Marine Invertebrates of Bermuda

Fire Coral (Millepora alcicornis)

By Mitchell Doctor
Dr. James B. Wood- Editor

Abstract  Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links 

Fire Coral, Millepora alcicornis


Millepora alcicornis is a common member of Caribbean reefs and across the Bermuda platform. These hydrozoans are members of the family Milleporidae. These hydrocorals can be found in varying habitats including deep reefs, surf zone and reef flats but are restricted mainly to coral reefs. M. alcicornis gets its name fire coral from the painful stings it can inflict on SCUBA divers and snorklers. Setting Millepora apart from other hydrozoans is its ability to lay down a calcareous skeleton like those of the scleractinian corals. This ability makes Millepora the second most important reef builder in the reef ecosystem and lends the name of hydrocoral to Milleporidae. Millepores also have symbiotic zooxanthellae algae that lend their photosynthate as energy; however it makes the Millepores susceptible to coral bleaching. Millepora has pores at its outer layer that has individual parts called zooids which conduct specific tasks, like gastrozooid for feeding or the dactylozooids for defense. Colonies posses a varying range of morphologies dependent on habitat. Colonies may also grow attack branches that can directly overgrow nearby gorgonians (sea fans). Colonies reproduce by asexual fragmentation and are infrequent sexual breeders.


Phylum: Cnidaria
  Class: Hydrozoa
    Order: Capitata
       Family: Milleporidae

The surface of Millepora alcicornis is covered in zooids (Cairns 1982). Each zooid has a particular job. The gastrozooid is particularly used for feeding. It is where the prey is digested and into the body. These gastrozooids are surrounded by dactylozooids (Cairns 1982). These dactylzooids house the cnidarian stinging cells. The painful stings of M. alcicornis are inflicted using the cnidae, stinging thread, which are released from its cnidoblast. The cnidae stinging cells are a unique trait present in cnidarians and are well represented within the class Hydrozoa. The cnidae are mainly used for prey capture and defense. Prey will be stung and immobilized with these cnidae and the tentacles of the gastrozooid will carry the food to the mouth.

The morphology of Millepora alcicornis is also highly variable. It has the greatest range of growth morphologies of any other Millepora (Lewis 1989). It is believed that Millepora starts all of it growth forms with a universal beginning, encrusting (Lewis 1989). These encrustations occur at all depths and will appear to grow on any suitable substrate, like dead corals, ship wrecks, and the reef. Morphologies of M. alcicornis differ with the varying habitats it is found. For example, in a habitat with heavy surf, like on the edge of reefs, M alcicornis will exist in either a robust, plate-like, or bladed colonies (Lewis 1989). In lagoons, sheltered deep water reefs, and places where water movement is calm, M. alcicornis can be found as leafy, delicately branched and upright colonies that all flourish (Lewis 1989). M. alcicornis follows the overall trend of changing from plate-like forms at the shallow and exposed sites to thick-branching forms at deeper and sheltered locations (Kaandorp 1999). Colonies found to be living on vertical surfaces had larger encrusting bases, and longer perimeters and lower densities of branches than compared to those colonies on horizontal surfaces (Edmunds 1999). The growth rates on these two surfaces also vary due to the vertical surfaces increased area and the horizontal surfaces reduced area because of the spread of the Millepora (Edmunds 1999). The branches of the Millepora were not specialized in comparison to the encrusting bases in terms of the density of the dactylozooids and the gastrozooids (Edmunds 1999). The encrusting bases on vertical surfaces are likely to be large because of greater age instead of faster growth due to its high resistance to wave action and storm damage (Edmunds 1999). Those colonies on horizontal surfaces are likely to be small because they can be considered relatively young and short lived due to they possibility of their branches breaking off due to high wave action (Edmunds 1999). These findings show that the high success of M. alcicornis is a result of the consequences of their morphology. (Edmunds 1999). This supports that the slow growth and resistance to wave damage of encrusting bases exists on vertical surfaces and that the use of branches broken off can be used as asexual means of reproduction (Edmunds 1999).


Millepora are common members of Caribbean reefs (Stromgen 1976). More specifically, they occupy shallow coral reefs where they cover many different substrates (Edmunds 1999). Millepora is found at depths ranging from less than one meter to as deep as forty meters (Lewis 1989). Millepora alcicornis has the greatest depth range of any other Millepora. It can be found at depths as deep as fifty meters (Lewis 1989). M. alcicornis can be found throughout the reef system in Bermuda; at the forereef, lagoon, patch reef, bank reef, within the surf zone, and the reef flat (Lewis 1989).


