The filter-feeding marine animals, sea squirts, salps and appendicularians are collectively known as tunicates. The scientific name refers to their protective cellulose covering or ‘tunic’.
Tunicates are distant relatives of vertebrates – animals with a backbone – and zoologists have traditionally grouped them together. At some time in their life cycle, both tunicates and vertebrates have gill slits, a nerve cord, and a supporting rod running down their backs. Tunicate larvae look like microscopic tadpoles and possess these vertebrate features. However, as adults they show little resemblance to fish or other mobile backboned animals. Many settle permanently and grow anchored to solid surfaces.
Sea squirts are the best known of the tunicates. New Zealand has at least 166 species, some of which are solitary, while others form colonies. Individuals are flask shaped, with a siphon that takes in sea water. The water is passed through gills to extract oxygen and filter out food particles. Water and waste products are squirted out of the body through a discharge siphon. One conspicuous species around the South Island is the stalked sea tulip (Boltenia pachydermatina or kāeo), which is common on rocks and wharf pilings at low-tide level.
About 12 species have recently arrived in New Zealand waters as hitchhikers on ships’ hulls or stowaways in ballast water. Two have created problems because of their rapid spread into marine farming areas. Ciona intestinalis outgrows the mussels cultivated on rope lines, preventing them from feeding. Didemnum vexillum, which arrived attached to a barge in the Marlborough Sounds in 2001, forms colonies on artificial surfaces – it is only a matter of time before they spread to mussel lines.
Salps were collected by Joseph Banks in the first piece of fieldwork conducted in New Zealand. On board the Endeavour, on 7 October 1769, he dipped his net into the waters of Poverty Bay and reported retrieving salps, bryozoans and seaweeds.
Salps and appendicularians
There are two groups of tunicate living as plankton in the upper ocean. Salps are transparent barrel-shaped animals that may live singly or in colonies. The colonial species join together in chains which are usually a few centimetres long, although 20-metre colonies of fire salps have been observed in summer in waters around northern New Zealand. Salps reproduce rapidly when the plant plankton they eat is abundant. The salps, in turn, become food for fish, marine mammals and seabirds. There are 19 known salp species in New Zealand waters.
Less well known are appendicularians, of which five species have been found in New Zealand. These creatures are very small (3–5 millimetres long), and drift through the sea in a mucus house. They construct these houses around them by secreting mucus from glands in their heads. The houses are inflated with water and serve to catch food.