The Loud and Exciting Audible World of Coral Reefs

Written by Evan Quinter

Among the many reasons to love coral reefs, we often adore these underwater oases for their visual beauty. Carpeted by corals of all shapes and sizes, coral reefs offer some of the most vibrant and cosmic color displays in the natural world; it’s clear why the reef tourism industry capitalizes on the enticing aesthetics of the reef. But as humans are drawn to the reef’s sights, marine organisms follow another sensory cue, sound. 

Coral reefs are alive with the babel of thousands of marine creatures, from tiny shrimp to large groupers. Fish, the singers of this aquatic ensemble, vocalize through two methods: contracting and expanding the sonic muscle around their air bladder (drumming) and sudden changes in direction and velocity when swimming (hydrodynamics). Additionally, they can rub two pieces of hard skeleton or teeth together to produce noise (stridulation). 

Invertebrates also add to the reef’s audible world, primarily through stridulation. Spiny lobsters pull their soft antennas across each other to elicit sound like a violin, which helps them stay noisy even when molting. Crabs are the percussionists, as they place a claw on the ground and scratch their claws together to send vibrations through the substrate. One of the loudest reef creatures, the Snapping Shrimp (a.k.a. the Pistol Shrimp), slams its pincers together with such speed that it emits shockwaves so explosive that they stun their prey and camouflage submarines from sonar!

Fig. 1 Tim Gordon, lead author of the 2019 study, placing speakers in a coral reef, photo credit to T. Gordon

 This underwater symphony has important ecological purposes, mainly in reef animal recruitment. In 2019, researchers from Australia and the U.K. played the sounds of healthy reefs over a degraded coral system and compared community results to deserted, silent reefs. The difference was clear, as the acoustically enriched reef experienced a doubling in overall biological abundance and a 50% increase in species richness. With fish returning to the reef at higher volumes and diversity, their results indicate the restorative potential of acoustic enrichment to improve juvenile fish recruitment. Coral larvae (planulae) also swam toward healthy reef sounds, demonstrating the necessity of acoustic cues for recruitment across various marine taxa.

Fig. 2 demonstrating the possibility of acoustic enrichment improving harmful effects of coral reefs, photo credit to Gordon, T. et al., 2019.

The importance of naturally clamorous reefs is also highlighted when reef sounds are obstructed. As unhealthy reefs are quiet from lack of activity, lower recruitment levels may slow or stunt the revitalization of reefs. On top of this, anthropogenic noise and boat motor pollution can lead to various issues with the reef organisms. Planulae were observed to have difficulty finding suitable habitats within close proximity to boat engines. Reef fish behavior to predatorial signals were altered, with slower response times and even increased space use, as opposed to hiding. Cortisol (related to stress) and hormonal levels also escalated during the first few days of noise pollution exposure, although acclimated tolerance to louder oceans may result in behavioral and community shifts. 

Coral reefs are brilliant ecosystems, rich with a diverse and sonorous soundscape. The soundwaves flowing out from reef communities are vital for recruitment of reef organisms, as well as behavior and predator-prey interactions. When we discuss conservation and activism, we need to remember our impact on the physical, ecological, and acoustical levels of reef ecosystems. 

If you want to hear what a healthy reef sounds like, or certain vocalizations of reef organisms, check out the links below:

Sounds of the coral reef: https://www.youtube.com/watch?v=LYFbMnH8zsc

What Coral Reef Fish Sound Like: https://www.youtube.com/watch?v=9LW4VWHw-uI

Goliath Grouper “Barking”: https://www.youtube.com/watch?v=wFWIyceZyX8

Featured Image – Photo credit to Harry Harding, University of Bristol