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Underwater hearing in sea snakes (Hydrophiinae): first evidence of auditory evoked potential thresholds
Abstract
The viviparous sea snakes (Hydrophiinae) are a secondarily aquatic radiation of more than 60 species that possess many phenotypic adaptations to marine life. However, virtually nothing is known of the role and sensitivity of hearing in sea snakes. This study investigated the hearing sensitivity of the fully marine sea snake Hydrophis stokesii by measuring auditory evoked potential (AEP) audiograms for two individuals. AEPs were recorded from 40 Hz (the lowest frequency tested) up to 600 Hz, with a peak in sensitivity identified at 60 Hz (163.5 dB re. 1 µPa or 123 dB re. 1 µm s-2). Our data suggest that sea snakes are sensitive to low-frequency sounds but have relatively low sensitivity compared with bony fishes and marine turtles. Additional studies are required to understand the role of sound in sea snake life history and further assess these species' vulnerability to anthropogenic noise.
... Particle acceleration can be measured using capacitive, piezoresistive or piezoelectric accelerometers, while particle velocity can be measured using geophones, all of which are proof-mass instruments (a proof mass is a known quantity of mass used in a measuring instrument as a reference for the measurement of an unknown quantity) that are becoming more readily available (Nedelec, 2021). Particle acceleration can also be measured using a pressure gradient between hydrophone pairs (Chapuis et al., 2019). Finally, in simplified acoustic conditions (deep water and far from the source relative to wavelength), particle velocity magnitude but not direction can be estimated from pressure measured by a single hydrophone (Nedelec, 2021). ...
Within the set of risk factors that compromise the conservation of marine biodiversity, one of the least understood concerns is the noise produced by human operations at sea and from land. Many aspects of how noise and other forms of energy may impact the natural balance of the oceans are still unstudied. Substantial attention has been devoted in the last decades to determine the sensitivity to noise of marine mammals—especially cetaceans and pinnipeds—and fish because they are known to possess hearing organs. Recent studies have revealed that a wide diversity of invertebrates are also sensitive to sounds, especially via sensory organs whose original function is to allow maintaining equilibrium in the water column and to sense gravity. Marine invertebrates not only represent the largest proportion of marine biomass and are indicators of ocean health but many species also have important socio-economic values. This review presents the current scientific knowledge on invertebrate bioacoustics (sound production, reception, sensitivity), as well as on how marine invertebrates are affected by anthropogenic noises. It also critically revisits the literature to identify gaps that will frame future research investigating the tolerance to noise of marine ecosystems.
... For example, Crotalus atrox (Rattlesnakes) suspended in a steel mesh basket responded to airborne sounds (emitted from speakers not hard-mounted to the wall but 'held in position by the surrounding acoustic foam') between 200 and 400Hz [12]. Hydrophis stokesii (Sea Snakes) also exhibited responses to sounds (via an underwater speaker) between 40 and 600Hz, peaking at 60Hz and 500Hz [13], and royal pythons (Python regius) had the greatest sensitivity to substrate vibration and sound-pressure at 80-160 Hz [4]. By contrast, human hearing is most sensitive at 2,000-5,000 Hz: more than 10x higher than snakes [14]. ...
Evidence suggests that snakes can hear, but how snakes naturally respond to sound is still unclear. We conducted 304 controlled experiment trials on 19 snakes across five genera in a sound-proof room (4.9 x 4.9 m) at 27ºC, observing the effects of three sounds on individual snake behavior, compared to controls. We quantified eight snake behaviors (body movement, body freezing, head-flicks, tongue-flicks, hissing, periscoping, head fixation, lower jaw drop) in response to three sounds, which were filtered pink-noise within the following frequency ranges: 0-150Hz (sound 1, which produced ground vibrations, as measured by an accelerometer), 150-300Hz (sound 2, which did not produced ground vibrations), 300-450Hz (sound 3, which did not produced ground vibrations). All snake responses were strongly genus dependent. Only one genus (Aspidites, Woma Pythons) significantly increased their probability of movement in response to sound, but three other genera (Acanthophis (Death Adders), Oxyuranus (Taipans), and Pseudonaja (Brown Snakes)) were more likely to move away from sound, signaling potential avoidance behavior. Taipans significantly increased their likelihood of displaying defensive and cautious behaviors in response to sound, but three of the five genera exhibited significantly different types of behaviors in sound trials compared to the control. Our results highlight potential heritable behavioral responses of snakes to sound, clustered within genera. Our study illustrates the behavioral variability among different snake genera, and across sound frequencies, which contributes to our limited understanding of hearing and behavior in snakes.
