Monogenean infection reduces thermal tolerance and alters thermal preference in Colossoma macropomum: physiological costs in an Amazonian fish host.

Parasitism can impose significant physiological stress on host organisms, potentially altering their behaviour and metabolic functioning. This study investigates the effects of Monogenean parasitism on thermal preference, heat stress tolerance, and physiological responses in Colossoma macropomum. Using a controlled laboratory thermal choice system, we compared parasitized and non-parasitized individuals regarding their spatial use of thermal zones and critical thermal maxima (CTmax). Lipid profiles were assessed to evaluate metabolic impacts, with no significant differences in triglyceride or cholesterol levels between groups. Additionally, we analyzed the relative expression of key stress and immune-related genes (hsp70, hsp90, and il-1β). Parasitized fish showed greater variation in zone occupancy and reduced CTmax, indicating lower thermal tolerance. Expression of hsp70 remained stable, while hsp90 and il-1β were significantly upregulated in infected individuals. These results indicate activation of cellular stress and immune pathways. Our findings demonstrate that Monogenean parasitism compromises heat resilience by reshaping thermal behaviour, reducing CTmax, and activating stress–immune pathways. This integrative evidence underscores the potential for host–parasite interactions to shape ectotherm vulnerability under climate warming, with direct implications for both natural populations and aquaculture management.