Noisy kina

Kina are a particular species of sea urchin endemic to New Zealand. Most of us can identify a kina shell found on a beach as a mottled green globe (sometimes intact) with a hole in the base.

When alive, this green globe is covered with moveable spikes to protect the kina from predators. Kina like to live on shallow rocky reefs where they graze on seaweed and kelp. Kina are an important part of the reef ecosystem – and are very, very noisy.

How kina make sound

Kina eat seaweed through a hole in the base of their shell. Five small teeth extend out of the small gap in the shell and scrape and chew seaweed off the rocks. The mouth and teeth are often called ‘Aristotle’s lantern’ because Aristotle first described this sea urchin structure in a book in approximately 380BC.

When the kina are scraping, chewing and processing the seaweed, the teeth move in and out of the body generating noise. This is a low-frequency crunching sound. You probably wouldn’t expect to be able to hear a kina munching away on a piece of seaweed, especially under water, but researchers have recorded something that they think is kina feeding.

Recordings show an increase in noise levels of around 20dB when the kina start feeding around dusk – a phenomenon referred to as the ‘evening chorus’. A 20dB increase would be perceived by a human ear as a four-fold increase in volume. That is a lot of noise from a very small kina, so how does a kina eating generate such a huge noise?

Kina and Helmholtz resonators

Dr Craig Radford at the Leigh Marine Laboratory has been able to prove that the shell of the kina acts as a sound amplifier. When the kina eats, the sound bounces around the inside of the shell, causing resonance

Anything that can vibrate has particular frequencies (called natural frequencies) at which it vibrates best. Resonance happens when the waves of sound from kina chewing match the natural frequencies for vibration within the kina. The sound vibration builds up, which amplifies the sound, making it louder.

The body and shell of a kina form a special type of structure known as a Helmholtz resonator – a structure with a small neck or opening that is filled with fluid or air.

Pressure fluctuations over the opening causes the fluid or air inside the structure to resonate, forming an audible sound.

A good example is blowing across the top of a bottle, which generates a sound of a certain frequency (pitch). Kina act in the same way. As the teeth move in the opening of the shell, they form a pressure fluctuation that resonates inside the fluid-filled shell.

Kina of different sizes have different resonant (natural) frequencies. The smaller the shell, the higher the resonant frequency, and the larger the shell, the lower the resonant frequency. This is the same as blowing across bottles of different sizes filled with air. The action of the teeth, the size of the shell and the resonant effect means that kina produce a range of noises that can be measured between 400–4,000Hz.

Testing the theory

Scientists have suspected for some time that the noise of the ‘evening chorus’ was due to the increased feeding activity of kina, and in 2008, Dr Radford was able to finally put this to the test.

He designed a laboratory experiment that would allow kina to feed in a tank, which meant he could record the sound they made over time. He proposed that, if kina were indeed working as a Helmholtz resonator, smaller kina would generate a higher frequency sound than larger kina.

When he compared the noise generated by kina of different sizes, he was able to conclusively show, for the first time, that kina do produce noise at the predicted frequency and at the predicted frequency related for their size. This directly correlated to the theoretical calculations based on a Helmholtz resonator. Also, Dr Radford proved that kina can generate enough noise to account for the evening chorus, at the frequencies and loudness detected in the ocean.

Kina are noisy, noisy critters.