Fig. 7. Structural specificity of the sperm response to L-tryptophan. Red abalone sperm were assayed for behavioral responses to natural egg-conditioned sea water (ESW), and to synthetic standards tested at 10^-7 mol l^-1 concentration. Both the natural L-enantiomer (L-trp) and the unnatural D-enantiomer (D-trp) of tryptophan were tested to determine the stereospecificity of sperm response. Serotonin, or 5-hydroxytryptamine, is a neuroactive metabolite of L-tryptophan known to activate sperm in other molluscan species. Filtered sea water (FSW) was used as a negative control. Values are means ± S.E.M.; means not linked by a horizontal bar differed significantly (one-way ANOVA with post-hoc Scheffé test, P<0.05). (A) Change in sperm swim speed in response to tested solutions. (B) Change in cell density due to sperm chemotaxis.