Morphological variation of the digestive tract: a feeding behaviour response in a freshwater fish species

Plasticity of resource use represents an important strategy for fish species living in unstable freshwater environments. Trophic polymorphism (i.e. variation in the morphology of the alimentary tract) has been observed in dozens of vertebrate species in response to diet variation. The digestive tract has been reported as one of the most reactive systems. The Digestion Theory predicts that consumption of food with a high proportion of indigestible material would produce an increase in the length of the digestive tract. This response allows individuals to get energy enough even when consuming a low-quality diet. We evaluated the diet—using classical stomach content analysis and stable isotopes (nitrogen and carbon)—and the length of digestive organs of an abundant and ubiquitous fish species, the La Plata croaker ( Pachyurus bonariensis Steindachner, 1879). A total of 95 individuals were captured at three sites of the lower Uruguay River, with different food supply during two sampling campaigns. We found larger digestive organs (stomach and intestine) at the site with higher sediment vegetal matter availability and, consequently, with the highest vegetal matter consumption. However, these differences in food consumption were not observed with the stable isotope analysis, probably due to the different temporal resolution of this technique. The results presented herein support one prediction of the Digestion Theory, confirming digestive flexibility as an intraspecific compensatory mechanism in a freshwater fish species.