Hexacorallia: six-fold symmetric anthozoans including stony corals and anemones

Key characteristics

Hexacorallians are defined by a set of anatomical and developmental traits. Each individual is a polyp with tentacles typically arranged in single rows and an internal structure that reflects six-fold (or a multiple of six) symmetry. Many secrete a hard skeleton of calcite or a related calcium carbonate form, which in colonial species produces lasting reef frameworks. The life history commonly involves a mobile, planktonic larval stage followed by settlement into a sessile adult stage—part of a broader life cycle typical of cnidarians.

Representative groups and ecology

The subclass includes familiar animals such as stony corals, many of which are colonial and contribute to tropical reef building, as well as solitary and colonial forms like sea anemones, tube anemones, and zoanthids. Hexacorallians occupy ecological roles as reef architects, predators of small plankton and symbiotic hosts: several coral species harbour photosynthetic algae, which can supply significant energy to the host under favorable light conditions.

Classification and evolutionary history

Living hexacorallians are divided among several orders; current treatments recognize six living orders that encompass both reef-forming and solitary taxa. The group also has a long fossil record: extinct orders such as rugose and tabulate corals were major reef builders in the Palaeozoic era. This deep history shows both continuity and turnover, and highlights Hexacorallia's longstanding importance in marine ecosystems and sedimentary geology.

Distinctions and notable facts

Hexacorallia is contrasted with the other anthozoan subclass, Octocorallia, which have eight-fold symmetry and typically eight pinnate tentacles. In Hexacorallia the single-row tentacle arrangement and six-oriented internal septa are consistent morphological markers. Many hexacorallians are sessile as adults but retain a dispersive planktonic phase, whereas some taxa show reproductive and life-history adaptations that reduce dispersal and favour local persistence.

Importance and human relevance

Hexacorallians, especially stony corals, are central to coastal biodiversity and provide ecosystem services, including habitat formation, shoreline protection and tourism value. They are also indicators of environmental change: stressors such as warming, acidification and pollution can cause coral bleaching and mortality. Conservation and restoration efforts often focus on vulnerable hexacorallian species and the reefs they build.

• Typical features: polyp form, single tentacle rows, six-fold symmetry (symmetry).
• Life cycle stages: planktonic larvae to sessile adults.
• Economic and ecological roles: reef construction (reef), biodiversity, coastal protection.

For more specialized taxonomic treatments, fossil records and conservation status updates, consult dedicated resources and databases maintained by marine research institutions and taxonomic authorities (see Anthozoa and related entries for starting points).