Evidence that 1-methyl-l-tryptophan is a food-related odorant for the Senegalese sole (Solea senegalensis)

Nocturnal animals often rely heavily on olfactory cues to locate their food. This is especially true for fish whose prey live buried in the substrate. The aim of the current study was to identify non-polar odorants released by the ragworm (Hediste diversicolor) that are detected by the olfactory system of the Senegalese sole (Solea senegalensis) and therefore may be food-related cues. Ragworm-conditioned water was passed through a C18 solid-phase extraction cartridge (to remove polar compounds) and the eluate fractionated by reverse phase high-performance liquid chromatography (HPLC). The fractions were then tested for olfactory activity in the sole by the electro-olfactogram (EOG). Most olfactory activity was found in the first two fractions to elute from the HPLC column (0–4 min). Liquid chromatography-mass spectrometry (LC/MS) of both fractions revealed a base peak with a molecular mass of 219.4 Da and a fragment of 205.3 Da. These data are consistent with a methylated form of tryptophan; synthetic 1-methyl-l-tryptophan had a similar HPLC retention time (2.8 min) and similar LC/MS spectra. Furthermore, sole showed olfactory sensitivity to 1-methyl-l-tryptophan with the lower (left) epithelium being more sensitive than the upper (right). Cross-adaptation (using EOG recording) suggested that the olfactory receptors responding to 1-methyl-l-tryptophan are different in the lower epithelium from the upper. These results suggest that ragworms release 1-methyl-l-tryptophan, or similar tryptophan derivative, and that sole may use this chemical cue to locate and/or identify one of their main prey species. This may help the formation of artificial feeds that are more attractive to sole.