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

The review, conducted by a single investigator using PubMed and Google Scholar, began with 1,608 publications and, after rigorous screening against modified PRISMA 2020 guidelines, distilled the evidence down to 34 peer-reviewed studies, comprising 31 research papers and three case studies. Nearly all of the included work was published between 2014 and 2025, underscoring how recently the topic has captured scientific attention. The United Kingdom and the United States together produced almost two-thirds of the studies, with additional contributions from Australia, Brazil, New Zealand, China, Pakistan, Portugal and Singapore. Mammals dominated the species roster, with non-human primates studied most often, but birds were also well represented, including flamingos, penguins, hornbills, kiwi and free-flight aviary collections, alongside a handful of reptiles such as an American alligator, a Siamese crocodilian and semi-aquatic turtles. Notably, no study examined teleost fish, and half of all investigations evaluated two or fewer individual animals, a small sample size that limits how broadly the results can be generalized.

Behavioural observation was by far the most common assessment tool, deployed in 28 of the studies through species-specific ethograms that catalogue activities such as resting, foraging, pacing and social interaction. The single most frequently documented response was increased vigilance, reported in ten studies across macaques, gibbons, capuchins, peccaries, orangutans, giraffes, hornbills, cheetahs and mixed aviary species. Vigilance, the scanning behaviour animals use to monitor their environment for threats, is widely interpreted as a sign of heightened arousal or anxiety. Other documented responses included retreating away from visitors or out of sight, decreased rest, increased self- or social grooming, increased pacing, reduced appetite, increased swimming, altered circadian activity patterns, greater time spent underwater in a Siamese crocodile, and even increased protectiveness of infants in orangutans. Importantly, six studies found no statistically significant behavioural change linked to noise, and no publication reported overt abnormalities such as self-inflicted trauma or serious aggression.

Physiological measurement, by contrast, was rare. Only six studies assessed glucocorticoid concentrations, the hormonal fingerprints of activity in the hypothalamic-pituitary-adrenal axis, with five pairing faecal or urinary hormone analysis with behavioural observation. Three of the six reported elevated glucocorticoids: one in okapis attributed to visitor presence, one in orangutans linked to ventilation system noise, and one in a giant panda later attributed to normal physiological variation unrelated to noise. The remaining studies found no elevation or no correlation with behaviour. The review’s authors note that the broader literature increasingly cautions against interpreting glucocorticoids as stand-alone welfare indicators, since levels can fluctuate with reproductive status, circadian rhythms and sampling methods, and argue that pairing hormone data with complementary measures, such as the cognitive bias testing used in one primate study, would produce far more robust welfare conclusions.

Perhaps the most surprising finding concerns the sources of the noise itself. Visitor noise was the focus of 27 studies, but construction noise appeared in five and concerts in two. Yet in two publications, researchers suspected that ventilation systems, not visitors, were the real acoustic culprit, because decibel levels did not rise appreciably when crowds grew. A third study found that behavioural changes tracked a habitat’s waterfall feature more closely than visitor noise. One investigation concluded that reducing visitor numbers or limiting opening hours did not guarantee a quieter environment, highlighting the persistent hum of mechanical infrastructure in zoo soundscapes. At the same time, enclosure acoustics can work in animals’ favour: one study showed that a habitat’s soundscape substantially buffered short-term fluctuations in external noise, suggesting that thoughtful acoustic design can be a protective factor rather than a hazard.

The objective acoustic data gathered across the studies spanned an extraordinary range, from 51.5 decibels to a peak of 122.2 decibels recorded near a sloth habitat in a walk-through enclosure. Concert noise reached a maximum of 88 decibels and construction noise 76 decibels. One study measured construction and non-construction days at different frequencies in hertz, while another compared how A-weighted and C-weighted decibel filters affected penguin species, finding that only southern rockhoppers responded positively to pool use under elevated C-weighted levels. These technical details matter because sound meters positioned on the public-facing side of a habitat may not accurately represent what an animal actually perceives, a limitation the review flags as a significant obstacle to drawing welfare conclusions from sound pressure levels alone.

When it came to characterizing welfare outcomes, the literature revealed a striking inconsistency. Eleven studies suspected a negative welfare impact from anthropogenic noise based on observed behavioural changes, nine could not determine whether welfare was affected, seven reported no substantial impact, and seven did not address welfare as an outcome at all. Welfare assessment protocols were rarely described in a standardized way, and many studies acknowledged confounding factors, including the visual presence of visitors, the sight of construction workers, species-level and individual differences in noise sensitivity, and even the acoustic interference of precipitation. Under the Five Domains model of animal welfare, which integrates nutrition, physical environment, health, behavioural interaction and mental state, these complications underscore that noise cannot be evaluated in isolation. The ventilation system example illustrates the dilemma: mechanical noise may be stressful, yet reducing ventilation could compromise thermal comfort and air quality, producing a net welfare loss across multiple domains.

Fourteen publications offered mitigation recommendations, clustering around three practical strategies. Public education, delivered through signage or trained staff, was the most frequently cited, and one study found that ‘watching eyes’ signage requesting quietness measurably reduced noise levels. The second strategy centred on giving animals choice and control over their location, an established contributor to positive welfare, with several studies documenting animals retreating to indoor or off-exhibit spaces to escape noise. Additional suggestions included controlling visitor numbers and timing, habituation training, adding soothing background sound, increasing vegetation, redesigning enclosures to dampen sound and continuing behavioural monitoring during disruptive periods. Operational planning, such as relocating noise-sensitive species during extended construction, could preserve visitor engagement while protecting animal welfare.

The review concludes that anthropogenic noise alone may not harm welfare in every case, since some studies could not disentangle acoustic from visual disturbances, and that species and individuals differ markedly in their responses. Individual-level welfare assessments, the authors argue, allow institutions to plan more appropriately and may prevent costly habitat reconstruction by identifying minor, targeted modifications before welfare is seriously compromised. As zoo accreditation standards evolve, the authors suggest that noise deserves more formal inclusion in welfare assessment frameworks, and that future research should explore how animals respond to different human-generated frequencies, whether reactions differ between chronic and acute exposure, and which documented behaviours genuinely reflect negative welfare states. For a field barely a decade old, the message is clear: the soundscape of the modern zoo is not just background noise, but a variable that zoological institutions can measure, design around and ultimately manage to the benefit of the animals in their care.

Subject of Research: The effects of anthropogenic noise on animal welfare in zoological settings

Article Title: A Scoping Review: The Effect of Anthropogenic Noise on Animal Welfare in Zoological Settings

Article References: Kruse, T. N., & Flint, M. (2026). A Scoping Review: The Effect of Anthropogenic Noise on Animal Welfare in Zoological Settings. Veterinary Medicine and Science, 12(5), Article e71197. https://doi.org/10.1002/vms3.71197

Image Credits: AI Generated

DOI: 10.1002/vms3.71197

Keywords: anthropogenic noise, zoo animal welfare, animal behaviour, soundscape, glucocorticoids, visitor effects, Five Domains model, noise mitigation, scoping review, captive animals, vigilance, zoo acoustics