Amazonian waters harbour an ancient freshwater Ceratomyxa lineage (Cnidaria: Myxosporea)

- A new Myxozoan species is described parasiting the gallbladders of a cichlid fish from Amazon basin.
- The new parasite has worm-like form plasmodia and exhibits a gradient of development.
- Freshwater Ceratomyxa former an early divergent clade within the broader Myxozoan marine lineage.
- The myxozoan radiation from marine to freshwater environments and the reverse may had happened in multiple occasions.
- The periods of marine incursion in the geological history of the Amazon may have played an important evolutionary role in Ceratomyxa genus.

A new species of Ceratomyxa parasitizing the gall bladder of Cichla monoculus, an endemic cichlid fish from the Amazon basin in Brazil, is described using morphological and molecular data. In the bile, both immature and mature myxospores were found floating freely or inside elongated plasmodia: length 304 (196-402) μm and width 35.7 (18.3-55.1) μm. Mature spores were elongated and only slightly crescent-shaped in frontal view with a prominent sutural line between two valve cells, which had rounded ends. Measurements of formalin-fixed myxospores: length 6.3 ± 0.6 (5.1-7.5) μm, thickness 41.2 ± 2.9 (37.1-47.6) μm, posterior angle 147°. Lateral projections slightly asymmetric, with lengths 19.3 ± 1.4 μm and 20.5 ± 1.3 μm. Two ovoid, equal size polar capsules, length 2.6 ± 0.3 (2-3.3) μm, width 2.5 ± 0.4 (1.8-3.7) μm, located adjacent to the suture and containing polar filaments with 3-4 turns. The small subunit ribosomal DNA sequence of 1605 nt was no more than 97% similar to any other sequence in GenBank, and together with the host, locality and morphometric data, supports diagnosis of the parasite as a new species, Ceratomyxa brasiliensis n. sp. Maximum parsimony and maximum likelihood analyses showed that C. brasiliensis n. sp. clustered within the marine Ceratomyxa clade, but was in a basally divergent lineage with two other freshwater species from the Amazon basin. Our results are consistent with previous studies that show Ceratomyxa species can cluster according to both geography and host ecotype, and that the few known freshwater species diverged from marine cousins relatively early in evolution of the genus, possibly driven by marine incursions into riverine environments.

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