Behavioural responses of glass eels of Anguilla anguilla to non-protein amino acids

Glass eels reacted chemotactically towards five non-protein constitutional amino acids, D-glutamine, D-asparagine, D-glutamic acid, D-alanine and β -alanine, dissolved in fresh water or salt water, with behavioural thresholds ranging from 10^-9 M for the most effective (D- and β ;-alanine) to 10^-7 M for the other three. With the exception of D-asparagine in fresh water and D-alanine at concentrations ≥10^-7 M, these amino acids were strong attractants. The results are compared with previous findings on the respective L-isomers. The non-protein amino acids may influence behaviour in the search for food and the recognition of conspecifics.