The Sounds of Life Underwater

Dr Anjani Ganase, marine biologist, asks us to use our sense of hearing even when exploring under the sea. The sounds of life are some of the clues that scientists can detect on healthy coral reefs. 

When we snorkel and dive on coral reefs, the cues are mostly visual. We observe coral, sponges and soft corals of different shapes and sizes, and if we accidentally brush up against a fire coral we are quickly reminded what touch feels like. But have we thought about the sounds we hear from coral reefs? To our untrained ears, the sounds we hear – the cracking and the snapping – are like background radio static. This noisy discord comes from the busy underwater activity from the many reef residents. Reefs are incredibly noisy places and it may be difficult to distinguish particular noises, since some are outside the range of hearing for humans. 

Coral reefs with higher coral cover tend to be noisier and have more diverse reef sounds. Turks and Caicos Islands. Photo Credit The Ocean Agency/ XL Catlin Seaview Survey

Next time you go diving or snorkelling, listen carefully to the reef because you may be able to hear the low grunts of a fish nearby, the snapping of a shrimp’s claw, the clicking of urchin spines or even the crunching of Parrotfish beaks as they graze the reef substrate. As happens on land, sounds in the marine world have many functional purposes for communication. The only difference is that sound is transmitted to farther distances underwater. The sounds on the reef scapes vary from place to place based on the type of coral reef communities, but the sounds may also change daily, between day and night. During the twilight periods, reef sounds may intensify as there are changing shifts, similar to morning and evening traffic, where the nocturnal animals are awakening while the daytime residents are retreating. 

Specific examples of “noisy” activities include spawning aggregations, courting behaviour, territorial behaviour, feeding activities and competition (Kaplan et al 2015). When male Bumphead Parrotfish actually butt heads to win the right to mate with a female, the sound of the clash rings like metal striking metal for the whole reef to hear!

Finding the way by sound

After spawning events, coral and fish larvae are projected into the water column and they use an array of environmental cues, including sound, to find their way back to the reef and to determine suitable living spaces on the reef. Scientists have shown that coral and fish larvae were attracted to recorded sounds of reefs and would swim in the direction of the sound, regardless of other cues such as light. Other sensory cues may take priority when the larvae are actually in the vicinity of the reef. There is also a clear association between the range and intensity of reef sounds and coral reef cover. This is because healthy coral reefs with more structural complexity tend to house higher numbers and arrays of fish and invertebrate species, which in turn contributes to the amount and types of sounds being produced. Using specific reef sound indices – loudness and range - as a proxy for reef health can be a simple tool for monitoring coral reef health. A study compared the reef sounds among protected and unprotected reef areas in Moorea, South Pacific, and discovered that protected areas were generally noisier corresponding to the higher coral composition and reef fish densities, while non-protected sites were generally quieter (Bertucci et al 2016). 

Reef degradation can occur as a result of human activity – over-harvesting of fish and crustaceans which contribute to the noises of the reef - or damage from natural disasters where inhabitants lose the homes and need to seek refuge elsewhere. These forms of disturbances are reflected in the changes in reef sound - it becomes quieter - or a shift in the types of reef sounds (Gordon et al 2018). 

Degraded coral reefs with less structure and inhabitants tend to be quieter. Lizard Island reef following coral bleaching event in 2016 resulted in significant loss in coral cover and reef residents. Photo credit: The Ocean Agency / XL Catlin Seaview Survey

What does this mean for coral reef recovery?  

Scientists have also shown that larvae are more likely to seek out healthier reefs because the quieter sounds of degraded reef may be more difficult to hear. Coral reefs with poor health and lower coral cover have a lower intensity and range of sound, which means that fish larvae have a harder time detecting the reef from the background oceanic noises which results in lower recruitment rates (up to 40 %; Gordon et al 2018). This means that degraded coral reefs may be at even higher risk of further degradation because of limited capacity to attract recruits to the reef community. At the same time, there are also more artificial sounds being created by humans. 

What about too much noise?

While reefs are becoming harder to hear as they become more degraded, the ocean has become an even noisier place because of human activity. There is a considerable amount of marine traffic in the ocean that makes it difficult for even larger marine mammals to communicate and navigate with sound. Along the shallow coastlines, much of the boat traffic overlaps with the same range of the sound frequencies as many reef fish species.  This overlapping of sound may make communication by sound on reefs extremely difficult. Let’s start thinking about all the environmental conditions, including the sound environment of coral reefs and marine habitats in general, when it comes to management of environmental conditions of the reef.

References:

Vermeij MJA, Marhaver KL, Huijbers CM, Nagelkerken I, Simpson SD (2010) Coral Larvae Move toward Reef Sounds. PLOS ONE 5(5): e10660.https://doi.org/10.1371/journal.pone.0010660

Bertucci F, Parmentier E, Lecellier G, Hawkins AD, Lecchini D. Acoustic indices provide information on the status of coral reefs: an example from Moorea Island in the South Pacific. Scientific reports. 2016 Sep 15;6:33326.

Gordon TA, Harding HR, Wong KE, Merchant ND, Meekan MG, McCormick MI, Radford AN, Simpson SD. Habitat degradation negatively affects auditory settlement behavior of coral reef fishes. Proceedings of the National Academy of Sciences. 2018 May 15;115(20):5193-8.