Ephemera danica (Ephemeroptera: Ephemeridae) As a Resource for Two Commensals: Ciliated Protozoans (Sessilida) and Chironomids (Diptera).

The distribution and coexistence of two unrelated commensals, the chironomid Epoicocladius ephemerae (Kieffer 1924) and ciliate Carchesium polypinum L. 1758, on one host species, Ephemera danica Muller 1764, sampled in two small lowland rivers in 2009, 2010 and 2011, were investigated. We analyzed 288 mayfly specimens from the Bzura River and 101 from the Mroga River. The number of commensals on a single mayfly specimen varied between 0 and 18 chironomids, and from 0 to 46 colonies of ciliates. Prevalences were >48% for chironomids and ∼30% for ciliates, whereas mean intensities were low (4.01±6.04 commensals on one host). The spatial distribution of each commensal species was investigated on different parts of the host body. Neither chironomids nor ciliates infected the whole mayfly body. The co-occurrence of these two commensals was not random and showed a negative association. Chironomids were most frequent on two or three parts of the body (two parts of the abdomen, with gills and without gills, and legs), whereas ciliates were found on two parts (the whole abdomen). Coexistence of the two commensal species led to partitioning of resources that was host body size dependent: small mayflies (optimal size 11.63 mm) were primarily settled by ciliated protozoans while larger specimens (optimal size 28.77 mm) were settled by chironomids.