Hydnophora rigida, commonly known as horn coral, are found in reefs and are in the genus ''

Hydnophora ''Hydnophora'' is a genus of large polyp stony corals in the family Merulinidae. List of species According to World Register of Marine Species : * '' Hydnophora bonsai'' Veron, 1990 * '' Hydnophora exesa'' (Pallas, 1766) * '' Hydnophora gran ...

''. They were first described by Dana in 1846. Their color is naturally green and brown, or sometimes cream. They can also become fluorescent green and cyano-red emission.

Distribution and background

''H. rigida'' was described by Dana in 1846. One of the most common species of Hydnophora is the ''H. rigida''. Other common names are horn coral or spine coral. They are commonly found in shallow reef environments around Australia and the Philippines, Fiji, Indonesia, Samoa,

Solomon Islands Solomon Islands is an island country consisting of six major islands and over 900 smaller islands in Oceania, to the east of Papua New Guinea and north-west of Vanuatu. It has a land area of , and a population of approx. 700,000. Its capita ...

, in the

Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, ...

North West Cape North West Cape is a peninsula in the north-west of Western Australia. Cape Range runs down the spine of the peninsula and Ningaloo Reef runs along the western edge. It is in the Gascoyne region and includes the town of Exmouth. History In 1618, ...

and the

Coral Sea The Coral Sea () is a marginal sea of the South Pacific off the northeast coast of Australia, and classified as an interim Australian bioregion. The Coral Sea extends down the Australian northeast coast. Most of it is protected by the Fre ...

. They can also be found on the east coasts of Africa and around Madagascar. ''H. rigida'' tends to inhabit

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') a ...

s and protected reef slopes.

Description

''H. rigida'' has thin branches that are cream or green without the encrusting bases. The horn coral also has a green

fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

or a cyano red emission. They can be seen at depths from 1–30 m (3–100 ft). The colonies are bushy with small conical mounts called monticules that are unique because they form where the

corallite A corallite is the skeletal cup, formed by an individual stony coral polyp, in which the polyp sits and into which it can retract. The cup is composed of aragonite, a crystalline form of calcium carbonate, and is secreted by the polyp. Corallit ...

walls of the adjacent polyp fuse together. The polyps that surround the base of monticules can seem brown and may contrast slightly. They also have tentacles that can come out at night.

Reproduction

The colonies of ''H. rigida'' spawn after the full moon of November. They are simultaneously

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...

coral that develop eggs earlier than the testis and there are some strong indications that many colonies of ''H. rigida'' carry out multiple gametogenic cycles each year. In the

Houtman Abrolhos The Houtman Abrolhos (often called the Abrolhos Islands) is a chain of 122 islands and associated coral reefs, in the Indian Ocean off the west coast of Australia, about west of Geraldton, Western Australia. It is the southernmost true coral ...

Islands, the coral colonies would contain ripe eggs in March, so people inferred that they are part of the mass spawning event that happens in western Australia. It has been found that the population of ''H. rigida'' was always in a fertile condition from November to February and this indicates that the spawning season may spread over several months each year. This coral releases the eggs instead of the

planula A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores. Some groups of Nemerteans also produce larvae that are very similar to the planula, which ...

e larva, which indicates that they are broadcaster spawners. The size of their eggs is relatively small, about 220 µm. This is another indication that they may spawn several times a year unlike other coral species that only spawn once a year. Researchers have also found that most colonies contained ripe eggs before the full moon and ripe testes around the full moon of November since they found out there are significantly less fertile colonies found in December. Since they spawn in November, scientists have speculated that they have a biannual cycle when they also spawn in March and it is not overlapping. Once the planula larvae are formed from the fertilized egg, the larvae settle onto the substrates and then become planters. Then they begin to form tiny polyps that excrete calcium carbonate and slowly develop into a coral.

References

{{Taxonbar, from=Q3932073 Merulinidae Cnidarians of Australia Cnidarians of the Pacific Ocean Corals described in 1846