Fig. 3. The seasonal effect of 5-hydroxytryptamine (5-HT) on clearance rates. Data are the ratios of the treatment rate to the basal rate. The treatment rate is the clearance rate after a 5 min exposure to 5-HT, and the basal rate is the initial, unstimulated clearance rate (see text). Data are means ± s.e.m. Closed triangle (C) is the mean ratio for untreated controls: i.e. gills that were not exposed to 5-HT during the treatment run. Sample sizes ranged from 4 to 9 at each concentration of 5-HT. At 10^–5 mol l^–1 5-HT the means are significantly different (P<0.05).