Stable isotopes indicate niche segregation between two sympatric species of Hypomasticus (Characiformes: Anostomidae) in a Neotropical Atlantic coastal basin

The consumption of different food resources is a key mechanism facilitating the coexistence of sympatric species, since dietary specializations tend to reduce antagonistic interactions such as competition. Differences in feeding strategies, in turn, are usually associated with morphological adaptations. The objective of this study was to evaluate whether the sympatric fish species, Hypomasticus copelandii and Hypomasticus mormyrops, which exhibit differences in oral morphology (terminal to subterminal mouth and downturned mouth, respectively), occupy different isotopic niches, avoiding interspecific competition for resources. To do so, we analyzed the carbon and nitrogen stable isotopes of individuals of both species sampled in the Santo Antônio River, which is an important tributary of the Doce River basin. Both species occupy distinct isotopic niches, and there is no overlap between them, indicating distinct resource use. Hypomasticus copelandii occupies a wider niche area than H. mormyrops, which may be explained by its wide mouth gape that facilitates the consumption of a greater variety of food resources. Our results provide further evidence of niche segregation between two congeneric species, suggesting that mouth position in the Anostomidae family likely reflects evolutionary responses to sympatric speciation driven by the exploration of different food resources.