Abstract
Parasitism is one of the most common forms of interaction in ecosystems, with increasing evidence that demonstrates the impact of parasites on ecological processes, including population dynamics, predator-prey interactions, and behavioural changes. Parasitic helminths, which often exhibit complex life cycles involving multiple host species and trophic transmission primarily through predation, can offer valuable insights into ecological food web dynamics. Many of these helminths have been shown to manipulate the behaviour of their intermediate hosts to increase predation risk and thereby enhance their transmission success. These behavioural changes may involve altering the expression level, repeatability, or associations among one or multiple behavioural traits simultaneously, exposing the intermediate host to a greater likelihood of predation by the definitive host of the parasite.
In Otago Harbour, the mottled triplefin (Forsterygion capito) harbours a high parasite diversity. The brain-encysting trematode Cardiocephaloides ovicorpus was one of the parasites identified from F. capito in high prevalence and intensity during a recent large-scale survey of parasite diversity and life cycles in the region. Cardiocephaloides ovicorpus has a complex life cycle involving transmission from F. capito to avian predators, the parasite’s definitive host. Following this discovery, my thesis further investigates the parasite community within F. capito and tests the potential behavioural effects of C. ovicorpus on the fish.
I conducted behavioural tests on F. capito naturally infected by C. ovicorpus to assess effects of the parasite on the expression and repeatability of fish personality traits. Further, I utilised molecular techniques to identify all parasite specimens recovered from fish dissections. Fourteen distinct parasite species were identified within the fish samples, including one species new to science and three new host records. This provides additional information on parasite diversity and establishes more predator-prey interactions within Otago Harbour by linking triplefins to the definitive hosts of the three parasites, which are newly recorded in the triplefins from this study. Cardiocephaloides ovicorpus was retrieved in exceptionally high abundance with 100% prevalence, suggesting its dominance within the parasite community. Intraspecific competition was also observed among Cardiocephaloides ovicorpus within the braincase of triplefins, as a significant negative correlation between parasite size and infection intensity was found. This occurs because a smaller body size implies reduced fitness and fecundity, resulting from increased competition for host resources as parasite numbers rise. Despite this competition, C. ovicorpus significantly influenced host behaviour, as positive correlations were detected between parasite intensity and both the frequency and repeatability of surfacing behaviour.
The findings suggest that C. ovicorpus may enhance its transmission to avian definitive hosts by inducing changes in surfacing behaviour of the fish. The increased frequency of surfacing likely raises the risk of exposure to predators while the decrease in behavioural repeatability may reduce the alignment between behavioural responses and environmental cues, such as predator presence, further elevating risk of predation to the host. Nevertheless, causality between behavioural changes and parasite infection remains unconfirmed, as experimental infection was not possible due to the first intermediate host of C. ovicorpus remaining unknown despite of extensive screening of possible species of gastropods. Future research should prioritise identifying the unknown first intermediate host to enable experimental infections and direct tests of the causal relationship between parasite infection and host behavioural changes. Overall, this research highlights the importance of parasites in shaping species interactions within ecosystems and contributes to our understanding of behavioural variation in wild animal populations.