... This led to the misunderstanding that these animals were deaf for a long time. However, the auditory function of turtles and snakes has been confirmed through combined behavioral and electrophysiological methods [12,13]. The electrical potentials in response to sounds have been recorded from the inner ear of many turtle species [14][15][16]. ...
An auditory ability is essential for communication in vertebrates, and considerable attention has been paid to auditory sensitivity in mammals, birds, and frogs. Turtles were thought to be deaf for a long time; however, recent studies have confirmed the presence of an auditory ability in Trachemys scripta elegans as well as sex-related differences in hearing sensitivity. Earlier studies mainly focused on the morphological and physiological functions of the hearing organ in turtles; thus, the gene expression patterns remain unclear. In this study, 36 transcriptomes from six tissues (inner ear, tympanic membrane, brain, eye, lung, and muscle) were sequenced to explore the gene expression patterns of the hearing system in T. scripta elegans. A weighted gene co-expression network analysis revealed that hub genes related to the inner ear and tympanic membrane are involved in development and signal transduction. Moreover, we identified six differently expressed genes (GABRA1, GABRG2, GABBR2, GNAO1, SLC38A1, and SLC12A5) related to the GABAergic synapse pathway as candidate genes to explain the differences in sexually dimorphic hearing sensitivity. Collectively, this study provides a critical foundation for genetic research on auditory functions in turtles.
... In aquatic snakes, cephalic sensilla may play an important ecological role, since the environmental shift from land to water has led to crucial changes in their sensory modalities (Crowe-Riddell et al., 2016). For example, the few studies of hearing sensitivity in snakes suggest that this is considerably lower in sea snakes compared to terrestrial snakes (Christensen et al., 2012;Chapuis et al., 2019); spectral sensitivities of visual pigments have undergone rapid diversification in marine snakes compared to terrestrial species (Simões et al., 2020); a reduction in the olfactory system has been reported among sea snakes (Shichida et al., 2013); and many fully aquatic marine snakes present larger and more pronounced cephalic sensilla compared to semi-aquatic or terrestrial snakes (Crowe-Riddell et al., 2016). Furthermore, increased size and higher coverage of sensilla has independently evolved in sea snakes and sea kraits on multiple occasions, indicating that these organs may function as hydrodynamic receptors that detect low-frequency water vibrations produced by predators or prey items (Westhoff et al., 2005;Dehnhardt & Mauck, 2008;Crowe-Riddell et al., 2016). ...
Understanding the roles of ecological and sexual selection in the variation of sensory systems may elucidate aspects of the natural history of organisms. Little is known about the evolution of mechanoreception in snakes and how the function and structure of mechanoreceptors vary between species or sexes. Here, we describe the internal and external morphology of cephalic mechanoreceptor sensilla and quantify inter- and intraspecific variation in four sensilla traits of two freshwater snake species that differ in their habitat and diet preferences, Helicops pastazae and Helicops angulatus, by combining scanning electron microscopy (SEM), histological techniques and image analyses. SEM showed sensilla as prominent evaginations of the epidermis surrounded by concentric rings, with H. pastazae having larger and more heterogeneous sensilla. In both species, histology showed a reduction in the outer epidermal layer above the sensilla with a grouping of dermally derived central cells below it. Higher values of sensilla traits were found in H. pastazae, except for the chin-shields. We also found that males of both species had significantly higher values of sensilla traits on all of the scales examined. We hypothesize that the variation in both qualitative and quantitative traits in scale sensilla might be a consequence of differences in foraging and/or reproductive strategies between species and sexes.
