Redescription and phylogenetic position of Myxobolus aeglefini and Myxobolus platessae n. comb. (Myxosporea), parasites in the cartilage of some North Atlantic marine fishes, with notes on the phylogeny and classification of the Platysporina
Myxobolus 'aeglefini' Auerbach, 1906 was originally described from cranial cartilage of North sea haddock (Melanogrammus aeglefinus), but has subsequently been recorded from cartilaginous tissues of a range of other gadoid hosts, from pleuronectids and from lumpsucker (Cyclopterus lumpus) in the North Atlantic and from a zoarcid fish in the Japan Sea (Pacific). We obtained partial small-subunit rDNA sequences of Myxobolus 'aeglefini' from gadoids and pleuronectids from Norway and Iceland. The sequences from gadoids and pleuronectids represented two different genotypes, showing 98.2% identity. Morphometric studies on the spores from selected gadids and pleuronectids revealed slight but statistically significant differences in spore dimensions associated with the genotypes, the spores from pleuronectids were thicker and with larger polar capsules. We identify the morpho- and genotype from gadoids with Myxobolus 'aeglefini' sensu Auerbach, and the one from pleuronectids with Sphaerospora platessae Woodcock, 1904 as Myxobolus platessae n. comb. The latter species was originally described from Irish Sea plaice (Pleuronectes platessa). Myxobolus albi Picon et al., 2009 described from the common goby Pomatoschistus microps in Scotland is a synonym of M. 'aeglefini'. The Pacific Myxobolus 'aeglefini' represents a separate species, showing only 97.4-97.6% identity to the Atlantic species. In phylogenetic analyses based on SSU rDNA sequences, these and some related marine chondrotropic Myxobolus spp. form a distinct well supported group. This clusters with freshwater and marine myxobolids and Triangula and Cardimyxobolus species, in a basal clade in the phylogeny of the Platysporina. Members of family Myxobilatidae, Ortholinea spp. (currently Ortholineidae) and sequences of some other urinary system infecting myxosporeans form a well supported clade among members of the suborder Platysporina. Based on phylogenetic analyses, we propose the following changes to the classification of Myxosporea: i) Ortholineidae is dismantled and Ortholinea spp. transferred to Myxobilatidae, and ii) Myxobilatidae is transferred from suborder Variisporina to Platysporina.