First description of Chloromyxum squali Gleeson and Adlard, 2012 (Myxozoa) in the Mediterranean Sea in a new host Squalus blainville (Chondrichthyes: Squalidae): morphological, ultrastructural and phylogenetic data

Chloromyxum squali Gleeson and Adlard, 2012 originally described from Squalus acanthias Linnaeus, 1758 is reported for the first time from the gallbladder of Squalus blainville (Risso, 1827) caught from the eastern coast of Tunisia. In the present study, this chloromyxid was described based on morphological and ultrastructural features combined with molecular analysis of 18S rDNA. Young plasmodia were found attached to the gallbladder, while mature plasmodia and myxospores were observed floating free in the bile. Mature plasmodia were polysporic, subspherical in shape, measured 97.8 ± 0.5 µm long and 63.4 ± 0.4 µm wide. Mature myxospores were ovoid with a pointed anterior end, measuring 10.2 ± 0.5 µm long and 8.3 ± 0.5 µm wide. Two asymmetrical shell valves adhered together along an S-shaped suture line. Each valve has 5–7 elevated surface ridges parallel to suture line. A bundle of long caudal filaments extended from the basal end of shell valves. Four pyriform polar capsules equal in size, measuring 3.1 ± 0.4 µm long and 2.5 ± 0.3 µm wide, were situated at the same level in the anterior pole of the myxospore, each with a polar filament coiled in 7–8 turns. Pairwise comparisons among the SSU rDNA sequences revealed significant similarity between Chloromyxum squali infecting S. acanthias with the sequence obtained in this study. Phylogenetic analysis revealed that C. squali clustered in the clade of Chloromyxum species infecting the gallbladder of marine Chondrichthyes. Chloromyxum squali showed a seasonal variation of prevalence with significantly higher prevalence noted in summer and in autumn and absence of infection in winter.