Morphological Differences in Feeding and Digestive Organs, the Diversity of Intestinal Microorganisms, and Variations in Digestive Enzyme Activity Promote the Differentiation of Nutritional Niches in Schizothoracinae Species

The mechanisms of nutritional niche differentiation among sympatric Schizothoracinae fishes play an important role in their adaptive evolution and should be considered in conservation strategies. To date, there have been no reports about the role of different feeding strategies in nutritional niche differentiation among five Schizothoracinae species (<i>Ptychobarbus leptosomus</i>, PL; <i>Gymnodiptychus pachycheilus</i>, GP; <i>Schizothorax kozlovi</i>, SK; <i>Schizopygopsis malacanthus</i>, SM; and <i>S. wangchiachii</i>, SW). This study explored the role of feeding strategy differences in nutritional niche differentiation among sympatric Schizothoracinae fishes. We analyzed feeding/digestive organ morphology, diet composition, intestinal digestive enzyme activity, and microbial diversity in five species from the middle Yalong River. The results revealed dietary divergence: PL consumed small fish, invertebrates, and algae; GP/SK fed on invertebrates and algae; and SM/SW primarily ate algae. Additionally, α-amylase activity was lower in PL/GP/SK than in SM/SW (<i>p</i> < 0.05), while lipase activity was lower in SM/SW than in PL/GP/SK (<i>p</i> < 0.05), and PL exhibited the highest trypsin activity (<i>p</i> < 0.05). The intestinal microbial composition/diversity also varied: the PL group differed significantly from the GP + SK and SM + SW groups (<i>p</i> < 0.05), with <i>Cetobacterium</i> being dominant in the PL group and <i>Clostridium</i> being dominant in the other groups; the α diversity was highest in the SM + SW group and lowest in the PL group. PICRUSt2 predictions revealed significant differences in nutrient metabolism pathways between the PL group and the other groups (<i>p</i> < 0.05). Overall, the morphological differences in feeding/digestive organs and recent food intake may drive the dietary divergence, inducing adaptive changes in digestive enzymes and microbial diversity and ultimately promoting nutritional niche differentiation among sympatric Schizothoracinae species.