Increased behavioural lateralization in parasitized coral reef fish

Preferential use of one side of the body for cognitive or behavioural tasks (lateralization) is common in many animals, including humans. However, few studies have demonstrated whether lateralization is phenotypically plastic, and varies depending on the ecological context. We studied lateralization (measured as a turning preference) in the bridled monocle bream (Scolopsis bilineatus). This coral reef fish is commonly infected by a large, ectoparasitic isopod (Anilocra nemipteri) that attaches to the left or right side of its host's head. Fish that were parasitized showed no turning bias with respect to the side on which the parasite had attached. On average, however, parasitised fish were significantly more lateralized (i.e. had a strong side bias) than unparasitized fish. The extent of lateralization declined significantly when we experimentally removed the parasite. Our results indicate that lateralization can vary with the ecological context. One possible explanation is that lateralization shortens the response time until fish flee after encountering a predator. A stronger side bias might be advantageous for parasitized individuals to overcome their recently documented lower maximum swimming speed.