Starfish belong to a group of spiny-skinned animals, or echinoderms, which have five-part, symmetrical bodies. They are known as kiri taratara in Māori. New Zealand’s 617 species live in a range of habitats, from intertidal rock pools to the deep sea floor.
Echinoderms are divided into six descriptively named classes:
- starfish (sea stars)
- brittle stars and basket stars
- sea lilies and feather stars
- sea daisies
- sea cucumbers
- sea urchins.
Only New Zealand and Bermuda have examples of all six classes of living echinoderms.
Echinoderms share some unusual features, unknown in other animal groups:
- adult bodies arranged in five equal parts
- internal canals and tubes carrying sea water. This hydraulic system regulates water pressure within their bodies, controlling movement, feeding and respiration.
- transparent tube feet connected to the hydraulic system, which pumps up the feet and helps the creature to walk. Often the feet have suckers for holding prey or anchoring.
- an internal skeleton of calcium carbonate crystals
- a mouth, but no head or brain, although they do have nerves.
Starfish, or sea stars, are common inhabitants of rocky shores. They are easily recognised by their (usually five) radiating arms. The mouth is on the underside and the anus is on top. Starfish have a leathery or spiny upper surface and a grooved undersurface covered with suctioning feet.
Starfish can readily regenerate parts of their body. The native 11-armed Coscinasterias calamaria can split in two, and then regrow missing tissue on both halves. Starfish with one or two large arms and three or four tiny new ones are known as sea comets.
Most starfish are predators, extruding the stomach from the mouth to engulf prey. After digesting the nutritious parts, the starfish swallows its stomach back into its body. The sun star or pātangaroa (Stichaster australis) uses its tubular feet to pull each side of a mussel shell apart, then inserts its stomach and begins digesting the shell’s soft contents.
New Zealand’s largest starfish, 75 centimetres in diameter, is the seven-armed Astrostole scabra, a predator of pāua (abalone), and kina (sea urchins).