Unravelling hidden battles: Genetic insights into parasite diversity and competition in mottled triplefin (Forsterygion capito).

Parasites play an important role in ecosystems, often exhibiting complex life cycles that involve two to four host species and transmission through trophic interactions. By tracking parasite life cycles, predator-prey links can be identified, offering valuable insights into food webs. In this study, we examined the helminth community of the mottled triplefin (Forsterygion capito), one of the most common intertidal fish species in New Zealand, using molecular techniques. We identified 14 parasite species, including one new to science and three new host records. These findings established new predator-prey links in the previously known local food web. We also provide evidence of parasite competition, focusing on the trematode Cardiocephaloides ovicorpus, the most prevalent and abundant parasite found in the brain case of F. capito. As the intensity of C. ovicorpus metacercariae increased, their individual size decreased, suggesting intraspecific competition for resources within the limited space of the brain case. Additionally, C. ovicorpus may indirectly outcompete other parasite species using F. capito by inducing host behavioural manipulation to enhance its transmission to avian hosts given its encystment site within the brain case of the fish. Overall, our findings emphasise the importance of obtaining information about parasites in order to improve our understanding of species interactions within ecosystems.