Til Böttner and Mareike Huhn are studying sea squirts. tunicates. These are small tunicates that live a sedentary existence, attaching themselves to the seabed, rocks, or other substrates.
Photo Credit: Courtesy of Ruhr-Universität Bochum
Scientific Frontline: Extended "At a Glance" Summary: Tunicate Perception of Underwater Noise
The Core Concept: Sea squirts are sedentary marine invertebrates that detect and react to anthropogenic underwater noise through substrate-borne vibrations rather than acoustic sound pressure.
Key Distinction/Mechanism: Unlike many marine animals that respond to waterborne acoustic waves, the sea squirt Halocynthia papillosa exhibits behavioral contractions exclusively in response to mechanical vibrations between 50 and 800 hertz, remaining unaffected by sound pressure levels exceeding 130 decibels.
Major Frameworks/Components:
- Vibroacoustic Stimuli: The complex physical interaction of sound pressure, particle motion, and substrate-borne vibrations in aquatic environments.
- Mechanoreception: The hypothesized use of specialized ciliated mechanoreceptor cells located in the coronal organ to detect local mechanical deformations or structural vibrations.
- Benthic Ecology: The study of bottom-dwelling organisms and their unique sensory adaptations to environmental stressors.
Branch of Science: Marine Biology, Bioacoustics, and Neurobiology.
Future Application: Reevaluating marine environmental impact assessments for coastal construction and shipping to actively include substrate vibration metrics, thereby improving the conservation of benthic ecosystems.
Why It Matters: Current acoustic assessments typically rely solely on waterborne sound pressure, which fails to capture the true impact of human-made noise on bottom-dwelling marine organisms.
Ships and construction projects fill our seas with noise, but stationary sea squirts perceive it much differently from what we previously thought. One study shows that it is not the noise itself that triggers reactions in them.
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 therefore 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 their findings in the scientific journal Marine Biology on August 18, 2026.
Sound Pressure Alone Does Not Suffice
“We examined the sea squirt Halocynthia papillosa to see which components of low-frequency, vibroacoustic stimuli elicit behavioral responses,” notes Til Böttner in 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 Hz. 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 dB, 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.
A Close Look at the Nervous System
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.
Questions Remain
Sound pressure, particle motion, and substrate-borne vibrations are physically closely intertwined underwater, especially at low frequencies. This makes it impossible to completely separate these factors, including in the current study. “However, the findings show that mechanical components of underwater sound have to be more closely considered in the examination of benthic organisms,” say the researchers in Bochum.
Future studies aim to identify which mechanical stimuli are actually perceived and how these are sensorially and neuronally processed. To achieve this, the research conducted at the Department of General Zoology and Neurobiology at Ruhr University Bochum combines marine biology, bioacoustics, and neurobiology to understand the effects of anthropogenic underwater noise in greater detail. The work has drawn international recognition: Böttner received the prize for the best presentation by an early-career researcher for presenting his research at the International Tunicate Meeting (ITM).
Published in journal: Marine Biology
Authors: Til Böttner, Lukas Hessel, René Ortmann, Wolfgang H. Kirchner, Stefan Herlitze, and Mareike Huhn
Source/Credit: Ruhr-Universität Bochum | Raffaela Römer
Edited by: Scientific Frontline
Reference Number: mb093026_01
