The ocean can hide an entire world beyond the reach of sight. For sharks, however, sound may offer an early warning system, carrying information about prey, predators and movement through the water long before anything comes into view. A new study from Florida Atlantic University has now quantified how far that acoustic world extends for one familiar coastal predator. According to Science Daily, researchers studying wild blacktip sharks found that the animals could detect low-frequency sounds from as far as 74 metres, or about 243 feet, away. Even more remarkably, the sharks could determine the sound’s origin and quickly turn away from its source. Scroll down to read the details.
Listening to sharks in the wild
Understanding shark hearing has traditionally been difficult. Laboratory experiments can reveal what frequencies an animal detects, but they do not always capture how it responds to sound while moving freely through the ocean. According to Science Daily, blacktip sharks offered researchers an unusual opportunity. Large numbers gather along Florida’s Palm Beach County coast during winter, with smaller groups present throughout the year. The area’s shallow, clear waters also made it possible to observe individual sharks from above without significantly disrupting their behaviour.The research team anchored a boat offshore and placed an underwater speaker in the water. The speaker was allowed to drift with the current, helping separate the experimental sound from noise generated by the boat, the report adds. Also, researchers played low-frequency sounds across three ranges: 100–200 hertz, 200–400 hertz and 400–800 hertz. A 10-kilohertz sound, outside the known hearing range of sharks, served as a control.A reaction from 243 feet awayThe team used calibrated hydrophones to measure sound levels at different points in the water. At the same time, a drone flying roughly 40 to 50 metres above the surface captured the sharks’ movements, the Science Daily report states. Researchers later analysed the footage frame by frame, tracking each shark’s position and changes in swimming direction when a sound was played. And the results were striking, as blacktip sharks responded to all three low-frequency ranges but showed no comparable response to the 10-kilohertz control. Some detected the sounds from as far as 74 metres away.The sharks also frequently made abrupt turns away from the underwater speaker. This suggested that they were doing more than simply noticing that a sound existed. They were able to determine its direction.More than 70% of the recorded responses occurred in what scientists describe as the acoustic far field, where sound behaves differently from the patterns found close to its source. The sharks were particularly sensitive to lower-frequency sounds, detecting them at greater distances and at lower sound levels, the report explains further.
How do sharks hear underwater?
A specialised sensory structure called the macula neglecta is thought to play an important role in detecting vibrations and particle movement associated with sound travelling through water.
The findings are especially intriguing because sharks do not have the gas-filled swim bladder found in many bony fish. Such an organ can assist sound detection by responding to changes in pressure. Instead, sharks rely largely on their inner ears. A specialised sensory structure called the macula neglecta is thought to play an important role in detecting vibrations and particle movement associated with sound travelling through water. The new research suggests that this sensory system is capable of picking up useful acoustic information even when a sound source is hundreds of feet away.
Why the ocean counts
The study also points to the importance of behavioural experiments in the natural environment. Sound does not behave the same way inside tanks, and the reflections from walls can create complicated acoustic patterns that can make an animal’s response difficult to interpret. In open water, researchers can see how sharks respond to sound without those artificial echoes. This gives a more complete picture of how hearing functions as part of a predator’s everyday sensory toolkit.
An acoustic map under the waves
For a shark cruising the ocean, sound might offer clues to things still well beyond its sight. The ability to detect and orient towards or away from distant acoustic signals may help these predators gain information about their environment, locate potential opportunities or respond to threats. The Florida study adds an important piece to the still incomplete picture of shark sensory biology. It also suggests that the underwater world may be far less silent than it appears from the surface, and that sharks are listening to much more of it than scientists once knew.Images Courtesy: istock