Millepora alcicornis is a member of the hydrozoans. They display all the traits of a hydrozoan. However, Millepora in general have one very different aspect of all other hydrozoans. It relates greatly to the many morphological and ecological similarities between the Millepores and hermatypic corals. Millepores can secrete a calcareous skeleton, just as the hermatypic scleractinian corals (Lewis 1989). Millepores are the second most important reef builder on the reefs next to hermatypic corals (Lewis 1989). Another thing in common between the Millepores and hermatypic corals is that the Millepores also have symbiotic zooxanthellae algae (Verwoot 1966). Just not in corals, the zooxanthellae conduct photosynthesis and act the same way within the Millepores as they do hermatypic corals.


Millepora alcicornis in its branched form grows in a distinctive pattern. M. alcicornis colonies pursue near by gorgonians (sea fan). M. alcicornis can redirect growth of varying numbers of specialized branches horizontally toward a target gorgonian (Wahle 1980). These attack branches elongate away from the colony and form hand like structures that once in contact with the gorgonian, they encircle and encrust the gorgonian (Wahle 1980). This will ultimately result in the overgrowing of the entire gorgonian colony and more Millepora (Wahle 1980). Wahle’s (1980) study showed a very interesting characteristic about these attack branches. The attack branches were produced exclusively toward nearby gorgonians (Whale 1980). The observation was that these attack branches were never found to grow into open water or toward any other suitable substrata like alive or dead corals, sponges, algae, or other Millepora (Whale 1980). This observed specificity and direction of attack branches can not be attributed to random extensions to find substrata, but a deliberate response to a nearby gorgonian detected by water flow over the Millepora (Whale 1980). In this study, Millepora was shown to only respond to only alive gorgonians; showing that the Millepora can detect the gorgonians due to water flow (Whale 1980). This extension of the Millepora may be physically separated from the original colony, but is genetically identical (Whale 1980). This will increase the colonies probability of genotypic survival, thus creating the potential for unlimited colony size, and therefore increased fecundity (Whale 1980).


Millepora alcicornis can reproduce in a couple different ways. One way is fragmentation. In a storm, 79% of the branches were lost from a colony of M. alcicornis, and 4% re-attached to the substratum to produce new colonies (Edmunds 1999). This study directly shows how a piece of Millepora can be detached from the parent colony and re-colonize a sufficient substratum with a new colony that will be genetically identical to the parent colony. M. alcicornis are infrequent breeders (Lewis 1989). They disperse short lived medusae bearing gametes into the water column (Lewis 1989). Once fertilized, a planulae is formed and the life cycle reflects that of all Cnidarians.

Zooxanthellae and Bleaching

Millepora alcicornis possess symbiotic zooxanthellae algae like those of the hermatypic corals (Verwoot 1966). Zooxanthellae exchange their photosynthate to the Millepora for use as energy and in return the zooxanthellae have somewhere to live and thrive. However, the possession of zooxanthellae also means that M. alcicornis is susceptible to bleaching. Coral bleaching occurs when a coral, or hydrocoral like Millepora lose its zooxanthellae and/or its photosynthetic pigmentation (Cook et al. 1990). A bleaching period of M. alcicornis occurred in Bermuda in 1988 (Cook et al. 1990). It coincided with the longest period of elevated sea water temperatures Bermuda in the thirty-eight previous years (Cook et al. 1990). 1988 was the first recorded bleaching event in Bermuda ever (Cook et al. 1990). It was directly related to the period of elevated sea water temperature (Cook et al. 1990). The most affected species in the bleaching survey done over August and September of 1988 was M. alcicornis (Cook et al. 1990). The bleaching was more prevalent at sites in the outer rim reef than in the lagoonal patch reefs (Cook et al. 1990). By the time a survey was done again in December, the incidents of bleaching in M. alcicornis at the rim reef were significantly decreased (Cook et al. 1990).

