The order Scleractinia, commonly known as stony corals or hard corals, comprises the cnidarians responsible for building coral reefs. Their most distinctive feature is the secretion of an external skeleton of calcium carbonate (aragonite), which provides structural support and protection to the polyp and, over thousands of years, accumulates to form vast reef structures. These organisms exhibit radial symmetry, a cylindrical polyp with stinging tentacles, and a close symbiotic relationship with dinoflagellate algae (zooxanthellae), which supply much of the energy required for their growth. Approximately 7,300 species of stony corals have been described, primarily distributed in tropical and subtropical waters, although some species inhabit cold and deep waters.
Morphologically, stony corals display great diversity of forms: from branching and bushy colonies, such as those of the genus Acropora, to massive and spherical forms, such as Porites, or large solitary species such as Fungia. Each polyp sits on a calcareous skeleton called a corallite, whose walls and septa constitute the main support structures. Tentacles, arranged around the mouth, possess cnidocytes with nematocysts that enable them to capture prey. Their coloration often depends on their symbiotic zooxanthellae, though coral pigments themselves can contribute to reddish, green, or blue hues depending on the species and environmental conditions.
In terms of feeding, stony corals are mixotrophic organisms. They obtain between 70% and 90% of their energy from the photosynthesis of zooxanthellae, which explains their predominance in clear, well-lit waters. However, they also capture plankton, larvae of other invertebrates, and dissolved organic matter using their tentacles and mucus secretions. Deep-water species, which lack zooxanthellae, rely exclusively on heterotrophic feeding, using ocean currents to obtain nutrients.
The sub-classification of the order stony corals reflects their ecological and morphological diversity. Currently, the order Scleractinia comprises 89 families, including Acroporidae and Poritidae, which form large branching or massive colonies that build tropical reefs. Massive and brain corals (families such as Faviidae and Mussidae) are characterized by slow growth and resistance to adverse environmental conditions. Finally, solitary and deep-water corals (families such as Caryophylliidae and Dendrophylliidae) include species such as Lophelia pertusa, capable of forming reefs in abyssal zones.
Hard corals play a crucial ecological role as ecosystem engineers, providing shelter, food, and breeding grounds for vast marine biodiversity. However, they face significant threats such as coral bleaching, caused by rising sea temperatures and ocean acidification, which disrupt their symbiosis with zooxanthellae. Studying them is essential not only for the conservation of coral reefs but also for understanding evolutionary and adaptive processes in changing marine environments.
