Changes in digestive traits and body nutritional composition accommodate a trophic niche shift in Trinidadian guppies

A trophic niche shift can occur as an adaptive response to environmental change such as altered resource quality, abundance or composition. Alterations in digestive traits such as gut morphology and physiology may enable these niche shifts and affect the persistence of populations and species. Relatively few studies, however, have assessed how niche shifts influence suites of digestive traits through phenotypic plasticity and evolutionary mechanisms, and how these trait changes can subsequently alter the nutrition, fitness and life history of organisms. We investigated how population divergence and plasticity alter the gut physiology of wild Trinidadian guppies (Poecilia reticulata), assessing whether variation in digestive traits correspond with enhanced nutrient assimilation under a pronounced dietary shift. We examined gut enzyme activity, and gut size and mass of wild guppies from both high-predation (HP) and low-predation (LP) habitats when reared in the laboratory and fed on high- or low-quality diets designed to reflect their dietary differences previously found in nature. After 10 weeks on the experimental diets, HP guppies maintained shorter and lighter guts than LP guppies on either diet. Guppies also differed in their digestive enzymatic profiles, more often reflecting nutrient balancing so that increased enzyme expression tended to correspond with more deficient nutrients in the diet. LP guppies had increased somatic phosphorus at the end of the experiment, possibly related to the higher alkaline phosphatase activity in their guts. Our results suggest that differences in gut physiology exist among populations of Trinidadian guppies that may reflect local adaptation to their disparate environments.