Can Oysters Hear?
An animal’s ability to detect acoustic signals, be it transmitted through the water or another substrate where they live, is a crucial sense. Unlike visual and chemical environmental stimuli, acoustic signals have unique advantages because they travel rapidly, and an animal can detect them irrespectively of the ambient light and the direction of the current.
A team of scientists from France and Morocco joined forces to determine, once and for all, whether oysters can hear. The research group based their work on the increasing concern that anthropogenic noise may have a significant impact on the marine environment. They explain that despite clear evidence on disturbed fish and aquatic mammals, there is still insufficient data for most invertebrates.
In this study of oysters, the researchers chose rapid valve closure as a sign of behavioural responses to various sound frequencies at different acoustic energies. The oysters were exposed to different pure tones through two different underwater speakers, and their responses were recorded using noninvasive sensors. The behavioural responses were assessed across all oysters and, naturally, showed some variation. Like humans, their sensitivity to noise varies on an individual basis.
Depending on the sound frequency, the number of responding oysters exhibiting a decrease in valve opening ranged from 0 to 100%. Besides the valve closure amplitude, the response delay varied, too. The calculated response curve shows that oysters are sensitive to frequencies ranging from 10 Hz to less than 1000 Hz. Here, the greatest response falls within the frequency range of 10 Hz to 200 Hz, with 60% to 95% of animals responding during the tests. Because the valve closure amplitude is a result of animals’ sound perception, it shows that low frequencies carry a meaning different from higher frequencies; oysters are most sensitive to sound at low frequencies. The response delay also confirms this: It is systematically faster in the lowest frequency range of 10 Hz to 80 Hz.
The researchers explain that in water-breathers, sound reaches the sense organ directly because their body has more or less the same density as the water. Their sense organ, along with the rest of the body, moves with the sound. Mollusks in particular detect particle motion in a manner similar to the hair cells in the ears of vertebrates.
Sounds and vibrations produced by breaking waves, currents, and even thunder claps are in the sensitivity range of mussels and oysters. Scientists suggest that such hearing abilities in oysters could play a role in synchronizing spawning events (i.e. knowing when a thunderstorm is coming) and influence spawning efficiency (i.e. anticipating tidal changes). Thus, hearing abilities could participate indirectly in oyster population dynamics. Interestingly, the results show that oysters can also hear swimming fishes and the movements of certain crabs and shrimps, both potential predators.
Most anthropogenic noise pollution in coastal zones inhabited by oysters and other bivalve mollusks produces low frequencies (below 1 kHz) and is caused by distant and nearby cargo boats scattered throughout the oceans. It is important to note that shipping and wind turbines are considered to produce sound continuously, and such noise is within the hearing ability of oysters, too. On the other hand, small recreational boats, jet skis, and water bikes, which produce higher frequency sounds at 1 kHz to 5 kHz, may go unnoticed.
Naturally, invertebrates are an important part of biodiversity. This study shows how important it is to understand their relationship with the world of sound, especially how they interact with and to what extent their behaviors and natural habitat are compromised by human activity. Animal advocates will benefit from this new knowledge when promoting the need to protect and respect these creatures. Understanding how oysters are affected by noise will undoubtedly invoke more human compassion for these wonderful animals of the deep.