Histo-anatomical atlas of Garra rufa and its thermal tolerance at human body temperature.

Garra rufa, the doctor fish, is a small freshwater cyprinid notable for its exceptional tolerance to high temperatures, surviving at the human body temperature of 37 °C, and has emerging potential as a novel laboratory model for human cancer xenotransplantation and infectious disease research. To establish a foundation for its experimental use, we conducted comprehensive histo-anatomical analyses across major organ systems. The overall body organization and tissue architecture of G. rufa are broadly similar to those of zebrafish (Danio rerio), indicating a conserved cyprinid body plan. However, several organ systems in G. rufa exhibit species-specific differences from zebrafish, including a well-developed adhesive disc around the oral region, a long and coiled intestine, and distinct dark pigmentation of the peritoneum. These species-specific traits may reflect ecological and behavioral adaptations of G. rufa, including benthic scraping in warm, flowing habitats. Physiological assays showed that G. rufa maintained high survival rates and total swimming distance at 37 °C, whereas zebrafish exhibited significant mortality and a reduction in total swimming distance under the same conditions. Collectively, this study provides a comprehensive histo-anatomical atlas of G. rufa and documents its marked tolerance to 37 °C. Although the underlying molecular mechanisms remain unresolved, these findings establish a foundation for future mechanistic studies of high-temperature tolerance and for evaluation of its suitability as an experimental fish species.