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 sea floor. A research team working alongside Dr. Mareike Huhn from the Department of General Zoology and Neurobiology at Ruhr University Bochum thus 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 the sound pressure so much as they do to vibrations. The team published their findings in the scientific journal Marine Biology from 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,” reports Til Böttner 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 underwatersound 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.

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 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 under water, 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: Til Böttner received the prize for the best presentation by an early-career researcher for his presentation of his research at the International Tunicate Meeting (ITM).