Venomous Corals: The Fire Corals

All Fire corals are calcareous hydrozoans and belong to the genus, Millepora. The other calcareous hydrozoans, including the azooxanthellate lace corals, species of Stylaster and Distichopora, are closely related to Millepora, but are put into a different taxonomic group. Being hydrozoans rather than anthozoans, Millepora are not true stony corals, although their abundance does allow them to be a major contributor to reef structure and sediments. Fire corals are found in tropical seas, worldwide, predominantly on reef crests and in shallow waters subject to high water movement. For an unknown reason, they are conspicuously absent from the reefs of Hawaii. Rapid proliferation by fragmentation allows rapid growth and encrustation of Millepora on the windward reef crests. Upright branching and vertical plating growth forms are found in areas removed from severe wave action.

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Millepora sp. in Ken Gosisnki's aquarium. These corals thrive in strong light and water flow in the aquarium.. Photo courtesy of Ken Gosisnki (mustang).

This Distichopora sp. resembles Fire coral, and is related to them. However, it does not have zooanthellae and lacks the potent sting. Photo by Eric Borneman.

Unfortunately, the taxonomy of the Fire corals is in need of revision and is not particularly useful. There are at least 48 reported species of Millepora throughout the world. However, because of inter-specific variations of form and color due to different lighting intensities, water movement, and other environmental factors, it is not known how many distinct species are present. In the Caribbean, three types of Fire corals predominate: a branching or crenelated form (M. alcicornis), a plate-like or flat-topped form (M. complanata), and an encrusting or box-like form (M. squarrosa). Similar morphological types are found in the Indo-Pacific regions, although the regional differences are more varied, and more species (both classified and unclassified) are reported from these oceans. Commonly seen Fire corals from these regions include encrusting, clavate, blade-like, upright, and branching calcareous growth patterns. A behavior common to encrusting Millepora species worldwide is a tendency to completely encrust living sessile organisms, particularly gorgonians. The result is a shape that is often taken to be representative of the Millepora species, when in fact it is representative of the shape of the encrusted object. Given the varied growth forms of these hydrozoans, it may be surprising to learn that the living animals are so similar.

A digitate or branched form of Millepora that very much resembles the Australian Millepora sp. in a photo below. However, this is one of the other three species of Millepora in the Caribbean, M. alcicornis. Here, it is found in protected waters of a seagrass covered lagoon at the rhomboid reefs of Belize. Photo by Eric Borneman.

Another Caribbean species of Millepora, M. complanata, shown here in its plastic-like growth form, beginning as an encrusting colony and forming upright vertical fronds. This is often the most common Millepora in the Caribbean and is found in exposed, as well as protected, areas. Here it is found on a patch reef near Portsmouth, Dominica. Photo by Eric Borneman.

One of the three Caribbean species, this is the most easily identified Millepora species, M. squarrosa. It is named for the squarish indentions on its surface. Its color is nearly always pinkish, and it tends to be found in areas where it very much resembles coralline algae. Upon touching, however, it is very apparent that it is not coralline algae. Photo by Eric Borneman.

Anatomy and Behavior

Although they are more closely related to the typical hydroids that produce jellyfish, the Fire corals certainly superficially resemble stony corals. They are calcareous, although their skeleton, unlike that of stony corals, almost completely encloses the living polyps. Despite the often-textured pattern of the skeleton, the actual skeletal surface is quite smooth. Upon close examination, tiny pinholes, or pores, can be seen scattered across the surface, or coenosteum. Understandably, Millepora means "many pores." There are three types of pores dotting the coenosteum: gastropores, dactylopores, and ampullae.

