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 o f the herbicides previously associated with fish kills in agricultural drai ns near the Sacramento/San Joaquin rivers and their Delta. This area is an important spawning ground for fish, and thus there are concerns over possib le toxic effects on early life stages of fishes. To define targets of thiob encarb embryotoxicity and to determine the degree of protection afforded by the chorion, medaka (Oryzias latipes) embryos were exposed under static no nrenewal conditions. Responses to exposures initiated at blastula or at ini tiation of heart beat (stages 10 and 23, respectively) were assessed. In ad dition, enzymatically dechorionated embryos (stage 13, gastrula) were expos ed and compared to responses in embryos with intact chorions. Embryos were observed daily for development and for gross abnormalities including: brady cardia, pericardial edema, hemostasis, poor yolk resorption, cephalic and s pinal deformities, and abnormal hatching. A subset was also evaluated for h istologic alterations. Based on gross abnormalities, the concentration of t hiobencarb affecting 50% (EC50) 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 sen sitive than their chorionated cohorts (LC50 = 2.5 vs. 1.0 mg/l). Liver hist ologic alterations were seen in chorionated embryos at EC50 levels and high er. Stage-specific toxicity was evident; nevertheless, the EC50 and NOEC va lues for embryos treated at stage 10 and stage 23 were similar. (C) 2000 El sevier Science B.V. All rights reserved.