hydrocorals

Hydrocorals are colonial marine cnidarians belonging to the hydrozoan families Milleporidae (fire corals) and Stylasteridae (lace corals). They secrete a calcareous skeleton and can resemble true reef-building corals, but they are not scleractinian corals. The polyps form colonies within a shared calcium carbonate framework and usually exhibit smaller, more numerous individuals than many hard corals.

The skeleton of hydrocorals is typically aragonitic and supports a variety of colony forms, including branching, fan-like, and plate-like structures. Each polyp lives in a calyx and extends tentacles to capture plankton. Fire corals in Milleporidae are especially known for their potent nematocysts, which can sting swimmers and cause a burning sensation, contributing to their common name. Lace corals (Stylasteridae) often have intricate, lacy skeletons and can occur at various depths.

Distribution and habitat vary by group. Hydrocorals occur in tropical and subtropical seas around the world. Milleporidae are found in warm, often shallow waters of both the Atlantic and Indo-Pacific, while Stylasteridae are highly diverse in the Indo-Pacific and western Atlantic. They contribute to reef structure in some regions and provide habitat for small reef organisms, though they generally occur less prominently than reef-building scleractinian corals.

Life history features reflect their hydrozoan affinity. Hydrocorals have a polyp-dominated colonial stage; many species reproduce sexually with planula larvae and also propagate asexually by budding. Some species may have reduced or absent free-swimming medusae as adults, distinguishing them from many true corals. Human interactions are most notable for the stinging fires of Milleporidae.