A new gallbladder parasite Ceratomyxa costaecola n. sp. (Cnidaria: Myxosporea) infecting the Goldblotch grouper Epinephelus costae (Serranidae) from Tunisian waters.

A new species of Ceratomyxa Thélohan, 1892 named Ceratomyxa costaecola n. sp. was recorded in the gallbladder of the Epinephelus costae (Steindachner) fish found in the Gulf of Tunis, Tunisia. Plasmodia of this species were disporic, ellipsoidal in shape and measured between 13 and 16 μm in length and 20-24 μm in width. The mature myxospores were elongated, crescent-shaped with convex anterior and concave posterior angle, measuring 6.3 ± 0.5 (5.8-7.0) μm in length and 24.7 ± 2.3 (22.0-27.0) μm in thickness. Myxospores with two unequal shell valves, the longer one with a rounded end and the shorter one with a pointed end. Polar capsules were pyriform, equal in size, measuring 2.5 ± 0.1 (2.4-2.8) μm in length and 2.2 ± 0.1 (2.1-2.4) μm in width, with 4-5 coils of polar filament measuring 14.7-26.0 μm long after extrusion. The binucleated sporoplasm filled the entire cavity of the myxospore. The phylogenetic analysis showed that C. costaecola n. sp. was closely related to Ceratomyxa species infecting Epinephelidae fishes and also other fishes families from different localities.