Science1 publisher2 min readPublished
Sea squirt contractions track seabed vibration more closely than sound pressure
Sea squirts in Ruhr University Bochum tests did not react comparably to sound pressure over 130 decibels while seabed vibration stayed below threshold. Noise checks that measure only pressure may miss what seafloor animals sense, though the study recorded behavior and did not measure harm.
The Scientist · Science desk
What happened
- The Bochum team tested the sea squirt Halocynthia papillosa both in field experiments in the Mediterranean and under controlled laboratory conditions.
- The animals contracted mainly in response to stimuli between 50 and 800 hertz, and those contractions coincided with increased vibration in the substrate.
- The sensing structure is unconfirmed, though ciliated mechanoreceptor cells around the siphons, especially in the coronal organ, are candidates.
- The study, by Til Boettner and colleagues working with Mareike Huhn at Ruhr University Bochum, appeared in Marine Biology in 2026.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint At the levels tested, a pressure reading on its own could not tell an assessor whether a seafloor animal like this one detected the noise.
- decision Noise studies of benthic species now have grounds to add substrate-vibration and particle-motion measurements to their pressure readings.
- constraint With vibration and particle motion not fully separated and only behavior recorded, the work cannot yet be the basis for a harm threshold for seabed vibration.
The most informative condition in these experiments is one where the animals did nothing. Underwater sound pressure went above 130 decibels while substrate vibration stayed under the reaction thresholds the team had determined, and the sea squirts showed no comparable reactions [7]. So pressure at those levels was not enough to set off the response.
That result does not show that vibration by itself is enough. At low frequencies, sound pressure, particle motion and substrate-borne vibration are physically intertwined, and the authors write that they could not completely separate them in this study either [11]. The positive half of the finding is an association [6]. The paper's title is worded to match, describing responses "associated with substrate-borne vibrations rather than sound pressure alone" [2].
Mareike Huhn, of Ruhr University Bochum's Department of General Zoology and Neurobiology [1], used the same qualifier. "The results show that sound pressure alone cannot explain the observed behavioral reactions," she said [8].
Which receptor responds determines what should be measured. Til Boettner, who did the work for his doctoral thesis [14], put the open question this way: "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," he said [10]. If the answer is the substrate, then for an animal fixed to the seabed or a rock [3], the exposure that counts is how much that surface moves.
Sound pressure is the quantity often examined when judging how shipping, construction and other human noise affects marine organisms, according to the phys.org report [4]. For Halocynthia papillosa, a high pressure reading did not predict a response [7]. The researchers' own conclusion is about method: "the findings show that mechanical components of underwater sound have to be more closely considered in the examination of benthic organisms," they said [12].
I think the evidence supports the claim that pressure-based assessments can miss what a seafloor animal senses. The measured outcome was a contraction [6]. The report does not give the number of animals tested, the vibration thresholds in physical units, or any effect on feeding, growth or survival.
What to watch
- Physiological recordings from the siphon mechanoreceptors showing whether they respond to particle motion in the water or to vibration arriving through the substrate and mantle.
- Whether the full Marine Biology paper gives vibration thresholds in physical units that can be compared with measured seabed vibration near shipping lanes and construction sites.
- Any follow-up linking repeated contractions at 50 to 800 hertz to feeding, growth or survival in attached sea squirts.