The importance of visual and chemical cues in infection detection and avoidance in a freshwater fish

The characteristics of infected animals, including their smell, appearance, behaviour or sound, can greatly differ from those of uninfected conspecifics. These differences can serve as cues to recognize and avoid infected individuals to minimize the risk of infection. Avoidance of infected conspecifics is a risk-sensitive behaviour that can be influenced by various factors such as sensory cues and environmental parasite load, which both remain poorly understood in fish. We investigated the ability of two populations of wild caught pumpkinseed sunfish ( Lepomis gibbosus ) to distinguish between conspecifics infected with parasitic worms versus uninfected individuals in two-choice experiments using visual and chemical cues separately. One population was from a lake without parasites (parasite naive) whereas the other population originated from a lake with a high prevalence of trematode and cestode worms (parasite experienced). Both populations preferred to be with conspecifics, regardless of their infection level, over being alone when given visual cues but avoided conspecifics and remained alone when given chemical cues, suggesting that visual and chemical cues are not redundant. Neither population showed any preference between infected and uninfected conspecifics when given visual cues. However, in the presence of chemical cues, there was a great interindividual variation: some fish preferred uninfected conspecifics while others preferred infected ones. On average, naive fish avoided infected conspecifics whereas experienced fish did not show any preferences, suggesting that fish from lakes with high prevalence of infection habituate to infection cues. We suggest that pumpkinseeds use chemical rather than visual cues to discriminate between infected and uninfected conspecifics and make a shoaling decision. Our study highlights the importance of considering different sensory cues as well as parasite load when studying avoidance and shoaling behaviours, especially in a time of modifying sensory landscape and parasites abundance of freshwater ecosystems through global changes.