Impacts of an eye invading parasite on the behaviour and ecology of its freshwater fish host

Abstract

Parasites are increasingly shown to impact ecosystem processes such as population dynamics and food web interactions. This can be accomplished by altering the host’s behaviour as a side-effect of infection, a response of the host to being infected, or through direct manipulation by the parasite. Some parasites, such as those that infect the sensory organs, are more likely to impact the behaviour of their host than others. Diplostomid trematodes often infect the eyes (e.g. lens, humours, or retina) of fish as larvae where they can impact various behaviours such as escaping predators and finding food. Tylodelphys darbyi is the only identified diplostomid in New Zealand. In its juvenile stage, this parasite resides in the humours of common bully, or toitoi, Gobiomorphus cotidianus, and is particularly abundant in the fish population of Lake Hayes. In this thesis, using a series of laboratory experiments, I address the impact T. darbyi has on its host’s 1) ability to react to predators, 2) choice of microhabitat, 3) ability to compete with conspecifics, 4) personality traits, as well as 5) determine the parasite’s distribution among South Island lakes. I demonstrate that there appears to be no impact of the parasite on the host’s ability to avoid predation, however as infection increases fish spend more time actively moving about away from shelter. The more T. darbyi are present in the eyes of fish, the more likely fish are to stay closer to a food source, share a shelter, and for males to be darker in colour. Further, infection with T. darbyi appears to impact personality traits (boldness, activity, exploration, and aggression) in fish, and based on a comparison with fish from another lake where T. darbyi is absent, the parasite might also drive population-level differences in behaviour. Overall, the influence of T. darbyi on fish behaviour is subtle rather than pronounced, but detectable across multiple behaviours. Finally, I demonstrated that T. darbyi has a broader distribution on the South Island than previously recorded, and I also used molecular markers to uncover 2-3 more species of diplostomids in New Zealand, all also found in G. cotidianus. These findings indicate that the parasite impacts the behaviour of bullies in the study population, i.e. Lake Hayes, that it has the potential to drive population-level differences in its host, and that it is only one of several species of diplostomid trematode parasites in the South Island, each of which may exert its own impacts on fish behaviour. Gobiomorphus cotidianus is a ubiquitous presence in New Zealand freshwaters and as such any factor that impacts their behaviour and ecology can have wide ranging implications. Finally, this thesis further brings to light the underappreciated role of parasites in generating inter-population and inter-individual differences in host behaviour.