When asked to picture a beautiful coral reef in their mind, most people would imagine the bursting colors of corals, fish darting to and fro as crabs and eels keep to themselves buried within the protective crevices of the reef, and they might even add a few dark shadows of sharks looming above. However, how many people would imagine a cacophony of sounds in the background to accompany the picturesque seascape? Not many. Strangely enough when humans dream of swimming below the serene ocean surface they picture diving under the water and entering a world of silence, save for splashing near the surface. In reality, below the surface is filled with sounds emanating from marine animals across the ocean; however, the most important sounds to marine life are inaudible sound waves that fill the ocean with signals guiding marine life. In fact, coral reefs can be heard from miles away! Though scientists know that sound is present in the ocean and must play some sort of ecological role, its impact is very understudied. For this reason, scientists from Woods Hole Oceanographic Institute sought to understand how these sounds play a role in directing marine animal larvae to coral reef hotspots.
From Open Ocean to Coral Reefs:
Nearly all marine animal larvae are born in the open ocean and have to somehow move to a reef or protected area in order to grow
and survive. Recently two WHOI scientists sought to find out how larvae know how relocate to coral reefs. They published their findings from a field experiment, which tested their hypothesis that sound drew larval fish and invertebrates to reefs, in the journal Scientific Reports. Surprisingly, their results suggested that sound is not a direct signal to larval marine life to go towards a reef; however, the scientists still believe that sound plays a large factor in determining which specific location on the reef an animal should inhabit. The magnitude of their research is not due to what they discovered but the methods used to reach this conclusion and the emergence of a new subfield of coral reef studies.
The Effect of Sound on Larval Marine Life
Previous studies showed that marine larvae responded to recorded biological sounds that were played back in an enclosed underwater setting. This gave WHOI researchers Max Kaplan and Aran Mooney the idea to further test how sound influences the lives of larval marine life. Previously, any studies regarding underwater sound used a hydrophone (underwater microphone) to gather data on pressure waves. Humans are familiar with pressure waves as the type of sound that pushes on a human’s eardrum. As it turns out, marine animals do not hear sound as we do, instead they feel it through a variety of biological mechanisms. In contrast to pressure waves, a type of sound called particle motion is the physical vibration of water as a sound wave travels through it. In most invertebrates, nerves running through their flesh pick up on these vibrations and they interpret it as sound. Bony fish have otoliths, minute bone-like structures, in their skulls that react to particle motion. The feeling particle motion triggers for marine animals is similar to how it feels to stand near a speaker at a rock concert, as the sound waves vibrate over and through human skin. Given that these physical features exist, scientists are trying to unravel the mystery of what biological function it serves
This study is the first of its kind to use both a hydrophone to measure pressure waves and a specially designed accelerometer to measure particle movement, using these tools they can get more accurate data sets instead of just a ballpark guess. In Maui, the researchers surveyed a shallow reef twice a day, at dawn and mid-morning, for 3 days collecting data from 0 to 1500 meters away from the reef. The results of this 3
day expedition was particle motion levels that were much lower than expected. Dr. Mooney stated that it, “seems unlikely [marine larvae] would be able to use sound to find the reef”.
Why Does this Study Matter?
So what is the big deal about the study if it doesn’t prove sound is used by laval marine animals to locate reefs? The ideas and methodology, not the results, is what has sparked other scientists and ocean enthusiasts around the world to take interest in this research project. First and foremost, the new method these researchers developed to measure particle motion will be implemented in field and labs studies to better understand how sound affects the life stages and behavioral patterns of marine animals. It also raises questions about how aquarium and wild fish react to sound and how integral the background noise of a rowdy coral reef is to the health of fish. Furthermore, should the impact of ocean sound patterns be considered when captive breeding reef fish? Most importantly, the ongoing and previous studies of this subject has opened up the doorway to debate how else can humans aid dying or damaged reefs. Some have suggested that playing recorded noise or imitating the sounds in the water column would entice fish and other marine life to return to the reef. Others believe that understanding how particle motion impacts reef fish and corals could lead to innovative new ways to keep coral reefs around the world healthy. The ingenuity of this study highlights how much there is still to learn about the ocean and how investigating these questions can lead to technological innovation and even conservation of critical coral reef habitats.