Hydrocorals are hydrozoans that build hard, coral-like skeletons of calcium carbonate. They look like corals and help build reefs, but they belong to the class Hydrozoa, alongside hydroids, siphonophores and many small jellyfish, rather than to the Anthozoa, the class of true stony and soft corals. The two main groups are the stinging fire corals (Millepora) and the delicate, colourful lace corals (Stylasteridae).

They are some of the reef’s most common cases of mistaken identity. Divers who would never touch a stony coral regularly brush a yellow-brown branch or a crusted wreck rail and discover they have met fire coral. In caves and under ledges, the bright purple and orange fans they photograph are often lace corals, not sea fans or bryozoans. And far below diving depth, on seamounts and island slopes, lace corals are among the most common hard corals of all, forming part of the deep-water “coral gardens” that shelter fish and invertebrates.

This page explains what sets hydrocorals apart from true corals, how the two main families live, feed and reproduce, why fire coral stings and lace coral does not, and why so many fire coral species are now on the IUCN Red List. It also serves as the index for the individual fire coral and lace coral guides on TheSea.

What is the difference between a hydrocoral and a true coral?

All are cnidarians with stinging cells, but they are built differently.

The biggest practical difference is the polyps. In a stony coral each polyp sits in its own visible cup, the corallite, with a ring of tentacles that both catches food and defends the colony. In a hydrocoral the polyps are tiny and specialised. Feeding polyps, the gastrozooids, sit in larger pores called gastropores; they are surrounded by long, thin stinging polyps, the dactylozooids, which emerge from smaller dactylopores. In fire corals the gastropore with its ring of dactylopores is called a cyclosystem. Nothing like a coral cup is visible, so the surface of a living hydrocoral looks smooth and finely pitted rather than sculpted. The genus name Millepora means “a thousand pores”, which is exactly what you see under a hand lens.

The polyps are linked by a network of fine tissue canals, the coenosarc, that runs through the skeleton, the coenosteum. When the dactylozooids are extended, a fire coral looks faintly furry or hazy in the water. This “fuzz” is the best field clue that you are looking at a hydrocoral and not at an encrusting stony coral or a sponge. For the anthozoan side of the comparison, see parts of a coral polyp.

Hydrocorals also have a different life cycle. Many hydrozoans alternate between a fixed polyp stage and a free-swimming medusa, the small jellyfish that carries the reproductive organs. Fire corals have kept a reduced version of that medusa. Lace corals have lost it and brood their young in skeletal chambers instead. Stony corals have no medusa stage at all. Classification reflects these differences: the World Register of Marine Species places both families in the hydrozoan order Anthoathecata, but fire corals (Milleporidae) belong to the suborder Capitata and lace corals (Stylasteridae) to the suborder Filifera. They are therefore not each other’s closest relatives, and their hard skeletons appear to have evolved separately.

What are fire corals?

Fire corals, genus Millepora, are the reef-building hydrocorals and the only genus in the family Milleporidae. Fifteen living species are currently recognised, from the Caribbean, Brazil and the Cape Verde Islands to the Red Sea, the Indian Ocean and the Pacific. Several more have been proposed, and a 2018 molecular study of Pacific Millepora by Takama and colleagues concluded that species boundaries in the genus need to be revisited, because shape alone is a poor guide to identity. Start with the Millepora genus page for the full species list.

Fire corals are usually mustard-yellow, ochre or tan, often with paler, almost white tips. The colour comes from the symbiotic algae, the zooxanthellae, in their tissues. Like reef-building stony corals, fire corals get much of their energy from these algae, which photosynthesise and pass sugars to their host, so fire corals are confined to clear, well-lit water. Most grow shallower than about 30 m, although the Caribbean branching fire coral, Millepora alcicornis, has been recorded to around 40–55 m. They also catch zooplankton, using the dactylozooids to sting prey and pass it to the gastrozooids.

The same species can look completely different from one reef to the next. Most colonies start as a thin crust over rock, dead coral or the hull of a wreck. In calm water they then grow into upright branches or lacy fans; branching fire coral can reach around 50 cm tall. In surf they form thick, upright blades or interlocking box-like plates that resist breaking waves. This plasticity confused early naturalists so badly that in 1898 the British zoologist Sydney Hickson argued that most Atlantic forms were a single variable species; that view has since been questioned, and blade fire coral (M. complanata) and box fire coral (M. squarrosa) are again treated as separate species.

Fire corals are fast-growing, competitive and opportunistic. They can encrust and smother neighbouring organisms, and M. alcicornis has been observed growing “attack” branches that reach out to encircle and overgrow gorgonian sea fans, then using the dead fan as a skeleton for new growth. Broken fragments readily reattach and grow, and blade fire coral is one of the first species to recolonise reefs damaged by storms. In the Red Sea, M. dichotoma forms clumps several metres across, and on many Caribbean reef crests blade fire coral is one of the dominant structures in the surf zone.

How do fire corals reproduce?

