By Paul Caiger.
Jacques Cousteau’s celebrated 1950s book and documentary about the oceans was called the The Silent World. However, the underwater world is anything but, something that even the great Cousteau did not disagree with later in life.
Underwater sound is produced by a range of sources, from the abiotic sounds of waves crashing and boulders tumbling, to the myriad of animal life that produces sound, be it intentional or incidentally as by-product of movement. With the knowledge that there is such a noisy world down there, there is an ever-growing field of exploration in this area.
The field of bioacoustics, or animal sounds, has been around for some time. The ancient scholars Aristotle and Pliny the Elder described some fish sounds over two thousand years ago. Understandably, the highly-specialised echolocation and sonar of cetaceans has fascinated humans for some time, and was the first to be looked at in detail, including by Cousteau himself. But dolphins and whales aren’t the only oceanic animals that can hear sound, and researchers have found that many fish, cephalopods, corals, crustaceans, gastropods and turtles can hear sound, and all of them show distinct behavioural responses to it.
There are estimated to be around 800 fish species that can produce sound, but this is likely a vast underestimate. Fish intentionally produce sound to vocalise, whether it be to communicate with each other, or to warn off aggressors. These sounds are made by rubbing fin spines together or grinding their teeth, or by using sonic muscles to push, pull and rub against their swimbladders, much like a drum. In fact, many fish are actually named for the sound they produce, such as ‘croakers’, ‘drums’ and ‘grunts’, and many soniferous fishes can be found in New Zealand’s waters.
Any fisherman that has caught a john dory or gurnard would be aware of the grunting noise made with the fish on board. Similarly, any diver who has ventured too close to a black angelfish defending his nest will be bombarded by a pulse of sharp knocking sounds accompanied by an angry, charging fish! Furthermore, the common perception that the ‘knocks’ or ‘clicks’ divers often hear are crayfish is actually more likely their cave-dwelling housemates, bigeyes (crayfish can make stridulating sounds but this is quieter and less frequent). Bigeyes have recently been found to be rather adept at communicating, and at night large schools of bigeyes can spread out from their caves in the darkness to feed, keeping in touch with each other by contact calls.
There are other less obvious animals that produce sound, and some of them are ecologically very important. Kina, or sea urchins, are one of the two most distinct biological sound-producing organisms on New Zealand coastal reefs. The hollow shell of these animals creates a chamber, and by acting as a Helmholtz resonator, amplify the sound of the urchins scraping algae to a loud and distinct sound.
The second important animals are also invertebrates – snapping shrimp. Snapping shrimp make loud snapping sounds with their claws, which is actually produced by a cavitation bubble imploding. A side effect of the imploding bubble is also the emission of a short burst of light (sonoluminescence) that momentarily reaches over 4,000 degrees Celsius! The jury is still out on whether the cavitation bubble is for communication or to stun prey, but either way, this is probably the most ubiquitous biotic sound around the coast, and is the crackling sound you will hear snorkelling in the shallows or when sleeping on a boat.
Together, sounds produced by these two invertebrates play a key role in reef ecology, and create distinct habitat sound characteristics that can travel tens of kilometres. (In fact, the loud and extremely low-frequency humpback songs have been heard across ocean basins!) The larvae of many reef animals have been shown to be attracted to these habitat-specific sounds, explaining how reef animals end up in the right place more often than not after weeks adrift in the open ocean.
Using hydrophones, we can hear these underwater sounds, and when this is studied over time, patterns begin to emerge. Sound levels are highest during the dawn and dusk periods, and lowest during midday and midnight, much the same as the dawn choruses in our native bush. These are produced by the many crepuscular activities of the reef’s inhabitants. And if we take a further step back, there are strong lunar and seasonal patterns in reef sounds, all generally connected to feeding and mating behaviours.
Synthesising all this information together to get an idea of an ecosystem-wide approach, comes into the realm of soundscape ecology, a recently emerging field. The study of sound throughout the whole environment has been likened to a canary in a coal mine – sound could be the first indicator of environmental changes. Current research at the University of Auckland headed by Dr. Craig Radford is studying the role sound plays in feeding ‘work-ups’, and how, when one group of animals start feeding, they attract the next group, setting off the wildly impressive spectacles that are work-ups.
Of course, not much of the natural world is free from the fingers of man, and underwater sound is no different. Anthropogenic noise, that is, loud noises produced from things such shipping, mining and seismic surveys, are increasingly overwhelming the biological sounds. Man-made noise in the oceans is estimated to be doubling in intensity every decade, and with it, masking sound signatures and drowning out the communication of marine animals. The exact side effects of this are only just beginning to be known, and whether animals can adapt is unclear. Sounds are a perpetual and dynamic property of all natural landscapes, and as underwater sound plays such a pivotal role in innumerable situations beneath the waves, it is surely something we need to think about. You needn’t worry though that the sounds from your scuba equipment will have a detrimental effect on marine life – although it might explain why some fishes are seen more frequently when using a rebreather.