Tropical coral reefs

Corals are animals that build colonies

A coral is an animal, related to sea anemones and jellyfish. The individual animal is a polyp: a small, soft, sac-like organism, usually only a few millimetres across, with a mouth at the top surrounded by a ring of tentacles. One polyp on its own could not build anything, so corals live in colonies of hundreds or thousands of genetically identical polyps joined together by living tissue. A coral head, a branching thicket or a flat plate on the reef is one colony, not one animal.

Polyps increase the size of a colony by budding: a new polyp grows out of the side of an existing one and stays attached to it, so the colony spreads outwards over the rock. Colonies also reproduce sexually, releasing eggs and sperm into the water; the larvae drift for days or weeks, then settle on a hard surface and develop into the first polyp of a new colony. That is how a reef spreads to new ground.

Most reef-building corals have photosynthetic microorganisms living in their tissues. These are single-celled algae called zooxanthellae, and they live inside the cells of the polyp. This is what makes a coral unusual: the colony is part animal and part plant at the same time, and it can obtain food in two different ways. It is also why reef-building corals are restricted to water shallow enough for light to reach them.

Each polyp also secretes a calcium carbonate skeleton beneath and around itself. The reef itself is the accumulated skeleton of countless generations of polyps, cemented together with the remains of coralline algae and the debris of other animals. A reef is therefore a structure built by living organisms: it is not a rock that animals have moved onto.

• Coral = an animal (a cnidarian), not a plant and not a rock.
• Polyp = the individual animal: a small sac with a mouth, tentacles and a stomach.
• Colony = many polyps joined by living tissue; what you actually see as a coral head, branch or plate.
• Budding = a new polyp grows from the side of an existing one and stays attached, so the colony grows.
• Zooxanthellae = single-celled photosynthetic algae living inside the polyp's tissues.
• Calcium carbonate skeleton = secreted by the polyp; the dead skeletons of many generations make up the reef.

The structure and functions of a coral polyp

A polyp is a cylinder of tissue standing in a cup of skeleton, with a ring of tentacles around the top. The tentacles are armed with stinging cells: each one is a tiny capsule containing a coiled thread with a barb, which fires when it is touched, injecting toxin into whatever triggered it. The tentacles catch small animals and plankton drifting past and push them towards the centre.

In the middle of the ring of tentacles is the mouth, which leads down into the stomach. The mouth is the only opening the polyp has, so food goes in and undigested waste comes back out through it. Digestion begins in the stomach, and because the polyps of a colony are joined by living tissue, the nutrients from one polyp's meal are shared through the colony. This matters: a colony can feed as a whole, and a damaged part can be supported by the rest.

Underneath and around the polyp is the calcium carbonate skeleton. It supports the soft body, gives the polyp somewhere to retreat into when it is disturbed or exposed at low tide, and provides the hard surface on which the colony grows upwards and outwards. The skeleton is secreted by the polyp itself, and each polyp sits in its own cup within it.

Feeding is not the polyp's only source of food. The zooxanthellae in its tissues photosynthesise in daylight, and many polyps keep their tentacles withdrawn during the day and extend them at night to catch plankton. A polyp can therefore feed like an animal and be fed by its algae, which is what allows a reef to support such dense life in water that contains so few nutrients.

The mutualistic relationship with zooxanthellae

The relationship between a coral polyp and its zooxanthellae is mutualistic: the two organisms live closely together and each one benefits. Neither does well without the other. The zooxanthellae live inside the cells of the polyp, particularly in the tissue lining the stomach, where they are in the light and bathed in the polyp's waste products.

What the coral gives the algae: a safe place to live inside its own tissues, where the algae are protected from being eaten and are carried into the light as the colony grows; carbon dioxide from the coral's respiration, which the algae need for photosynthesis; and nitrogen- and phosphorus-containing waste, which the algae use to make proteins and other compounds. For an alga, a coral is a well-lit home with a constant supply of fertiliser.

What the algae give the coral: oxygen from photosynthesis, which the coral uses for respiration; and sugars and other organic compounds made in photosynthesis, which the coral uses as food. For most reef-building corals this is the main food supply, not the plankton caught by the tentacles. The algae also take up the coral's nitrogenous waste, removing material that would otherwise be toxic, and they help the polyp to deposit its calcium carbonate skeleton.

Because so much of the coral's food comes from photosynthesis, the relationship explains where reefs can exist. It also explains what happens when the partnership breaks down: if the coral is stressed, for example by water that stays unusually warm for several weeks or by heavy sediment, it expels its zooxanthellae. The tissue turns white because the brown algae have gone, and if they do not return the coral slowly starves, because its main food supply has been lost.

