The reefs of New Zealand are noisy places, not the silent world that many of us imagine. The animals that live on reefs, such as shrimp, kina and fish, all have their distinctive sounds, and this combines to form a symphony of sound. We might not be able to hear it very well, but reefs are noisy places.
The first listening post
The sounds of the sea have been known since the Second World War (1939–1945) when the Navy started recording under water to listen for submarines and approaching ships. In New Zealand, when the war ended, navy researchers continued their interest in underwater noise. In 1958, a hydrophone (underwater speaker) was permanently sited at a Navy listening station off Great Barrier Island in the Hauraki Gulf. This allowed scientists to hear for the first time the rhythm and flow of underwater sounds.
The variety was astonishing – a ‘morning chorus’, the ‘barnyard chorus’ and an ‘evening chorus’ were all recorded and named. Imagine being the first researcher to hear a chorus of squeals, creaks, cries, barks, groans and whoops deep under New Zealand waters. Due to the similarity of these noises to a range of domestic animals, this pattern was named the ‘barnyard chorus’ in this early research.
Finding out how reef noise is generated
Scientists were quick to realise that many of these noises were caused by animals (biotic) rather than just the movement of the water and the earth (abiotic). The search was on for what could possibly be causing this enormous diversity of sound.
In the 1960s, kina were first suggested as the source for the sound pattern called the ‘evening chorus’. Researchers had noticed that, when they recorded under water, there was often a dramatic increase in the sound recorded in the evenings (particularly in the range 800–4,000Hz). In 2008, scientists at the Leigh Marine Reserve were finally able to prove that kina on local reefs were responsible for most of this sound.
However, there were lots of other noises – barking, creaking and groaning – and scientists wanted to find out what could be causing this.
Finding the noisiest animals
Over the past 50 years, researchers have identified a huge variety of sea-dwelling animals that make noise.
Internationally, the most well known are the snapping shrimp. These small creatures (only 3–7cm long) are capable of making a brief sound that can reach 218 decibels, whereas rock music is only around 150dB. However, these measurements aren’t quite comparable, as the decibel scale is dependent on a number of factors that are different under water. Adjusting to account for these differences means that, in air, the sound of the shrimp might be about 150dB – still a painfully loud rock concert!
New Zealand reefs are home to many different species of snapping shrimp, and we have our own native species of this incredible creature – the kōwhitiwhiti moana (Alpheuas novaezealandiae).
Like all snapping shrimp, they generate noise by snapping an enlarged front pincer together very rapidly. This causes water in the pincer to superheat. The resulting gas expands rapidly, causing a pressure wave to form, and this wave can be recorded as sound.
The snapping action is used as a defence or predatory mechanism by the shrimp. It can rapidly stun another animal with a mere snap of a pincer. While the sound of one claw snap might be short-lived, the sound of many hundreds of shrimps can be recorded while they hunt the reef and defend their territory.
Other noisy reef species
What about all those other sounds from the other animals of the barnyard? Research is still going on to figure out all the details, but we do know that there are a number of other noisemakers at home on New Zealand reefs.
Kina and snapping shrimp are responsible for most of the noise, but there are a range of other sounds. A lot of these come from fish. Gurnard have been recorded grunting, growling and groaning, while John Dory bark like a dog!
While listening to the reef, researchers have also recorded a huge variation of sound levels. The noisiest period seems to be around dusk when an increase of approximately 20dB can be heard above the normal ‘background’ noise of waves and wind – 20dB is equivalent to the difference in noise from a hairdryer (90dB) as compared to a chainsaw (110dB).
Researchers have found that both shrimp and kina are more active at dusk as opposed to the middle of the day or the middle of the night (a pattern referred to as ‘crepuscular’). This increased activity leads to a spike in the sound recording and forms the evening chorus that humans first heard back in the 1940s.
Nature of science
Science knowledge development often involves collaboration. An understanding of noise under water was first developed by Navy personnel during WWII and continued by navy researchers after the war. Since then, an international team of scientists has collaborated to investigate the range of noises found on reefs around the world. Scientists from the Leigh Marine Laboratory work with colleagues in Australia and further afield.