Ultrastructure and phylogeny of Ceratomyxa diplodae (Myxosporea: Ceratomyxidae), from gall bladder of European seabass Dicentrarchus labrax.

The myxosporean parasite Ceratomyxa diplodae Lubat et al. 1989 sensu Sitjà-Bobadilla & Álvarez-Pellitero, 1993, originally described from the annular seabream Diplodus annularis in the Adriatic Sea, has subsequently been reported from several other sparid hosts, and also a moronid fish, the European seabass Dicentrarchus labrax from the Mediterranean Sea. Here, molecular identity and additional morphological data are given for this parasite infecting the gall bladder of D. labrax in a southern Portuguese fish farm. In the bile, disporic plasmodia were spherical to subspherical with a smooth surface membrane. Most myxospores were crescent-shaped, 5.1 ± 0.5 (4.8-6.7) µm long (mean ± SD) and 21.9 ± 1.0 (20.4-23.9) µm thick; a few were more arcuate, 5.7 ± 0.4 (5.3-6.3) µm long and 17.3 ± 1.0 (16.3-19.1) µm thick. The wall consisted of 2 symmetrical valves united along a slightly curved suture line, with moderately tapering to rounded ends. Two spherical polar capsules, measuring 2.9 ± 0.3 (2.5-3.4) µm in diameter, contained a polar filament forming 8 to 9 coils organized in 2 rows. Host species, tissue tropism, and myxospore morphology supported species identification. Phylogenetic analyses of the small subunit ribosomal RNA sequence positioned the parasite among most sparid-infecting Ceratomyxa spp., suggesting the existence of a common ancestor for these species. The acquisition of molecular data from infections of C. diplodae in its original host and in other sparids is essential in order to ascertain if the morphological and biological variations found among reports of this parasite are intra- or inter-specific.