Sound travels through the ocean carrying clues about potential prey, predators and other animals, giving sharks an important way to detect what they cannot yet see. But exactly how far a free-swimming shark can hear a sound – and whether it can determine where that sound is coming from – has remained largely unknown.
Now, a new Florida Atlantic University study of blacktip sharks (Carcharhinus limbatus) is helping close that gap. Using an underwater speaker, an aerial drone and free-swimming sharks, researchers found that sharks can detect sounds from considerable distances and respond by dramatically changing direction away from the source – even when the sound source is nearly 250 feet away.
Results of the study, published in the journal Integrative Organismal Biology , provide the first quantified evidence that free-swimming sharks can detect and orient away from sound in the acoustic far field, offering new insight into one of the most fundamental yet least understood senses in these ancient marine predators.
Researchers chose blacktip sharks because their predictable seasonal aggregations off Southeast Florida provided a rare opportunity to observe free-swimming sharks in clear, shallow water. Each winter, large numbers of blacktip sharks gather along the Palm Beach County coast, while smaller groups remain in the area year-round.
"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," said Stephen Kajiura, Ph.D., senior author and a professor of biological sciences in FAU's Charles E. Schmidt College of Science .
The study was conducted in shallow nearshore waters throughout Palm Beach County. Researchers anchored a boat and deployed an underwater speaker which drifted with the current up to 19 meters away, which minimized the boat's influence on the sharks. They tested three ranges of low-frequency sounds – 100 to 200 Hertz, 200 to 400 Hertz and 400 to 800 Hertz – along with a 10-kiloHertz control sound outside the known hearing range of sharks. They played the sounds at a high intensity to startle the sharks rather than try to attract them.
Researchers used calibrated hydrophones to map sound levels at different distances so they could calculate exactly what the sharks heard when they responded. An aerial drone tracked free-swimming sharks from 40 to 50 meters above the water as researchers presented control and experimental sounds. Frame-by-frame video analysis was used to measure the sharks' response distance and changes in swimming direction.
The sharks responded to all three experimental sound ranges but not to the 10-kiloHertz control. They detected low-frequency sounds from up to 74 meters (243 feet) away, considerably farther away than previously demonstrated under free-swimming conditions. Interestingly, the sharks rapidly changed course away from the source, indicating they could determine the direction of the sound. More than 70% of responses occurred in the acoustic far field, and the sharks were more sensitive to lower frequencies, detecting them from farther away and at lower sound levels.
"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."
This finding is intriguing because sharks do not have the gas-filled swim bladder found in many bony fish, an organ that can help detect sound pressure. Sharks are instead thought to rely on their inner ears, including a specialized structure called the macula neglecta, to detect movement and vibrations produced by sound traveling through the water.
The findings also highlight the value of studying shark hearing in the wild, where researchers can observe responses to sound without the reflections and other limitations of laboratory tanks.
"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. This is why it is so important to do these types of experiments in the ocean with wild sharks to get a natural response" said Caroline Sullivan, lead author who conducted this work as part of her master's degree in biological sciences.
Study co-author is Edmund Gerstein, Ph.D., a research director, Charles E. Schmidt College of Science.
"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 awarded to Kajiura, and the National Save the Sea Turtle Foundation awarded to Sullivan.
- FAU -
About Florida Atlantic University:
Florida Atlantic University is one of the nation's fastest-rising public research universities, serving more than 32,000 students in South Florida. Ranked among the Top 100 Public Universities by U.S. News & World Report, Florida Atlantic is also one of only 13 institutions nationwide to hold Carnegie Foundation designations for R1 research, opportunity and community engagement. Guided by its strategic plan, "2031FAU: Where Tomorrow Begins," the university is focused on delivering career-ready education and experiential learning, driving scholarly inquiry that creates healthier, safer and more prosperous communities, strengthening institutional excellence, and elevating its impact across South Florida and beyond. Florida Atlantic continues to advance its position as Florida's first quantum university, integrating research, education and industry partnerships around next-generation computing technologies. Through other signature strengths in neuroscience and healthy aging, environmental, ocean and coastal innovation, and national defense and autonomous systems, Florida Atlantic expands knowledge, fuels economic opportunity and fulfills its mission as South Florida's hometown university.
