Hydrocorallina

Hydrocorallina is an informal taxonomic group comprising hydrozoans that secrete a calcareous skeleton, resembling corals. These colonial cnidarians are found in marine environments worldwide. While often called "hydrocorals," they are not true corals, which belong to the class Anthozoa. Hydrocorallina colonies are characterized by their rigid, branching, or encrusting skeletons.

The group is traditionally divided into two orders: Milleporida and Stylasterida. Milleporida, commonly known as fire corals, include species like Millepora alcicornis. They possess dactylopores, which house the feeding polyps (dactylozooids), and gastrozooids within their skeletons. Stylasterida, or lace corals, exhibit a more delicate and often branching skeletal structure. These colonies can be brightly colored and are found in both shallow and deep-sea habitats.

Reproduction in Hydrocorallina is complex, involving both sexual and asexual phases. They play a significant role in reef ecosystems, contributing to reef structure and providing habitat for other marine organisms. Some species, particularly fire corals, can deliver a painful sting to humans due to nematocysts present on their feeding polyps. Their skeletal remains can also contribute to the sedimentary budget of coral reefs.