The research, conducted by scientists at Florida Atlantic University and published in the journal Integrative Organismal Biology, focused on blacktip sharks, Carcharhinus limbatus, a species whose predictable winter aggregations along the Palm Beach County coast created an unusual scientific opportunity. Each winter, large numbers of blacktips gather in the shallow, clear nearshore waters of the region, while smaller groups remain year-round. That combination of abundance, accessibility, and water clarity allowed the team to observe unrestrained, free-swimming sharks from above while simultaneously presenting controlled underwater sounds, something that is extraordinarily difficult to achieve in most shark habitats and virtually impossible in a laboratory tank.

Their abundance and accessibility made it possible to observe the animals from above without disturbing their natural behavior while also presenting controlled underwater sounds, explained Stephen Kajiura, Ph.D., senior author of the study and a professor of biological sciences in Florida Atlantic University’s Charles E. Schmidt College of Science. By working with sharks in their natural environment rather than in captivity, the researchers could be confident that the behaviors they measured reflected genuine auditory capabilities rather than artifacts of confinement.

The experimental design was carefully constructed to separate the sound stimulus from any influence of the research platform. The team anchored a boat and deployed an underwater speaker that drifted with the current, at times up to 19 meters away from the vessel, minimizing the chance that the boat itself, rather than the speaker, was what the sharks were reacting to. Three ranges of low-frequency sounds were tested: 100 to 200 Hertz, 200 to 400 Hertz, and 400 to 800 Hertz. These frequencies fall within the range that sharks are known to detect. As a control, the researchers also played a 10-kiloHertz tone, a frequency lying outside the known hearing range of sharks, to confirm that any observed reactions were genuinely auditory rather than responses to some other cue. The sounds were played at high intensity with the deliberate aim of startling the sharks rather than luring them in, allowing the team to measure avoidance and orientation responses.

Quantifying what the sharks actually heard required careful acoustic calibration. The researchers used calibrated hydrophones to map sound levels at different distances from the speaker, building a picture of how the sound field decayed with range. This calibration step meant that whenever a shark visibly reacted, the team could calculate precisely what sound level and frequency the animal had detected at that moment. Above the water, an aerial drone flying at an altitude of 40 to 50 meters recorded the sharks as control and experimental sounds were presented. Frame-by-frame analysis of the drone video then allowed the researchers to measure two critical variables: the distance at which each shark responded, and the change in swimming direction that followed.

The results were striking. The sharks responded to all three experimental sound ranges but showed no reaction to the 10-kiloHertz control tone, confirming that the responses were driven by audible sound. More remarkably, the animals detected the low-frequency signals from up to 74 meters away, a distance considerably greater than any previously demonstrated under free-swimming conditions. Even more intriguing was the direction of the response: rather than turning toward the sound, the sharks rapidly changed course away from it. That behavioral pattern indicated that the animals were not merely detecting that a sound existed, but were able to determine the direction from which it originated, a capability known as sound localization.

More than 70 percent of the observed responses occurred in what acousticians call the far field, the region beyond the acoustic near field where sound behaves differently than it does close to its source. Near a source, sound is dominated by complex local flows of water particles; in the far field, the sound propagates as a traveling wave, and its properties change in fundamental ways. The sharks’ demonstrated sensitivity in the far field suggests they are detecting the particle motion associated with sound even at considerable distances from the source, something that had not previously been demonstrated in free-swimming sharks, Kajiura noted. This distinction matters because it implies that shark hearing operates effectively not just in the immediate vicinity of a sound producer but across the long ranges at which acoustic information is actually useful in the open ocean.

The finding is particularly fascinating from a comparative anatomy perspective. Many bony fish possess a gas-filled swim bladder, an organ that can be compressed by sound pressure waves and thereby convert pressure fluctuations into signals the fish can detect, often greatly extending hearing sensitivity. Sharks lack any such structure. Instead, they are thought to rely on their inner ears, including a specialized sensory region called the macula neglecta, to detect the movement and vibrations that sound generates as it travels through the water. The new results suggest that this particle-motion-based system is far more capable at distance than earlier work had established, allowing sharks to extract directional information from faint, far-field acoustic signals without the pressure-sensing hardware that serves other fish.

The study also underscores why shark hearing research belongs in the ocean rather than the laboratory. Tanks are acoustically hostile environments for this kind of work: sound reflects off the walls, floor, and surface, creating interference patterns and echoes that bear little resemblance to natural sound fields. Trying to do hearing experiments in a tank results in the sound bouncing off the walls, which causes complex and confusing signals; it is like being in a house of mirrors, said Caroline Sullivan, lead author, who conducted the work as part of her master’s degree in biological sciences. This is why it is so important to do these types of experiments in the ocean with wild sharks to get a natural response, she added. By combining open-water acoustics with drone-based behavioral observation, the team effectively built a field laboratory capable of measuring sensory performance in animals that were free to ignore the experiment entirely, which makes their consistent, measurable responses all the more convincing.

The broader implications reach beyond basic sensory biology. The ocean is an acoustic environment, and sharks are clearly tuned into it in ways scientists are only beginning to understand, Kajiura said. Being able to detect and respond to sounds from hundreds of feet away gives these predators an important source of information about their surroundings, he noted, adding that the next question is how their sensory system allows them to pick up and interpret these distant sounds. Answering that question could reshape understanding of how sharks hunt, navigate, and avoid threats, and it carries practical weight as well: as human activities such as shipping, construction, and sonar introduce ever more noise into coastal waters, knowing the distances and frequencies at which sharks hear and react becomes essential for assessing how anthropogenic sound might influence these ecologically important predators. The work was supported by the Colgan Foundation, awarded to Kajiura, and the National Save the Sea Turtle Foundation, awarded to Sullivan, with Edmund Gerstein, Ph.D., a research director in the Charles E. Schmidt College of Science, serving as co-author. For now, the study stands as a vivid reminder that the ocean’s most iconic predators perceive a world of sound that humans are only beginning to measure, and that some of the most fundamental questions about ancient senses are best answered where the animals live, in clear water, under an open sky, with a drone hovering quietly overhead.

Subject of Research: Sound detection and directional orientation in free-swimming blacktip sharks

Article Title: Sharks can hear sounds nearly 250 feet away and find the source

Article References: Sharks can hear sounds nearly 250 feet away and find the source. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: blacktip sharks, shark hearing, underwater acoustics, far field sound, macula neglecta, Florida Atlantic University, Integrative Organismal Biology, drone observation, sound localization, marine predators, Palm Beach County, sensory biology