Not True Corals at All
Hydrocorals are one of the reef’s most common cases of mistaken identity. They look like corals — they form calcium carbonate skeletons, they grow on reefs, and they contribute to reef structure. But they are not corals. Hydrocorals are hydrozoans, members of the class Hydrozoa, which places them closer to jellyfish and Portuguese man-of-war than to true stony corals.
The distinction matters because hydrocorals have different biology, different stinging cells, and — in the case of fire coral — a far more potent sting than most true corals.
The Two Main Groups
Millepora (fire coral) is the most ecologically and medically significant hydrocoral. Despite its name, it is not a coral. Fire coral forms branching, plate, or encrusting colonies covered in a smooth, yellowish-brown surface with tiny pores. The pores house two types of polyps: feeding polyps (gastrozooids) and stinging polyps (dactylozooids). The stinging polyps contain nematocysts — stinging cells — that are significantly more potent than those of most true corals. Contact with fire coral causes an immediate, intense burning sensation (hence the name), often followed by red welts and localized swelling.
Fire coral is extremely common on Indo-Pacific and Caribbean reefs, often colonizing dead coral skeletons and reef structures. It is responsible for more diver and snorkeler stings than almost any other reef organism, primarily because it is abundant, widely distributed, and easily confused with true coral.
Stylasteridae (lace coral) are the second major hydrocoral group — delicate, branching colonies that resemble sea fans or branching corals. They are found in deeper water (typically 20–200+ meters) on overhangs, in caves, and on steep reef walls. Unlike fire coral, lace corals do not have potent stinging cells and pose no threat to divers. They are often brilliantly colored — purple, pink, orange, white — and are prized subjects for underwater photographers.
Role on the Reef
Fire coral is an important reef builder in its own right, contributing calcium carbonate to reef structure. It is also an early colonizer of damaged reef areas and is often one of the first organisms to reestablish on bleached or broken reef surfaces. Its resilience to disturbance makes it ecologically significant in an era of increasing reef stress.
Frequently Asked Questions
Is fire coral dangerous?
Contact causes a painful burning sting and welts that can last days. It is not typically life-threatening to healthy adults but can cause significant discomfort and localized skin reactions. People with allergies should be cautious.
How do you treat a fire coral sting?
Rinse with seawater (not fresh water, which can activate remaining nematocysts), remove any visible tentacle fragments, apply hydrocortisone cream for inflammation, and take antihistamines if available. Avoid rubbing the affected area.