Study Finds Sea Squirts React to Substrate Vibrations Rather Than Sound Pressure

Researchers at Ruhr University Bochum have found that sedentary sea squirts react primarily to vibrations in the seabed rather than sound pressure caused by underwater noise, according to a study published in the journal Marine Biology on August 18, 2026.

The research, led by Dr. Mareike Huhn from the university's Department of General Zoology and Neurobiology, investigated how the sea squirt species Halocynthia papillosa responds to anthropogenic underwater noise, such as sounds generated by shipping and construction. Sea squirts are stationary marine tunicates that attach themselves to rocks, the seabed, or other submerged surfaces.

Doctoral researcher Til Böttner conducted field experiments in the Mediterranean as well as controlled laboratory tests to analyze the animals' behavioral reactions to low-frequency vibroacoustic stimuli. The researchers observed that the creatures reacted with contractions primarily to frequencies between 50 and 800 Hertz. These behavioral responses were linked to increased vibrations in the substrate.

According to the study, the sea squirts showed no comparable contraction responses even when underwater sound pressure levels exceeded 130 decibels, as long as substrate vibrations stayed below specific reaction thresholds. The researchers noted that sound pressure alone cannot account for the behavioral reactions observed in benthic organisms.

The specific sensory structures responsible for detecting the mechanical stimuli remain unconfirmed. However, the researchers noted that sea squirts have specialized ciliated mechanoreceptor cells in their siphon region, particularly within the coronal organ, which may detect mechanical deformations or local water movements. Böttner stated that further physiological and neurobiological research is needed to determine whether these receptors are triggered by particle motion in the water or by vibrations traveling through the substrate and mantle.

Because sound pressure, particle motion, and substrate vibrations are physically intertwined at low frequencies, the authors noted that isolating these factors entirely remains difficult. The Bochum team concluded that mechanical components of sound must be evaluated more thoroughly when assessing the impact of human-made noise on seafloor organisms.

Source