Effects of tributyltin on medaka ( Oryzias latipes) embryos at different stages of development

Abstract

Potential mechanisms of action for the toxicity of tributyltin (TBT) were studied in the freshwater fish embryos of medaka ( Oryzias latipes). Toxic concentrations of TBT have been found in estuaries and freshwater systems, presumably due to their use as biocides in boat, antifoulant paints and in industry for plastics production. Medaka embryos were exposed to a single concentration of TBT at developmental stages that corresponded to the formation of structures and/or organs which might be potential targets. Times of exposure included day 0, oviposition, day 3, completion of somite formation, and day 5, liver formation. Endpoints for evaluating toxicity were acute embryo lethality (96 h), rate of embryo development, hatching success, gross abnormalities, as well as hatchling eye diameter and number of somites. The clear chorion of medaka embryos allowed staging and in ova observations. Results showed that the acute toxicity of TBT was stage related. The 96 h LC 50 (LC 50: lowest concentration to cause 50% lethality in the test population) for embryos exposed on day 0 was 159 nM, which was lower than that for days 3 and 5, 360 and 340 nM, respectively. Subchronic endpoints showed that toxicity was concentration related and that embryos exposed on day 0 were more sensitive than those exposed on days 3 and 5. Lowest observable effect levels (LOELs) for hatching success were 36 nM for day 0 and 143 nM for days 3 and 5. LOELs for the combined effects of hatching success and gross abnormalities were 36 nM for day 0 and 71 nM for days 3 and 5. Developmental rate was slowed by TBT in a concentration-related manner; however, embryos treated with 36 and 71 nM were able to recover and hatch at the same time as controls. Types of gross abnormalities were similar regardless of day of exposure and consisted of tails bent at the tip, curled, and/or shortened. These abnormalities corresponded with statistically significant reductions in numbers of somites in all three age groups exposed to 71 nM ( P<0.05). Although day 0 embryos were the most sensitive, the similar abnormalities for all 3 days of exposure indicated that TBT's toxicity was not due to effects on an age-dependent target but one present throughout embryo development.