Parasite infection induces size-dependent host dispersal: consequences for parasite persistence

Host dispersal is now recognized as a key predictor of the landscape-level persistence and expansion of parasites. However, current theories treat post-infection dispersal propensities as a fixed trait, and the plastic nature of host's responses to parasite infection has long been underappreciated. Here, we present a mark-recapture experiment in a single host-parasite system (larval parasites of the freshwater mussel Margaritifera laevis and its salmonid fish host Oncorhynchus masou masou) and provide, to our knowledge, the first empirical evidence that parasite infection induces size-dependent host dispersal in the field. In response to parasite infection, large fish become more dispersive, whereas small fish tend to stay at the home patch. The observed plasticity in dispersal is interpretable from the viewpoint of host fitness: expected benefits (release from further infection) may exceed dispersal-associated costs for individuals with high dispersal ability (i.e. large fish) but are marginal for individuals with limited dispersal ability (i.e. small fish). Indeed, our growth analysis revealed that only small fish hosts incurred dispersal costs (reduced growth). Strikingly, our simulation study revealed that this plastic dispersal response of infected hosts substantially enhanced parasite persistence and occupancy in a spatially structured system. These results suggest that dispersal plasticity in host species is critical for understanding how parasites emerge, spatially spread, and persist in nature. Our findings provide a novel starting point for building a reliable, predictive model for parasite/disease management

Environmental factors affecting the invasion success and morphological responses of a globally introduced crayfish in floodplain waterbodies

Floodplain ecosystems that are characterized by high habitat heterogeneity and hydrological connectivity are considered hotspots for freshwater biodiversity. However, these biodiversity-rich areas have been seriously threatened by biological invasions. The signal crayfish (Pacifastacus leniusculus) is listed as 100 of the world's worst invasive species, and is a major threat to freshwater biodiversity and ecosystem functioning. Here, we examined environmental factors relating to the invasion success of signal crayfish and their morphological responses in floodplain waterbodies. Classification and regression tree analyses showed that most of the influential factors differed between tributary and lake populations. In floodplain tributaries, the occurrence of crayfish was positively related with water temperature and abundance of leaf cover, while crayfish abundance was highest where large-wood was abundant. In floodplain lakes, crayfish were absent at oxygen-poor sites, and abundant at sites with high connectivity to a main channel. These results indicate that conservation practitioners should consider different environmental factors in accordance with strategies for invasive species management (i.e., offensive or defensive management). Furthermore, we demonstrated morphological differences between tributary and lake populations, with tributary crayfish having wider chelae. These morphological differences might have resulted from the physical differences between the two types of waterbodies, facilitating the rapid invasion of signal crayfish to floodplain waterbodies. Our study showed that invasion-risk assessments should consider both environmental factors and morphological responses to new environments to understand invasion ecology and to form effective conservation plans and to prioritize management actions

A delayed effect of the aquatic parasite Margaritifera laevis on the growth of the salmonid host fish Oncorhynchus masou masou

Parasitic species often have detrimental effects on host growth and survival. The larvae of the genus Margaritifera (Bivalvia), called glochidia, are specialist parasites of salmonid fishes. Previous studies have reported negligible influences of the parasite on their salmonid hosts at natural infection levels. However, those studies focused mainly on their instantaneous effects (i.e., during the parasitic period). Given the time lag between physiological and somatic responses to pathogen infections, the effect of glochidial infection may become clearer during the post-parasitic period. Here, we examined whether the effect of glochidial infections of Margaritifera laevis on its salmonid host Oncorhynchus masou masou would emerge during the post-parasitic period. We performed a controlled aquarium experiment and monitored fish growth at two time intervals (i.e., parasitic and post-parasitic periods) to test this hypothesis. Consistent with previous observations, the effects of glochidial infection were unclear in the middle of the experiment (day 50; parasitic period). However, even with a natural glochidial load (48 glochidia per fish), we found a significant reduction in growth rates of infected fish in the extended period of the experiment (day 70; post-parasitic period). Our results suggest that examining only instantaneous effects may provide misleading conclusions about mussel-host relationships

Data from: Parasite infection induces size-dependent host dispersal: consequences for parasite persistence

Host dispersal is now recognized as a key predictor of the landscape-level persistence and expansion of parasites. However, current theories treat post-infection dispersal propensities as a fixed trait, and the plastic nature of host’s responses to parasite infection has long been underappreciated. Here, we present a mark-recapture experiment in a single-host parasite system (larval parasites of the freshwater mussel Margaritifera laevis and its salmonid fish host Oncorhynchus masou masou) and provide the first empirical evidence that parasite infection induces size-dependent host dispersal in the field. In response to parasite infection, large fish become more dispersive, whereas small fish tend to stay at the home patch. The observed plasticity in dispersal is interpretable from the viewpoint of host fitness: expected benefits (release from further infection) may exceed dispersal-associated costs for individuals with high dispersal ability (i.e., large fish) but are marginal for individuals with limited dispersal ability (i.e., small fish). Indeed, our growth analysis revealed that only small fish hosts incurred dispersal costs (reduced growth). Strikingly, our simulation study revealed that this plastic dispersal response of infected hosts substantially enhanced parasite persistence and occupancy in a spatially structured system. These results suggest that dispersal plasticity in host species is critical for understanding how parasites emerge, spatially spread, and persist in nature. Our findings provide a novel starting point for building a reliable, predictive model for parasite/disease management

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Electronic Supplementary Material from Parasite infection induces size-dependent host dispersal: consequences for parasite persistence

Host dispersal is now recognized as a key predictor of the landscape-level persistence and expansion of parasites. However, current theories treat post-infection dispersal propensities as a fixed trait, and the plastic nature of host's responses to parasite infection has long been underappreciated. Here, we present a mark–recapture experiment in a single host–parasite system (larval parasites of the freshwater mussel <i>Margaritifera laevis</i> and its salmonid fish host <i>Oncorhynchus masou masou</i>) and provide the first empirical evidence that parasite infection induces size-dependent host dispersal in the field. In response to parasite infection, large fish become more dispersive, whereas small fish tend to stay at the home patch. The observed plasticity in dispersal is interpretable from the viewpoint of host fitness: expected benefits (release from further infection) may exceed dispersal-associated costs for individuals with high dispersal ability (i.e. large fish) but are marginal for individuals with limited dispersal ability (i.e. small fish). Indeed, our growth analysis revealed that only small fish hosts incurred dispersal costs (reduced growth). Strikingly, our simulation study revealed that this plastic dispersal response of infected hosts substantially enhanced parasite persistence and occupancy in a spatially structured system. These results suggest that dispersal plasticity in host species is critical for understanding how parasites emerge, spatially spread, and persist in nature. Our findings provide a novel starting point for building a reliable, predictive model for parasite/disease management