Hidden diversity: parasites of stream arthropods

Parasite communities of aquatic macroinvertebrates have so far rarely been studied, even if these host organisms are of high relevance for the ecology of freshwater systems and their parasites likely affect the host populations and communities. Therefore, this study addresses this ‘hidden diversity’ in aquatic arthropods of a stream ecosystem in North Rhine‐Westphalia, Germany. Samples of benthic organisms were taken, and the most abundant host species of major invertebrate groups (amphipods and larvae of chironomids, beetles, caddisflies, mayflies, stoneflies) were tested by PCR for different parasite taxa [microsporidians, acanthocephalans (only amphipods), trematodes, nematodes]. Furthermore, the possible link between prevalence and host feeding type was investigated for each parasite group using a newly developed ‘feeding type score’. In total, 10 species of aquatic insect larvae and two amphipods of the genus Gammarus spp. were tested and 16 different isolates of microsporidians, three acanthocephalan species, six species of trematodes and one nematode were found. Microsporidians were present in all host species with prevalences ranging from 20 to 100%. Only one presumably specific host–microsporidian association was found for the mayfly Ephemera danica and three isolates were detected exclusively in amphipods. Three acanthocephalan species were detected in the amphipods with prevalences of up to 5.3%. All tested host species were infected with trematodes (prevalences 25–100%), except the caddisfly Sericostoma sp. Nematodes were detected in five host species with prevalences ranging from 14 to 80%. For several positive samples, no sequencing result could be obtained, especially for trematodes and nematodes, therefore, the actual parasite diversity might be even higher. Active filter feeding showed the highest feeding type prevalence score for microsporidia and nematodes, and passive filter feeders for trematodes, but these results have to be interpreted cautiously as only one active and one passive filter feeder species each were present in the sample. The impressive number of 26 parasite species (12 microsporidians and 10 helminths) were detected in 12 host species, mostly with surprisingly high prevalences. These results illustrate the ‘hidden diversity’ in aquatic ecosystems and highlight the importance of parasites for aquatic ecological research and the need for studies on the effects of mixed parasite infections on the host populations and communities.