Fig. 3. The concentration–response relationships for spermine, spermidine and agmatine reveal detection thresholds for each of these stimuli in the 1 µmol l^–1 range. For spermine and spermidine, data from a total of three fish were pooled; for agmatine, data from 20 fish was analyzed. Each fish was used for only one concentration–response series. Odorants were tested as an ascending concentration series. The average responses to the 100 µmol l^–1 L-glutamine and AFW controls are shown as single points and as a dotted line (AFW only). Values are means ± S.E.M. Responses to spermine, spermidine and agmatine that are significantly greater than the response to the AFW control are designated with a, b and c, respectively (one-way ANOVA; Dunnett's post hoc t-test; P<0.05). Abbreviations as in Fig. 1.