Potential for acoustic masking due to shipping noise in the European lobster (Homarus gammarus)

Marine traffic is the most pervasive underwater anthropogenic noise pollution which can mask acoustic communication in marine mammals and fish, but its effect in marine invertebrates remains unknown. Here, we performed an at sea experiment to study the potential of shipping noise to mask and alter lobster acoustic communication. We used hydrophones to record buzzing sounds and accelerometers to detect lobster carapace vibrations (i.e. the buzzing sounds' sources). We demonstrated that male individuals produced carapace vibrations under various ambient noise conditions, including heavy shipping noise. However, while the associated waterborne buzzing sounds could be recorded under natural ambient noise levels, they were masked by shipping noise. Additionally, lobsters significantly increased their call rates in presence of shipping noise, suggesting a vocal compensation due to the reduction of intraspecific communication. This study reports for the first time the potential acoustic masking of lobster acoustic communication by chronic anthropogenic noise pollution, which could affect ecologically important behaviors.

Sound detection by the American lobster (Homarus americanus)

Although many crustaceans produce sounds, their hearing abilities and mechanisms are poorly understood, leaving uncertainties regarding whether or how these animals use sound for acoustic communication. Marine invertebrates lack gas-filled organs required for sound pressure detection, but some of them are known to be sensitive to particle motion. Here, we examined whether the American lobster (Homarus americanus) could detect sound and subsequently sought to discern the auditory mechanisms. Acoustic stimuli responses were measured using auditory evoked potential (AEP) methods. Neurophysiological responses were obtained from the brain using tone pips between 80 and 250 Hz, with best sensitivity at 80-120 Hz. There were no significant differences between the auditory thresholds of males and females. Repeated controls (recordings from deceased lobsters, moving electrodes away from the brain and reducing seawater temperature) indicated the evoked potentials' neuronal origin. In addition, AEP responses were similar before and after antennules (including statocysts) were ablated, demonstrating that the statocysts, a long-proposed auditory structure in crustaceans, are not the sensory organs responsible for lobster sound detection. However, AEPs could be eliminated (or highly reduced) after immobilizing hairfans, which cover much of lobster bodies. These results suggest that these external cuticular hairs are likely to be responsible for sound detection, and imply that hearing is mechanistically possible in a wider array of invertebrates than previously considered. Because the lobsters' hearing range encompasses the fundamental frequency of their buzzing sounds, it is likely that they use sound for intraspecific communication, broadening our understanding of the sensory ecology of this commercially vital species. The lobsters' low-frequency acoustic sensitivity also underscores clear concerns about the potential impacts of anthropogenic noise.

Group behavior and tolerance of Eurasian minnow (Phoxinus phoxinus) in response to tones of differing pulse repetition rate

Behavioral guidance systems are commonly used in freshwater fish conservation. The biological relevance of sound to fish and recorded responses to human-generated noise supports the viability of the use of acoustics as an effective stimulus in such technologies. Relatively little information exists on the long-term responses and recovery of fish to repeated acoustic exposures. In a controlled laboratory study, the response and tolerance of Eurasian minnow (Phoxinus phoxinus) shoals to tonal signals (150 Hz of 1 s pulse duration) differing only in temporal characteristics ("continuous," "slow," "intermediate," or "fast" pulse repetition rate) were investigated. In comparison to independent control groups, fish increased their mean group swimming speed, decreased inter-individual distance, and became more aligned in response to the onset of all four acoustic treatments. The magnitude of response, and time taken to develop a tolerance to a treatment differed according to pulse repetition rate. Groups were found to have the greatest and longest lasting response to tone sequences tested in this study when they were pulsed at an intermediate rate of 0.2 s^-1. This study illustrates the importance of understanding the response of fish to acoustic signals, and will assist toward the development of longer-term effective acoustic guidance systems.

Acoustic behaviour of male European lobsters (Homarus gammarus) during agonistic encounters

Previous studies have demonstrated that male European lobsters (Homarus gammarus) use chemical and visual signals as a means of intraspecific communication during agonistic encounters. In this study, we show that they also produce buzzing sounds during these encounters. This result was missed in earlier studies because low-frequency buzzing sounds are highly attenuated in tanks, and are thus difficult to detect with hydrophones. To address this issue, we designed a behavioural tank experiment using hydrophones, with accelerometers placed on the lobsters to directly detect their carapace vibrations (i.e. the sources of the buzzing sounds). While we found that both dominant and submissive individuals produced carapace vibrations during every agonistic encounter, very few of the associated buzzing sounds (15%) were recorded by the hydrophones. This difference is explained by their high attenuation in tanks. We then used the method of algorithmic complexity to analyse the carapace vibration sequences as call-and-response signals between dominant and submissive individuals. Even though some intriguing patterns appeared for closely size-matched pairs (<5 mm carapace length difference), the results of the analysis did not permit us to infer that the processes underlying these sequences could be differentiated from random ones. Thus, such results prevented any conclusions about acoustic communication. This concurs with both the high attenuation of the buzzing sounds during the experiments and the poor understanding of acoustic perception by lobsters. New approaches that circumvent tank acoustic issues are now required to validate the existence of acoustic communication in lobsters.

Noise Impact on European Sea Bass Behavior: Temporal Structure Matters

Anthropogenic sounds come in different forms, varying not only in amplitude and frequency spectrum but also in temporal structure. Although fish are sensitive to the temporal characteristics of sound, little is known about how their behavior is affected by anthropogenic sounds of different temporal patterns. We investigated this question using groups of Dicentrarchus labrax (European sea bass) in an outdoor basin. Our data revealed that the temporal pattern of sound exposure is important in noise impact assessments.