Characters: Colonies are spherical, 70-100 millimetres across. Corallites are polygonal, 2.5-3.5 millimetres diameter. Septa and walls are as described for Siderastrea savignyana except that there are 40-48 septa per corallite.
Colour: Light reddish-brown.
Similar Species: Siderastrea savignyana, which does not form spherical colonies and has fewer septa.
Habitat: Rocky substrates.
Taxonomic Note: Evidence is mounting that this species is Siderastrea siderea introduced from the Caribbean. For discussion see Forsman, Guzman, Chen et al. (2005), Forsman, Concepcion, Haverkort et al. (2010), LaJeunesse, Forsman and Wham (2016). Introduced species will be treated separately in a later version of this website.