Noisy sea urchins answer marine mystery
24 Jul 2008
In case you didn’t know - underwater ambient noise around New Zealand’s coast gets louder twice a day. And now scientists have discovered why.
Auckland University researchers have discovered that kina, a common sea urchin or sea egg considered a delicacy by Māori, make a lot of noise when they eat which creates a dawn and dusk chorus beneath the waves.
Craig Radford and Andrew Jeffs, the scientists who researched the marine mystery, recorded the sounds made by reef animals, then compared them with the background sound in the natural reef. That’s when they found that kina were the culprits.
They discovered that grazing sea urchins produced the noise as they scraped algae off rocks. The hard, dome-shaped bodies of the animals act like resonance chambers, amplifying the sound of their chewing.
The urchins hide in crevices during the day, out of sight from predators, and emerge to feed at dusk.
When they first come out I guess they're hungry, so they're eating with lots of gusto and making lots of munching noises," says Andrew Jeffs. He suggests that the peak in urchin feeding just before dawn may be their "supper", before they tuck themselves back into crevices for the day.
This regular noise could even help the larvae of fish and crabs find their way to reefs, says Jeffs, as previous studies have found that some larvae can orient towards sound.
New Zealand species
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 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 that acts like 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 off the northwest coast of New Zealand to the Snares south of Stewart Island, and around the Chatham Islands.
Kina spawn from November to March, and have a free-swimming larval stage that lasts up to three 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.
Fearsome spines afford some protection from predators, but small kina are no match for large rock lobsters, snapper or the seven-armed starfish.
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.
Kina quick facts
Age: may reach up to 20 or more years
Size: average diameter 85-150mm
Colour: green and brown
Food: large algae and other seaweed
Predators: a wide range of fish and invertebrate predators such as rock lobster and starfish
Habitat: rocky shores around New Zealand
Availability for fishing: best harvested August - January
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