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.
The five classes
Evidence taken from fossils and living molecules has allowed scientists to group cnidarians into five classes:
- Anthozoa (corals and anemones)
- Scyphozoa (swimming jellyfish)
- Staurozoa (stalked jellyfish)
- Cubozoa (box jellies)
- Hydrozoa (hydroids and siphonophores).
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.
Where to find cnidarians
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.