Fire Corals: Sneaky, Painful & Masters of Disguise

What are fire corals? Well, they’re not true corals, although their talent for stinging certainly lives up to the fire part of its name. The tiny animals behind them are hydrocorals, hydroids that build calcium carbonate dwellings. They’re closer to jellyfishes than to the stony corals they sort of resemble. 

Blade fire corals Millepora complanata
A master of disguise, fire coral assumes many shapes. This stuff, photographed in Bonaire, is probably blade fire coral (Millepora complanata). 

THE FIRST THING TO SAY ABOUT FIRE CORALS IS A WARNING: THEY LOOK TOTALLY INNOCENT. But touching them, presumably accidently, has caused many a diver pain.

Basically, they sit on the reef in plain sight, presenting smooth surfaces seemingly devoid of threatening tentacles or any other features. Encrusting versions are good at assuming the shapes of the organisms they overgrow, like sponges, sea rods and sea fans. And, coming mostly in hues of tan, mustard brown and greenish-yellow, they’re easily overlooked. Until you touch them.

Fire corals’ genus name Millepora – Latin for “a thousand pores” –  illustrates their divergence from stony corals. Stony corals’ polyps are each ensconced in their own easily seen corallite. But examine a fire coral closely and you’ll see a smooth calcareous exoskeleton riddled with pinprick-sized holes with barely visible hair-like threads protruding.

And, those threads are packed with powerful nematocysts, the tiny, barb-shaped structures that inject stinging toxins into the tissues of anything that comes its way.

Fire coral often lurks around other reef denizens, but sometimes it produces impressive structures of its own, like this stand of blade fire coral in Bonaire.


To be clear, fire corals’ goal with their stingers is to capture the microscopic zooplankton and phytoplankton that come their way – their nutritional targets – not to stalk humans.

Stony corals have the same technique of stunning and reeling in diatoms, fish larvae and the like. But while stony corals’ toxins are calibrated to match those tiny prey, fire corals’ stings are outsized and painful to animals much larger than the little organisms that comprise their planktonic diet.

Like stony corals, fire corals are colonial polyp animals that live within exoskeletons of calcium carbonate. But as much as they sort of look like stony corals on the outside, they’re different animals underneath.


Fire coral and stony coral are both members of Phylum Cnidaria (from the Greek word for “nettle”), the group of stinging  animals that includes corals, gorgonians and sea anemones (Class Anthozoa, “flower animal”), jellyfishes (Class Scyphozoa, “cup-shaped animal”) and box jellies (Class Cubozoa, “cube-like animal”).

But fire corals are in Class Hydrozoa (“water animal,”) along with stinging hydroids and siphonophores (“tube bearers”) like the Portuguese Man of War. Like stony corals, hydroids have cup-like bodies that serve as both mouth and anus, encircled by stinging tentacles.

tubularian hydroids (Tubularia crocea)
Fire corals are only one portion of the planet’s 2,700 varieties of hydroids. Hydroids as a whole come in body shapes ranging from the hard surfaces of  lace corals to these flower-like tubularian hydroids (Tubularia crocea), photographed in Boston’s outer harbor. 

Hydrozoans as a whole number about 2,700 species worldwide, in shapes that resemble algae, branching leaves, featherstars, sea fans and many other variations. Most hydroids are colonial and sessile – animals that live in interconnected groups, permanently anchored to the bottom. Some, like jellyfishes, are solitary and free-swimming. In terminology, bottom-dwellers are polyps, free swimmers are medusas.


With their penchant for generating stony-like structures, fire corals are referred to as hydrocorals – again, different from stony corals. Fire corals, in Family Milleporidea, Genus Millepora, amount to perhaps some 50 species worldwide, most of them in the Indo-Pacific-Red Sea basin. Actually, estimates of species are all over the place, from 15 to 24 to 48 reported species.

Theoretically, they come in several basic types, like blades, branches, ridges and boxes. But they’re aggressive and adaptable. Complicating identification is the fact that they have a maddening ability to overgrow existing structures, encrusting and assuming the shape of the underlying sea rod, sea fan, sponge, whatever.

encrusting hydrocoral
Encrusting fire coral assumes the shape of the organism it overgrows. This fire coral, mixed in with other sea rods, got me in the Bahamas. 

Out of the 50 species, the Caribbean has only a handful. For reasons nobody understands, Hawaii has none at all.

What all fire corals have in common, outwardly, is the coloration of the calcium carbonate superstructures – tans, yellow-greens, mustard-browns, generally with whitish edges or tips. They all have those smooth surfaces and those zillions of pin-size holes. And they all sting.

