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

This article investigates how predation pressure influences parallel phenotypic divergence in gut morphology in two congeneric poeciliid fish species, *Phalloceros harpagos* and *Phalloceros anisophallos*. By comparing populations from high predation (HP) and low predation (LP) environments with similar abiotic conditions, the study isolates predation effects on trophic traits. Results show that HP populations have lower fish densities but higher per-capita availability of high-quality resources (macroinvertebrates), correlating with shorter gut lengths indicative of a more carnivorous diet, while LP populations exhibit longer guts associated with consumption of lower-quality food such as algae. The parallel divergence in gut morphology across both species suggests that predation indirectly shapes resource availability and diet, driving convergent ecological adaptations. The study highlights the role of both evolutionary processes and phenotypic plasticity in these patterns and recommends further research to disentangle genetic versus plastic contributions to gut morphology variation.