Story: Starfish, sea urchins and other echinoderms
Page 2 – Sea urchins
New Zealand has about 70 sea urchin species; most are deep-water dwellers, but 11 are found around coastal reefs.
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.
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.
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.