Parasites shape community structure and dynamics in freshwater crustaceans

Parasites directly and indirectly influence the important interactions among hosts such as competition and predation through modifications of behaviour, reproduction and survival. Such impacts can affect local biodiversity, relative abundance of host species and structuring of communities and ecosystems. Despite having a firm theoretical basis for the potential effects of parasites on ecosystems, there is a scarcity of experimental data to validate these hypotheses, making our inferences about this topic more circumstantial. To quantitatively test parasites' role in structuring host communities, we set up a controlled, multigenerational mesocosm experiment involving four sympatric freshwater crustacean species that share up to four parasite species. Mesocosms were assigned to either of two different treatments, low or high parasite exposure. We found that the trematode Maritrema poulini differentially influenced the population dynamics of these hosts. For example, survival and recruitment of the amphipod Paracalliope fluviatilis were dramatically reduced compared to other host species, suggesting that parasites may affect their long-term persistence in the community. Relative abundances of crustacean species were influenced by parasites, demonstrating their role in host community structure. As parasites are ubiquitous across all communities and ecosystems, we suggest that the asymmetrical effects we observed are likely widespread structuring forces.

Do different parasite species interact in their effects on host fitness? A case study on parasites of the amphipod Paracalliope fluviatilis

SUMMARYThere is a gap in our understanding of the relative and interactive effects of different parasite species on the same host population. Here we examine the effects of the acanthocephalan Acanthocephalus galaxii, an unidentified cyclophyllidean cestode, and the trematodes Coitocaecum parvum and Microphallus sp. on several fitness components of the amphipod Paracalliope fluviatilis, using a combination of infection surveys and both survival and behavioural trials. In addition to significant relationships between specific parasites and measures of amphipod survival, maturity, mating success and behaviour, interactions between parasite species with respect to amphipod photophilia were also significant. While infection by either A. galaxii or C. parvum was associated with increased photophilia, such increases were negated by co-infection with Microphallus sp. We hypothesize that this is due to the more subtle manipulative effect of A. galaxii and C. parvum being impaired by Microphallus sp. We conclude that the low frequency at which such double infections occur in our sampled population means that such interactions are unlikely to be important beyond the scale of the host individual. Whether or not this is generally true, implying that parasitological models and theory based on single parasite species studies do generally hold, requires cross-species meta-analytical studies.

Mysid predation on amphipods and Daphnia in a shallow coastal lake: prey selection and effects of macrophytes

Interactions between mysids and invertebrates other than pelagic zooplankton have not been well studied. We tested the hypothesis that the mysid Tenagomysis chiltoni preys on the co-occurring amphipod Paracalliope fluviatilis in laboratory experiments. In prey size preference experiments with single prey species, male and female T. chiltoni consumed more small than large amphipods in the absence of the macrophyte Elodea canadensis, but were not selective when macrophytes were present. When offered small and large Daphnia, T. chiltoni males consumed more small than large Daphnia, whereas female T. chiltoni did not show any preference. When amphipods and Daphnia of similar size were presented together, male T. chiltoni strongly selected Daphnia in the presence and absence of E. canadensis, whereas female mysids did so only in the presence of macrophytes. The total number of prey consumed by each sex did not differ between macrophyte treatments. However, female T. chiltoni, which are larger than males, consumed more prey than males. These data suggest that mysids may be an important source of mortality to the amphipod P. fluviatilis. Mysids may also prey on amphipods in other aquatic systems where they co-occur.

Life histories of benthic invertebrates in a kauri forst stream in northern New Zealand

Life histories of the following 12 benthic invertebrate species were investigated at four sites in the Waitakere River: Potamopyrgus antipodarum (Gastropoda : Hydrobiidae); Paracalliope fluviatilis (Amphipoda : Eusiridae), Zephlebia (Neozephlebia) sp. and Deleatidium spp. (Ephemeroptera : Leptophlebiidae), Hydora nitida (Coleoptera : Elmidae), Maoridiamesa harrisi, ?Austrocladius sp. and Paratanytarsus agameta (Diptera : Chironomidae), Austrosimulium australense (Diptera : Simuliidae), Aoteapsyche colonica (Trichoptera : Hydropsychidae), Oxyethira albireps (Trichoptera : Hydroptilidae), and Olinga feredayi (Trichoptera : Conoesucidae). All species had life cycles which were non-seasonal according to the Hynes model. Comparison with recent studies in southern North Island and South Island streams suggests that non-seasonal life cycles predominate in New Zealand streams.

Life cycle of Coitocaecum anaspidis Hickman, a New Zealand digenetic trematode

The life cycle of Coitocaecum anaspidis involves three important stages. The adult occurs in Galaxias brevipennis, G. attentuatus, Gobiomorphus gobioides, and in specimens of Anguilla spp. less than 30 cm. long. Potamopyrgus antipodum and P. badia are the molluscan hosts of the sporocyst and cercaria. In New Zealand the cercaria attacks and encysts in Paracalliope fluviatilis, and in Australia in Anaspides tasmaniae. A metamorphosis of the cercarial larva into a progenetic metacercaria takes place in the amphipod.Paracalliope does not become infected with Coitocaecum till the crustacean has reached 2·0 mm. in length. The thigmotropism of the Paracalliope fits exactly with the ground-living habit of the stumpy-tailed cercaria. The small river fish feeding in slack water acquire Coitocaecum from Paracalliope.