Recent Research

Banaszak et al. (2003) studied two colonies of Millepora alcicornis in the Mexican Caribbean. One colony was exposed to high environmental light and another nearby colony was exposed to 5.9 times lower light levels (Banaszak et al. 2003). The colony in lower light levels was due to shadowing from a dock (Banaszak et al. 2003). The colony in high light was bleached white as a result of a 10 day period in which seawater temperatures in the lagoon rose 1.5 degrees Celsius above the 6-year August average (Banaszak et al. 2003). However, the colony in low levels of light did not experience any bleaching at this time (Banaszak et al. 2003). This also shows how highly susceptible M. alcicornis is. It also shows that only elevated temperature cannot cause bleaching by itself, but also needs the factor of high irradiance to cause bleaching.

Dutra et al. (2003) used M. alcicornis in a sediment accumulation study. It was found that the average diameter of M. alcicornis was lowest at sedimentation levels that were greater than or equal to 10 (Dutra et al. 2003). M. alcicornis was also found to be 48% occurrence more relative to other coral species within the data site (Dutra et al. 2003).

Akler et al. (2004) used M. alcicornis in a study to try and see the response of sea fans, Gorgonia ventalina. M. alcicornis was found to induce necrosis, the death of cells, in the sea fans. In general, sea fans responded differently to M. alcicornis than to the abiotic control factor (Akler et al. 2004). M. alcicornis attachment resulted in an increase in the relative proportion of both purple sclerites and spindles in the sea fan (Akler et al. 2004).

Commercial Importance

Millepora alcicornis has no commercial importance

Bermuda Laws

Millepora alcicornis falls under the Coral Reef Preserve act of 1966. The law states that there will be no removal or willful harm to any part of a coral reef within the coral reef preserve as set in the law. There is also a duty when leaving the country on all corals. As stated in the 1997 amendment of the tariff act, that a duty of $71 Bermudian is to be paid with the removal of any coral off the island.

Personal Interest

I became familiar with Millepora alcicornis over the past couple months. Over the summer on a diving trip to the Florida Keys, I was stung for the first time by Millepora, better known as fire coral. It was a quick sting and I moved my hand away fast. Once I was here in Bermuda and started diving, I knew better to stay away from it. So on a dive the second week here in Bermuda, I was carried by a burst of current that pushed me into a large colony of fire coral and stung my upper arm. I actually received a scar after the sting had healed.

This episode led me to find out more about Millepora alcicornis. I wanted to know as much as I could so I could tell people what gave me this cool scar from SCUBA diving in Bermuda. I found that fire coral is quite an interesting organism through my research and can definitely explain what it is, and how it can be quite a pain when working underwater.


Alker AP, Kiho Kim, Danielle H. Dube, and C. Drew Harvell. Localized induction of a generalized response against multiple biotic agents in Caribbean sea fans. 2004. Coral reefs, 23: 397.

Banaszak AT, Ayala-Schiaffino BN, Rodriquez-Roman A, Enriquez S, and Iglesias-Prieto R. 2003. Response of Millepora alcicornis (Milleporina: Milleporidae) to two bleaching events at Puerto Morelos reef, Mexican Caribbean. Rev. Biol. Trop., 51, 4:57.

Cairns SD. 1982. Stony Corals (Cnidaria: Hydrozoa, Scleractinia) of Carrie Bow Cay, Belize. Smithson. Contr. Mar. Sci., 12:271.

Cook Clayton B., Alan Logan, Jack Ward, Brian Luckhurst, and Carl J. Berg Jr. 1990. Elevated Temperatures and Bleaching on a High Latitude Coral Reef: The 1988 Bermuda Event. Coral Reefs, 9:45.

Edmunds P.J. 1999. The role of colony morphology and substratum inclination in the success of Millepora alcicornis on shallow coral reefs. Coral Reefs, 18, 2:133.

Kaandorp J.A. 1999. Morphological analysis of growth forms of branching marine sessile organisms along environmental gradients. Marine Biology, 134:295.

Lewis John. B. 1989. The ecology of Millepora. Coral Reefs, 8:99.

Stromgen Tor. 1976. Skeleton Growth of the Hydrocoral Millepora complanata Lamarck in Relation to Light. Limnology and Oceanography, 21:157.

Verwoot W. 1966. Skeletal Structure in the Solanderiidae and its bearing on hydroid classification. Symp. Zool. Soc. London, 16:23.

Wahle Charles, M. 1980. Detection, Pursuit, and overgrowth of Tropical Gorgonians by Milleporid Hydrocorals: Perseus and Medusa Revisited. Science, 208:689.


"Nothing Feels Like Fire, Than to Touch a Milleporina..."
Cnidarian Life Cycle