The Ocean’s Hidden Language: How Sound Shapes the Sea
Sound is a crucial stimulus that allows humans to perceive and interpret the world around us. It helps us understand our surroundings by picking up on vibrations, enabling us to hear speech, detect danger, and sense other important auditory cues that guide our actions and decision-making. Similarly, sound plays a critical role in underwater ecosystems. Just as sound facilitates communication and navigation on land, it becomes even more vital in the ocean, traveling roughly five times faster than it does in air. This unique property allows marine animals to communicate over vast distances, perceiving sounds from farther away than terrestrial creatures can.
Other stimuli, such as sight and smell, are also important for perceiving underwater surroundings. However, these senses function at a reduced capacity compared to how they operate on land. Water absorbs light more quickly than in air, limiting visibility, and scents dissipate faster, making it harder to detect odors. As a result, the diminished effectiveness of sight and smell underwater, coupled with the enhanced movement of sound, make sound the most efficient means of communicating for many species.
Interestingly, despite the importance of sound in both terrestrial and aquatic ecosystems, for decades the ocean was popularly imagined as a vast, silent expanse—an idea that gained traction through Jacques Cousteau’s influential film, The Silent World (1956). However, this misconception began to shift after declassified naval reports from World War II and the Cold War raised concerns about “biological noise” in the ocean. The use of underwater microphones, formally called hydrophones, offered researchers their first glimpse into the bustling, noisy world beneath the waves.
Over the years the notion of a silent ocean has been dispelled as researchers and enthusiasts alike have been drawn to the complex vocalizations of cetaceans and the calming sounds of rain beneath the surface. Scientists now classify underwater sounds into three categories: biological, geological, and anthropogenic (human-made). These sounds often overlap, creating what we call the ocean’s soundscape. Healthy coral reefs, for example, host some of the most complex soundscapes, filled with hundreds of sound-creating organisms, breaking waves, and humming engines—making reefs one of the noisiest ecosystems on Earth.
Sound is so important to ocean life that a reef’s very existence is in part due to noise. Many reef organisms begin life as larvae, drifting in the open ocean. Current research shows that these larvae use biological sound cues from coral reefs to find suitable habitats for growth. These cues help organisms distinguish between healthy and degraded reefs, as quieter reefs often signal fewer animals and ultimately an unhealthy ecosystem. Because these degraded reefs are less attractive to juveniles, scientists have begun using acoustic enrichment— playing healthy reef sound recordings at degraded ecosystems—to boost recruitment and aid in recovery. Experiments have shown success, leading to increases in the number and types of species in the reef.
While the overall soundscape of a coral reef provides essential cues for organisms to navigate, each inhabitant contributes its own distinctive vocalizations. Let’s explore some of the fascinating creatures whose sounds play a critical role in shaping the vibrant underwater world of a coral reef.
Snapping Shrimp
Credit: NOAA
Snapping shrimp (Alpheus spp.) are among the loudest biological sound producers in the ocean. Often, these organisms can be found living in large colonies within coastal ecosystems like coral reefs and rocky shores. Their most distinctive feature is a large claw that snaps shut quickly, creating a sharp “snap” sound. A bubble forms as the claw is rapidly opened, and pops as the claw closes. Despite their small size of about 2 inches, large groups of Alpheus spp. make so much noise, they are capable of disrupting sonar and underwater communication.
Spiny Lobster (Palinuridae)
Credit: Sheila Patek & the Patek Lab
Spiny lobsters are a family of about 60 species (Palinuridae), and are easily recognized by their long, spiny antennae and the absence of claws on their first four pairs of walking legs. Unlike most arthropods that make sound by rubbing a hard pick against a series of bumps, spiny lobsters produce sound differently. They rub a soft part of their antennae, called the plectrum, against a smooth surface beneath their eyes. This friction creates a rasping sound, like the bowing of a violin. Scientists call this “stick-slip” sound production. It is believed that these sounds help spiny lobsters deter predators.
Black Grouper
Credit: NOAA
Black grouper, Mycteroperca bonaci, are found in the western Atlantic, spending the majority of their adult lives along rocky bottoms like coral reefs. As one of the largest grouper species, these fish can grow up to five feet long, weigh up to 180 pounds, and are distinguished by an olive-gray body with black blotches. M. bonaci make low frequency thumping or pulsing noises that are thought to be associated with courtship and spawning behaviors.
The Diversity of Fish Sounds
Credit: Tim Lamont, Lancaster University
This audio clip features an edited compilation showcasing the diversity that can be found in sound production by fish. Fish use a wide range of vocalizations to communicate, navigate, and interact with their environment. These sounds vary greatly between species, ranging from clicks and grunts to whistles and drumming noises. Fish can produce sounds through various mechanisms, such as rubbing body parts together, vibrating their swim bladders, or even grinding their teeth.
Bottlenose Dolphins
Credit: NOAA
Dolphin sound production has been extensively studied and is well documented, making them one of the most researched marine animals in terms of acoustic behavior. Dolphins produce a wide range of sounds, including whistles and clicks, which are vital for communication, navigation, and hunting. Bottlenose dolphins (Tursiops spp.) use echolocation to emit high-frequency clicks and listen for the returning echoes, which helps them locate prey in murky waters. Furthermore, these marine mammals live in close knit groups and develop unique high-pitched whistles to communicate their identity within the pod.
Human-made Noise
The ocean’s natural soundscape is shaped by the calls of marine life, movements of water, and Earth’s geological processes, but human activity now contributes to underwater acoustics. From the hum of boat engines to the deep rumble of industrial machinery, human produced noise has become an increasingly dominant part of the ocean’s acoustic environment.
Small Vessel (NOAA)
Large Vessel (NOAA)
Pile Driving: Offshore Wind Energy Construction (NOAA)
Artificial noise has increased exponentially since the industrial revolution, with the introduction of large, powered vessels designed to transport goods and services. These cargo ships, equipped with roaring engines and whirring propellers, have dramatically altered the ocean's soundscape. However, they are not the only sources of human-made noise in the water. Oil exploration, sonar, and recreational vessels, among others also contribute to the rise of anthropogenic noise. Despite its growing presence, noise pollution remains one of the least understood threats to marine life, especially when compared to more extensively studied issues such as overexploitation and habitat degradation.
Like other harmful influences on marine ecosystems, noise pollution has serious consequences for marine life. Effects can range from temporary hearing loss to the abandonment of critical migratory paths, and in extreme cases—death. One major concern for coral reefs is the disruption of their natural soundscape. This phenomenon, known as masking, has the potential to prevent marine organisms from detecting crucial biological sounds. If larval fish and invertebrates cannot detect a coral reef’s natural soundscape, they may fail to reach the ecosystem, negatively affecting the reef’s overall health.
As we continue to uncover the complexities of ocean sound, it is clear that the impact of anthropogenic noise is far-reaching, affecting not only marine life but the health of entire ecosystems. Protecting the ocean’s natural soundscape requires a united effort to reduce harmful noise pollution and preserve the delicate balance of life beneath the waves. The future of our oceans depends on our ability to listen, understand, and take meaningful action to safeguard the bustling world that sustains planet Earth.