Feeling the noise: behavioural responses in ascidians are associated with substrate-borne vibrations rather than sound pressure alone

Abstract Marine anthropogenic noise pollution has increased substantially in recent decades, raising concerns about its effects on marine organisms. While research has primarily focused on marine mammals and – to a smaller extent – on fish, comparatively little is known about how sessile benthic invertebrates perceive and respond to underwater vibroacoustic stimuli. In this study, behavioural responses of the ascidian Halocynthia papillosa were investigated under field and laboratory conditions to assess the relative contribution of sound pressure and substrate-associated mechanical cues. Specimens were collected from Mediterranean coastal sites near Pula, Croatia (44.83° N, 13.84° E), and Elba, Italy (42.76° N, 10.40° E), between 2022 and 2024. Individuals were exposed to low-frequency stimuli between 50 and 1500 Hz under experimental conditions differing in the relative dominance of sound pressure and substrate-borne vibration. Contraction responses occurred primarily at frequencies between 50 and 800 Hz and were consistently associated with substrate-borne vibration amplitudes exceeding frequency-dependent threshold ranges. In contrast, elevated sound pressure under reduced substrate-borne vibration did not elicit significant contraction responses under the tested conditions. Repeated exposure to vibroacoustic stimulation resulted in reduced contraction duration, relaxation time, and contraction intensity, indicating habituation of the sensory-motor response. Together, these findings suggest that mechanically coupled low-frequency cues associated with elevated substrate-borne vibration play an important role in the behavioural responses of H. papillosa . The study further highlights the importance of incorporating substrate-associated mechanical components into marine bioacoustic experiments involving benthic invertebrates.