Corals

This Open University course extract explains how coral forms

By: The OpenLearn team (Programme and web teams)

  • Duration 10 mins
  • Updated Wednesday 9th August 2006
  • Introductory level
  • Posted under Natural History
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Coral Copyrighted image Copyright: Production team

In the Rough Science programme Shipwrecked, the Rough Scientists build an underwater explorer to view the coral reef around Zanzibar and its rich diversity of wildlife. The coral is not simply a lump of rock but a living organism made by small sea creatures, they also provide some of the most diverse and productive communities on Earth.

To find out more about coral and how it grows, read the following extract from the second level course Biology: Uniformity and Diversity (S204)

Most of the animals known as corals are anthozoans. There ‘stony’ corals resemble sea anemones, but their outer layer secrets hard, calcareous material and many form colonies of numerous joined polyps. Some feed in a similar way to sea anemones, obtaining particles of food from water currents. Many, including almost all of those that build reefs, also obtain some or all of their nourishment from photosynthetic algae (mostly dinoflagellates, kingdom Protoctista) held in the cells lining the enteron. These symbolic associations are broadly similar to that of Hydra. The cnidarian cells partially digest the cell walls of their internal symbiots, so up to 80% of all the sugars synthesized by the algae leak into their host’s cells, and are used for energy production. The photosynthetic and other pigments of symbiotic algae also contribute to the brilliant colours of living coral.

The metabolism of the symbiont algae is crucial to the formation of the calcareous base as well as to the nutrition of the soft tissues. The removal of bicarbonate ions from the sea water surrounding the coral by algal photosynthesis favours the precipitation of calcium carbonate, so aiding the formation of the hard, calcareous material that over many years builds up to form huge reefs.

Reef-building corals are also restricted to warm waters, though other species that do not have obligatory symbiosis with protoctists occur in deeper water and at high latitudes, including around Britain.

Under ideal conditions of light and temperature and with a full complement of healthy algae, corals deposit calcium carbonate fast enough for their reefs to grow upwards at up to 0.5ìmh-1. But the symbiosis is easily disrupted by turbulence or sediment that make the water opaque and smother or break the coral, by a small change in sea temperature, or by excessive predation.

The variety of shapes and patterns of the colonies arise from the type of budding and the degree of separation of the resultant polyps. In the ‘brain coral’ (Meandrina sp) the polyps do not separate, and thus form rows of small mouths surrounded by rows of tentacles; in life, the entire structure would be covered with living tissue with numerous small tentacles. A large colony consists of hundreds of intersecting rows that resemble the surface of a mammalian brain.

Sea-pens and precious coral Corallium belong to another group of corals in which the skeleton consists particularly or entirely of collagen-like extracellular proteins impregnated with inorganic salts, which may confer brilliant colours. They are predators or filter feeders and do not harbour photosynthetic symbionts. Many therefore live in deep water and other dark inhabitats.

Most cnidarians take larger particles than do sponges, and some trap and kill living animals, but for them, as for sponges, flowing water is the main source of nourishment and a potential hazard.

 
Coral Copyrighted image Copyright: Production team

The mechanical properties of the mesogloea of sea anemones and hydroids determine what kind of water flow the animal can withstand and hence where it can live and what shapes it can assume. Short, squat sea anemones such as Actinia equine are common in shallow rock pools on beaches, usually in small crevices or under ledges. They remain upright and open with their stubby tentacles exposed even when pounded by waves and tidal surges, because their mesogloea is relatively rigid but elastic, storing the energy of deformation and recoiling when released, like a tennis ball. Tall sea anemones such as Metridium sp. are restricted to calm waters below the regions exposed by tides, where they can project their crown of long, delicate tentacles into slow-flowing currents. Slight differences in the composition and arrangement of the macromolecules make their mesogloea more plastic, enabling them to extend slowly from a compressed to a greatly elongated shape. Being on a beach in a storm would rip them to shreds.

As well as enabling them to detect predators attacking from any direction, circular animals can withstand buffeting by water currents from many different directions.

Both the body form and the chemical composition and crystal structure of the secreted skeleton determine the corals’ ability to withstand wave action and tides. Sever storms and tidal waves, or even just he wash from passing ships, often break up coral skeletons or smother the living tissues in sand and rubble.

Many corals collect particulate food and/or prey on small animals at night and during dull weather. Non-predatory species lack tentacles, but all corals have numerous cnidocytes on their surface, some of which, as divers have found, contain toxins that can ‘sting’ people quite badly.

Like plants, sea anemones and corals (and other sessile animals, including sponges) are vulnerable to grazing animals. Several kinds of echinoderms and fish, including the parrot fish, eat the coral itself. Reefs support a rich variety of animals. Many fish that live in the open ocean as adults breed on reefs: their eggs and juveniles need the shelter and abundant food that reefs provide.

Most natural coral reefs are near coasts or surround islands and have been polluted and /or badly over-fished by the people who live there. However, underwater cables, offshore oilrigs and sunken ships and aircraft also make ideal substrata for sessile invertebrates including corals, sea anemones and sponges. Many such artificial reefs support thriving colonies of other kinds of invertebrates and fish. Harvests of many kinds of fish are noticeably better near old wrecks sunk in shallow water.