Zschokkella hildae Auerbach, 1910: phylogenetic position, morphology, and location in cultured Atlantic cod

The myxozoan Zschokkella hildae Auerbach, 1910, was detected with a prevalence of 100% in cultured Atlantic cod, Gadus morhua L. aged 1+ from a culture facility on the west coast of Scotland. Sporogonic stages of Z. hildae, plasmodia producing 2-5 mature spores, were located predominantly in the collecting ducts and ureters of the kidney, and spores were present in the urine collected from the bladder. Less frequently, plasmodia were detected in the interstitial tissue of the kidney. The parasite prevalence in cultured fish was considerably higher than reported in wild fish but no obvious signs of pathology were detected. SSU rDNA sequencing and phylogenetic analysis showed that Z. hildae is closely related to a Sinuolinea sp. from the urinary system of turbot, Psetta maxima (L.), and that these two species, together with other myxozoans from the urinary system of marine fish cluster together in a sub-clade of the recognised marine clade of myxozoans. This sub-clade is characterised by a specific linear expansion segment, helix E23_15 in the secondary structure of variable region V4 of the SSU rDNA. Z. hildae and Sinuolinea sp. show extraordinary large linear expansion segment in both V4 and V7 and an important number of complementary base changes in the conservative regions of the SSU rDNA, indicating considerable evolutionary changes in the SSU rDNA of these species when compared with other myxozoans from the marine environment.