Feeding Morphology Supports Carnivorous Habits in Algansea lacustris: A Multitrait Approach

Accurate classification of fish trophic strategies based solely on gut contents can be misleading, especially when plant material is ingested incidentally during predatory benthic foraging. The Pátzcuaro chub (Algansea lacustris) is a critically endangered cyprinid endemic to Central Mexico. It has historically been described as omnivorous with a tendency toward algivory, despite limited anatomical evidence. In this study, integrated anatomical, morphometric, and functional approaches were used to reassess the feeding strategy of A. lacustris and inform conservation-oriented aquaculture. Double-staining techniques revealed a specialised filtering and crushing branchial–pharyngeal system adapted to capture and process animal prey. Relative intestinal length (RIL) was measured from freshly dissected intestines. Intestinal transit time was experimentally evaluated using a formulated diet and live Artemia. Algansea lacustris exhibited a short intestine (RIL = 0.86 ± 0.10) and rapid intestinal transit (<30 min), both of which are characteristics of carnivorous teleosts. These results provide consistent anatomical and physiological evidence that A. lacustris is primarily adapted to a low-trophic carnivorous or insectivorous feeding strategy, with important implications for its ecological characterisation. Moreover, intestinal transit was faster after ingestion of live Artemia than after the formulated diet, likely due to differences in moisture content. The observed short transit times indicate the need for more frequent feeding and support the refinement of diet formulation and feeding strategies in conservation aquaculture programmes.