Morphological and molecular characterization of Myxobolus mucosus sp. n. (Myxosporea: Myxobolidae) with basifilamental sporulation in two cyprinid fishes, Rutilus rutilus (L.) and Leuciscus leuciscus (L.) in Russia

A new Myxobolus species, Myxobolus mucosus sp. n., was found in the basifilamental location of roach, Rutilus rutilus, and common dace, Leuciscus leuciscus, during the investigation of fish myxosporean fauna in Finnish Bay of Baltic Sea and Lake Ladoga in 2014. Plasmodia from the two hosts share similar morphometric and morphological features which are whitish and slightly elongated oval and measured 300–500 μm in diameter. Mature spores of M. mucosus sp. n. are ellipsoidal in frontal view and lemon shaped in sutural view, measuring 13.6 ± 0.62 (12.8–14.2) μm in length, 10.7 ± 0.51 (10.3–11.4) μm in width, 7.8 ± 0.41 (7.2–8.3) μm in thickness and 13.0 ± 0.89 (11.5–14.2) μm in length, 10.8 ± 0.77 (10.2–11.5) μm in width, 7.7 ± 0.52 (7.3–8.0) μm in thickness from roach and common dace, respectively. Polar capsules were pyriform, equal in size, converging anteriorly, with 5.9 ± 0.22 (5.7–6.3) μm in length and 3.9 ± 0.3 (3.5–4.1) μm in width and 5.6 ± 0.38 (5.1–6.2) μm in length and 3.8 ± 0.24 (3.5–4.0) μm in width from roach and common dace, respectively. Spore valves are relatively thin, symmetrical, and smooth. The most remarkable morphological character of this new Myxobolus species is the distinct mucous envelope, surrounding the full myxospore valves, up to 25 and 32 μm, respectively, in roach and common dace. The partial 18S ribosomal DNA (rDNA) sequence variation of myxospores from two hosts is very tiny (below 0.8 %) but significantly differing from all available sequences of myxosporeans in GenBank. Therefore, it can be concluded that this myxosporean from these two closely related cyprinids are conspecific by combining morphological characteristics, tissue specificity, and molecular data. Phylogenetic analysis showed that this newly described species cluster with several gill-infecting Myxobolus species from cyprinid fish inhabiting in Eastern Europe.