Fine scale features and organelles found among Symbiodinium. (A) Close-up of divided cells of a tetrad from a culture of “S. corculorum” that is genetically similar to S. pilosum (both are type A2). This particular lineage is unusual among described Symbiodinium by possessing mucocysts, and chloroplasts with parallel and peripheral thylakoid arrangements. Flagella are seen in cross section in the space between daughter cells. (B) A thick cellulosic cell wall is observed for many cultured isolates. (C) TEM of a cross-section through a chloroplast lobe at the periphery of the cell with parallel and peripheral thylakoids grouped in sets of three.