A new species of Myxidium (Myxosporea: Myxidiidae), from the western chorus frog, Pseudacris triseriata triseriata, and Blanchard's cricket frog, Acris crepitans blanchardi (Hylidae), from eastern Nebraska: morphology, phylogeny, and critical comments on amphibian Myxidium taxonomy.

During March 2001-April 2004, 164 adult anurans of 6 species (47 Rana blairi, 35 Rana catesbeiana, 31 Hyla chrysoscelis, 31 Pseudacris triseriata triseriata, 11 Bufo woodhousii, and 9 Acris crepitans blanchardi) from Pawnee Lake, Lancaster County, Nebraska, were surveyed for myxozoan parasites. Of these, 20 of 31 (65%) P. triseriata triseriata and 1 of 9 (11%) A. crepitans blanchardi were infected with a new species of Myxidium. Myxidium melleni n. sp. (Myxosporea) is described from the gallbladder of the western chorus frog, P. triseriata triseriata (Hylidae). This is the second species of Myxidium described from North American amphibians. Mature plasmodia are disc-shaped or elliptical 691 (400-1,375) x 499 (230-1,200) x 23 (16-35) microm, polysporic, producing many disporic pansporoblasts. The mature spores, 12.3 (12.0-13.5) x 7.6 (7.0-9.0) x 6.6 (6.0-8.0) microm, containing a single binucleated sporoplasm, are broadly elliptical, with 2-5 transverse grooves on each valve, and contain 2 equal polar capsules 5.2 (4.8-5.5) x 4.2 (3.8-4.5) microm positioned at opposite ends of the spore. Myxidium melleni n. sp. is morphologically consistent with other members of Myxidium. However, M. melleni n. sp. was phylogenetically distinct from other Myxidium species for which DNA sequences are available. Only with improved morphological analyses, accompanied by molecular data, and the deposit of type specimens, can the ambiguous nature of Myxidium be resolved. Guidelines for descriptions of new species of Myxidium are provided.