... While some locations with high boat traffic such as Broome Port have very few sea snakes, others such as Shark Bay and Darwin Harbour support large populations of sea snakes. Sea snakes are sensitive to mechanical vibrations (Romanov, 1991) and to low-frequency sounds (peak sensitivity at 60 Hz) but have lower sensitivity than bony fishes and marine turtles (Chapuis et al., 2019). Collectively, very little is known on the impact of vibrations and sound on sea snakes. ...
Over the past decade, vertebrate populations globally have experienced significant declines in distribution and abundance. Understanding the reasons behind these population declines is the first step in implementing appropriate management responses to improve conservation outcomes. Uncovering drivers of extirpation events after the fact, however, requires a careful forensic approach to prevent similar declines elsewhere. The once abundant and species-rich sea snake fauna of Ashmore Reef Marine Park, in the Timor Sea, collapsed dramatically in the early 2000s. No such decline has occurred on surrounding reefs. We synthesise the evidence for this collapse and the subsequent slow recovery and evaluate the plausibility of potential drivers for the declines, as well as provide evidence against certain explanations that have been proposed in the past. Our systematic review shows that of seven possible hypotheses considered, at least three are credible and require additional information: (1) stochastic environmental events may have increased the snakes’ susceptibility to pathogens, (2) a resurgence in the abundance of top predators may have induced a localised change in trophic structure, and (3) an acute increase in local boat traffic may have had negative physical impacts. One or more of these factors, possibly acting in combination with as yet other unidentified factors, is the most plausible explanation for the precipitous decline in sea snake populations observed. Based on this position, we identify future research directions with a focus on addressing critical gaps in knowledge to inform and prioritise future management actions.
... Travel time and character of signals reflected from density discontinuities in the seabed provide information on the layering of strata and potential hydrocarbon traps [1,2]. The frequencies produced by seismic sources fall within the hearing sensitivity of fishes [3,4], many invertebrates [5], reptiles [6,7], and marine mammals [8]. The combination of frequency spectra, intensity, and the extended duration of seismic survey operations (often weeks to months) can result in varying degrees of acute and chronic impacts on marine taxa [5,[8][9][10][11][12][13][14][15][16]. ...
An experimental marine seismic source survey off the northwest Australian coast operated a 2600 cubic inch (41.6 l) airgun array, every 5.88 s, along six lines at a northern site and eight lines at a southern site. The airgun array was discharged 27,770 times with 128,313 pressure signals, 38,907 three-axis particle motion signals, and 17,832 ground motion signals recorded. Pressure and ground motion were accurately measured at horizontal ranges from 12 m. Particle motion signals saturated out to 1500 m horizontal range (50% of signals saturated at 230 and 590 m at the northern and southern sites, respectively). For unsaturated signals, sound exposure levels (SEL) correlated with measures of sound pressure level and water particle acceleration (r2= 0.88 to 0.95 at northern site and 0.97 at southern) and ground acceleration (r2= 0.60 and 0.87, northern and southern sites, respectively). The effective array source level was modelled at 247 dB re 1µPa m peak-to-peak, 231 dB re 1 µPa2 m mean-square, and 228 dB re 1 µPa2∙m2 s SEL at 15° below the horizontal. Propagation loss ranged from −29 to −30log10 (range) at the northern site and −29 to −38log10(range) at the southern site, for pressure measures. These high propagation losses are due to near-surface limestone in the seabed of the North West Shelf.
... In particular, amphibians communicate extensively using sounds (i.e. chorus frogs) [107], insects demonstrate hyperacuity in directional hearing [108], reptiles (in particular snakes) and spiders can feel vibrations [109][110][111][112]. ...
