Integrated digestive and metabolic traits underlie trophic differentiation in juvenile fish assemblages

Physiological traits related to digestion and energy allocation play a central role in shaping trophic strategies and niche differentiation among coexisting fish species. Here, we evaluated digestive enzyme activities, intestinal coefficient (IC) and energy reserves in three juvenile fish species inhabiting a coastal lagoon system (Mugil liza, Odontesthes argentinensis and Oligosarcus jenynsii) with contrasting feeding habits and trophic strategies. Clear interspecific differences were observed: M. liza exhibited higher amylase activity and intestinal coefficient values, whereas O. argentinensis and O. jenynsii showed greater proteolytic activity. Energy reserve patterns also differed markedly, with O. argentinensis characterized by elevated hepatic triglycerides and O. jenynsii associated with glycogen and protein-related variables. Principal component analysis revealed clear multivariate differentiation among species, indicating coordinated variation across digestive and metabolic traits. These findings highlight that digestive physiology, intestinal morphology and energy allocation form an integrated functional axis underlying trophic differentiation and suggest that such physiological integration may facilitate species coexistence in coastal lagoon ecosystems.