Fire corals reproduce both asexually and sexually. Asexual reproduction is mostly by fragmentation: when a storm or a careless fin snaps off a branch, the piece can cement itself to the bottom and grow into a new, genetically identical colony. Fragmentation is probably the most frequent way fire corals spread locally.

Sexual reproduction is where fire corals show their hydrozoan roots. Colonies are either male or female. During the breeding season, small cup-like chambers called ampullae form in the skeleton, and inside them the colony buds off tiny medusae, miniature jellyfish less than a millimetre across. The medusae are released into the water, usually around dusk, and they carry the eggs or sperm. They do not feed and live only briefly; their job is to spawn. Fertilised eggs develop into planula larvae that drift in the plankton before settling and founding new colonies.

The most detailed recent study, by Lachan Roth, Tom Shlesinger and Yossi Loya, followed three fire coral species in the Gulf of Aqaba in the northern Red Sea for six consecutive years (2016–2021) and was published in Coral Reefs in 2024. They found that reproductive structures appeared only between June and August, and that each medusa cohort took roughly two to three weeks to develop. Unlike most stony corals, which spawn once a year, M. dichotoma bred four to six times in a single season, and M. exaesa and M. platyphylla once or twice. Release started at sunset and was tied to the moon: M. dichotoma mostly bred around new and full moon, M. exaesa a few days before them and M. platyphylla around the first quarter. The largest medusae measured about 0.67 mm in M. dichotoma and 0.4 mm in M. exaesa. Larvae already carried zooxanthellae, and some were still swimming ten days after fertilisation.

The same study noted that fire coral abundance at two of the three depth zones it surveyed in Eilat appears to have been broadly stable for about 50 years, a useful contrast with the steep declines reported elsewhere.

What are lace corals?

Lace corals, family Stylasteridae, are delicate, brittle hydrocorals in pink, purple, orange, red, yellow and white. They are the second most species-rich of the 77 hydrozoan families. In his 2011 global review in PLoS ONE, Stephen Cairns of the Smithsonian Institution counted 247 valid living species, plus 21 species known only as fossils from the Paleocene onwards. The largest genus, Stylaster, had 77 species; Distichopora had 25.

Unlike fire corals, lace corals have no zooxanthellae. They feed on plankton and organic particles carried by currents, which is why on reefs they grow under overhangs, in caves and on current-swept walls rather than in full sun. Most of them live far deeper than divers go. Cairns found stylasterids from Antarctica to the Arctic Circle and from the surface to 2,789 m, but most common at 200–400 m around islands, and 90% of species live deeper than 50 m. The deepest genus, Crypthelia, ranges from about 128 m to the family’s depth record of 2,789 m. There are no stylasterids in the Arctic Ocean itself and only one known species in the Mediterranean.

Diversity is highest in the western Pacific. The temperate southwest Pacific region around New Zealand had 57 species in Cairns’s compilation, the tropical southwest Pacific around Indonesia, the Philippines and New Caledonia had 45, and the tropical northwest Atlantic, including the Caribbean, had 42. Many species have tiny ranges. Distichopora anceps, described from off Laysan in the northwestern Hawaiian Islands in 1978, could not be found again despite searches by submersible and remotely operated vehicle until researchers returned to the exact type locality, where it turned out to be the dominant invertebrate across what may be its entire range of about 10 square kilometres.

The two lace corals most divers meet are the first two ever described, both named by the German naturalist Peter Simon Pallas in 1766: the rose lace coral Stylaster roseus of the Caribbean and the violet Distichopora violacea of the Indo-Pacific. On tropical reefs, Distichopora forms stubby, flattened branches with rows of pores along their edges; Stylaster forms finer, more fan-like colonies. Off California, the purple California hydrocoral, Stylaster californicus, grows from the shallows to about 110 m.

How do lace corals live and reproduce?

Lace corals have the same basic polyp arrangement as fire corals, with gastrozooids surrounded by dactylozooids, but the details of the skeleton vary greatly between genera and are the main way species are identified. Some genera have a lid that can close over the gastropore; some have a spine, the gastrostyle, projecting up inside it. Cairns’s review included an illustrated glossary of 53 skeletal characters used to tell them apart.

Reproduction is quite unlike that of fire corals. Lace corals have lost the free medusa. Colonies are usually either male or female, rarely hermaphroditic, and the sex can often be read from the skeleton, because eggs and sperm develop in ampullae that differ in shape between the sexes. These appear as small blisters on the surface or as chambers inside the branches. Eggs are fertilised and brooded inside the female ampullae and releases them as well-developed planulae. Studies of Stylaster californicus by G.L. Ostarello found that its larvae spend very little time in the plankton, sometimes none at all, often settling not far from the parent colony. Such short dispersal helps explain why so many stylasterids have small, isolated ranges.

Lace corals are also homes for other animals. Tiny ovulid snails of the genus Pedicularia, just 4–6 mm long, live only on stylasterids, matching the colour of their host so closely that they are almost invisible. Spionid and polynoid polychaete worms build tunnels and cage-like tubes along the branches, and nemerteans, sea spider larvae and barnacles have also been recorded.