Background
Ecological research now deals increasingly with the effects of noise pollution on biodiversity. Indeed, many studies have shown the impacts of anthropogenic noise and concluded that it is potentially a threat to the persistence of many species. The present work is a systematic map of the evidence of the impacts of all anthropogenic noises (industrial, urban, transportation, etc.) on biodiversity. This report describes the mapping process and the evidence base with summary figures and tables presenting the characteristics of the selected articles.
Methods
The method used was published in an a priori protocol. Searches included peer-reviewed and grey literature published in English and French. Two online databases were searched using English terms and search consistency was assessed with a test list. Supplementary searches were also performed (using search engines, a call for literature and searching relevant reviews). Articles were screened through three stages (titles, abstracts, full-texts). No geographical restrictions were applied. The subject population included all wild species (plants and animals excluding humans) and ecosystems. Exposures comprised all types of man-made sounds in terrestrial and aquatic media, including all contexts and sound origins (spontaneous or recorded sounds, in situ or laboratory studies, etc.). All relevant outcomes were considered (space use, reproduction, communication, etc.). Then, for each article selected after full-text screening, metadata were extracted on key variables of interest (species, types of sound, outcomes, etc.).
Review findings
Our main result is a database that includes all retrieved literature on the impacts of anthropogenic noise on species and ecosystems, coded with several markers (sources of noise, species concerned, types of impacts, etc.). Our search produced more than 29,000 articles and 1794 were selected after the three screening stages (1340 studies (i.e. primary research), 379 reviews, 16 meta-analyses). Some articles (n = 19) are written in French and all others are in English. This database is available as an additional file of this report. It provides an overview of the current state of knowledge. It can be used for primary research by identifying knowledge gaps or in view of further analysis, such as systematic reviews. It can also be helpful for scientists and researchers as well as for practitioners, such as managers of transportation infrastructure.
Conclusion
The systematic map reveals that the impacts of anthropogenic noises on species and ecosystems have been researched for many years. In particular, some taxonomic groups (mammals, birds, fishes), types of noise (transportation, industrial, abstract) and outcomes (behavioural, biophysiological, communication) have been studied more than others. Conversely, less knowledge is available on certain species (amphibians, reptiles, invertebrates), noises (recreational, military, urban) and impacts (space use, reproduction, ecosystems). The map does not assess the impacts of anthropogenic noise, but it can be the starting point for more thorough synthesis of evidence. After a critical appraisal, the included reviews and meta-analyses could be exploited, if reliable, to transfer the already synthesized knowledge into operational decisions to reduce noise pollution and protect biodiversity.
... The environment of Shaw's sea snake may also change its ability to sense sound. Studies on the hearing sensitivity of fully marine sea snakes (including Shaw's sea snake) discerned they are sensitive to low-frequency sounds and can sense water motions (such as those generated by prey objects), probably via an expanded scale sensilla (larger than land snakes) on their head (Westhoff et al. 2005;Crowe-Riddell et al. 2016;Chapuis et al. 2019;Crowe-Riddell, Williams, et al. 2019). In Shaw's sea snake, 129 genes in expanded gene families have enrichment in sensory perception of sound (GO:0007605). ...
