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Understanding the ecological assembly of parasite communities is critical to characterise how changing host and environmental landscapes will alter infection dynamics and outcomes. However, studies frequently assume that (a) closely related parasite species or those with identical life-history strategies are functionally equivalent, and (b) the same factors will drive infection dynamics for a single parasite across multiple host species, oversimplifying community assembly patterns. Here, we challenge these two assumptions using a naturally occurring host–parasite system, with the mussel Anodonta anatina infected by the digenean trematode Echinoparyphium recurvatum, and the snail Viviparus viviparus infected by both E. recurvatum and Echinostoma sp. By analysing the impact of temporal parasite dispersal, host species and size, and the impact of coinfection (moving from broader environmental factors to within-host dynamics), we show that neither assumption holds true, but at different ecological scales. The assumption that closely related parasites can be functionally grouped is challenged when considering dispersal to the host (i.e. larger scales), while the assumption that the same factors will drive infection dynamics for a single parasite across multiple host species is challenged when considering within-host interspecific competition (i.e. smaller scales). Our results demonstrate that host identity, parasite identity and ecological scale require simultaneous consideration in studies of parasite community composition and transmission. [ABSTRACT FROM AUTHOR]

However, these models can only be used for one parasite at a time, and so possible interactions between parasites need to be indirectly estimated by including other parasites as a predictor in separate models for each parasite studied (e.g. Markle et al. [18]; Obiegala et al. [21]). This example is particularly pertinent; while the mobility of parasite life history stages has been demonstrated as important in driving infection dynamics and parasite infracommunity structure in terms of both presence and abundance (e.g. Lhorente et al. [17]; Mazé-Guilmo et al. [19]), the relative mobilities of different hosts, and specifically the mobile-immobile contrast, are an underdeveloped area of research. More generally, we recommend careful consideration of how host and parasite traits as well as the environment affect parasite prevalences and parasite abundances and an acknowledgement that these effects may not be the same. For example, whether parasites are directly transmitted between conspecifics (i.e. microparasites) or environmentally transmitted (i.e. macroparasites) can alter the risk of a parasite infection in the same host population (Han et al. [12]), though whether this extended to parasite intensity was not explored. [Extracted from the article]

To persist in oligotrophic waters, reef-building corals rely on nutritional interactions with symbiotic dinoflagellates of the family Symbiodiniaceae, but the true diversity of this family remains poorly characterised. In this paper, we assess Symbiodiniaceae richness at Atauro Island (Timor-Leste) as well as on reefs of the neighbouring Timor mainland, using direct sequencing of three gene regions: cob gene, mitochondrion; ITS2 region, nucleus; and psbAncr region, chloroplast; in addition to a highly multiplexed application of next-generation sequencing. These geographic sites are among the most biodiverse in the world, but have never had their symbiont communities studied. Despite their proximity, our results reveal symbiont richness 1.25 times higher at Atauro Island than the Timor mainland, a result evident in dominant sequences. In contrast, Timor had a significantly richer background sequence diversity. Although sampling was restricted to shallow sites only, symbiont richness at Atauro Island was also higher than comparative reefscapes globally, after standardising for number of taxa sampled. While Atauro and Timor have related symbiont populations, with the same novel types recorded at both sites, there were also clear differences in symbiont composition between the two geographic regions, with Timor displaying a consortium more characteristic of stressed reef environments (proportionally hosting twice as many Durusdinium sequences, formerly clade D, as Atauro). These results reveal a symbiont richness that matches the high biodiversity of these reefs, but also potentially negative effects of proximal human populations on Symbiodiniaceae, even when previous studies have shown corals to be largely unaffected. [ABSTRACT FROM AUTHOR]

Reef-building corals are nutritionally dependent on the symbiotic alga Symbiodinium and may therefore select for high-performing symbiont lineages. However, the effects of fine-scale genetic variation within symbiont populations on host performance are not well understood. Applying quantitative PCR (qPCR) to Pocillopora damicornis colonies from Lord Howe Island (Australia), we estimated the relative abundance of intra-genomically variable (IGV) Symbiodinium cells and those with homogeneous rDNA sequence repeats. We then analysed their physical status and photosynthetic performance at both ambient and extreme summer temperatures (25 and 29 °C, respectively) in a closed-system oxygen flux chamber. The proportion of IGV Symbiodinium was negatively correlated with photosynthetic production and efficiency, and under thermal stress, colonies dominated by IGV cells could only meet respiratory demands at high irradiance (> 250 µmol photons m−2 s−1). IGV Symbiodinium were also estimated to be 50% larger by volume, occurred at lower density, and had higher chlorophyll c2/a ratios. The comparatively poor performance of heterogeneous Symbiodinium cells suggests either a direct deleterious effect of IGV or reduced performance in putative inter-lineage recombinants, whose genomes feature ITS2 variants from both parental ribotypes. While it remains unclear whether the mixed rDNA arrays result from somatic mutation or recombination, our results demonstrate that the beneficial capacity of symbionts can vary within Symbiodinium populations, with potential implications for host fitness. [ABSTRACT FROM AUTHOR]