Miliolidium n. gen, a New Symbiodiniacean Genus Whose Members Associate with Soritid Foraminifera or Are Free‐Living

Abstract The dinoflagellate family Symbiodiniaceae comprises numerous divergent genera containing species whose ecologies range from endosymbiotic to free‐living. While many associate with invertebrates including corals, sea anemones, jellyfish, giant clams, and flatworms, others occur within the cytoplasm of large protists, most notably benthic foraminifera in the sub‐family Soritinae. Recent systematic revisions to the Symbiodiniaceae left out formal naming of some divergent lineages because each lacked a representative type species to erect new genus names. Here we provide genetic, morphological and ecological evidence to describe a new genus and species. Miliolidium n. gen. is closely related to the genus Durusdinium and contains several genetically divergent ecologically distinct lineages found in distant geographic locations indicating an Indo‐Pacific wide distribution. One of these, Miliolidium leei n. sp., is represented by an isolate cultured from Amphisorus sp. originally collected in the Gulf of Eilat, northern Red Sea. Its peripheral chloroplast extensions are uniquely petal‐ or lobe‐shaped, and cells possess a pyrenoid with three stalks connecting to chloroplasts, and without thylakoid intrusions. It is related to an isolate cultured from an azooxanthellate sponge from Palau and another that is commonly harbored by the soritid Marginopora vertebralis in shallow reef habitats from Guam. Research on Symbiodiniaceae diversity including free‐living species in benthic habitats and those mutualistic with soritid foraminifera remains extremely limited as does our knowledge of their diversity, physiology, biogeography, and ecology.