Scleractinian

Scleractinia, commonly known as stony or hard corals, are an order of marine cnidarians within the class Anthozoa. They secrete calcium carbonate skeletons, forming the rigid framework of most coral reefs (the skeletons are primarily aragonite). Most species are colonial polyps arranged in discrete cup-shaped corallites, often with sixfold symmetry, though solitary forms occur.

Many scleractinians host symbiotic dinoflagellates in the family Symbiodiniaceae (zooxanthellae) within their tissues. The symbionts perform photosynthesis, supplying energy that supports rapid growth in well-lit, shallow tropical waters. Non-photosynthetic species or those living in deep or murky habitats feed by capturing plankton and detritus.

Reproduction occurs both sexually and asexually. Sexually, many species release eggs and sperm in synchronized spawning events or brood larvae; asexual reproduction includes budding and fragmentation, enabling colony expansion and recovery. The life cycle often involves a free-swimming larval stage (planula) before settlement.

Ecologically, reef-building scleractinians are the principal architects of tropical reefs, creating three-dimensional habitats that support diverse marine life. They have a long fossil record, appearing in the Middle Triassic and subsequently becoming major reef builders through the Mesozoic and Cenozoic.

Threats include climate change and ocean acidification, which reduce calcification and can cause coral bleaching when heat stress drives away symbionts. Other pressures include pollution, overfishing, and disease. Conservation efforts focus on protecting reef habitats, reducing local stressors, and restoration to preserve scleractinian diversity and reef structure.