A new coelozoic Myxozoan species, Ceratomyxa tunisiensis n. sp., was found infecting the gallbladders of two carangid fish, Caranx rhonchus and Trachurus trachurus (Perciforme, Carangidae), from the Gulf of Gabès, on the southern coast of Tunisia. The parasite develops in spherical mono-, diplo-, or polysporic tropozoites in the gallbladder of the hosts. Mature spores are typical of the genus Ceratomyxa. They are transversely elongated and narrowly crescent-shaped with a slightly convex anterior and concave posterior, and measure 23 ± 0. 27 (20-25) μm width × 6 ± 0.26 (5-8) μm in length. Spore shell valves are symmetrical with rounded ends. Two spherical polar capsules situated on either side of the sutural line measure 2.2 μm (2.0-3.0) in diam. Periodical sampling of C. rhonchus and T. trachurus from Marsh 2012 to February 2013 showed that infection due to C. tunisiensis occurs in 59% and 69% of the examined fish, respectively. Molecular analysis based on the small subunit (SSU) rRNA sequence shows high genetic divergence with all other ceratomyxid species. A Maximum Likelihood phylogenetic tree shows association with the species C. leatharjecketi Fiala, kova, Kodadkova, Freeman, Bartošova-Sojkova, and Atkinson, 2015 reported from the gallbladder of Aluterusmonoceros (L.) caught in the Andaman Sea, off Malaysia. Nonetheless, the SSU rRNA sequences of C. tunisiensis and C. leatharjecketi have only a 90% similarity.
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