The sea cucumber-infecting parasite Apostichocystis gudetama gen. nov. sp. nov. expands marine-host-specific clade of Apicomplexan Parasites

Unknown ellipsoid bodies, later classified as apicomplexan cysts, are heavily prevalent in the ovaries of hatchery-reared Japanese Sea Cucumbers (Apostichopus japonicus), where they can lead to lower fecundity in infected individuals and adverse effects to hatchery operations, impacting recovery and aquaculture efforts for this endangered species. Apicomplexans are widespread and essential to marine environments, where they can significantly affect the health and fitness of host populations. We performed deep genomic sequencing of recovered cysts to gain more ecological and evolutionary information on this cryptic parasite. From this, we were able to recover the complete rrn operon, the entire mitochondrial genome, and a partial apicoplast (relic chloroplast) genome of this apicomplexan. The rrn operon phylogeny revealed this parasite as being closely related to coccidian-like parasites of marine fish (ichthyocolids) and cnidarians (corallicolids), while organelle phylogenomics hint at a closer relation to the protococcidian Eleutheroschizon. Using this new phylogenetic context along with previous morphological descriptions, we describe this parasite as Apostichocystis gudetama gen. nov. sp. nov. Mining publicly available microbiomes reveal the presence of Apostichocystis spp. beyond its host range, alluding to other potential hosts or cryptic, closely related lineages. Its phylogenetic placement has important implications concerning the evolution of parasitism within Apicomplexa and the divergence of a marine-host-specific clade of coccidian-like parasites.