Diet shifts alter the activity and distribution of digestive enzymes in an herbivorous fish.

Digestion is primarily performed by digestive enzymes. Here, we examined the activity levels of seven digestive enzymes along the digestive tract of the herbivorous fish, Cebidichthys violaceus. We reared C. violaceus on carnivore, omnivore, and herbivore diets in the laboratory for nine months and compared the digestive enzyme activities among the fish on the different diets and with wild-caught fish consuming their natural foods. Enzymatic activities were generally lower in the laboratory than in wild-caught fish. The marked anterior-to-posterior amylase activity gradient along the gut in wild-caught fish was absent in the lab-fed fish. We hypothesize that the dampened enzymatic activity may have been caused by reduced food intake in the laboratory in comparison to the wild fish. N-acetyl-β-D-glucosaminidase (NAGase) activity (degrades chitin breakdown products) peaked in the distal intestines of the lab-fed fish, but not the wild fish. The role of this enzyme in the digestive process remains unknown since the lab diets contained no chitin, and its origin may have been microbial. Overall, C. violaceus can tolerate diets with a wide range of protein and carbohydrate levels. However, the totality of our data suggests that live algal diets may be best for this herbivorous fish in a captive setting, especially for aquaculture.