hexacorallia

Hexacorallia is a major subgroup of Anthozoa within the phylum Cnidaria. It includes a variety of sea-daring, colonial, and sessile marine animals such as sea anemones, hard corals, corallimorphs, tube anemones, and related groups. Members are generally polyps that retain a simple body plan throughout life and lack a free-swimming medusa stage in the life cycle.

A defining feature of Hexacorallia is hexamerous organization. Adult bodies typically have six or multiples of six tentacles and a radial arrangement of internal septa (the mesenteries) in multiples of six. This hexameral symmetry contrasts with the octomerous organization seen in the other major anthozoan group, Octocorallia. Many hexacorallians also possess a gastrovascular cavity subdivided by these septa, aiding digestion and circulation. Several members form calcareous skeletons, while others are soft-bodied.

The group encompasses several important lineages. Scleractinia includes stony corals that build reefs; Actiniaria comprises solitary and occasionally colonial sea anemones; Corallimorpharia, often called corallimorphs, resemble corals but lack a rigid skeleton; Zoantharia includes various zoanthids and related colonial forms; Ceriantharia are tube-dwelling anemones with long, flexible bodies. Ecologically, many reef-building scleractinians harbor photosynthetic symbionts (zooxanthellae) in shallow waters, contributing to primary production and habitat structure. Reproduction occurs via sexual spawning and asexual methods such as budding or fragmentation, varying by lineage.

Taxonomically, classification of Hexacorallia has varied with advances in morphology and molecular data, but the group is consistently recognized as a key component of Anthozoa.