The transition of terrestrial snakes to marine life approximately 10 million years ago (Ma) is ideal for exploring adaptive evolution. Sea snakes possess phenotype specializations including laterally compressed bodies, paddle-shaped tails, valvular nostrils, cutaneous respiration, elongated lungs and salt glands yet knowledge on the genetic underpinnings of the transition remain limited. Herein, we report the first genome of Shaw's sea snake (Hydrophis curtus) and use it to investigate sea snake secondary marine adaptation. A hybrid assembly strategy obtains a high quality genome. Gene family analyses date a pulsed coding-gene expansion to about 20 Ma, and these genes associate strongly with adaptations to marine environments. Analyses of selection pressure and convergent evolution discover the rapid evolution of protein-coding genes, and some convergent features. Additionally, 108 conserved non-coding elements appear to have evolved quickly, and these may underpin the phenotypic changes. Transposon elements may contribute to adaptive specializations by inserting into genomic regions around functionally related coding genes. The integration of genomic and transcriptomic analyses indicates independent origins and different components in sea snake and terrestrial snake venom; the venom gland of the sea snake harbours the highest PLA2 (17.23%) expression in selected elapids and these genes may organize tandemly in the genome. These analyses provide insights into the genetic mechanisms that underlay the secondary adaptation to marine and venom production of this sea snake.
... Evoked potentials have been recorded from the midbrain of the sea snake Hydrophis (Lapemis) curtus in response to a vibrating sphere Hz, peak sensitivity at 100 Hz), but no nervous response was successfully recorded directly from a scale organ [52]. However, more recently, auditory evoked potentials were recorded from the midbrain of A. laevis and H. stokesii in response to tone bursts from 40 to 600 Hz (peak sensitivity at 60 Hz) [53]. This work showed that some species of sea snakes are capable of detecting low amplitude water motion, pressure and/or particle motion. ...
The evolution of epidermal scales was a major innovation in lepidosaurs, providing a barrier to dehydration and physical stress, while functioning as a sensitive interface for detecting mechanical stimuli in the environment. In snakes, mechanoreception involves tiny scale organs (sensilla) that are concentrated on the surface of the head. The fully marine sea snakes (Hydrophiinae) are closely related to terrestrial hydrophiine snakes but have substantially more protruding (dome-shaped) scale organs that often cover a larger portion of the scale surface. Various divergent selection pressures in the marine environment could account for this morphological variation relating to detection of mechanical stimuli from direct contact with stimuli and/or indirect contact via water motion (i.e. 'hydrodynamic reception'), or co-option for alternate sensory or non-sensory functions. We addressed these hypotheses using immunohistochemistry, and light and electron microscopy, to describe the cells and nerve connections underlying scale organs in two sea snakes, Aipysurus laevis and Hydrophis stokesii. Our results show ultrastructural features in the cephalic scale organs of both marine species that closely resemble the mechanosensitive Meissner-like corpuscles that underlie terrestrial snake scale organs. We conclude that the scale organs of marine hydrophiines have retained a mechanosensory function, but future studies are needed to examine whether they are sensitive to hydrodynamic stimuli.
Noise is a sound that is unwanted due to its frequency and amplitude, and it may be causally associated with stress. Noise adversely affects both captive and wild animals, including reptiles, whose hearing ranges are particularly sensitive to low-frequency sounds, and they can easily be affected by noise in general. Reptiles are also particularly sensitive to light, because they utilise a broader spectrum than many or most other animals. This is difficult to adequately replicate in captivity. Light can also greatly affect reptiles’ physiology and well-being. Light pollution particularly affects foraging behaviour, activity patterns and orientation; hence it is important to maintain an appropriate light environment that considers UV-B and infrared provision in a gradient that allows individuals to behaviourally regulate vitamin D production and temperature. In this chapter, the basic characteristics of reptile sound, noise and light perception are reviewed. Noise and light conditions encountered in captive environments, including transport of lizards for the pet trade, are described, and recommendations are provided to minimise stress caused by noise and inappropriate light conditions.KeywordsSoundLightNoiseThird eyeStressReptileCaptivityWelfare
Snakes evince the ability to detect substrate-borne mechanical waves (through a variety of substrates) and surface mechanical waves; exactly how these specialized vertebrates accomplish this remains largely unknown. Behavioral and neurophysiological studies in snakes have struggled to differentiate the modalities, mechanisms, and central pathways for the airborne and ground-borne detection of mechanical waves. The snake cochlea is the best-known component of this sensory system; previous studies have shown that the snake cochlea has a rather consistent frequency response range, some intriguing differences in sensitivity, and a mechanical coupling to the middle ear ossicle. How pressure waves reach the middle ear ossicle/cochlea is not clear; whether or not there are pathways (perhaps utilizing the lung) to the cochlea that bypass the ossicle, and the relative role of the snake’s vestibular system in the detection of mechanical waves (if any), remain a mystery. The pathway by which neural signals transduced in the cochlea reach higher brain centers has not been determined in snakes. Perhaps most intriguing, we do not know how pressure stimuli encoded at the cochlea are integrated with stimuli encoded elsewhere on the snake’s body.
