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Starfish, sea urchins and other echinoderms

by  Maggy Wassilieff

The stars that live in the ocean are just as fascinating as those in the night sky. Starfish, sea lilies, feather stars, sea urchins and other colourfully named creatures belong to the group known as echinoderms (meaning spiny skinned). They share a distinctive five-segment symmetry and other features that range from practical to bizarre.


Spiny-skinned animals

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.

Distinguishing features

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

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.

Sea comets

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.

Feeding

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).


Sea urchins

New Zealand has about 70 sea urchin species; most are deep-water dwellers, but 11 are found around coastal reefs.

Kina

Kina or the common sea urchin or sea egg (Evechinus chloroticus) is the best-known species – commercially valuable and considered a delicacy by Māori. Resembling a curled-up green hedgehog, kina has a nearly spherical shell (or test) protecting its internal organs. Projecting from the shell are long and short movable spines and tube feet. Its mouth, on the underside, contains a five-sided limy structure known as Aristotle’s lantern. This acts like a set of jaws and teeth, grinding up food into digestible pellets.

These small creatures (5–10 centimetres in diameter) are endemic to New Zealand, found on shallow water reefs from the Three Kings Islands to the Snares, and around the Chatham Islands. They spawn from November to March, and have a free-swimming larval stage that lasts for up to 3 months. They can live for 20 years or more.

Kina are important members of rocky reefs. They are omnivorous but prefer to eat large brown seaweeds, especially the common kelp Ecklonia radiata. Dozens may gather and eat out all the seaweed at a site, leaving it barren. Their fearsome spines afford some protection from predators, but small kina are no match for large rock lobsters, snapper or the seven-armed starfish.

Toxic tips

The short spines of the deep-sea Tam O’Shanter urchin (Araeosoma thetidis), found in New Zealand waters, are tipped with poison glands. Although no one has been injured in New Zealand, it has caused painful wounds on people in Australia and New Caledonia.

Kina roe

Like most echinoderms, kina have separate sexes, but it is difficult to tell them apart unless the shell is cracked open. During early summer, their five sets of sex glands become very swollen. These are the highly prized kina roe, traditionally eaten raw by Māori. Some 600–700 tonnes of wild kina are commercially harvested each year, most being sold on the domestic market. New Zealand fishermen would like to export kina to Japan, where top-quality roe sells for NZ$500 a kilogram. However, the quality of wild roe is variable – some may be bitter or brown, and a reliable export market has not yet developed. Scientists have started raising kina in aquaculture facilities to investigate whether different diets enhance growth or roe quality.

How to eat a kina

There is nothing sophisticated about dining on wild kina. First the animal has to be caught. They are gathered by free diving (no underwater breathing apparatus is permitted), or picked from rocky shores. Gloves are advisable. Once ashore with your catch, it is a matter of cracking open the shell with a rock and pulling out the roe. The flavour has been described as a creamy combination of sweet, sour, salty and bitter.

Sand dollars and heart urchins

Sand dollars (also known as snapper biscuits) and heart urchins are flattened versions of the typical sea urchin, adapted for burrowing in sand and mud. They have smaller and more numerous spines than sea urchins. These animals swallow quantities of sand and mud to extract organic particles for their food. When sand dollars die, the skeleton usually breaks into five triangular segments that are often cast up onto west coast surf beaches. New Zealand scientists use sand dollar embryos to test for toxic chemicals in effluent and sea water.


Other echinoderms

Brittle stars

Brittle stars look like thin starfish. They usually have five long skinny arms – but sometimes as many as eight – connected to a central disc. As their common name suggests, the arms are fragile and break off easily. Like starfish, brittle stars are able to regrow damaged or severed limbs; unlike starfish, they have no anus or intestine. Some of the oddest looking specimens have arms that repeatedly branch, perhaps up to 20 times. These animals, such as Astroba granulatus, are also known as basket stars.

New Zealand has at least 165 species of brittle star, but they are often overlooked – most live in deep waters, and some are quite small. Those in shallow waters usually retreat under rocks and fine sands, or weave themselves among seaweeds. The snake star Astrobachion constrictum hides in black coral.

Brittle stars use their flexible, spiny arms to move through the water, and to seek and capture food. The common mottled brittle star or weki huna (Ophionereis fasciata) secretes mucus along its arms, which trap food particles as they sweep over the sea floor.

Sea lilies and feather stars

Sea lilies are stalked echinoderms resembling feather dusters. They are of ancient lineage and more likely to be found as fossils than as living animals. The 13 New Zealand species inhabit deep waters offshore, living a sedentary life attached to the rocky sea floor or underwater mountains.

Feather stars are brightly coloured, mobile creatures that look like shuttlecocks. They walk on the tips of their arms and can swim. New Zealand has 36 different species.

Both groups are suspension feeders. They stretch out their branching arms, the sticky tube feet on the undersides trapping nutritious titbits. The food is then passed along to the mouth.

Sea daisies

In 1983, nine little creatures were discovered living in sunken wood on the deep sea floor off Castlepoint in the North Island, and off Hokitika in the South Island. They had the distinction of being placed in a class of their own. In scientific terms this is significant – the equivalent of finding a new group of animals such as birds or mammals or fishes. Disc-shaped and edged with a ring of spines, they were clearly echinoderms, but differed from any that were known. They had no arms, mouth, gut, or anus. Scientists named them Xyloplax medusiformis. A second Xyloplax has subsequently been found in decomposing wood off the coast of Bermuda. Resembling flowers, these oddities are known as sea daisies.

Since 2000 their unique position has been questioned. DNA analysis of echinoderm groups suggests that sea daisies may be very modified starfish.

Sea cucumbers

Warty, brainless animals that breathe through their rear end and lose their guts at the slightest disturbance do not sound attractive culinary prospects, but sea cucumbers are considered a delicacy by some. There are about 100 sea cucumber types in New Zealand waters and, in places, they may be the most conspicuous animal of the very deep sea floor. Some show parental care and hold babies under their bodies.

Strawberry fields

Masses of the beautiful red and white-spotted strawberry sea cucumber (Squamocnus brevidentis) gather on rocky walls around the southern coast of the South Island, reaching densities of 1,000 animals in a square metre. One site in Preservation Inlet, Fiordland, is known as the strawberry fields.

Stichopus mollis, or rori, is a grey, soft-bodied, sausage-shaped sea cucumber. It favours sheltered coastal waters to a depth of 40 metres all around New Zealand, growing to 20 centimetres in length. Using its three rows of tube feet, it moves slowly over the sea floor in search of food. Stichopus is a detritus feeder, sifting through mud and sand for organic matter to digest. This animal has been harvested on a small scale in recent years in New Zealand and is being investigated for its aquaculture potential. Sea cucumbers are boiled, gutted, reboiled, and then smoked before use.


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How to cite this page: Maggy Wassilieff, 'Starfish, sea urchins and other echinoderms', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/starfish-sea-urchins-and-other-echinoderms/print (accessed 17 July 2019)

Story by Maggy Wassilieff, published 12 Jun 2006