Chemical reactor theory (CRT) suggests that the digestive tract functions as a chemical reactor for processing food. Presumably, gut structure and function should match diet to ensure adequate nutrient and energy uptake to maintain performance. Within CRT, dietary biochemical composition is the most important factor affecting gut structure and function in vertebrates. We fed Danio rerio (zebrafish) diets ranging from high- to moderate- to low-quality (i.e., ranging from high-protein, low-fiber to low-protein, high-fiber), and observed how gut length and surface area, as well as the activity levels of digestive enzymes (amylase, maltase, trypsin, aminopeptidase, and lipase) shifted in response to these dietary changes. Fish on the low-quality diet had the longest guts with the largest intestinal epithelial surface area and enterocyte cellular volumes. Fish on the moderate-quality diet had intermediate values of most of these parameters, and fish on the high-quality diet, the lowest. These data largely support CRT. Digestive enzyme activity levels were generally elevated in fish fed the moderate- and low-quality diets, but were highest in the fish fed the moderate-quality diet, suggesting that a diet with protein levels closest to that of the natural diet of D. rerio (they are omnivorous in nature) may elicit the best gut performance. However, fish fed the carnivore diet reached the largest terminal body size. Our results support CRT in terms of gut structure; however, our enzyme results do not necessarily agree with CRT and largely depend on which enzyme is discussed. In particular, the evidence for lipase activities being elevated in the fish fed the low-protein, high-fiber diet perhaps reflects a lipid-scavenging mechanism in fish consuming high-fiber foods rather than CRT.
Natural dietary shifts offer the opportunity to address the nutritional physiological characters required to thrive on a particular diet. Here, we studied the nutritional physiology of Podarcis siculus , populations of which on Pod Mrčaru, Croatia, have become omnivorous and morphologically distinct (including the development of valves in the hindgut) from their insectivorous source population on Pod Kopište. We compared gut structure and function between the two island populations of this lizard species and contrasted them with an insectivorous mainland outgroup population in Zagreb. Based on the Adaptive Modulation Hypothesis, we predicted changes in gut size and structure, digestive enzyme activities, microbial fermentation products (SCFAs), and plant material digestibility concomitant with this dietary change. The Pod Mrčaru population had heavier guts than the mainland population, but no other differences in gut structure. Most of the enzymatic differences we detected were between the island populations and the outgroup population. The Pod Mrčaru lizards had higher amylase and trehalase activities in their hindguts compared to the Pod Kopište population, the Pod Kopište lizards had greater SCFA concentrations in their hindguts than the plant-eating Pod Mrčaru population. Interestingly, the differences between the Pod Mrčaru and Pod Kopište populations are primarily localized to the hindgut and are likely influenced by microbial communities and a higher food intake by the Pod Mrčaru lizards. Although subtle, the changes in hindgut digestive physiology impact digestibility of plant material-Pod Mrčaru lizards had higher digestibility of herbivorous and omnivorous diets in the laboratory than did their source population.
Some of the most disturbed and imperilled habitats (and by extension, the inhabitants found therein) in North America are the freshwaters of southern California. With its arid climate, increasing drought conditions and large human population, there are no naturally occurring freshwater systems in southern California that are not impacted directly by humans (Richmond, Backlin, Galst-Cavalcante, O'Brien, & Fisher, 2018). One of the most discussed and litigated naturally occurring residents of southern California freshwater systems is the Santa Ana sucker (Catostomus santaanae), which is a federally threatened (U.S. Fish and Wldlife Service, 2000) native of the Santa Ana, Los Angeles, San Gabriel and Santa Clara River drainages (Richmond et al., 2018) (Figure 1). Catostomus santaanae is an herbivorous fish (Greenfield et al. 1970; Saiki, Martin, Knowles, & Tennant, 2007), and similar to other suckers (family Catostomidae) and minnows (family Cyprinidae) with sub-terminal mouths (Figure 2), C. santaanae is a benthic grazer, subsisting on periphyton
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