Stimulants of feeding behavior in fish: Analyses of tissues of diverse marine organisms

Analyses of the free amino acids, quaternary amines, guanido compounds, nucleotides, nucleosides, and organic acids in extracts of tissues from 10 species of marine teleost fishes and 20 species of invertebrates are reported. With multidimensional scaling techniques, the relative concentrations of the above chemicals in fishes, molluscs, and crustaceans are shown to cluster into separate taxon-specific groups. The greatest differences are between the fishes and the two groups of invertebrates. Similarities are more evident between the molluscs and crustaceans where eight of the nine most abundant substances are identical: i.e., betaine, taurine, trimethylamine oxide, glycine, alanine, proline, homarine, and arginine. The major tissue components in the fishes and invertebrates are correlated with compounds previously shown to stimulate feeding behavior in 35 species of fish. Glycine and alanine are major tissue components and are also the two most frequently cited feeding stimulants in the 35 species. Molluscs and crustaceans each contain high concentrations of five of the most frequently cited stimulants (glycine, alanine, proline, arginine, and betaine); these substances all occur in much lower concentrations in fish. Some minor tissue components, such as tryptophan, phenylalanine, aspartic acid, valine, and uridine 5'-monophosphate, are, however, important feeding stimulants for some fish species. Stimulants for herbivores and carnivores are often different. Several major feeding stimulants are substances that serve as ''compensatory solutes,'' stabilizing enzymes and structural proteins.