Wild blacktip sharks swimming freely off the Florida coast can detect low-frequency sounds from nearly 250 feet away, and instantly turn away from wherever the noise came from, according to a new study from Florida Atlantic University published in the journal Integrative Organismal Biology.
Researchers had long assumed sharks could hear reasonably well, but testing exactly how far, and whether a free-swimming shark can tell where a sound is coming from, is difficult to do outside a lab. Palm Beach County's shallow, clear water and its predictable wintering blacktip population gave the team, led by FAU biology professor Stephen Kajiura, an unusual opportunity: they anchored a boat, let an underwater speaker drift up to about 60 feet away, and played three bands of low-frequency sound plus a high-frequency control tone the sharks shouldn't be able to hear at all. A drone hovering overhead filmed the sharks' reactions frame by frame.
The sharks ignored the control tone but reacted to all three low-frequency ranges, responding from as far as 74 meters, about 243 feet, away, sharply changing direction rather than merely startling in place. More than 70% of those responses happened in what's called the acoustic "far field," where sound behaves differently than it does close to the source, which told researchers the sharks weren't just picking up on nearby vibration.
"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."
— Stephen Kajiura, Florida Atlantic University
The finding is notable partly because of what sharks lack: unlike many bony fish, they have no gas-filled swim bladder to help detect sound pressure, relying instead on their inner ears and a structure called the macula neglecta. Lead author Caroline Sullivan, who ran the experiments as part of her master's degree, said doing the work in the ocean rather than a tank was essential to getting a clean result:
"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."
— Caroline Sullivan, Florida Atlantic University
The team, whose work was also covered by ScienceDaily, now wants to pin down exactly how sharks' sensory systems process sound from that far away — useful both for understanding shark behavior and, eventually, for anyone designing quieter boats or fishing gear.
