Linking anatomical and histological traits of the digestive tract to resource consumption and assimilation of omnivorous tetra fishes

This study explores the interplay between digestive tract traits, food
intake, and assimilation in omnivorous tetra fishes (Psalidodon
bifasciatus, P. aff. gymnodontus, and Bryconamericus ikaa) from the Iguaçu
River basin, an ecologically significant region known for high endemism.
We hypothesize that variations in digestive tracts across species would
associate with differences in diet, isotopic composition in fish tissues,
and overall diet assimilation. To test this, we employed stereoscopic and
light microscopy to characterize the gross anatomy, histomorphology, and
histochemistry of fish digestive tracts. Additionally, the study used
stomach content and stable isotope analyses to trace fish diets. While
these tetra fishes shared histological structures, disparities were noted
in anatomical digestive traits and diet preferences. The smallest species,
B. ikaa, with a shorter intestine, had fewer pyloric caeca and primarily
consumed animal-based diets. Conversely, P. bifasciatus and P. aff.
gymnodontus, with longer intestines, displayed numerous pyloric caeca and
consumed a balanced mix of animal and plant items. Despite anatomical and
dietary differences, all three species predominantly assimilated
animal-origin food. Histological variations were found among digestive
tract segments, with the esophagus having the thickest muscular layer,
gradually thinning towards the posterior intestine. The final portion of
the intestine exhibited a significant expansion in lumen perimeter, while
the esophagus had the smallest lumen area. Goblet cells were most
concentrated in the posterior intestine for all species. The gross anatomy
of these tetra fishes aligns with their ingestion of animal or plant
diets, while diet assimilation is dominated by animal-origin food. These
findings provide crucial insights into the structural and tissue
characteristics of their digestive systems, laying the groundwork for
deeper exploration into the physiological aspects of their digestive
tracts and enhancing our understanding of their feeding strategies.