Thiobencarb-induced embryotoxicity in medaka (Oryzias latipes): stage-specific toxicity and the protective role of chorion.

Thiobencarb (S-(4-chlorobenzyl)-N,N-diethyl thiol carbamate) has been one of the herbicides previously associated with fish kills in agricultural drains near the Sacramento/San Joaquin rivers and their Delta. This area is an important spawning ground for fish, and thus there are concerns over possible toxic effects on early life stages of fishes. To define targets of thiobencarb embryotoxicity and to determine the degree of protection afforded by the chorion, medaka (Oryzias latipes) embryos were exposed under static nonrenewal conditions. Responses to exposures initiated at blastula or at initiation of heart beat (stages 10 and 23, respectively) were assessed. In addition, enzymatically dechorionated embryos (stage 13, gastrula) were exposed and compared to responses in embryos with intact chorions. Embryos were observed daily for development and for gross abnormalities including: bradycardia, pericardial edema, hemostasis, poor yolk resorption, cephalic and spinal deformities, and abnormal hatching. A subset was also evaluated for histologic alterations. Based on gross abnormalities, the concentration of thiobencarb affecting 50% (EC(50)) of embryos exposed at blastula was 3.6 mg/l, while the putative no observable effect concentration (NOEC) was 1.0 mg/l. For embryos exposed at onset of heart beat (stage 23), these values were 4.1 and 2.5 mg/l, respectively. Dechorionated embryos tended to be more sensitive than their chorionated cohorts (LC(50)=2.5 vs. 1.0 mg/l). Liver histologic alterations were seen in chorionated embryos at EC(50) levels and higher. Stage-specific toxicity was evident; nevertheless, the EC(50) and NOEC values for embryos treated at stage 10 and stage 23 were similar.