Corals, sea anemones and jellyfish belong to a group of animals called cnidarians (pronounced ‘nid-air-e-ans’). There are two others in the cnidarian group: hydroids, known collectively as sea firs; and siphonophores, such as the Portuguese man-of-war, which are not single creatures, but colonies of many specialised individuals.
With 1,048 marine species, cnidarians are one of the largest groups of invertebrates in New Zealand waters. Although they may look quite different from each other, they share a common ancestry.
A feature of cnidarians is that they may have two forms. In one, the medusa or jellyfish phase, it is free swimming; in the other, it attaches to a surface and is called a polyp.
Cnidarians have a simple sac-like body, with a single opening surrounded by a ring of tentacles. Their body is made of two distinct layers of tissue, separated by a thick gelatinous substance called mesoglea.
All cnidarians have specialised stinging or nettle cells in their tentacles (their name comes from the Greek ‘knide’, meaning nettle). Rather like a harpoon, the nettle cells shoot out tiny barbed threads that penetrate tissue and release poisons. Cnidarians are carnivores, using their nettle cells to paralyse small animals, which they then grasp with their tentacles and eat.
Three nasty cnidarian stingers lurk in New Zealand waters. In summer the lion’s-mane jellyfish may be encountered in harbours and on the coast. Portuguese men-of-war or bluebottles frequently wash up on beaches, where inquisitive children, attracted by their blue floats, may touch their tentacles and be stung. In the Hauraki Gulf, the popularly known ‘long stringy stingy thingy’, Apolemia uvaria, is responsible for a number of stinging incidents.
Evidence taken from fossils and living molecules has allowed scientists to group cnidarians into five classes:
Anthozoa have only the polyp phase. The others either have both phases in their life cycle, or just the medusa phase. When both phases are present, sex organs develop in the medusa phase.
Most New Zealanders encounter cnidarians when fossicking or snorkelling around coastal rocks, where sea anemones and mussel’s beard live. Diversity increases in the warmer waters of the northern offshore islands. Corals and anemones attach themselves to the sea floor or to shells and seaweeds; jellyfish and Portuguese men-of-war drift or swim in surface waters. There are only five species of freshwater cnidarians in New Zealand.
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:
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.
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.
Like corals, hydroids can be solitary or animals which live in colonies. Both have soft and hard-bodied forms. A familiar species is mussel’s beard (Amphisbetia bispinosa), the hairy growth on mussel shells, which actually consists of chains of tiny polyps. Another hydroid is the brilliant red coral (Errina novaezelandiae), which lives in New Zealand’s fiords and other southern localities. It has a rigid skeleton up to 30 centimetres tall, and is classed as a hydrocoral.
Few people other than divers or professional biologists encounter siphonophores. The exceptions are the Portuguese man-of-war (Physalia physalis) and, in the warmer waters of the Hauraki Gulf, Apolemia uvaria, popularly called the ‘long stringy stingy thingy’.
Siphonophores are the most complex organisms in the open ocean. They resemble a jelly, but are actually colonies of specialised individuals. The name refers to the stomach (or siphon) shape of some of these individuals.
Most siphonophores are found below the ocean’s surface. There is generally a float with a trailing stem, to which are attached many, in some cases thousands, of individuals that perform particular functions such as propulsion, feeding and digestion, protection and food capture (by stinging), and reproduction. The length of this stem ranges from a few millimetres to tens of metres, depending on the species, and some are probably the longest animals in the world. A few, like Portuguese men-of-war, are exceptional because they live on the sea surface, having a large gas-filled float. The gas is poisonous carbon monoxide. Most other siphonophores have a small float, inadequate for buoyancy, which may serve as a sense organ, telling the animal which way is up.
Worldwide, there are about 160 named species of siphonophores, 49 of which have been found in New Zealand waters. They can be abundant and are major predators of fish larvae and small crustaceans.
Jellyfish are cnidarian medusae. Jellyfish are members of the class Scyphozoa, which, although widespread in the world’s oceans, includes only about 250 species.
Sudden swarms of jellyfish can be devastating. They can clog the pipes of leisure boats, ships, and power plants. They consume massive amounts of the larvae of commercial fish and shellfish and their food. High densities of jellyfish can literally suffocate the stock in commercial fish farms.
Swimming jellyfish (scyphozoans) live in sea water and have a reduced or absent polyp phase. New Zealand species have never been properly studied, and several species are probably new to science.
The best-known species around New Zealand include the stinging lion’s-mane jellyfish (Cyanea species) and the moon jelly (Aurelia species). The latter can wash up in large numbers on beaches. It is quite transparent, with four purple sex organs visible through the jelly. Touching the upper surface of the jelly-like bell is safe – it is the tentacles that should be avoided. Moon jellies are responsible for the death of farmed fish in New Zealand, when they are drawn into bays where the fish are penned. The actual cause of death is not certain, but it is thought to be due to nettle-cell irritation and jellyfish mucus coating the fish gills.
Little is known about the biology of stalked (‘upside-down’) jellyfish, which belong to the class Staurozoa. They can creep and somersault, but cannot swim in typical jellyfish fashion. Only a single species, Craterolophus macrocystis, has been documented in New Zealand waters. Originally collected from Port Chalmers, attached to bladder kelp, it has apparently never been found again. It was about 2.5 centimetres tall and dark green when alive, and therefore well camouflaged. A second stalked jellyfish, Depastromorpha africana, was recently reported in New Zealand waters and an unidentified species of Lipkea has been found on cave walls at the Poor Knights Islands. Fossil members, called conulariids, are known from Paleozoic rocks in New Zealand.
As their name indicates, box jellies are muscular, square-shaped animals. Members of the class Cubozoa, they differ from true jellyfish, having a life cycle that involves total metamorphosis of the polyp into the medusa (jellyfish stage). They also have highly developed eyes. New Zealand does not have a problem with box jellies, but they are a nuisance in Queensland, where the species Chironex fleckeri is the most venomous animal in the world. Only one box jelly species is known from New Zealand – the diminutive Carybdea sivickisi, seen in Cook Strait.
Batson, Peter. Deep New Zealand: blue water, black abyss. Christchurch: Canterbury University Press, 2003.
Cairns, S. D., and others. ‘Phylum Cnidaria – corals, medusae, and hydroids.’ In The New Zealand inventory of biodiversity: a species 2000 symposium review, edited by D. P. Gordon. Christchurch: Canterbury University Press, 2005.
Powell, A. W. B. Powell’s native animals of New Zealand, edited by B. J. Gill. 4th ed. Auckland: David Bateman, 1998.