Myxosporean parasites of Ceratomyxa merlangi and Myxidium gadi in whiting Merlangius merlangus: a comparative epizootiological analysis based on samples from two localities off southern and northern coasts of the Black Sea

Morphological and genetic analysis of Ceratomyxa saurida Zhao et al. 2015 and Ceratomyxa mai sp. nov. (Myxozoa: Ceratomyxidae) from the East China Sea

We describe Ceratomyxa saurida Zhao et al. 2015 and Ceratomyxa mai sp. nov. (Myxozoa: Ceratomyxidae) from the East China Sea. C. saurida was found in the gallbladders of 3/13 specimens of its type host, Saurida elongata Temminck and Schlegel 1846 (Aulopiformes). Myxospore characters were consistent with the original description to which we have added small subunit (SSU) rRNA gene data. C. mai sp. nov. was found in gallbladders of 3/13 specimens of S. elongata and 5/13 specimens of Neobythites sivicola Jordan and Snyder 1901 (Ophidiiformes). Mature myxospores of C. mai sp. nov. were crescentic in sutural view, with a deeply concave posterior angle 142.2±8.2° (125.8‒158.2°) and an arched anterior side. Shell valves were smooth and equal, 20.9±1.9 (17.3‒24.7) µm thick and 9.2±0.5 (8.1‒9.9) µm long, and joined at a straight, thin sutural plane passing between two nematocysts (polar capsules). The nematocysts were equal-sized, pyriform, 2.6±0.2 (2.4‒2.9) µm long and 2.7±0.2 (2.4‒3.3) µm wide, with their tapered ends pointed toward each other, located in the anterior third of the spore. Sequences of the SSU rRNA gene and internal transcribed spacer 1 showed that the isolates of C. mai sp. nov. obtained from S. elongata and N. sivicola were identical. The SSU rRNA gene sequence of C. mai sp. nov. was distinct from all known myxosporeans and clustered with C. saurida, and then with Ceratomyxa filamentosi Kalatzis, Kokkari and Katharios 2013, both of which also infect Aulopiformes fishes.

Molecular and morphological description of Ceratomyxa scophthalmi sp. nov. (Myxozoa) infecting Scophthalmus maeoticus and first report of Myxidium finnmarchicum in the Black Sea

In the present study, 2 species of myxozoan parasites, the novel Ceratomyxa scophthalmi sp. nov. and Myxidium finnmarchicum, were documented from the gallbladder of Scophthalmus maeoticus inhabiting the Turkish Black Sea coast at Sinop. C. scophthalmi sp. nov. had mostly spherical, clavate, and fine-grained polymorphic plasmodia that ranged in diameter from 16.0 to 29.0 µm. Spores were arcuate with a convex anterior margin and concave posterior. Valves of spores were highly elongated and unequal in thickness with one valve 1.1-1.3 times thicker than the other. Valves narrowed gradually toward slightly truncated ends. The mean (range) dimensions of spores were 9.4 (8.5-10.0) µm in length and 49.6 (44.5-55.0) µm in thickness, with polar capsules that were 3.1 (2.7-3.5) µm in length and 3.2 (2.7-3.5) µm in width. Polar filaments were coiled with 4-5 turns. Large numbers of immature forms of C. scophthalmi sp. nov., which were often 'crumpled', and mature forms were found together in the gallbladder bile. Along with morphological differences, phylogenetic analysis based on 18S rDNA, including pairwise nucleotide similarities with other related Ceratomyxa species, suggests C. scophthalmi as a novel species. Moreover, this report provides the first morphological and molecular descriptions of M. finnmarchicum outside of its original geographical location and type host species.

Host size influences prevalence and severity of Kudoa thyrsites (Cnidaria: Myxosporea) infection in Atlantic salmon Salmo salar

Kudoa thyrsites is a cosmopolitan myxozoan parasite of marine fish. The infection causes an economically important myoliquefaction in farmed Atlantic salmon in British Columbia, Canada. Laboratory exposure of Atlantic salmon smolts to infectious seawater was used to test the hypothesis that infection with K. thyrsites is more severe in age-matched, smaller salmon. In each of 2 trials approximately 4 mo apart, smolts were graded into small (80 and 68 g), medium (117 and 100 g) and large (142 and 157 g) initial weight groups (IWGs) and concurrently exposed to infectious seawater. The effects of IWG and time on fish size and infection severity were assessed by linear mixed-effects models. The fish were screened for infection by histological examination at intervals following exposure. Increases in mean length and weight were statistically significant in all IWG during both trials. The infection was detected in fish in both trials, and in Trial 2, the prevalence was significantly greater in larger fish 1000 degree-days (DD) after exposure. The severity of infection (plasmodia mm-2 muscle) was significantly higher in larger smolts: between medium and large IWGs at 2500 DD in Trial 1 and between small and medium IWGs at 1500 and 2000 DD in Trial 2. The hypothesis is rejected and possible explanations for the greater occurrence of K. thyrsites in larger smolts are discussed.