Gut responses of Trinidadian guppies to different diets

Background/Question/Methods      

            When animals experience environmental change or habitat shifts, they can be exposed to different resources or increased competition. In such circumstances, altering their trophic niche can provide a fitness advantage and help them coexist with other organisms. Such dietary shifts can generate strong selection for traits that maximize the acquisition, digestion and assimilation of nutrients and energy in novel dietary items. We investigate how phenotypic plasticity and rapid evolution alter gut physiological characteristics that may help increase energy and nutrient extraction in response to a dietary shift. Our study focused on fish from wild Trinidadian guppy (Poecilia reticulata) populations. Guppies from high predation habitats have colonized low predation environments, and have evolved a suite of life history adaptations in their new environments. Additionally, fish in low predation environments consume lower quality diets consisting of detritus, while fish in high predation habitats consume predominantly invertebrates. We examined gut enzyme activity, gut transit time, and gut length of guppies from both high (HP) and low predation (LP) habitats when fed on a high quality invertebrate diet and low quality spinach diet. In this experiment, we examine population divergence of Trinidadian guppy gut physiology and morphology and determine the plasticity of these gut attributes in relation to dietary quality.

Results/Conclusions

After eight weeks on the experimental diets, HP guppies maintained significantly shorter guts than LP guppies on both diets; however, guppies from both populations had longer guts when reared on spinach diets. It appeared that LP fish retained digesta in their guts for longer periods of time, but egested more often, especially on spinach diets. Habitat of origin (HP vs. LP) and diet were found to have effects on enzyme activities. Specifically, among three enzymes tested, HP guppies generally had higher levels of activity. Differences were significant for beta-glucosidase and marginally significant for N-acetyl glucosaminidase on both diets. Alpha-glucosidase activity was higher in HP guts, but only on the spinach diet. However, LP guppies had greater phosphatase activity than HP guppies, and activity levels for both guppy groups decreased on a spinach diet. Although contrary to the trends in the three other enzymes tested, the elevated phosphatase activities in LP fish correspond to increased phosphorus excretion rates among LP fish in the field. These results suggest that, despite dietary plasticity, population level differences in gut physiology exist in Trinidadian guppies from different predation regimes, possibly reflecting local adaptation to their disparate environments.