Divergent effect of predator presence on gut morphology shows parallel patterns in congeneric species

Abstract Understanding the mechanisms driving parallel evolution across species and populations is fundamental to evolutionary biology. We investigated the parallel phenotypic divergence of trophic morphology in response to predation pressure in two recently described congeneric species of poeciliid fish, Phalloceros harpagos and Phalloceros anisophallos. By comparing populations from high and low predation environments with similar environmental characteristics, we aimed to isolate the effect of predation on trophic traits while controlling the effect of other sources of environmental variation. We found that populations experiencing high predation pressure had shorter guts, which is indicative of a more carnivorous diet, compared to populations in low predation environments. This parallel divergence in trophic morphology suggests that predation indirectly influences per-capita resource availability, driving convergent ecological characteristics in different species of Phalloceros. Our study highlights the importance of considering indirect effects of predation on trophic traits and provides insights into the mechanisms underlying parallel phenotypic divergence. This is one of the few field studies that has directly tested parallel phenotypic divergence, within and between species, focusing on one agent of selection and minimizing the confounding effect of other environmental sources of variation.