Myxozoan survey of thicklip grey mullet Chelon labrosus reinforces successful radiation of Myxobolus in mugiliform hosts

A myxozoan survey was performed on specimens of thicklip grey mullet Chelon labrosus (Risso) captured from the Douro River estuary, northern Portugal. Eleven new species, all belonging to the genus Myxobolus Bütschli, 1882 (M. abdominalis n. sp., M. aestuarium n. sp., M. caudalis n. sp., M. chelonari n. sp., M. cucurbitiformis n. sp., M. douroensis n. sp., M. intestinicola n. sp., M. invictus n. sp., M. labicola n. sp., M. peritonaei n. sp., and M. pinnula n. sp.) are described based on microscopic and molecular data, confirming the known high radiation of these myxozoans in mullets. Additionally, Myxobolus pupkoi Gupta et al., 2022 is reported for the first time from C. labrosus, bringing forth a novel case of morphological plasticity between geographic isolates. We consider that molecular-based comparisons are imperative for the description of mugiliform-infecting Myxobolus, with distance estimation further matching two of the novel Myxobolus spp. with sphaeractinomyxon types previously reported from another Portuguese estuary. This finding supports sphaeractinomyxon as specific life cycle counterparts of Myxobolus that infect mullets. Phylogenetic analyses of 18S rDNA retrieved a monophyletic clade of mugiliform-infecting myxobolids comprising well-supported lineages of species parasitizing mullets from the genera Chelon, Mugil, Crenimugil, and Planiliza. The existence of more than one Chelon- and Planiliza-infecting lineage reveals that myxobolids parasitized members of these genera multiple times during their evolution. Lastly, the elevated number of unmatched sphaeractinomyxon sequences included in the Chelon-infecting lineages clearly shows that Myxobolus diversity hosted by this genus remains underrated.