Understanding Coral Biology
Coral reefs are among the most complex ecosystems on earth, and the organism at their foundation — the reef-building coral — is itself a composite entity: an animal, a garden of algae, and a carbonate factory simultaneously. Understanding what coral is and how it functions makes every dive more legible and every conservation choice more grounded.
What a coral is: the Anthozoa context
Reef-building corals belong to the class Anthozoa within the phylum Cnidaria — the same phylum as jellyfish and anemones. The order Scleractinia contains all the hard, reef-building species. Anthozoa are exclusively polyp-forming: unlike jellyfish, which have both polyp and medusa stages, anthozoans remain in polyp form throughout their lives. There is no free-swimming medusa stage.
A coral colony is a clone. The individual units are polyps — cylindrical animals with a ring of tentacles surrounding a central mouth that opens directly to the gastrovascular cavity, which serves both digestive and circulatory functions. In reef-building species, adjacent polyps are connected by living tissue called the coenosarc, which runs across and between them and through which nutrients and chemical signals pass. When you look at the surface of a brain coral or a table coral, you are looking at thousands of genetically identical polyps.
Polyp anatomy
Each polyp consists of two cell layers: the ectoderm (outer) and the endoderm (inner), separated by a gelatinous middle layer called the mesoglea. The ectoderm contains stinging cells called cnidocytes, which carry nematocysts — spring-loaded harpoon structures that discharge on contact or chemical stimulus. Coral nematocysts are used primarily to capture zooplankton prey, though they are also used defensively and in aggressive interactions with competing corals.
Within the endoderm, occupying specific cells called symbiosomes, live the zooxanthellae — the dinoflagellate algae of the family Symbiodiniaceae whose photosynthesis provides the majority of the coral's energy. The internal location of the algae is protective; the coral tissue provides CO2 and waste nitrogen as raw material for the algae's photosynthesis, and the algae return oxygen and carbon-fixed sugars. The entire reef's productivity rests on this exchange.
The aragonite skeleton
Below the living tissue of the polyp is the corallite — the calcium carbonate structure secreted by the polyp's base. Reef-building corals use aragonite, a crystalline form of calcium carbonate that is metastable compared to calcite but denser and better suited to the structural demands of a reef framework. Polyps extract calcium ions and carbonate ions from the surrounding seawater and precipitate them at the base of the ectoderm in a precisely organised crystal structure that accumulates over time.
Ocean acidification — the reduction in seawater pH caused by dissolved CO2 from the atmosphere — reduces the saturation state of aragonite in seawater, making skeleton deposition energetically more costly and the resulting skeleton more porous and weaker. Aragonite undersaturation at the current trajectory is projected to affect large areas of high-latitude reef before mid-century, making ocean acidification a parallel stressor to thermal bleaching rather than a secondary concern.
Soft corals — octocorals of the order Alcyonacea — produce sclerites instead of a continuous skeleton: microscopic, spicule-like calcium carbonate elements embedded in the tissue. Soft corals are flexible rather than rigid and do not contribute to the reef framework in the same structural way, though they cover enormous proportions of reef surface, particularly in current-rich areas like the Somosomo Strait.
Nutrition: three routes to energy
The coral polyp is not solely dependent on zooxanthellae photosynthesis. It also feeds actively as a predator. At night, polyps extend their tentacles into the water column and capture zooplankton — copepods, amphipods, fish larvae, and dissolved organic matter — using nematocysts to immobilise and the mouth to ingest. Some coral species are better predators than others: mushroom corals Fungia sp. and large-polyp stony corals like Goniopora can capture prey substantially larger than their polyp diameter.
The third nutritional pathway is dissolved organic carbon uptake directly through the surface of the tissue — absorbing sugars, amino acids, and other compounds from the water column. The relative importance of these three routes varies by species, light level, water column productivity, and season.
Sexual reproduction and broadcast spawning
Most reef-building corals reproduce sexually once a year in a co-ordinated mass spawning event triggered by water temperature, lunar phase, and day length. In the Indo-Pacific, the dominant trigger is the full moon following the warmest water temperature of the year, typically one to three nights after the full moon. Corals release buoyant bundles containing eggs and sperm simultaneously — broadcast spawning — and fertilisation occurs in the water column.
The spawning event is visible as a blizzard of pink and cream bundles rising through the water. Larvae, called planulae, develop in the plankton over days to weeks before settling on hard substrate and metamorphosing into the first polyp of a new colony. Settlement is not random: planulae respond chemically to surfaces colonised by specific crustose coralline algae, which indicate suitable substrate. Survival from egg to settled polyp is less than one percent under natural conditions; from settled polyp to reproductive adult can take three to ten years, depending on the species.
Asexual reproduction and colony competition
Corals also reproduce asexually. Fragmentation — whether by storm damage, fish biting, or intentional fragmentation in restoration programmes — produces fragments that can survive if they settle on hard substrate. Some species produce asexual polyp buds that drift and settle. Budding within a colony produces new polyps as the colony grows.
At the colony boundaries, aggressive interactions regulate space. Brain corals Diploria strigosa and star corals Orbicella annularis extend mesenterial filaments across the tissue surface — reaching outside the polyp through the mouth or through temporary openings in the body wall — to digest adjacent competing coral tissue. These chemical-and-physical boundary wars are invisible to the casual diver but produce the geometric boundaries visible between adjacent colonies on a reef flat.
The benthic community and water-column ecology
A reef is not only the coral itself. The benthic zone — the reef surface and substrate — supports a community of crustose coralline algae, turf algae, fleshy macroalgae, sponges, echinoderms, worms, and crustaceans that interact constantly with the coral framework. Herbivory is fundamental to reef health: parrotfish Scarus sp. and surgeonfish Acanthurus sp. graze algae that would otherwise overgrow coral tissue; sea urchins Diadema antillarum perform a similar function in the Caribbean. Loss of herbivores — through overfishing or the mass die-off of Diadema following a Caribbean-wide pathogen event in 1983 — shifts the competitive balance toward algae and away from coral.
Above the benthic community, the water column supports the plankton community that supplies the filter feeders, the larval fish, and the food chain running up to the apex predators. The interconnection between the reef framework, the benthic community, and the water column is the ecology that makes reef diving infinitely productive to observe.
Exploring reefs with new understanding
The reef biology described here applies to every coral site on the interactive map. Knowing whether a site is dominated by broadcast- spawning Acropora, massive brain corals with complex colony competition, or soft coral Dendronephthya growing in current shapes what you look for and what you will remember.