Many species get significant nutrition boosts from symbiotic zooxanthellae that photosynthesize solar energy into glucose. Due to their propensity for generating hard calcium matter, hydrocorals are considered important contributors to reef building.

The only other group of hydrocorals besides fire corals is lace corals, which don’t sting. They’re marked by intricate branch designs and red, pink and purple hues. This specimen was spotted in Indonesia. 


Like stony corals, hydrocorals live in colonies of interconnected polyps behind their solid exteriors. But the polyps of stony coral colonies are identical clones of each other, each several millimeters in size and occupying its own corallite.

Hydrocoral polyps are more likely to be one millimeter in size. And a hydrocoral structure is more often described by the three types of pores scattered across its smooth surface: dactyolpores (“fingers”), specializing in stinging and capturing prey; gastropores (“stomachs”), specializing in feeding; and ampullae (“flask-shaped”), specializing in reproduction.

Typically, five to nine dactyolpores surround each feeding polyp, fostering feeding efficiency. The stingers extend their tentacles, detect and stun passing planktonic prey with their toxins, cover them with a sticky mucus and pass them into the gastropores and the short, plump feeding polyps within, which themselves rarely emerge. Coenosarc canals connect them all, facilitating nutrient sharing.

While in true corals the photosynthetic zooxanthellae are embedded in polyps’ tissues, in hydrocorals they’re located in the coenosarcs rather than in the polyps themselves.

blade fire coral
Strangely, while fire coral’s mission is to sting and paralyze other reef organisms, some small animals actually make their homes in the protection of fire coral structures – without harm. Look closely and you’ll see a tiny blenny, species uncertain, at the base of this blade fire coral structure.


Reproduction begins with asexual budding by the ampullae – the reproductive polyps – which release small medusa into the water column. The free-swimming male and female medusa only live for a few hours but in that time release sperm and eggs. Fertilization results in free-swimming planula that eventually settle on suitable substrates. Each new polyp begins cloning itself by budding, starting a new colony.

How it grows is often reflects its environment: An area with light current is likely to see the growth of long strands of fire coral. Stronger currents result in thicker branches or blades. Very strong currents encourage encrusting over other structures.

Millepora dichotoma
Two fire corals from the Indo-Pacific basin. Above:, “net fire coral” (Millepora dichotoma). At right: “branching fire coral,” (M. alcicornis).

branching fire coral," (M. alcicornis)









Most often, a fire coral sting is painful but not serious. As with all cnidarians, the mechanism involves their tentacles’ release of nematocysts, barbed darts embedded by the thousands in tentacle tissues. Like arrowheads, they stick into the prey, injecting toxin to paralyze their victim’s tissues. While the nematocysts are drawn back into their capsules, the polyp’s tentacles move the captured prey to its feeding polyp.

Humans, of course, are not vulnerable to being drawn into fire coral gastropores but the stings still hurt. Depending on the degree of exposure, symptoms can range from mild itching, burning or blistering to extreme pain. The effect can last a few days to a week.


The first rule of dealing with fire coral would avoid it – stay far enough off the reef to minimize the possibility of contact. Be aware of its appearance and be alert. And, wear a wet suit, dive skin or rash guard to minimize exposed skin.

Secondly, skip all the folk remedies like soaking the affected area in seawater or urine or treating it with shaving cream. They don’t work and could make things worse.

Instead, first, avoid rubbing the area – it helps spreads the venom. The Mayo Clinic recommends rinsing the area with vinegar, carefully plucking out any visible tentacles with tweezers if possible, and soaking the skin in hot water.

Vinegar and a commercial spray called Sting No More® work to inhibit the spread of cnidae discharge. Of course, you have to think ahead to have these available. A small percentage of individuals may experience severe allergic reactions, like symptoms of shortness of breath, swelling of the tongue or throat. They should seek immediate medical attention.

You can read more about cnidarian stings in Nematocysts & the Science of Sting and How to Treat Stings: Jellyfish & Fire Coral.

PRINCIPAL SOURCES: Reef Coral Identification, Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Reef Creature Identification Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Les Wilk; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Marine Life of the North Atlantic, Andrew Martinez; Coral Reef Animals of the Indo-Pacific, Terrence Gosliner, David Behrens, Gary Williams; Fire Coral, Lamar University Dept. of Biology; Venomous Corals: The Fire Corals, Reefkeeping; Caribbean Coral Diaries – Fire Coral,; Cnidaria-Hydrozoa, Marine Education Society of Australasia; Fire Coral, Millepora alcicornis, Millepora complanata, Millepora dichotoma, Hydroid,, Wikipedia. Wikimedia Commons images are used under Creative Commons Attribution-Share Alike peermissions. 

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