Abiotic factors that limit where corals grow

Temperature. Reef-building corals need warm water, roughly between 20 °C and 29 °C, and they cannot survive prolonged cold or a sudden drop. This is the main reason reefs are confined to tropical and subtropical seas between the tropics, and to the warm currents that spread beyond them. Temperature also has an upper limit: if the water stays a few degrees above its normal summer maximum for weeks, the coral expels its zooxanthellae and may die.

Light intensity. The zooxanthellae have to photosynthesise, so corals need strong light and grow only in the shallow, sunlit water of the photic zone, usually within about the top 50 m. Where the water is very clear, light reaches further down and reef-building corals can grow deeper; where it is cloudy, the reef is shallow or cannot form at all.

Clear water. Corals need water that is not loaded with suspended sediment, for two reasons: sediment particles scatter and absorb light, cutting the supply to the algae, and particles that settle on the polyp smother it, so the coral has to spend energy cleaning itself. Fresh water and low salinity are also a problem, which is why reefs are absent at the mouths of large rivers.

Hard substrate. A coral larva needs a hard, stable surface to settle on and cement itself to, and the colony needs firm foundations to build a skeleton on. There are no coral reefs on soft mud or shifting sand. Rock, dead coral skeleton and the limestone of an old reef all provide the surface that a new colony needs.

• Temperature: about 20 °C to 29 °C; cold water excludes corals, and water that stays too warm makes them lose their zooxanthellae.
• Light intensity: the algae must photosynthesise, so corals are restricted to shallow water, usually the top 50 m.
• Clear water: sediment blocks light and smothers polyps; fresh water and low salinity also exclude corals.
• Hard substrate: larvae must settle and cement onto a hard, stable surface; soft mud and shifting sand will not do.

Why reef water is clear and low in nutrients

The water around a coral reef is famously clear, and the reason is that it is low in nutrients such as nitrate and phosphate. Clear water is not clean because the corals have filtered it; it is clear because there is very little dissolved nutrient in it, and therefore very little phytoplankton to cloud it. Where nutrients are plentiful, the water turns green with microscopic algae and the light is lost.

The nutrients that do exist are not floating in the water - they are held within the organisms living there. Each polyp's zooxanthellae take up the coral's nitrogenous waste directly from inside its cells. Anything released into the water is taken up quickly by sponges, algae, bacteria and filter feeders, and anything that grows is eaten almost at once by the grazers on the reef. The result is a tight, fast cycle in which nutrients are passed from organism to organism rather than accumulating in the water.

This is not a coincidence but a condition for reef growth. Because the reef recycles its nutrients so efficiently, it can support an enormous amount of life in water that would be considered too poor to grow anything. The reef is an efficient nutrient-trapping system sitting in a desert of a sea.

It is also fragile for exactly that reason. If extra nutrients arrive - from sewage, from fertiliser washed off the land, or from disturbed sediment - the balance tips. Fast-growing algae multiply, cover the rock and shade the coral, phytoplankton cloud the water and cut the light, and the corals are outcompeted on the space they need. Clear, nutrient-poor water is therefore both a cause of the reef's richness and a requirement for its survival.

Biotic factors: parrot fish and the crown-of-thorns starfish

Parrot fish are grazers with teeth fused into a beak, and they are one of the most important influences on what grows on a reef. They scrape algae off the coral rock and off the dead skeleton, which keeps the algae from growing over the living coral and competing with it for space and light. On a reef where parrot fish and other grazers are abundant, coral tends to dominate; where they have been removed by overfishing, fleshy algae take over and coral cover falls. In this way a fish that eats algae decides where coral can grow and how much of the reef it can occupy.

Parrot fish also eat coral. Their grinding teeth break off pieces of coral and grind them up, so the fish digests the living tissue and the algae inside the skeleton and passes the powdered calcium carbonate out as sand. This is bioerosion: it opens up bare space for new coral larvae to settle, it produces much of the white sand of tropical beaches, and it prevents any one colony from dominating. Heavy grazing therefore both helps and damages a reef, which is why the effect depends on how many fish there are.

The crown-of-thorns starfish (Acanthaster planci) is a coral predator. It pushes its stomach out through its mouth, spreads it over the coral, digests the living tissue and leaves the white skeleton behind. Normally it is uncommon and its grazing does little harm, but in an outbreak, when thousands of them occur together, they can strip the living coral from a large area of reef within months.

The aftermath of an outbreak is worse than the feeding itself. The dead skeleton is colonised within weeks by filamentous algae, which prevents new coral larvae from settling on it, so the reef cannot regrow until the algae and the dead coral have been broken down. Coral cover may stay low for a decade or more. Outbreaks appear to be more likely where nutrient-enriched water allows more of the starfish larvae to survive, which links this biotic factor back to the nutrient supply of the reef.

