Resolving complex life cycles of parasites is a major goal of parasitological research. The aim of this study was to analyse the life cycle of two species of the genusProfilicollis,the taxonomy of which is still unstable and life cycles unclear. We extracted individuals ofProfilicollisfrom two species of crustaceans (intermediate hosts) and four species of seagulls (definitive hosts) from sandy-shore and estuarine habitats along the south-east Pacific coast of Chile. Mitochondrial DNA analyses showed that two species ofProfilicollisinfected intermediate hosts from segregated habitats: whileP. altmanilarvae infected exclusively molecrabs of the genusEmeritafrom fully marine habitats,P. antarcticuslarvae infected the crabHemigrapsus crenulatusfrom estuarine habitats. Moreover,P. altmanicompleted its life cycle in four seagulls,Chroicocephalus maculipennis, Leucopheus pipixcan, Larus modestusandL. dominicanus,whileP. antarcticus, on the other hand, completed its life cycle in the kelp gullL. dominicanus. Accordingly, our results show that two congeneric parasites use different and spatially segregated species as intermediate hosts, and both are capable of infecting one species of definitive hosts. As such, our analyses allow us to shed light on a complex interaction network.

Background: Parasites are essential components of natural communities, but the factors that generate skewed distributions of parasite occurrences and abundances across host populations are not well understood. Methods: Here, we analyse at a seascape scale the spatiotemporal relationships of parasite exposure and host body-size with the proportion of infected hosts (i.e., prevalence) and aggregation of parasite burden across ca. 150 km of the coast and over 22 months. We predicted that the effects of parasite exposure on prevalence and aggregation are dependent on host body-sizes. We used an indirect host-parasite interaction in which migratory seagulls, sandy-shore molecrabs, and an acanthocephalan worm constitute the definitive hosts, intermediate hosts, and endoparasite, respectively. In such complex systems, increments in the abundance of definitive hosts imply increments in intermediate hosts’ exposure to the parasite’s dispersive stages. Results: Linear mixed-effects models showed a significant, albeit highly variable, positive relationship between seagull density and prevalence. This relationship was stronger for small (cephalothorax length >15 mm) than large molecrabs (<15 mm). Independently of seagull density, large molecrabs carried significantly more parasites than small molecrabs. The analysis of the variance-to-mean ratio of per capita parasite burden showed no relationship between seagull density and mean parasite aggregation across host populations. However, the amount of unexplained variability in aggregation was strikingly higher in larger than smaller intermediate hosts. This unexplained variability was driven by a decrease in the mean-variance scaling in heavily infected large molecrabs. Conclusions: These results show complex interdependencies between extrinsic and intrinsic population attributes on the structure of host-parasite interactions. We suggest that parasite accumulation—a characteristic of indirect host-parasite interactions—and subsequent increasing mortality rates over ontogeny underpin size-dependent host-parasite dynamics.

Thorny-headed acanthocephalan worms of the genusProfilicollisare widely distributed in the oceans of the world and present complex life cycles with intermediate and definitive hosts. The genus is still poorly known, with an unstable taxonomy and, for most species, incompletely characterized geographical distributions. In this study, based on molecular and morphological evidence, we report that the speciesProfilicollis altmaniis also distributed along the South American Atlantic coast, using the mole crabEmerita brasiliensisas an intermediate host. As such, our record shows thatP. altmanihas a Pan-American distribution where five species ofEmeritaare utilized as intermediate hosts.