Cryoxcellia borchgrevinki gen. nov., sp. nov., a new parasitic X-cell species in an Antarctic nototheniid fish, the bald notothen Trematomus borchgrevinki

X-cells were first described as an unknown cell type in northern hemisphere flatfish in 1969. Almost a decade later they were described in an Antarctic fish, the bald notothen Trematomus borchgrevinki , thus demonstrating their global distribution. Since this time, X-cells from various northern hemisphere fish species and from three other Antarctic fishes, the emerald notothen Trematomus bernacchii , the crowned notothen Trematomus scotti , and the painted notothen Nototheniops larseni have been identified as perkinsozoan parasites of the Family Xcellidae. Currently there are seven X-cell species described within this family. Here we report the morphology of X-cells isolated from the gill filaments of the bald notothen and include details of some of its division forms. Using short-read high-throughput DNA sequencing technology we have sequenced, assembled, and verified a 5347-bp region of the X-cell rRNA repeat unit that includes the complete 18S gene. In all cases, phylogenetic analyses identified this sequence as a distinct taxon and placed it among the perkinsozoan alveolates alongside other previously identified species in the X-cell family. Using a combination of morphological and genetic evidence we now describe a new X-cell genus and species, Cryoxcellia borchgrevinki gen. nov., sp. nov., from McMurdo Sound, Antarctica.