An anthropogenic cacophony
Sound travels faster and farther in water than in air. Over evolutionary time, many marine organisms have come to rely on sound production, transmission, and reception for key aspects of their lives. These important behaviors are threatened by an increasing cacophony in the marine environment as human-produced sounds have become louder and more prevalent. Duarte et al. review the importance of biologically produced sounds and the ways in which anthropogenically produced sounds are affecting the marine soundscape.
Science , this issue p. eaba4658
The evolution of epidermal scales was a major innovation in lepidosaurs, providing a barrier to dehydration and physical stress, while functioning as a sensitive interface for detecting mechanical stimuli in the environment. In snakes, mechanoreception involves tiny scale organs (sensilla) that are concentrated on the surface of the head. The fully marine sea snakes (Hydrophiinae) are closely related to terrestrial hydrophiine snakes but have substantially more protruding (dome-shaped) scale organs that often cover a larger portion of the scale surface. Various divergent selection pressures in the marine environment could account for this morphological variation relating to detection of mechanical stimuli from direct contact with stimuli and/or indirect contact via water motion (i.e. 'hydrodynamic reception'), or co-option for alternate sensory or non-sensory functions. We addressed these hypotheses using immunohistochemistry, and light and electron microscopy, to describe the cells and nerve connections underlying scale organs in two sea snakes, Aipysurus laevis and Hydrophis stokesii. Our results show ultrastructural features in the cephalic scale organs of both marine species that closely resemble the mechanosensitive Meissner-like corpuscles that underlie terrestrial snake scale organs. We conclude that the scale organs of marine hydrophiines have retained a mechanosensory function, but future studies are needed to examine whether they are sensitive to hydrodynamic stimuli.
Viviparous sea snakes (Elapidae: Hydrophiinae) are fully marine reptiles distributed in the tropical and subtropical waters of the Indian and Pacific Oceans. Their known maximum diving depth ranges between 50 and 100 m and this is thought to limit their ecological ranges to shallow habitats. We report two observations, from industry‐owned remotely operated vehicles, of hydrophiine sea snakes swimming and foraging at depths of approximately 250 m in the Browse Basin on Australia's North West Shelf, in 2014 and 2017. These observations show that sea snakes are capable of diving to the dim‐lit, cold‐water mesopelagic zone, also known as the ‘twilight’ zone. These record‐setting dives raise new questions about the thermal tolerances, diving behaviour and ecological requirements of sea snakes. In addition to significantly extending previous diving records for sea snakes, these observations highlight the importance of university‐industry collaboration in surveying understudied deep‐sea habitats.
Marine snakes represent the most speciose group of marine reptiles and are a significant component of reef and coastal ecosystems in tropical oceans. Research on this group has historically been challenging due to the difficulty in capturing, handling, and keeping these animals for field-and lab-based research. Inexplicable declines in marine snake populations across global hotspots have highlighted the lack of basic information on this group and elevated multiple species as conservation priorities. With the increased interest in research on marine snakes, we conducted a systematic survey of experts to identify twenty key questions that can direct future research. These questions are framed across a wide array of scientific fields to produce much-needed information relevant to the conservation and management of marine snakes.
