Dinoflagellate Endosymbiont of the Soft Coral Stichodactyla haddoni Saville-Kent Collected from the Coast of Hormuz Island, Strait of Hormuz

An endosymbiotic dinoflagellate isolated from the soft coral Stichodactyla haddoni from the coast of Hormuz Island, Strait of Hormuz, Persian Gulf, has been characterized morphologically for the first time. Spherical and egg-shaped cells 6–13 μm in diameter (8–13 μm during mitosis) were recorded. Transmission electron microscopy showed the presence of intracellular organelles including a nucleus, a mitochondrion, chloroplasts, a pyrenoid, calcium oxalate crystals, starch granules, lipid droplets, and cell periplast. Abundant stored substances such as starch and fat droplets, as well as the calcium oxalate crystals, in the studied species were a response to the harsh environmental conditions (high salinity and temperature). The composition and arrangement of the intracellular organelles in Symbiodinium cells led to the conclusion that the studied dinoflagellate was cytologically very close to Symbiodinium goreaui Trench & Blank (clade C). In contrast to previous records, chloroplasts were widely dispersed throughout the cell. A molecular analysis has confirmed that the endosymbiotic dinoflagellate identified as S. goreaui is a member of clade C of the genus Symbiodinium . It is the first record of a symbiont species coexisting with marine invertebrates on the Iranian coast of the Persian Gulf.