Digestive Enzyme Activities in Herbivorous and Carnivorous Prickleback Fishes (Teleostei: Stichaeidae): Ontogenetic, Dietary, and Phylogenetic Effects

We measured the activities of eight digestive enzymes in four species of herbivorous and carnivorous prickleback fishes and determined the effects of ontogeny, diet, and phylogeny on these enzyme activities. Of the four species,Cebidichthys violaceusandXiphister mucosusshift to a more herbivorous diet as they grow (≥45 mm SL [standard length]), whereasXiphister atropurpureusandAnoplarchus purpurescensremain carnivores throughout life. Digestive enzyme activities of small (30–40 mm SL) carnivorous juveniles were compared with those of larger (60–75 mm SL) wild‐caught juveniles that had consumed a natural diet and larger (60–75 mm SL) juveniles raised on a high‐protein animal diet.Cebidichthys violaceusand both species ofXiphistershowed ontogenetic changes in digestive enzyme activities, whereasA. purpurescensdid not. Despite dietary differences betweenX. atropurpureusandX. mucosus, these sister taxa displayed the most similar digestive enzyme activities from ontogenetic and dietary perspectives (high α‐amylase and lipase and low trypsin and aminopeptidase activities), and both were more similar toC. violaceus, a member of the same largely herbivorous clade, than either was toA. purpurescens, a member of an adjacent, carnivorous clade. The results support the hypothesis that phylogeny influences digestive enzyme activities in these fishes.Anoplarchus purpurescens, a carnivore with a diverse diet, showed great plasticity in enzyme activity, especially trypsin and aminopeptidase, which were elevated in this species to the highest level among the four species after consuming the high‐protein diet. These results support the hypothesis that fishes with relatively broad diets can modulate digestive enzyme activities in response to changes in dietary composition.