Wild blacktip sharks (Carcharhinus limbatus) detected low-frequency underwater sounds from up to 74 meters — roughly 243 feet — and turned sharply away from the source, according to field experiments conducted off the Palm Beach County coast in Florida. The study, published September 29, 2026, in Integrative Organismal Biology, provides the first quantified evidence that free-swimming sharks can detect and orient to sounds in the acoustic far field.

For a profession that often translates sensory research into clinical and behavioral context, the work is a reminder of how poorly understood basic shark senses remain. Until now, scientists had little direct evidence showing how far a shark swimming freely in the ocean can hear, or whether it can localize a distant sound.

Researchers at Florida Atlantic University, led by lead author Caroline Sullivan and senior author Stephen Kajiura, Ph.D., a professor of biological sciences in FAU's Charles E. Schmidt College of Science, chose blacktip sharks because they aggregate predictably off Southeast Florida. Large numbers congregate along the Palm Beach County coast each winter, and smaller groups remain there year-round. Clear, shallow water allowed the team to monitor individual sharks from above while minimizing interference with natural behavior.

"Their abundance and accessibility made it possible for us to observe them from above without disturbing their natural behavior, while also presenting controlled underwater sounds," Kajiura said. Study co-author Edmund Gerstein, Ph.D., a research director in the same college, completed the team.

The experimental design was deliberately field-based. After anchoring a boat, the researchers placed a drifting underwater speaker as far as 19 meters from the vessel to reduce the chance that the boat itself influenced shark behavior. They played three bands of low-frequency sound — 100–200 Hz, 200–400 Hz, and 400–800 Hz — plus a 10 kHz control tone that lies beyond the known hearing range of sharks. Rather than luring animals toward the speaker, they played the sounds loudly enough to provoke a startle response.

Calibrated hydrophones measured sound levels at multiple locations, allowing the team to calculate what each shark was hearing at the moment it reacted. A drone flying 40 to 50 meters above the water recorded the sharks' movements during both experimental and control trials. The researchers then analyzed the footage frame by frame to determine each animal's distance from the speaker and the sharpness of its directional change.

The results were unambiguous. Sharks responded to all three low-frequency bands and showed no response to the 10 kHz control. Detection distances of up to 74 meters substantially exceed anything previously demonstrated in freely swimming sharks. More than 70% of recorded responses occurred in the acoustic far field, and the animals were most sensitive to lower frequencies, which they detected from greater distances and at lower sound levels.

The sharp turns away from the speaker matter as much as the detection distances. The sharks were not simply registering that a sound existed; they could determine the direction it came from.

"What makes this finding particularly interesting is that the sharks were responding to sounds beyond the acoustic near field, where the sound behaves differently than it does close to the source," Kajiura said. "This suggests that they are detecting the particle motion associated with sound even at considerable distances from the source — something we have not previously been able to demonstrate in free-swimming sharks."

The finding carries particular weight because sharks lack the gas-filled swim bladder that helps many bony fish detect sound pressure. Sharks are believed instead to rely largely on their inner ears, including a specialized sensory structure called the macula neglecta, which may detect the motion and vibrations generated as sound travels through water.

The study also makes a methodological argument: ocean-based hearing experiments yield data that laboratory tanks cannot. Inside a tank, sound reflects repeatedly off the walls, creating acoustic patterns that complicate interpretation of an animal's response.

"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," Sullivan said, 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."

The broader implication is that sound may supply sharks with actionable information about events hundreds of feet away — prey, predators, and other animals — long before those targets enter their field of view. With implications for understanding shark behavior around vessel noise, fishing activity, and coastal development, the researchers now aim to define more precisely how the shark sensory system detects and processes these distant acoustic signals.

"The ocean is an acoustic environment, and sharks are clearly tuned into it in ways we 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. The next question is how their sensory system allows them to pick up and interpret these distant sounds."

The work was supported by the Colgan Foundation and the National Save the Sea Turtle Foundation.