An experiment conducted at sea has shown that free-ranging blacktip sharks can detect underwater sounds from up to approximately 74 meters away. The sharks tended to move away quickly from the sound source, indicating they can determine its direction. This finding adds new understanding to the biology of these sharks, which are still not fully understood, and their ecological adaptations. Hearing is a crucial sense for many vertebrates, enabling them to perceive their surroundings, detect prey or predators, and communicate with others of their species. This ability developed as vertebrates evolved, with their earliest ancestors living in water. In many bony fish, a gas-filled sac called the swim bladder helps transmit sound pressure to their auditory structures. However, sharks lack a swim bladder. Despite this, they are still able to respond to sounds and may be able to locate their source. Researchers believe sharks mainly use their inner ear for sound detection, including a specialized sensory structure called the macula neglecta, which may help detect particle movements caused by sound. However, the exact role of this structure remains unclear. Most previous studies on shark hearing have been conducted in controlled environments, such as tanks, where sound can reflect off the walls and create complex patterns that make it difficult to interpret the animals' responses. "Trying to conduct auditory experiments in a tank causes sound to reflect off the walls, creating complex and confusing signals; it's like being in an ice palace. That's why it is so important to conduct this type of experiments at sea with wild sharks to obtain a natural response," explains Caroline Sullivan, a researcher at the Florida Atlantic University (FAU), in a blog post. In a study published in the journal Integrative Organismal Biology, Sullivan and her colleagues measured the distance at which free-ranging blacktip sharks respond to underwater sounds. The team conducted the experiment off the coast of Palm Beach, in southeast Florida, where blacktip sharks (Carcharhinus limbatus) gather predictably. This species is well-suited for study because a large part of the population returns to the area each winter, and some individuals live there year-round. The clear, shallow waters allowed researchers to observe the sharks from the surface while minimizing disturbances to their natural behavior. The researchers used a boat anchored off Palm Beach and submerged an underwater speaker, which they allowed to drift with the current up to a distance of 19 meters. They played three series of low-frequency sounds through the speaker, covering the ranges of 100 to 200 Hz, 200 to 400 Hz, and 400 to 800 Hz. They also used a 10 kHz test sound, which is beyond the known auditory range of sharks. The sounds were played at a volume sufficient to provoke a startle reaction, rather than to attract the sharks. A drone hovered above the area to film the sharks and record their movements during the sound broadcasts. The videos were analyzed frame by frame to measure the sharks' distance from the speaker and their sudden changes in direction. The sharks responded to the three low-frequency sound ranges but not to the 10 kHz test sound. They reacted to the low-frequency sounds from up to 74 meters away, a greater distance than previously reported for sharks in their natural environment. The sharks moved quickly away from the sound source, suggesting they can determine its direction. They were also more sensitive to low frequencies, which they could detect from greater distances. "What makes this discovery particularly interesting is that the sharks responded to sounds located beyond their near acoustic field, where sound behaves differently than near the source," notes Stephen Kajiura, a co-author of the study. "This suggests that they detect the movement of particles associated with sound even at considerable distances from the source - something we had not been able to demonstrate before in wild sharks." These observations suggest that underwater sounds can inform sharks about events occurring at a distance, even before they can see the source. The researchers now plan to study more precisely how the sharks' sensory system detects and processes these distant acoustic signals.