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The Explorer's Guide contains a treasure trove of aquatic science resources developed for use in K-12 classrooms. Back to Explorer's Guide.

Fire Coral Scientific Name: Millepora sp.
Fire coral gets its name for a good reason. It is not a diver’s best friend. Brushing up against fire coral causes a lot of pain. Tiny barbs in the coral get launched into anything that touches fire coral and they cause a short term intense burning. Some people and animals are more sensitive to the effects of the stinging cells of this coral.

Region: Philippines

This white-tipped mustard colored animal is called a coral, but really isn’t. Fire coral is actually a relative of coral called a hydrozoan or hydroid or an animal related to coral that has two life forms—the polyp and the medusa. And as a hydrozoan, fire coral is more closely related to sea jellies than true coral.

This animal can take many different forms, depending on where it lives. In areas where the current isn’t very fast, it will grow into thin branches that form a lattice. If the current is somewhat strong, it will be found as thick staghorn-like branches or thick columns. Where there is a raging current or heavy wave action, fire coral will form a thin encrusting layer.

Fire corals have a symbiotic relationship or partnership needed for survival with zooxanthellae or algae that live just under the clear skin of the animal. The algae make food for the fire coral using the energy from the sun. The algae feed off the animal’s waste. The coral gives the tiny algae oxygen and a safe place to live.

Unlike many of the corals, which have large polyps or tiny coral animals, fire coral polyps are so small they are almost microscopic. The polyps are deep within the skeleton where they are linked by a network of minute canals. There are two different kinds of canals on the surface of the fire coral that house polyps. From one of the holes are hair-like polyps that have nematocysts or stinging barbs inside their cells and are used for protection and feeding. The other holes contain polyps that sexually reproduce.

The polyps are used to protect the entire colony from being overgrown by other reef animals. There is only a limited amount of space to live. Many animals that grow fast will grow over other animals that need sunlight for their zooxanthellae. This will kill the slower growing animal. Fire corals use one set of their polyps to shoot nematocysts at any animals that try to grow over them. Fire coral also grows really fast so it can also outgrow other animals.

The branches of fire corals are hollow and brittle. Inside the open space, this hydrozoan stores oxygen. This may be a defense strategy. If something bumps into it, the fire coral breaks off and the animal looses stability and may then bump into other fire corals.

Fire coral lives in shallow reef areas exposed to high light and varying currents. In areas where the current is strong and quick, thick masses of colonies of fire coral may form. They will live on the edge of reefs, on the top of steep dropoffs, or on the reef face.

Fire coral lives in the warm parts of all the oceans.

Fire corals eat just like other corals. They reach out their tentacles and sift the water for zooplankton or tiny animals and phytoplankton or tiny plants, which drift on ocean currents. Nematocysts shoot down the animal or plant, then fine hairs and mucus carry it to the animal’s mouth.

Fire corals rely on the zooxanthellae under their skin for the majority of their food.

Fire corals reproduce sexually. Like corals and sea jellies, hydroids go through a metamorphosis or change in shape as they grow. Their lifecycle begins when a female or male medusa or adult free swimming form of the animal leaves its polyp colony by budding, which is when a polyp divides in half to create a new individual. The medusa swims away from the colony and sexually reproduces. It releases eggs or sperm, which are fertilized by another fire coral medusa and a hairy bean-shaped planula. The planula swims until it finds a reef to live on. Then the planula grows into a flower-shaped polyp. The polyp attaches to a solid surface. Here the polyp goes through asexual reproduction through which it makes an exact copy of itself without eggs and sperm. The polyp makes these identical animals by budding. These new polyps keep dividing and form an entire fire coral colony.

>A species or group of organisms that is in danger of extinction or disappearing from the face of the earth in the near future if its situation is not improved.

A species that can be found throughout its natural range but is declining in number and may become endangered in the absence of special protection efforts.

A species that is not declining in number but is of special concern because it is sensitive to pressure by particular human activities or natural events.

A species that is not declining in number and is not sensitive to pressures by human activities or natural events.

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Threats and Management
Many home hobbyists don’t keep fire coral in their tanks. Generally, the tanks are too small to give fish and other animals to keep their distance from the pain- inducing hydroid. Fire corals also grow quickly on the reef. These reasons have kept the population stable.

Did You Know?
Fire corals have a hard skeleton like hard corals.

Print Publications:
Bender, L. 1988. Invertebrates. New York, New York: Gloucester Press. 36p.
ISBN# 0-531-17092-2.

Perrine, D. 1997. Mysteries of the Sea. Lincolnwood, Illinois: Publications International, Ltd. 312p. ISBN# 0-7853-2430-5.

Reader’s Digest. 1984. Reader’s Digest Book of the Great Barrier Reef. Sydney, Australia: Reader’s Digest Services Pty Limited. 384p. ISBN# 0-949819-41-7.

Veron, J. 2000. Corals of the World. Volume 3. Townsville, Australia: Australian Institute of Marine Science. 490p. ISBN# 0-642-32238-4.

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Vocabulary Words

Asexual Reproduction – Reproduction that does not involve the union of egg and sperm; results in an exact copy of the original animal.

Budding – A form of asexual reproduction where the animal grows a new individual that is an exact copy out of the side of it.

Extinction – An organism that has not been present on the face of the earth for over 50 years.

Hard coral – The members of the order Scleractinia, which secrete a heavy, external, calcareous skeleton, and many of which are primary contributors to the building of coral reefs.

Hydrozoan or hydroid – An animal that is a member of the Phylum Cnidaria. It is related to coral and has two life forms—the polyp and the medusa.

Medusa – The adult, free-swimming form which has an umbrella-shaped bell, tentacles and oral arms; reproduces sexually.

Metamorphosis – A change in the body form of an animal when changing from egg to adult.

Organism – A living thing.

Nematocyst – Stinging barbs like tiny harpoon-like stingers which deliver a paralyzing poison sting to stun or kill prey.

Pet trade – An industry or business in which animals are taken from the wild and sold to pet stores, zoos, and aquariums.

Phytoplankton – Microscopic plants that float at the mercy of the ocean currents.

Planula – The fertilized egg produced by the medusa; a free swimming, flat pear-shaped organism which moves with the aid of tiny hairs called cilia.

Polyp – An animal that does not move because it is attached to a hard substrate; resembles a flower with the bottom of its stalk attached to a stable surface and tentacles and oral arms facing upward; reproduces asexually.

Species – A group of organisms capable of breeding and producing fertile offspring; organisms that share the same gene pool.

Symbiotic relationship - Symbiosis – In order to survive, certain creatures form partnerships with other kinds of animals or plants for feeding, housing or protecting one another.

Zooplankton – Microscopic animals that float at the mercy of the ocean currents and have weak swimming ability.

Zooxanthellae – Tiny single-celled algae called dinoflagellates that have a symbiotic relationship with an organism like the giant clam or corals. In exchange for a safe place to live under the organism's skin, the zooxanthellae provides its host with nutrients it makes using the sun.

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Branching fire coral (Millepora alcicornis) Copyright Shedd, Jan Kanter
Branching fire coral (Millepora alcicornis) Copyright Shedd, Jan Kanter

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