Blacktip Sharks Detect Low-Frequency Sounds from 243 Feet

Blacktip sharks detected low-frequency sounds from distances of up to 74 meters (243 feet) and sharply turned away from the source. This finding provides the first evidence of wild sharks detecting particle motion from sound at ecologically relevant distances, reshaping the understanding of shark sensory ecology. Sharks responded to all three experimental low-frequency sound ranges but ignored a high-frequency control sound beyond their known hearing range. More than 70% of recorded responses occurred in the acoustic far field, a detection distance considerably farther than previously demonstrated under free-swimming conditions.

Sharks were most sensitive to lower-pitched noises, detecting them from greater distances and at quieter volumes. The three ranges of low-frequency sounds tested were:

A 10 kiloHertz control sound, which is beyond the known shark hearing range, was also used and was ignored by the sharks.

Methods avoided tank artifacts to capture natural shark responses

Researchers used drifting underwater speakers and drone monitoring in clear, shallow waters off Palm Beach County to observe free-swimming sharks without reflective interference. The study was conducted during the seasonal influx when blacktip sharks gather in huge numbers off southeast Florida each winter. An anchored boat deployed an underwater speaker that drifted with the current up to 19 meters away to minimize boat influence. Calibrated hydrophones mapped sound levels, while an aerial drone tracked sharks from 40 to 50 meters above the water, allowing frame-by-frame video analysis of response distance and directional changes. Caroline Sullivan, the lead researcher, explained that conducting hearing experiments in a tank results in sound bouncing off walls, creating complex and confusing signals akin to a house of mirrors. This is why it is critical to do these experiments in the ocean with wild sharks to get a natural response. Sounds were played at high intensity to startle sharks rather than attract them.

Findings highlight shark inner-ear adaptation for distant sound detection

Despite lacking gas-filled swim bladders, which many bony fish use for sound detection, sharks detected particle motion via sensitive inner-ear structures. Scientists believe sharks rely on these structures, including the macula neglecta, to pick up vibrations as sound waves travel through water. Professor Stephen Kajiura noted that what makes this finding particularly interesting is that the sharks were responding to sounds beyond the acoustic near field, where sound behaves differently than it does close to the source. He stated that being able to detect and respond to sounds from hundreds of feet away gives these predators an important source of information about their surroundings. This suggests they are detecting particle motion even at considerable distances, something not previously demonstrated in free-swimming sharks.

Research supported by foundations and conducted at Florida Atlantic University

The study was published in the journal Integrative Organismal Biology. Lead author Caroline Sullivan conducted the work as part of her master's degree in biological sciences at Florida Atlantic University. Senior authors were Professor Stephen Kajiura and research director Edmund Gerstein. The study was supported by the Colgan Foundation and the National Save the Sea Turtle Foundation, with materials provided by Florida Atlantic University.

Each winter, large numbers of blacktip sharks gather along the Palm Beach County coast, with smaller groups remaining year-round. Sharks consistently veered away from the sound source even when it was hundreds of feet away, and many swiftly changed direction after hearing sounds, suggesting they could pinpoint the noise origin. Professor Kajiura remarked that the ocean is an acoustic environment, and sharks are clearly tuned into it in ways we are only beginning to understand. The next question is how their sensory system allows them to pick up and interpret these distant sounds.