Coral is the name loosely given to a variety of animals which grow in colonies made of calcium carbonate. True corals belong to the Anthozoa class of cnidarians, but there are also members called corals in another class of cnidarians – the hydroids. To complicate matters, an entirely different group of animals, bryozoans, includes colonies that are known as lace corals.
The typical coral is a colony of numerous polyps, tiny individuals that secrete a delicate skeleton of lime and protein. Polyps feed by extending their tentacles in search of prey. When disturbed, they withdraw into the surface of the coral. There are exceptions to this typical form: some corals are solitary animals and some have soft bodies.
True corals are divided into two types:
- octocorals (with eight tentacles and body partitions)
- hexacorals (with tentacles and partitions in multiples of six).
New Zealand has over 275 species of octocorals, but few scientists study them. A large number of species still await formal description. Gorgonians or fan corals, the most diverse group, have a tree-like framework. In some species, like bamboo corals, the frame may be jointed. Octocorals also include soft forms such as dead men's fingers (Alcyonium species) and sea pens (Pteroeides bollonsi).
The largest invertebrate sea-floor species on the planet lives in New Zealand waters. This is the bubblegum coral (Paragorgia arborea). One specimen in the collection at the National Institute of Water and Atmospheric Research (NIWA), Wellington, has a trunk 42 centimetres in diameter. Before it was hauled to the surface in a trawling net, it had probably grown 7 metres high in its habitat on the edge of the Campbell Plateau, 800 metres deep. Carbon-14 dating gave it an age of 300–500 years.
Some live as solitary polyps, others live in colonies where their body tissues and nerves are connected. There are soft and hard forms. Most soft hexacorals are anemones. Hard hexacorals include black and stony corals. In stony corals, lime extends into the body of the polyps and when the individual animals die, a rigid framework remains.
Black corals are colonies of hexacorals with tiny polyps. Some are bushy and tree-like. Mostly they live in deep water, but in Fiordland the black coral Antipathella fiordensis can live in relatively shallow depths, where peat acids in the water restrict sunlight from penetrating. New Zealand’s 58 species comprise one-third of known Indo-Pacific black corals. Black corals have a lustrous skeleton that can be polished, and some overseas species are harvested for jewellery. None of the New Zealand species has proved suitable, and in any case, they are strictly protected.
Coral under threat
Several stony corals contribute to coral banks on the Chatham Rise and Campbell Plateau, to the east and south of New Zealand, where they are common and potentially threatened from sea-floor mining. Their growth rate is not known, but similar species in the North Atlantic take 200–360 years to grow a metre-high colony. Deep-water trawling for orange roughy fish has damaged some coral banks – centuries will pass before their habitat recovers.
Stony corals are characteristic of the tropics, and so it may be thought remarkable that temperate New Zealand has 127 living species (and many fossil species). Seventeen species contain photosynthetic algae (zooxanthellae) in their tissues, which produce food for the coral polyps. These species are restricted to warm subtropical waters around the Kermadec Islands. Unlike most corals with algal partners, they do not form coral reefs. Two species of stony coral, Culicea rubeola and Monomyces rubrum, are found in shallow water, although most (80%) occur at depths of 200–1,000 metres.
Sea anemones make up most of the 106 soft hexacoral species in New Zealand waters. Children exploring rock pools may enjoy prodding anemone stomach cavities to feel the gentle tug of the nettle cells – the poisons are too weak to be felt as a sting. Anemones can live singly or in chains. Most are permanently submerged, but some survive above low tide for a few hours in damp shade. The wandering sea anemone (Phlyctenactis tuberculosa) only loosely attaches itself, and is generally found floating among seaweed. The colourful jewel anemone (Corynactis australis) has tentacles with terminal knobs, each studded with nettle cells.