Coral reefs are wave‑resistant carbonate structures produced predominantly by colonies of reef‑building stony corals (order Scleractinia) whose polyps secrete aragonite skeletons that accrete over decades to millennia. NOAA Fisheries describes how these colonial animals cement together with calcium carbonate to form complex, three‑dimensional habitats that harbor about a quarter of all marine species while covering less than one percent of the ocean floor.
GCRMN estimates reefs occupy roughly 0.2% of the seafloor yet support at least 25% of marine species and critical ecosystem services.
Types and geomorphology
- –Reefs occur primarily as fringing, barrier, and atoll formations, with patch/platform reefs also recognized by many authorities. The National Oceanic and Atmospheric Administration outlines the three classical forms—fringing, barrier, and atoll—based on position relative to shore and intervening lagoons.
NOAA Ocean Service;
NOAA CoRIS. Encyclopedic treatments also note platform or patch reefs alongside the classical trio.
Britannica.
- –The subsidence theory proposed by Charles Darwin explains the sequence from fringing to barrier to atoll as volcanic islands slowly sink while corals grow upward; Darwin’s 1842 monograph set out the mechanism and remains a touchstone in reef science.
Darwin, C. R. (1842);
Distribution and environmental setting
- –Modern shallow‑water coral reefs are concentrated between roughly 30°N and 30°S in clear, warm, well‑lit, low‑nutrient waters; they are found in more than 100 countries and territories worldwide.
GCRMN;
- –The Coral Triangle in the Indo‑Pacific is the global epicenter of reef biodiversity, hosting about three‑quarters of the world’s coral species and extraordinary fish diversity, with major implications for regional food security and economies.
WWF.
Biology and ecology
- –Reef‑building corals form an obligate symbiosis with photosynthetic dinoflagellates (family Symbiodiniaceae, commonly called zooxanthellae) that translocate most of their photosynthate—often up to ~90%—to coral hosts, fueling high productivity in oligotrophic seas.
Current Biology;
PubMed.
- –Reef communities include corals, crustose coralline algae, sponges, echinoderms, mollusks, fishes, and myriad microbes, forming intricate food webs and habitat structures across distinct zones such as reef crest, fore‑reef, and back‑reef.
NOAA Education.
Reproduction and life history
- –Many broadcast‑spawning corals release buoyant egg–sperm bundles during highly synchronized events cued by temperature and lunar cycles; on the Great Barrier Reef, mass spawning typically occurs a few nights after full moons in late spring, producing larvae (planulae) that settle and calcify into new polyps.
GBRMPA/Reef Authority;
PNAS.
Ecosystem services and human benefits
- –Reefs reduce wave energy and help protect coasts; meta‑analyses indicate coral reefs can dissipate on the order of 70–97% of incoming wave energy, with reef crests providing the greatest attenuation.
Nature Communications via ScienceDaily;
Science Advances;
National Academies Press.
- –Coral reef–associated tourism has been valued at roughly US$36 billion per year globally, underscoring conservation and sustainable‑use incentives.
Threats
- –Ocean warming has increased the frequency and severity of mass bleaching, in which corals expel symbionts and turn white under thermal stress, risking starvation and mortality if stress persists.
NOAA AOML;
NOAA Flower Garden Banks NMS;
- –Since April 15, 2024, NOAA has confirmed the planet’s fourth global bleaching event; as of May 10, 2025, bleaching‑level heat stress had affected about 83–84% of the world’s reef area across at least 83 countries and territories, making it the largest on record.
NOAA Coral Reef Watch;
Washington Post.
- –The IPCC projects that even if warming is limited to 1.5°C, 70–90% of present warm‑water coral reefs are expected to decline; at 2°C, losses exceed 99% (very high confidence).
IPCC SR1.5 SPM;
- –Ocean acidification reduces aragonite saturation state, impairing coral calcification; aragonite saturation below ~3 stresses many calcifiers, with continued declines projected under high‑CO₂ scenarios.
NOAA Ecosystem Indicators;
NOAA SOS.
- –Stony Coral Tissue Loss Disease (SCTLD), first reported off Florida in 2014, has spread widely through the Caribbean, affecting >20 species with high lethality and unprecedented spatial extent; agencies have launched multi‑year response plans and research into treatments and transmission pathways.
NOAA CDHC;
- –Local pressures—including pollution, sedimentation, destructive fishing, and overexploitation—compound climate risks and hinder recovery.
Conservation, governance, and trade
- –Numerous reef systems are protected within marine protected areas and World Heritage properties administered by UNESCO; the Great Barrier Reef was inscribed in 1981 and remains a focus of international monitoring and advice.
Queensland SoE 2020;
Australian Government DCCEEW.
- –International trade in stony corals is regulated: the entire order Scleractinia is listed on Appendix II of CITES, requiring export permits to ensure sustainability.
CITES taxonomy entry.
- –The IUCN’s 2024 global reassessment reports that 44% of warm‑water reef‑building coral species are threatened with extinction, reflecting escalating climate and local pressures.
IUCN press release.
Restoration and adaptation research
- –Restoration approaches include coral gardening and microfragmentation (rapid growth via fusion of small conspecific fragments), assisted sexual reproduction (“coral seeding”), and selective breeding or microbial/algal symbiont manipulations framed as assisted evolution. Peer‑reviewed work shows micro‑colony fusion can accelerate tissue expansion in massive corals, and seeding methods can reduce costs and scale outplanting.
PeerJ;
SECORE/Phys.org;
SECORE Technology. Assisted‑evolution concepts for enhancing thermal tolerance have been proposed and explored in laboratory and early field studies.
Cold‑water and deep‑sea coral reefs
- –Beyond tropical shallows, deep‑sea corals—including framework‑forming species such as Desmophyllum (Lophelia) pertusum—create biodiverse biogenic reefs from ~200 to >2,000 m depth without photosymbionts; these habitats are slow‑growing and vulnerable to trawling, energy infrastructure, and acidification.
NOAA Fisheries—Deep‑Sea Corals;
USGS Lophelia II;
NOAA Ocean Explorer. Emerging research indicates sensitivity of cold‑water corals to warming with potential microbiome‑linked disease dynamics.
Environmental Microbiome.
Climatic drivers and outlook
- –Marine heatwaves associated with anthropogenic warming and oscillations such as El Niño are primary drivers of recent mass bleaching; risk escalates with cumulative heat stress and shortened recovery intervals.