, hatch success, and larval swimming behavior were monitored. Additionally, larvae were exposed to Cd during embryonic, hatching, and yolk sac stages and exposed to diazinon during a yolk sac stage to monitor larval swimming behavior (swimming speed). Cadmium exposure increased median spawning time and male gonadosomatic index (GSI); decreased fecundity, fertilization success, and female GSI; and altered larval swimming behavior. Diazinon exposure decreased male and female gonadal DHP concentrations, spermiogenesis, and altered larval swimming behavior. However, diazinon, at the sublethal concentrations I measured, did not impair reproduction. Larval swimming speeds in diazinon experiments appeared to follow an hormetic response: low doses increased swimming speeds, but higher doses decreased speeds. Cadmium also inhibited and stimulated larval swimming speed. The most significant swimming alterations in cadmium exposed larvae occurred in the youngest larvae tested (five days post-fertilization) and during hatching. Whereas cadmium decreased zebrafish reproductive success and diazinon did not, both contaminants affected larval swimming behavior. Swimming behavior is vital to larval survival and altered behavior could increase predation or decrease foraging success. Therefore, a small contaminant-induced alteration in swimming behavior could result in a large decrease in survival.