Fig. 1. Thiol levels in Geukensia demissa gills as a function of time. Gills were exposed to 100 µmol l^-1 Na₂S at time zero. Thiols pertaining to the left y-axis are thiosulfate (filled circles), glutathione (filled diamonds) and cysteine (filled upright triangles), connected with solid lines. Thiols pertaining to the right y-axis are sulfide (filled squares) and sulfite (filled inverted triangles), connected with broken lines. Lines serve only to connect data points. Open symbols shown near the 50 min mark depict levels of the same thiols measured in control tissue after 50 min. Values are means ± S.D. (N=6). To eliminate overlap of error bars, some points have been offset by 0.25–0.5 min. Asterisks designate a significant difference from the value at 0 min (Bonferroni test). (A) Gills exposed to 0.5 µmol l^-1 5-HT. (B) Gills exposed to 10 µmol l^-1 5-HT.