Sea Squirts Feel the Seafloor Shake, Not the Sound Itself

Beneath the surface of the Mediterranean, one of the ocean’s most unassuming animals has just rewritten a chapter of how scientists think about underwater noise. The sea squirt Halocynthia papillosa, a bright red ascidian anchored to rocks and reefs, has no ears, no swim bladder, and no obvious apparatus for hearing. Yet new research from Ruhr University Bochum shows that these filter-feeding tunicates respond in striking ways to low-frequency vibroacoustic stimuli, and that the key to their reactions is not the sound pressure traveling through the water column at all. Instead, the animals appear to react to vibrations transmitted through the very substrate to which they are permanently attached. The finding, published in the journal Marine Biology, carries implications that stretch far beyond one species, touching on how the growing chorus of human-generated noise in the ocean is assessed for its effects on life on the seafloor.

The study was led by Til Böttner as part of his doctoral thesis at the Department of General Zoology and Neurobiology at Ruhr University Bochum. As he explains, the team set out to determine which components of low-frequency, vibroacoustic stimuli actually elicit behavioral responses in the sea squirt. To answer that question, the researchers combined two complementary approaches: field experiments conducted in the Mediterranean, where Halocynthia papillosa lives naturally, and controlled laboratory conditions that allowed them to isolate and manipulate individual components of acoustic stimulation. This dual design matters, because in the real ocean, sound pressure, particle motion, and substrate-borne vibrations are physically intertwined, and teasing apart their relative contributions is one of the hardest problems in modern bioacoustics.

The behavioral results were clear and, in one respect, surprising. The sea squirts responded with contractions mainly to stimuli in a frequency band between 50 and 800 Hertz, a range that overlaps substantially with the low-frequency noise produced by shipping, construction, and other human activities at sea. Interestingly, these reactions were associated with increased vibrations in the substrate, the solid material on which the animals sit. In other words, when the seafloor itself trembled, the sea squirts noticed, contracting in a measurable defensive response. The frequency specificity of the reaction suggests a genuine sensory process rather than a generic startle, and it places these seemingly primitive animals within the growing roster of marine invertebrates known to detect and react to anthropogenic noise.

The most striking result, however, was what the animals did not do. Even when the underwater sound pressure levels exceeded 130 decibels, a level that would be considered significant by many standards used in noise-impact assessments, the sea squirts showed no comparable reactions, provided that the substrate vibrations remained below the thresholds the team had determined experimentally. As researcher Huhn concludes, the results show that sound pressure alone cannot explain the observed behavioral reactions. That single sentence carries considerable weight. Much of the regulatory and scientific framework for underwater noise relies on sound pressure measurements, and this study demonstrates that for at least one benthic species, such measurements may simply miss the stimulus that matters.

The physics behind this distinction is worth unpacking. Underwater sound is conventionally described by two related but distinct quantities: sound pressure, the oscillating compression of the water, and particle motion, the back-and-forth displacement of water molecules, which can be characterized in terms of displacement, velocity, or acceleration. Most fish and many invertebrates are thought to be sensitive to particle motion rather than pressure, yet pressure is far easier to measure and remains the standard metric in noise monitoring. To complicate matters further, at low frequencies a portion of the acoustic energy couples into the seabed and propagates as seismic waves, producing substrate-borne vibrations. For an animal cemented to a rock, like an ascidian, the substrate may effectively be the most direct pathway through which mechanical energy from a distant noise source reaches its body.

What sensory machinery might these animals be using? The answer is not yet certain, and the researchers are careful on this point. However, sea squirts possess specialized ciliated mechanoreceptor cells in the region of their siphons, the twin openings through which they draw in and expel water for filter feeding. These cells are particularly concentrated in a structure known as the coronal organ, and they may register local movements in the water or mechanical deformations of the body surface. Böttner notes that 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, the tough outer covering of the animal. Resolving that question will require direct recordings from the sensory cells and their associated neural pathways, work that is now on the research agenda in Bochum.

The coronal organ itself is a fascinating structure in evolutionary terms. Ciliated mechanoreceptor cells lining the siphons of ascidians are considered homologous, in a broad sense, to the hair cells of vertebrate ears, making tunicates a valuable model for understanding the deep evolutionary origins of mechanosensation. If Halocynthia papillosa is indeed detecting noise through these cells, the study would suggest that sensitivity to low-frequency mechanical disturbance is an ancient trait, present in animals whose lineage split from ours hundreds of millions of years ago. It would also imply that the coronal organ, long studied for its role in detecting water flow and potential threats as the animal adjusts its pumping behavior, may serve double duty as a vibroacoustic sensor tuned to the frequency range where human noise is loudest.

The researchers are candid about the limits of the current work. Sound pressure, particle motion, and substrate-borne vibrations are physically closely intertwined under water, especially at low frequencies, which makes it impossible to completely separate these factors, including in this study. Every acoustic stimulus delivered to an animal in a tank or on a reef carries some mixture of all three components, and even the most careful experimental design can only approximate the isolation of one pathway. Nevertheless, the findings show, as the researchers in Bochum put it, that the mechanical components of underwater sound have to be more closely considered in the examination of benthic organisms, the animals that live on or in the seafloor. For a field that has historically focused on fish, whales, and other free-swimming species, that is a significant redirection of attention toward the sessile majority of marine life.

The broader context makes this redirection urgent. Anthropogenic underwater noise, from commercial shipping and seismic surveys to pile driving and coastal construction, has increased dramatically over recent decades and is now recognized as a global environmental concern. Most impact assessments rest on sound pressure thresholds derived from studies of vertebrates, and the possibility that attached invertebrates respond primarily to substrate vibrations suggests that current metrics may systematically underestimate or mischaracterize the exposure of benthic communities. Since ascidians, mussels, corals, and other sessile animals cannot swim away from a noise source, their behavioral responses, such as the contractions observed in this study, may represent one of the few defenses available to them, and repeated disturbances could carry energetic or ecological costs that have yet to be quantified.

Future studies at Ruhr University Bochum aim to identify which mechanical stimuli are actually perceived by the sea squirts and how these stimuli are sensorially and neuronally processed, integrating marine biology, bioacoustics, and neurobiology into a single research program dedicated to understanding the effects of anthropogenic underwater noise in greater detail. The work has already drawn international recognition: Til Böttner received the prize for the best presentation by an early-career researcher at the International Tunicate Meeting for his presentation of this research. For now, the message of the study is both simple and disruptive to conventional thinking. An animal without ears can still feel the noise of the ocean, and it feels it through the ground. Any serious effort to understand, predict, or mitigate the impact of underwater noise on marine ecosystems will need to listen not only to the water, but to the seafloor beneath it.

Subject of Research: Behavioral responses of the sea squirt Halocynthia papillosa to underwater vibroacoustic stimuli and substrate-borne vibrations

Article Title: How sea squirts perceive underwater noise

Article References: How sea squirts perceive underwater noise. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: sea squirts, ascidians, underwater noise, substrate vibrations, sound pressure, particle motion, mechanoreceptors, coronal organ, bioacoustics, marine biology, anthropogenic noise, benthic organisms