Molecular Phylogeny and Surface Morphology of <italic>Thiriotia hyperdolphinae</italic> n. sp. and <italic>Cephaloidophora oradareae</italic> n. sp. (Gregarinasina, Apicomplexa) Isolated from a Deep Sea <italic>Oradarea</italic> sp. (Amphipoda) in the West Pacific

Abstract: In an effort to broaden our understanding of the biodiversity and distribution of gregarines infecting crustaceans, this study describes two new species of gregarines, <italic>Thiriotia hyperdolphinae</italic> n. sp. and <italic>Cephaloidophora oradareae</italic> n. sp., parasitizing a deep sea amphipod (<italic>Oradarea</italic> sp.). Amphipods were collected using the ROV Hyper‐Dolphin at a depth of 855 m while on a cruise in Sagami Bay, Japan. Gregarine trophozoites and gamonts were isolated from the gut of the amphipod and studied with light and scanning electron microscopy, and phylogenetic analysis of 18S rDNA. <italic>Thiriotia hyperdolphinae</italic> n. sp. was distinguished from existing species based on morphology, phylogenetic position, as well as host niche and geographic locality. <italic>Cephaloidophora oradareae</italic> n. sp. distinguished itself from existing <italic>Cephaloidophora</italic>, based on a difference in host (<italic>Oradarea</italic> sp.), geographic location, and to a certain extent morphology. We established this latter new species with the understanding that a more comprehensive examination of diversity at the molecular level is necessary within <italic>Cephaloidophora</italic>. Results from the 18S rDNA molecular phylogeny showed that <italic>T. hyperdolphinae</italic> n. sp. was positioned within a clade consisting of <italic>Thiriotia</italic> spp., while <italic>C. oradareae</italic> n. sp. grouped within the Cephaloidophoridae. Still, supplemental genetic information from gregarines infecting crustaceans will be needed to better understand relationships within this group of apicomplexans. [ABSTRACT FROM AUTHOR]