The gastropores contain the gastrozoids, or feeding polyps. These polyps are "short and plump, containing from four to six tentacles that are reduced to nematocyst knobs (Hyman, 1940)." The gastrozoids are completely retracted into the skeleton, and rarely emerge. If seen at all, they tend to form a white fuzzy film over the coral surface. The gastrozoids are connected beneath the skeleton in a network or canal system (secreta) connected by plates, called coenosarcs which allow nutrient movement within the colony. That nutrient is provided predominantly from the capture of small planktonic animals. The coenosarcs and coenosteum are areas of Fire coral heavily laden with zooxanthellae. While these symbiotic algae may be found elsewhere on the coral surface and within the living polyps, the concentration around these areas greatly enhances the ability of the polyps to meet their energy requirements in a most efficient manner since this is the area most constantly exposed to sunlight. Approximately 75% of the daily carbon required by Millepora is provided by its symbionts.

The surface of a Millepora skeleton, lacking the corallites of stony corals. The pores that are visible are mainly those housing the hair-like feeding polyps, the dactylozooids. Cyclosystems are not present on this specimen. Photo by Eric Borneman.

The most often visible structures of Fire corals are the short, thin, hollow, potent stinging tentacles of the mouthless dactylozoids. Dactylozoids, looking like fine transparent hairs, are very important functionally; these nematocyst-laden tentacles provide for both the corals' defense and their primary method of food capture. They are also a primary means of aggressive takeover of territory occupied by other species, along with the overshadowing and crowding out of nearby competitors through rapid growth. Although sometimes randomly scattered across the surface, there is often a pattern of five to nine dactylopores that surround each gastropore. This arrangement is called a cyclosystem, and it serves to increase the efficiency of nutrient transfer by allowing the surrounding feeding tentacles to flex and bring prey to the central gastrozooid. The dactylozoids are equipped with three types of nematocysts: stenoles, isorhizas and the unique macrobasic mastigophores, found only in Millepora species. Because of this limited distribution, macrobasic mastigophores are the definitive characteristic used in their taxonomic classification.

The many varied forms of Millepora sp., shown here as skeletons. Top left, M. complanata; top right, M. squarrosa; bottom left, M. dichotoma; bottom right, M. alcicornis. Photo by Eric Borneman.

The identification of Fire corals by color is not always easy. They tend to look like dead corals, since there is no visible tissue, fleshy polyps, or mucus. Furthermore, there are no defined cups (corallites) as found on the true stony corals (scleractinians). However, all species tend to have a characteristic white edge or tip that serves to warn the unwary that these corals are not to be touched. Blue coral (Heliopora coerulea), related hydrocorals such as the Lace corals (Stylaster spp. and Distichopora spp.), and several other true stony corals may have similar white edges or smooth surfaces. Some common colors seen in Fire corals are cream, brown, green, yellow, and purple, but mustard brown is by far the most common variation.

A bladed growth form of Millepora sp. in an atoll lagoon at 20m depth, Sulawesi, Indonesia. Here, large fields of isolated colonies were the predominant coral on the sandy sloping bottom. This area receives tremendously fast water flow during tidal changes. Photo by Eric Borneman.

For the most part, reproduction of Fire corals takes place sexually. Asexual reproduction by budding results in small medusae with four or five nematocyst knobs being released by the colony from the third pore type, the ampullae. The medusae die within hours of free life, but only after sex cells are formed and released. The union of sperm and egg results in free swimming planulae, which soon attach to a substrate to form new colonies. Fragmentation of the main colony is another possible method of asexual reproduction, and most imported specimens arrive as pieces broken off a larger colony.

Captive Care

Fire corals have gained popularity in reef aquariums, although they are not commonly offered and many aquarists who have them, find that they spread quickly and can be difficult to control. Intense light and high current are necessary for maximum growth rate and survival. The hair-like tentacles will extend night and day to feed on plankton, but can be completely retracted into the skeleton if needed. In fact, waving an object over a colony will often cause complete retraction of all stinging hairs, and will thus eliminate any potential for the dreaded "burn" of Fire coral. Given their capacity to sting, it may be surprising to observe how many fish and shrimp take refuge among the recesses and branches of Fire coral. Hawkfish, because of their "skinless" pectoral fins, are often found perched high atop Fire coral colonies with utter disregard for the potentially dangerous nematocysts. For humans, however, the sting can be quite painful to sensitive skin. Yet, touching these corals with the hand or fingers usually does not cause any burning sensation, since the nematocysts cannot penetrate thicker skin.