A 2008 study by Alberto Lindner, Stephen Cairns and Clifford Cunningham reversed a long-standing assumption about where reef animals come from. Their family tree of stylasterids showed that the group originated and diversified in the deep sea, then invaded shallow tropical waters at least three separate times, with one further invasion of shallow temperate water. It was among the first robust evidence that an important group of shallow-water marine animals evolved from deep-sea ancestors rather than the other way round.

What are hydrocoral skeletons made of?

Like stony corals, hydrocorals build their skeletons from calcium carbonate, but lace corals use two different mineral forms. In a study by Cairns and Ian Macintyre, about 76% of stylasterid species built skeletons of aragonite, the same form used by all living stony corals, while the other 24% were wholly or partly calcite, a less soluble form. Six of the seven Aleutian Islands stylasterids they analysed were calcitic. Cairns later pointed out that in the North Pacific the aragonite saturation horizon, the depth below which aragonite tends to dissolve, is unusually shallow, under 150 m, which may favour calcite skeletons there. This matters for the future, because ocean acidification is pushing that horizon shallower, and deep-water corals that build aragonite are expected to be among the first affected.

Are hydrocorals dangerous?

Fire corals are among the most common causes of minor stings in tropical divers and snorkelers. The dactylozooids carry potent nematocysts, and the sharp skeleton also scrapes skin, which helps venom in. Contact causes immediate burning pain, redness and raised welts. Pain can last from two days to two weeks, and inflammation sometimes flares up again after treatment. Stings are not lethal to people, but they can be very uncomfortable, and broad contact with bare skin, for example a forearm or leg dragged along a reef crest in surge, can produce a large, painful rash.

The usual first aid is to leave the water, rinse the area with seawater rather than fresh water, which can make remaining stinging cells fire, and gently remove any fragments of skeleton. Many guides recommend vinegar to deactivate leftover nematocysts, followed by cold packs and an over-the-counter hydrocortisone cream or antihistamine. Clean scrapes well, because coral cuts can become infected. Seek medical help for spreading redness, swelling or a severe reaction. The best prevention is good buoyancy, a full-length wetsuit or rash guard, and keeping hands and knees off the reef.

Lace corals and most other hydrocorals do not sting people noticeably. If you keep a reef tank, the stinging hitchhikers you are more likely to meet are small hydroids.

Are hydrocorals endangered?

Many fire corals now are. In IUCN Red List reassessments published between 2021 and 2024, the Caribbean blade fire coral M. complanata and box fire coral M. squarrosa (both assessed in 2021) and Brazil’s M. braziliensis and M. nitida (2022) were listed as Critically Endangered. Branching fire coral, M. alcicornis, was listed as Endangered in 2024, and the Indo-Pacific M. dichotoma and M. platyphylla as Endangered in 2022. Most of these listings rest on criterion A3c: a projected decline in habitat quality over the coming decades, driven mainly by ocean warming.

The reason is bleaching. Fire corals are often among the first reef builders to bleach in a marine heatwave, because their symbiosis with zooxanthellae is sensitive to high temperature and they live in the shallow, sunlit water that heats fastest. The best-documented case comes from Panama. In 1991 Peter Glynn and Willem de Weerdt reported in Science that two of the twelve reef-building coral species in Panama’s Pacific waters had been eliminated from their former ranges by the prolonged high temperatures of the 1982–83 El Niño, one of them the newly described Millepora boschmai. They called it the first documented case of species eliminations caused by the 1980s bleaching events, and pointed to three risk factors: high sensitivity to warming, a small geographic range, and restriction to the top 20 m of water. A year later Glynn and Feingold reported that the species was not extinct after all, and the IUCN now lists M. boschmai as Data Deficient.

Other pressures add to the heat. Fire corals are brittle and easily broken by anchors, divers and fishers, and nutrient pollution, sediment and overfishing favour seaweeds that can overgrow them. Lace corals face different threats. In the 1970s Stylaster californicus was collected and sold as curios and made into jewellery, and over-collecting led California to protect it in the Farnsworth Bank Ecological Reserve off Santa Catalina Island. In deep water, stylasterid gardens are easily destroyed by bottom trawls and, because the colonies are slow-growing, slow to recover. All stylasterids and all Millepora species have been listed on Appendix II of CITES since 1990, so international trade needs permits. See coral reef bleaching.

Can you keep hydrocorals in an aquarium?

Rarely, and usually not on purpose. Fire coral needs strong light and flow, stings tankmates and hands, and is difficult to keep long-term; it mostly arrives as a hitchhiker on live rock. Lace corals need frequent feeding with fine plankton because they have no algae to sustain them, and they usually waste away in standard reef tanks. Specialist aquarists who keep non-photosynthetic corals sometimes manage short-term success with intensive feeding, but because both families are CITES-listed and slow to recover in the wild, they are best enjoyed on the reef.