Hard coralsScleractinia

Hard corals (Scleractinia) are marine cnidarians that secrete a rigid, calcareous external skeleton, forming the diverse structures of reefs. Each animal is a polyp, typically a few millimeters wide in colonial forms, or up to 25 cm (10 in) in diameter in solitary species. Polyps have a cylindrical body with an oral disc, a central mouth, and surrounding tentacles. Colonies build complex formations, ranging from branching and encrusting to massive boulders and plate-like structures, often spanning several meters in diameter or height. Their colours vary, appearing brown, green, yellow, or pink, often due to symbiotic algae.

These corals constitute the dominant structural component of most shallow reefs. They thrive from sunlit reef flats to deeper slopes, occurring down to depths of 60 m (200 ft). Most species host symbiotic zooxanthellae, which live within their tissues and provide the majority of the coral's energy through photosynthesis. Polyps supplement this diet by extending their tentacles, primarily at night, to capture plankton from the water column.