Blacktip Sharks Hear Sounds From Nearly 250 Feet Away

A Florida Atlantic University study provides the first quantified evidence that free-swimming sharks detect and orient away from underwater sounds up to 74 meters off.

Florida Atlantic University researchers report the first quantified evidence that free-swimming sharks can detect and respond to underwater sounds from distances extending nearly 250 feet, according to the university's newsdesk. Working with blacktip sharks (Carcharhinus limbatus) off Palm Beach County, the team anchored a boat and deployed an underwater speaker that drifted with the current up to 19 meters away, minimizing the boat's influence. The study, published in Integrative Organismal Biology, found the sharks rapidly changed course away from the source, indicating they could determine its direction. FAU reports the findings challenge assumptions about how these predators perceive their acoustic environment.

The surprise is that more than 70% of responses occurred in the acoustic far field, where sound behaves differently than it does close to the source. FAU's senior author Stephen Kajiura said this suggests the sharks are detecting the particle motion associated with sound even at considerable distances from the source — something not previously demonstrated in free-swimming sharks. This is intriguing because sharks lack the gas-filled swim bladder that helps many bony fish detect sound pressure; they are instead thought to rely on inner-ear structures including the macula neglecta. According to EurekAlert, the sharks also proved more sensitive to lower frequencies, detecting them from farther away and at lower sound levels.

Each winter, large numbers of blacktip sharks gather along the Palm Beach County coast, giving researchers a rare chance to observe free-swimming sharks in clear, shallow water without disturbing natural behavior. The team tested three low-frequency ranges — 100–200 Hz, 200–400 Hz and 400–800 Hz — plus a 10-kiloHertz control outside the known hearing range of sharks, played at high intensity to startle rather than attract. An aerial drone tracked sharks from 40 to 50 meters above the water, and frame-by-frame video analysis measured response distances and direction changes. Lead author Caroline Sullivan, cited by FAU, compared tank experiments to being in a house of mirrors, since reflections create confusing signals.

Sound travels through the ocean carrying clues about prey, predators and other animals, giving sharks a way to detect what they cannot yet see — and hearing from hundreds of feet away adds a major information channel for these ancient predators, Kajiura told FAU. The work also underscores the value of studying hearing in the wild rather than in reflective laboratory tanks, and points to a next question: how the sharks' sensory system picks up and interprets such distant sounds. FAU reports the work was supported by the Colgan Foundation and the National Save the Sea Turtle Foundation.