Scleractinia: stony corals and the builders of coral reefs

Biology and key characteristics

Scleractinian polyps resemble small sea anemones and feed with tentacles bearing stinging cells. Many species are colonial, producing successive skeletal cups that remain attached to one another. The rigid skeleton is primarily aragonite, a form of calcium carbonate, secreted by the soft tissues over the animal's lifetime.

Most reef-building stony corals live in symbiosis with photosynthetic dinoflagellates, commonly called zooxanthellae (often assigned to the genus Symbiodinium). This symbiosis supplies much of the coral's energy and explains why many reef corals live in shallow, sunlit water. When stressed, corals may expel their symbionts, a process known as bleaching that reduces growth and can cause mortality.

Reproduction, growth and ecological role

Scleractinians reproduce both sexually and asexually. Many release gametes in synchronized spawning events, producing planktonic larvae (planulae) that disperse and settle to form new colonies; locally they expand by budding. Over generations their skeletons accumulate into the complex three‑dimensional structures that shelter countless fishes and invertebrates and support high biodiversity.

History, classification and distinctions

Fossil scleractinians appear in the geologic record from the Triassic period onward, making them important for palaeoenvironmental studies. Their nearest modern relatives are sea anemones (Actiniaria), while soft corals and 'fire corals' belong to different groups and are only more distant relatives. Contemporary taxonomies recognize roughly 1500 species distributed among about 35 families.

Human importance and threats

• Ecological: reefs provide habitat, food and coastal protection.
• Economic: fisheries, tourism and cultural value.
• Threats: ocean warming, acidification, pollution, overfishing and disease.

Conservation and restoration efforts seek to reduce local stressors, protect reef habitats and in some cases propagate corals for reef rehabilitation. Understanding scleractinian biology and their symbioses remains central to efforts to preserve these keystone marine organisms.