Coral reefs as areas of high species richness

Species richness means the number of different species present in an area. Coral reefs are areas of high species richness: a single reef holds far more species than almost any other part of the ocean of the same size, including many kinds of coral, fish, sponges, molluscs, crustaceans, sea urchins and algae. High species richness is not the same as a large number of individuals; a school of a thousand identical fish is one species.

The main reason is the structure of the reef. A reef is not a flat surface: it is a three-dimensional landscape of branching thickets, flat plates, boulders, crevices, caves and overhangs, and each of these offers a different combination of light, water movement and shelter. A crevice in a coral head is a different habitat from the open reef top a metre above it, so many species with different requirements can live close together without directly competing.

The reef is also a stable, long-lived and productive habitat. It has been there for thousands of years, so there has been time for many species to specialise; it produces food in the form of algae, plankton and coral tissue; and it sits at the edge of the open sea, so animals can shelter on the reef and feed in the water beyond it. The corals themselves add to the richness by providing surfaces for other organisms to grow on and live in.

High species richness also means intense competition for space, because the reef surface is limited and every organism needs somewhere to attach. Species survive by dividing up the reef: different feeding methods, different times of day, different parts of the reef, different sizes of prey. A reef is therefore rich in species precisely because it is crowded and divided into many small, different habitats.

Adaptations of reef organisms: trigger fish, parrot fish and nudibranchs

Trigger fish. A trigger fish has an adapted dorsal fin: the first spine is stout and can be locked upright by a second, shorter spine behind it, like a catch or a trigger. The fish backs into a crevice, raises the spine, and the locked fin jams against the roof of the gap so that the fish is held tightly in place. A predator that finds it cannot pull it out. The spine can be folded down again only by releasing the trigger, so the fish can swim away when the danger has passed. Its body is also laterally flattened and tough-skinned, which suits a life wedged into gaps in the reef.

Parrot fish. A parrot fish has a specialised set of grinding teeth - the teeth at the front of the jaws are fused into a hard, beak-like plate, and there is a second set of heavy grinding teeth in the throat. With the beak it scrapes algae and coral off the reef; with the throat teeth it grinds the coral fragments to powder. It can therefore eat coral, digest the living tissue and algae, and pass the ground limestone out as sand. No other fish on the reef can exploit that food source in the same way, so the adaptation gives the parrot fish a diet that is not competed for.

Nudibranchs. A nudibranch is a sea slug: soft-bodied and with no shell to hide in, so it needs a different defence. Many species eat prey that is itself toxic, such as certain sponges and soft corals, and then absorb the toxins from the prey into their own tissues instead of breaking them down. The nudibranch becomes toxic or distasteful to anything that eats it, so a predator that tries one learns to leave the rest alone. Their bright colours advertise the fact, which is why an animal with no armour can sit in the open on the reef.

Read the three together and the pattern is clear: each adaptation solves the problem of living on a crowded reef, and each answer needs the structure, what it does, and the advantage it gives. The trigger fish avoids predators, the parrot fish reaches a food supply others cannot, and the nudibranch turns its prey's chemistry into its own protection.

The Darwin-Dana-Daly theory of atoll formation

An atoll is a ring-shaped coral reef enclosing a shallow central lagoon, with no land inside it. The Darwin-Dana-Daly theory explains how one forms, and the whole theory depends on one idea: the island sinks while the coral grows upward and keeps pace with it. Get that direction right and the sequence writes itself.

Stage 1 - fringing reef. A volcanic island rises in warm tropical water, and its lava cools to hard rock, which is exactly the hard substrate coral larvae need. Larvae settle along the shoreline and grow outwards and upwards, forming a reef attached directly to the land, with little or no lagoon. This is a fringing reef. Reefs of this type fringe the coasts of the Red Sea and many volcanic islands today.

Stage 2 - barrier reef. The volcano becomes extinct. It begins to subside as the ocean floor beneath it cools and sinks, and its own weight and the sea erode it. The coral, however, keeps growing upwards at a rate that keeps pace with the sinking, so the reef surface stays near the surface of the sea where there is light, while the land behind it becomes lower and the gap between land and reef widens into a lagoon. A reef separated from the shore by a lagoon is a barrier reef, like the Great Barrier Reef off Queensland.

Stage 3 - atoll. Subsidence continues until the last of the volcanic island is below sea level. No land remains above the water, but the coral ring has kept growing upward, so all that is left at the surface is a narrow ring of reef enclosing a central lagoon: an atoll. The Maldives are a chain of atolls formed in exactly this way.

Daly's contribution. Daly added the effect of changes in sea level during the ice ages and of wave erosion. When sea level fell during a glacial period, the top of the reef was exposed and planed flat by waves; when the sea rose again, the coral grew back up to the new surface. This explains the flat reef top and the shallow lagoon better than subsidence alone, and it is why the theory carries three names rather than one.