Viviparous sea snakes are the most rapidly speciating reptiles known, yet the ecological factors underlying this radiation are poorly understood. Here, we reconstructed dated trees for 75% of sea snake species and quantified body shape (forebody relative to hindbody girth), maximum body length and trophic diversity to examine how dietary specialization has influenced morphological diversification in this rapid radiation. We show that sea snake body shape and size are strongly correlated with the proportion of burrowing prey in the diet. Specialist predators of burrowing eels have convergently evolved a ‘microcephalic’ morphotype with dramatically reduced forebody relative to hindbody girth and intermediate body length. By comparison, snakes that predominantly feed on burrowing gobies are generally short-bodied and small-headed, but there is no evidence of convergent evolution. The eel specialists also exhibit faster rates of size and shape evolution compared to all other sea snakes, including those that feed on gobies. Our results suggest that trophic specialization to particular burrowing prey (eels) has invoked strong selective pressures that manifest as predictable and rapid morphological changes. Further studies are needed to examine the genetic and developmental mechanisms underlying these dramatic morphological changes and assess their role in sea snake speciation.
Passive acoustic recording of marine noise has advanced considerably over recent years. For a long time, a lack of widely available technology limited the acquisition of long-term acoustic data sets to a small number of large, cabled installations mostly restricted to military use. For other users, recordings were limited by the available technology to short snapshots of minutes to possibly days of data at a time. As technology has improved, passive acoustic monitoring has shown marine soundscapes are filled with biotic and abiotic sounds that occur on a range of often unpredictable timescales. Thus, snapshot recordings can lead to biased data. In 1999, the Centre for Marine Science and Technology, together with Australia’s Defence Science and Technology Organisation, began developing remote underwater sound recorders to increase the duration and quality of recordings. As time passed, the sound recorders were developed significantly, have been deployed over 600 times at a variety of Australian and international locations and have identified a plethora of biological, geophysical and anthropogenic sound sources. This paper presents a brief history of the recorders’ development and characteristics, some examples of the information they have provided and future direction for their next generation.
Zooplankton underpin the health and productivity of global marine ecosystems. Here we present evidence that suggests seismic surveys cause significant mortality to zooplankton populations. Seismic surveys are used extensively to explore for petroleum resources using intense, low-frequency, acoustic impulse signals. Experimental air gun signal exposure decreased zooplankton abundance when compared with controls, as measured by sonar (~3-4 dB drop within 15-30 min) and net tows (median 64% decrease within 1 h), and caused a two- to threefold increase in dead adult and larval zooplankton. Impacts were observed out to the maximum 1.2 km range sampled, which was more than two orders of magnitude greater than the previously assumed impact range of 10 m. Although no adult krill were present, all larval krill were killed after air gun passage. There is a significant and unacknowledged potential for ocean ecosystem function and productivity to be negatively impacted by present seismic technology.
Marine seismic surveys produce high intensity, low-frequency impulsive sounds at regular intervals, with most sound produced between 10 and 300 Hz. Offshore seismic surveys have long been considered to be disruptive to fisheries, but there are few ecological studies that target commercially important species, particularly invertebrates. This review aims to summarise scientific studies investigating the impacts of low-frequency sound on marine fish and invertebrates, as well as to critically evaluate how such studies may apply to field populations exposed to seismic operations. We focus on marine seismic surveys due to their associated unique sound properties (i.e. acute, low-frequency, mobile source locations), as well as fish and invertebrates due to the commercial value of many species in these groups. The main challenges of seismic impact research are the translation of laboratory results to field populations over a range of sound exposure scenarios and the lack of sound exposure standardisation which hinders the identification of response thresholds. An integrated multidisciplinary approach to manipulative and in situ studies is the most effective way to establish impact thresholds in the context of realistic exposure levels, but if that is not practical the limitations of each approach must be carefully considered.
