Altered thermoregulation as a driver of host behaviour in glochidia-parasitised fish

Abstract

Parasites alter their host behaviour and vice versa as a result of mutual adaptations in the evolutionary arms race. One of these adaptations involves changes in host thermoregulation, which has the potential to harm the parasite and thereby act as a defence mechanism. We used a model of the brown trout Salmo trutta experimentally parasitised with ectoparasitic larvae called glochidia from the endangered freshwater pearl mussel Margaritifera margaritifera to reveal whether parasitation alters fish behavioural thermoregulation. A study based on radio telemetry temperature sensors was performed during almost one year M. margaritifera parasitic stage. Glochidia infested S. trutta altered its thermoregulation through active searching for habitats with different thermal regimes. General preference for lower temperature of infested fish varied, being sometimes above, sometimes below the temperature preferred by uninfested individuals. Infested fish also preferred different temperatures across localities, while uninfested fish maintained their thermal preference no matter which stream they inhabited. Glochidia further induced the expression of a behavioural syndrome among S. trutta personality traits, suggesting that it might increase the probability that the fish host would occur in the glochidia temperature optimum. Our findings present the first evidence that thermoregulation plays a fundamental role in the relationship of affiliated mussels and their fish hosts. Incorporating thermoregulation issue in the study of this relationship can help to interpret results from previous behavioural studies as well as to optimise management measures related to endangered mussels.