The Captive Gut: the impacts of long-term captivity on the intestinal digestive enzyme activities of a marine herbivorous fish

Given that digestion is largely a chemical process in fishes that do not masticate or triturate their food, digestive enzyme activities are frequently measured as indicators of what nutrients a fish can readily digest. Here, we explored what happens to digestive tract length and digestive enzyme activities in a marine herbivorous fish (Xiphister mucosus) fed in captivity for more than two years. Captivity is known to impact nearly all aspects of an animal's biology, but the gut microbiome has received more attention than digestive biochemistry in this regard. After consuming a prepared diet primarily composed of algae collected from the same site from which the fish were collected for over two years, the lab-fed fish showed a marked reduction in amylase, trypsin, and aminopeptidase activities in comparison to wild-caught fish. Moreover, the gradient of activity moving along the intestine (activities decreasing distally for amylase and trypsin, increasing for aminopeptidase) completely disappeared in the lab-fed fish. The relative gut length decreased by about 18 % in the lab-fed fish, suggesting that they ate less than their wild counterparts, which likely led to slower gut transit and more time for enzymes to interact with substrates. Hence, digestive enzyme activities were lower in the lab-fed fish. A formulated diet in the laboratory presents a different gut environment for the herbivorous X. mucosus, and perhaps feeding them a live algal diet would better replicate the wild-gut phenotype if this fish is to be held for lengthy periods of time for display or aquaculture purposes.