Come with me if you want to live: sympatric parasites follow different transmission routes through aquatic host communities

Community composition, including the relative density of each host species, plays a vital role in the transmission of parasites or disease in freshwater ecosystems. Whereas some host species can effectively transmit parasites, others can act as dead ends (non-viable transmission routes), accumulating large numbers of parasites throughout their life, thus becoming important sinks for parasite populations. Although population sinks have been identified in certain host-parasite systems, robust field estimates of the proportions of parasites that are lost to these hosts are lacking. Here, we quantified the distribution of encysted larval hairworms (phylum Nematomorpha), common parasites in lotic ecosystems, in two subalpine stream communities of New Zealand. With parasite and host population densities calculated per m², we identified which host species most likely contributed to the transmission of three sympatric hairworm morphotypes identified in both streams, and which species acted as population sinks. We also tested for seasonal patterns and peaks in the abundance of each morphotype in the two communities over the sampling season. Finally, we tested whether hosts emerging from the streams had comparable abundances of hairworm morphotypes throughout the sampling period. For each morphotype, different key sets of host species harboured more hairworms on average (abundance) than others, depending on the stream. For one morphotype in particular, two species of hosts were found to be important population sinks that inhibited over a third of these parasites from completing their life cycle. We also observed a clear peak in abundance for another hairworm morphotype during summer. Our data suggest that hosts emerging from the streams matched their aquatic counterparts with respect to hairworm abundance, indicating no infection-dependent reduction in emergence success. Our findings suggest that, depending on relative community composition, sympatric parasites follow different host transmission pathways, some of which lead to dead ends that potentially impact overall infection dynamics. In turn, this information can help us understand the spread or emergence of disease in both freshwater and terrestrial environments, since hairworms infect terrestrial arthropods to complete their life cycle.