Euduboscquella costata n. sp. (Dinoflagellata, Syndinea), an Intracellular Parasite of the Ciliate Schmidingerella arcuata: Morphology, Molecular Phylogeny, Life Cycle, Prevalence, and Infection Intensity.

The syndinean dinoflagellate Euduboscquella costata n. sp., an intracellular parasite of the tintinnid ciliate Schmidingerella arcuata, was discovered from Korean coastal water in November of 2013. Euduboscquella costata parasitized in about 62% of the host population, with infection intensity (= number of trophonts in a single host cell) ranging from 1 to 8. Based on morphology and nuclear 18S ribosomal RNA gene sequences, the parasite is new to science. Euduboscquella costata n. sp. had an infection cycle typical of the genus, but had morphological and developmental features that distinguished it from congeneric species. These features include: (1) episome of the trophont with 25-40 grooves converging toward the center of the shield; (2) a narrow, funnel-shaped lamina pharyngea extending from the margin of the episomal shield to the nucleus; (3) persistence of grooves during extracellular development (sporogenesis); (4) a single food vacuole during sporogenesis; (5) separation of sporocytes early in sporogenesis, regardless of type of spore formed; and (6) dinospore size (ca. 14 μm in length) and shape (bulbous episome with narrower, tapering hyposome). After sporogenesis, E. costata produced four different types of spore that showed completely identical 18S rRNA gene sequences. The gene sequence was completely identical with a previously reported population, Euduboscquella sp. ex S. arcuata, from Assawoman Bay, USA, indicating that the two populations are likely conspecific. Favella ehrenbergii, a widely recorded tintinnid known to host Euduboscquella spp., co-occurred with S. arcuata, but was not infected by E. costata in field samples or during short-term, cross-infection experiments.