Fire Coral - The Bane of Divers

Fire coral, the bane of water enthusiast who does not avoid contact with the reef, can make your undersea experience painful. But, on the positive side, it is a good sentinel that can help protect the fragile reef organisms from damage by snorkelers and scuba divers.

Fire coral is a master of disguise with the shape of colonies varying from encrusting, to massive, to blade-like or branching. These growth forms are the result of differences in between species, the type of substrate larvae settle upon and/or the hydrodynamic conditions of the local area. The best way to identify generic, don't-touch-me fire coral in the field is, in my opinion, to look for brown or yellow objects bearing white tips.

Fire corals and hard corals (stony coral, hermatypic coral, and reef-building coral are synonyms) are in two different Cnidarian subphyla, despite both producing skeletons of calcium carbonate and having a symbiotic relationship with zooxanthellae. Fire corals are classified as members of subphylum Medusozoa (jellyfish and hydrozoans), class Hydrozoa, order Anthoathecata while hard corals are members of the subphylum Anthozoa, class Hexacorallia, order Scleractinia.

The genus name Millepora, meaning “thousands of pores”, reflects the fact that the smooth surface of fire coral colonies are covered by thousands of tiny pores. There are three types of pores; gastropores, dactylopores, and ampullae. Dactylopores house the dactylozooids, the tall, thin polyps containing nematocysts. These stinging cells capture zooplankton and protect the colony. When expended, their hair-like bodies give fire coral a fuzzy appearance. Once captured by the dactylozooids, prey are transferred to the mouths of the gastrozooids, who are responsible for digesting the food. The ampullae act as developmental chambers for the jellyfish-like medusa produced by asexual budding of polyps. All the polyps in the colony are interconnected by canals that run just under the surface of the skeleton.

Like the majority of hydrozoans, fire coral have a life cycle that alternates between benthic polyps that reproduce asexually and planktonic medusae which reproduce sexually. Mature medusa are release by the colony into the water, where they quickly release eggs and sperm for the sexual phase of fire coral reproduction. The fertilized eggs develop into planula larvae which eventually settle on to a hard substrate where the colony develops by way of asexual reproduction.

Photograph of branching Millepora alcicornis colony by Nhobgood (talk) Nick Hobgood - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11449048 
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Photograph of branching Millepora alcicornis, dactylozoides by Fernándo Herránz Martín, GFDL <http://www.gnu.org/copyleft/fdl.html>, via Wikimedia Commons
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Millepora sp. (Encrusting Fire Coral) by Nhobgood (talk) Nick Hobgood

​Dactylozooids of Millepora alcicornis by Fernándo Herránz Martín