Microhabitat segregation and fine ecomorphological dissimilarity between two closely phylogenetically related grazer fishes in an Atlantic Forest stream, Brazil

Habitat segregation is considered the most prevalent resource-partitioning mechanism for stream fishes and the species morphology can be a strong predictor of their spatial distribution. However, most studies addressing morphology-habitat relationships have defined the space in physiognomically homogeneous units (i.e., mesohabitat), probably not detecting segregation among several closely related species. Here we investigated the ecomorphology and the use of habitat in a fine spatial scale (i.e., microhabitat) by two closely phylogenetically related grazer fishes (the loricariids Parotocinclus maculicauda and Hisonotus notatus), syntopic in an Atlantic Forest stream. We conducted standardized underwater observations in two 50 m long stream sections differing in canopy condition, totaling 273 individual microhabitat records. We clearly detected microhabitat segregation between the species. In both sample sites, H. notatus remained near the stream banks and closer to shelters, while P. maculicauda predominantly occurred in more hydrodynamic microhabitats, facing higher focal current velocities and water turbulence. Differences in focal elevation and water depth (i.e., vertical segregation) were exclusively detected in the deforested site. The spatial segregation was congruent with slight interspecific morphological differences, being in accordance with hypotheses about form-function relationships previously reported for fishes. Given that the diel activity and diet of these grazer species were strongly overlapping, we believe that the observed microhabitat segregation favors resource partitioning between P. maculicauda and H. notatus, facilitating their coexistence in high abundances in the studied system. This study illustrates how the assessment of fine-tuned ecological processes can provide subsidy to management strategies aiming the conservation of tropical stream biodiversity.