This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Shipping, construction work and other human activities are altering the acoustic conditions in our planet's seas. Sound pressure is often examined to determine the effects that this underwater noise has on marine organisms.
However, these observations may not be thorough enough for creatures that live on the seafloor. A research team working alongside Dr. Mareike Huhn from the Department of General Zoology and Neurobiology at Ruhr University Bochum wanted to find out how sea squirts respond to underwater noise.
Sea squirts are small tunicates that live a sedentary existence, attaching themselves to the seabed, rocks or other substrates. The researchers discovered that these animals do not respond to sound pressure so much as they do to vibrations. The team published its findings in Marine Biology.
"We examined the sea squirt Halocynthia papillosa to see which components of low-frequency, vibroacoustic stimuli elicit behavioral responses," reports Til Böttner about the research for his doctoral thesis. "In field experiments in the Mediterranean and under controlled lab conditions, the creatures responded with contractions mainly to stimuli between 50 and 800 hertz. Interestingly, these reactions were associated with increased vibrations in the substrate." Surprisingly, the sea squirts showed no comparable reactions even at underwater sound pressure levels exceeding 130 decibels, provided that substrate vibrations remained below the determined reaction thresholds.
"The results show that sound pressure alone cannot explain the observed behavioral reactions," Huhn concludes. It is not yet certain which sensory structures perceive the mechanical stimuli. However, sea squirts possess specialized ciliated mechanoreceptor cells in the region of the siphons, particularly in what is known as the coronal organ.
These may register local movements in the water or mechanical deformations. "Further physiological and neurobiological studies are required to determine whether these receptors are stimulated by particle motion in the water or by vibrations transmitted via the substrate and the mantle," says Böttner. Sound pressure, particle motion and substrate-borne vibrations are physically closely intertwined underwater, especially at low frequencies.
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