Microscopic and molecular procedures are used to describe a new myxosporean species, Chloromyxum clavatum n. sp., infecting the cartilaginous fish Raja clavata Linnaeus, 1758 (Chondrichthyes: Rajidae), collected from the northwest Atlantic coast of Portugal. Young plasmodia and mature spores were found floating free in the gall bladder of R. clavata. Spores were spherical to subspherical with a pointed anterior end, measuring14.4 ± 0.5 μm (n = 25) in length, 11.9 ± 0.5 μm (n = 25) in width, and 9.4 ± 0.5 μm (n = 15) in thickness. The spore's wall was composed of 2 equally sized valves, each displaying 6–8 elevated surface ridges and a bundle of several tapering caudal filaments attached to the basal portion. Spores contained 4 pyriform equally sized polar capsules (5.5 ± 0.4 μm × 2.9 ± 0.5 μm) (n = 25), each possessing an obliquely arranged isofilar polar filament coiled in 7–8 coils. Morphological data, host specificity, tissue tropism, and molecular analysis of the SSU rDNA gene identify this parasite as a new species of Chloromyxum. Neighbor-joining and maximum likelihood further reveal the parasite clustering with other species of Chloromyxum infecting the gall bladder of marine cartilaginous fish to form a clade positioned at the base of the freshwater clade, therefore constituting an exception to the major division of the class Myxosporea into the freshwater and marine clades, while supporting the existence of a correlation between tissue tropism and myxosporean phylogeny.

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