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.