Gut specialization for lower trophic levels shapes morphological evolution in North American minnows and suckers

Organisms utilizing lower trophic levels consume nutrient-poor foods with little energy and must maximize nutrient extraction during digestion. In fishes, there is a strong inverse relationship between trophic level and intestine length and a positive relationship between body length and intestine length. Adaptation to lower trophic levels and concomitant increases in intestine length may influence either the size of the abdominal cavity region or overall body size, as the fish must accommodate longer intestines. This study evaluates these two alternative adaptive pathways in 122 North American species from the order Cypriniformes. We used traditional dissection and microcomputed tomography scans to measure intestine length and abdominal cavity volume. We used a suite of phylogenetic comparative methods to determine whether diet influenced intestine length, body size, and abdominal cavity volume. Our results show that the herbivore–detritivore trophic group has significantly longer intestines than at least one other trophic group and is evolving towards a larger optimal intestine length than all other trophic groups. Furthermore, we see a positive, significant relationship between abdominal cavity volume and body size. These results support the inverse trophic position–body length pattern seen in the group and further elucidate the evolutionary limitations based on body shape covariation in predicting diversification in freshwater fishes.