Massive Starlet Coral

Siderastrea siderea is a colonial coral that forms low domes or boulder-shaped structures with a smooth dimpled surface as much as 2 metres (6 ft 7 in) wide on the seabed. It can be encrusting when young. The corallites, the calcareous cup-shaped depressions in which the polyps sit, are about 5 millimetres (0.20 in) wide with about 50 or 60 little ridges called septa. The general colour is reddish brown. This species can be confused with the closely related lesser starlet coral (Siderastrea radians) but that is usually smaller and has deeper, more angular corallites, each with 30 to 40 septa.

Siderastrea siderea is found in the Caribbean Sea and the northern Gulf of Mexico and round the coasts of southern Florida, the Bahamas and Bermuda. It can occur at depths of up to 40 metres (130 ft) but is most common in less than 10 metres (33 ft) of water. It is found on rocks in various reef environments but not in tidal pools or muddy areas. In the IUCN Red List of Threatened Species, the massive starlet coral is listed as being critically endangered. It is moderately susceptible to coral bleaching but is capable of recovering when conditions improve.