The Largest Nematogenys inermis From a Small Stream in the Maipo Basin Once Considered Extinct: Perspectives on Body Size Evolution and Peripheral Habitats as Refuges

Freshwater fish body size evolution spans extremes from miniatures to giants, with patterns that are often lineage-specific. Nematogenys inermis, endemic to Chile and the sole extant representative of Nematogenyidae, is regarded as a ‘living fossil’ within Loricarioidei and stands among the largest native fishes of Pacific-draining Andean basins. We report the largest individual known to date (45.5 cm Total Length), found in a narrow tributary of the Maipo River, a basin where it was previously considered extinct. The specimen was captured in an atypical environment for adults: a small, shallow and isolated stream, separated from one of the most degraded river systems in Chile. This record highlights unexpected habitat use, raises questions about persistence in heavily impacted watersheds, and underscores the importance of peripheral tributaries as refugial habitats. It also offers new perspectives on body size evolution in basal loricarioids, suggesting reductions from a larger ancestral condition. Understanding the current status of N. inermis about its natural history, ecology and distribution is urgent to guide conservation strategies for one of the world's most distinctive and threatened catfishes.