Spine coral (Hydnophora rigida)

Classification (WoRMS)

Full classification as recorded by WoRMS: Kingdom: Animalia · Phylum: Cnidaria · Class: Hexacorallia · Order: Scleractinia · Family: Merulinidae · Genus: Hydnophora.

Accepted name: Hydnophora rigida · Authority: (Dana, 1846) · Taxonomic status: accepted · AphiaID: 207406.

Synonyms and superseded combinations recorded for this taxon: Clavarina composita Rehberg, 1892; Hydnophora (Clavarina) rigida (Dana, 1864); Hydnophora breviconus Nemenzo, 1971; Hydnophora columellata Rehberg, 1892; Hydnophora laxa (Dana, 1846); Hydnophora mayori Hoffmeister, 1925; Hydnophora ramosa Nemenzo, 1959; Hydnophora rigida var. ramosa Nemenzo, 1959; Hydnophora rigida var. solida Chevalier, 1975; Merulina laxa Dana, 1846; Merulina prolifera Quelch, 1886; Merulina rigida Dana, 1846.

Source: World Register of Marine Species (WoRMS) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=207406 (CC BY 4.0). Accessed 2026-09-23.

Biology and life history (SeaLifeBase)

Measured values published by SeaLifeBase for Hydnophora rigida. Only fields the database actually holds for this species are shown; these are database values, not aquarium guidance.

Depth range: 0–40 m · Habitat zone: reef-associated.

Source: SeaLifeBase species summary (SpecCode 45727) — https://www.sealifebase.se/summary/Hydnophora-rigida.html (CC BY-NC 3.0 — non-commercial use only). Accessed 2026-09-23.

Distribution and occurrence records (GBIF)

894 georeferenced occurrence records for Hydnophora rigida are published through GBIF from 27 countries or marine areas. The map above plots 188 distinct locations from a sample of 894 records, omitting 13 inland records that carry an institution's coordinates rather than a collection site; it shows where the species has been observed and reported, not a modelled range.

Most records by country or area: Australia (399); United States Minor Outlying Islands (103); Indonesia (73); New Caledonia (57); Japan (55); Fiji (44); Marshall Islands (26); Philippines (23); Malaysia (21); Kiribati (17); Palau (16); India (9); Micronesia (Federated States of) (7); Singapore (5); French Southern Territories (5).

Marine regions with a valid WoRMS distribution record: East Africa; Indo-West Pacific; Fijian Exclusive Economic Zone; Marshall Islands Exclusive Economic Zone; Indonesian Exclusive Economic Zone; Australian Exclusive Economic Zone; Philippine Exclusive Economic Zone; United States Exclusive Economic Zone.

Source: GBIF Occurrence Search (taxonKey 2259534) — https://www.gbif.org/occurrence/search?taxon_key=2259534&has_coordinate=true (records CC0 / CC BY); WoRMS distribution records (MarineRegions) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=207406 (CC BY 4.0); Natural Earth 110m basemap — https://www.naturalearthdata.com/ (public domain). Accessed 2026-09-23.

Conservation status (IUCN Red List)

The IUCN Red List assesses Hydnophora rigida (Dana, 1846) as Least Concern (code LC). No trend, population estimate or threat analysis is given here, because none is available from this source without the Red List API; follow the source link for the full assessment, including its date.

Source: IUCN Red List of Threatened Species (taxon 132881) — https://www.iucnredlist.org/species/132881/0; GBIF IUCN Red List category endpoint (taxonKey 2259534) — https://api.gbif.org/v1/species/2259534/iucnRedListCategory. Accessed 2026-09-23.

Description and natural history (Wikipedia)

H. rigida was first 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, in the Great Barrier Reef, North West Cape and the Coral Sea. They can also be found on the east coasts of Africa and around Madagascar. H. rigida tends to inhabit lagoons and protected reef slopes.

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

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

Source: Wikipedia: Hydnophora rigida — https://en.wikipedia.org/wiki/Hydnophora_rigida (CC BY-SA 4.0). Accessed 2026-09-23.

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