The influence of environmental factors on abundance and prevalence of a commensal ostracod hosted by an invasive crayfish: are 'parasite rules' relevant to non-parasitic symbionts?
Symbiosis represents a widespread and successful lifestyle, but research on symbiotic associations has been mainly focused on parasites. Three general patterns in parasite ecology have been proposed: (i) aggregation, (ii) positive and tight correlation between mean symbiont abundance and its variance and (iii) positive correlation between abundance and prevalence of symbionts. The factors affecting abundance and prevalence within symbiont species can be grouped into host features and environmental conditions. According to research in parasite ecology, environmental conditions seem to play a minor role. We investigated whether the three most recognised parasite patterns were also evident in a non-parasitic freshwater ectosymbiont and analysed the effects of host features (sex, body size, density and ecdysis) and environmental conditions (climate and water chemistry) on abundance and prevalence of the symbiont. Our species model was an exotic ectocommensal ostracod, Ankylocythere sinuosa, inhabiting the invasive crayfish Procambarus clarkii in Europe. We sampled 373 crayfish from 26 Spanish localities. We modelled both abundance per occupied host (i.e. intensity) and prevalence using zero-altered models and a backward model selection of host and environmental variables, and assessed model performance through graphical analyses of Pearson residuals and the relationship between observed and fitted values of the finally selected model. We observed all three parasite patterns in A. sinuosa. Abundance of A. sinuosa was most strongly related to conductivity (negative effect), followed by host density (positive) and ecdysis (negative), with moderate relationships to variables related to water chemistry (NH4+ concentration, alkalinity/(Cl-+SO42-) and Cl-/SO42- ratios). Prevalence was related mainly to crayfish density (positive effect) and also to climatic conditions (maximum temperature of warmest month, annual precipitation and precipitation seasonality). Our results suggest that some 'parasite rules' may actually be general 'symbiont rules'. Moreover, our symbiont model was markedly affected by climatic conditions at a regional scale acting on prevalence, and water chemistry at a local level acting on symbiont abundance. This supports the hypothesis of a prominent role of environmental conditions influencing population parameters of non-parasitic freshwater ectosymbionts. Physiological tolerances to highly unstable environmental factors, together with dependence on highly variable external food sources, probably underlie the high sensitivity to environmental conditions of this category of symbiont.