Does nutritional behavior affect gut endosymbiont prevalence? The case of Paramoebidium avitruviense (Mesomycetozoa) within Siphonoperla torrentium (Plecoptera) nymphs

Aquatic macroinvertebrates harbor a broad spectrum of gut endosymbionts, among them trichomycetes species (including fungi and protozoa) are predominantly associated with shredders, collector or scraper hosts, which only rarely may ingest some animal tissues. An exception to this rule may be the protozoan Paramoebidium avitruviense, documented within opportunistic-predator nymphs of the stonefly Siphonoperla torrentium. In the present work, we studied a population of S. torrentium bearing P. avitruviense during 21 months to describe the life cycle and feeding habits of S. torrentium. We tested for associations between host diet, process of infestation and the prevalence of P. avitruviense, and provide information about the temporal dynamics of both the stonefly and the protozoan. Siphonoperla torrentium presented a univoltine life cycle and displayed an omnivorous diet which changed toward an invertebrate-based diet in older nymphs. The endosymbiont P. avitruviense showed fluctuations in relation to the host nymphal size, and prevalence peaks concurred with the highest host growth rate. Binary logistic regressions revealed that the diet influenced the prevalence of the protozoan, at least for some of the gut content fractions. Whereas an increment of the animal fraction decreased the probability of being infested, the consumption of other fractions related to a gatherer-collector activity increased the probabilities of infestation. However, the most decisive factor affecting endosymbiont prevalence was the melanization grade of S. torrentium, whereby the higher the melanization of the nymph (= longer post-ecdysis period), the higher the infestation probability by P. avitruviense.