Because there are no visible living polyps, it can be difficult to ascertain the health of a specimen in a tank. Other than through growth and a visually maintained robust color without bleaching or algal overgrowth, little else may be apparent to ascertain the corals' health. Color changes may be related to improper light intensity or rapid change in light or water conditions. Because they possess zooxanthellae, proper acclimation is as important with these hydrozoans as with any stony coral.

Well equipped to capture prey, these corals will accept foods offered to them. However, prey sources are generally very small because of the tiny openings into which food must pass to reach the polyps. In this regard, they are not more or less difficult to care for than most scleractinians. In terms of predators and disease, Fire corals are very resistant to attack. Some fireworms (Hermadice carunculata) have been reported to graze on Millepora, although they prefer other corals. Other potential predators of Millepora include specialist nudibranchs of the genus Phyllidia, and filefishes from the genera Aluterus and Cantherhines.

It Burns, It Burns!!!!

Although it can be quite painful, a sting from Fire coral is rarely dangerous unless accompanied by an allergic reaction or anaphylactic shock. In fact, the most serious effects seen after extensive stings are possible nausea and vomiting for two to three hours afterwards. The sting caused by these animals is a result of the injection of a water-soluble, heat affected, proteinaceous toxin. The discharged nematocysts cause small welts on the skin with red lesions around the raised areas. Swelling, blisters, and pus-filled encystations may occur soon after being stung. However, all symptoms generally disappear after 24 hours. If stung, treatment consists of a breakdown of the protein by soaking the affected area in hot water, swabbing the welts with vinegar, or applying a paste of meat tenderizer. After initial treatment, topical anesthetics may be applied to ease the burning sensation. It may also ease suffering to repeatedly issue forth numerous expletives in a loud voice.

A digitate, or branched form of Millepora sp. on a protected shallow reef flat on the Great Barrier Reef, Australia. This particular species had an extremely strong sting, as I discovered from the pain of an exposed ankle accidentally brushing against a colony. Photo by Eric Borneman.

Several people have contacted me after having had itching or welts remaining on the skin for up to several weeks after being stung. If the venom is not quickly inactivated, it acts much as poison ivy does, causing a Type IV delayed hypersensitivity reaction. The rash can generally be treated with topical antihistamines like cortisone or Benadryl. Other complications that may arise are likely related to any cuts or scratches that may accompany the sting. As with all surfaces underwater, and especially corals, bacteria abound on them. Numerous species that are known human pathogens or capable of causing infection exist on these corals, and areas that become reddened, fail to heal, or red streaks visible under the skin are among a few symptoms that would indicate infection. In such cases, medical attention is imperative. I am not a physician, and offer the advice above based on many direct experiences with fire coral stings. Anyone should seek proper medical advice after marine injuries, and not depend on the treatment advice of untrained persons.

This Millepora sp., originally acquired from Larry Jackson, shows the encrusting nature of the coral. The original fragment was on a small square block seen in the lower center of the colony. It has since encrusted an entire rock and several smaller coral fragments. I keep it on a separate rock to help prevent it from spreading further, although it is not likely this will work in its current position. Photo by Eric Borneman.


Fire corals are a very natural addition to any reef tank, and will thrive if given proper conditions for their success and growth. The biggest problem with Millepora in the aquarium is the quick growth and ability of the corals to overgrow and encrust surfaces, with the subsequent difficulty of controlling or halting such growth and the loss of specimens which have been encrusted. Even if a careless brush with these corals can result in some unpleasantness, there is no real risk to most other tank inhabitants, such as fishes. In fact, the shelter for many creatures afforded by the colonies provides them with a natural safe haven. Because of their prolific nature and abundance in the wild, it seems almost inappropriate not to have a small colony as part of an indoor reef crest. They are often dominant hermatypes, easily propagated by fragmentation, and abundant so that little to no impact on wild populations occurs from wild collection. I am a big fan of Millepora in aquariums, just so long as my surfaces remain far away from its surfaces!

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Venomous Corals: The Fire Corals -