Anthropogenically driven environmental changes affect our planet at an unprecedented scale and are considered to be a key threat to biodiversity. According to the World Health Organization, anthropogenic noise is one of the most hazardous forms of anthropogenically driven environmental change and is recognized as a major global pollutant. However, crucial advances in the rapidly emerging research on noise pollution focus exclusively on single aspects of noise pollution, e.g. on behaviour, physiology, terrestrial ecosystems, or on certain taxa. Given that more than two-thirds of our planet is covered with water, there is a pressing need to get a holistic understanding of the effects of anthropogenic noise in aquatic ecosystems. We found experimental evidence for negative effects of anthropogenic noise on an individual's development, physiology, and/or behaviour in both invertebrates and vertebrates. We also found that species differ in their response to noise, and highlight the potential underlying mechanisms for these differences. Finally, we point out challenges in the study of aquatic noise pollution and provide directions for future research, which will enhance our understanding of this globally present pollutant.
Scale sensilla are small tactile mechanosensory organs located on the head scales of many squamate reptiles (lizards and snakes). In sea snakes and sea kraits (Elapidae: Hydrophiinae), these scale organs are presumptive scale sensilla that purportedly function as both tactile mechanoreceptors and potentially as hydrodynamic receptors capable of sensing the displacement of water. We combined scanning electron microscopy, silicone casting of the skin and quadrate sampling with a phylogenetic analysis to assess morphological variation in sensilla on the postocular head scale(s) across four terrestrial, 13 fully aquatic and two semi-aquatic species of elapids. Substantial variation exists in the overall coverage of sensilla (0.8–6.5%) among the species sampled and is broadly overlapping in aquatic and terrestrial lineages. However, two observations suggest a divergent, possibly hydrodynamic sensory role of sensilla in sea snake and sea krait species. First, scale sensilla are more protruding (dome-shaped) in aquatic species than in their terrestrial counterparts. Second, exceptionally high overall coverage of sensilla is found only in the fully aquatic sea snakes, and this attribute appears to have evolved multiple times within this group. Our quantification of coverage as a proxy for relative ‘sensitivity’ represents the first analysis of the evolution of sensilla in the transition from terrestrial to marine habitats. However, evidence from physiological and behavioural studies is needed to confirm the functional role of scale sensilla in sea snakes and sea kraits.
Seismic surveys are widely used in marine geophysical oil and gas exploration, employing airguns to produce sound-waves capable of penetrating the sea floor. In recent years, concerns have been raised over the biological impacts of this activity, particularly for marine mammals. While exploration occurs in the waters of at least fifty countries where marine turtles are present, the degree of threat posed by seismic surveys is almost entirely unknown. To investigate this issue, a mixed-methods approach involving a systematic review, policy comparison and stakeholder analysis was employed and recommendations for future research were identified. This study found that turtles have been largely neglected both in terms of research and their inclusion in mitigation policies. Few studies have investigated the potential for seismic surveys to cause behavioural changes or physical damage, indicating a crucial knowledge gap. Possible ramifications for turtles include exclusion from critical habitats, damage to hearing and entanglement in seismic survey equipment. Despite this, the policy comparison revealed that only three countries worldwide currently include turtles in their seismic mitigation guidelines and very few of the measures they specify are based on scientific evidence or proven effectiveness. Opinions obtained from stakeholder groups further highlight the urgent need for directed, in-depth empirical research to better inform and develop appropriate mitigation strategies. As seismic surveying is becoming increasingly widespread and frequent, it is important and timely that we evaluate the extent to which marine turtles, a taxon of global conservation concern, may be affected.
A recent survey lists more than 100 papers utilizing the auditory evoked potential (AEP) recording technique for studying hearing in fishes. More than 95 % of these AEP-studies were published after Kenyon et al. introduced a non-invasive electrophysiological approach in 1998 allowing rapid evaluation of hearing and repeated testing of animals. First, our review compares AEP hearing thresholds to behaviorally gained thresholds. Second, baseline hearing abilities are described and compared in 111 fish species out of 51 families. Following this, studies investigating the functional significance of various accessory hearing structures (Weberian ossicles, swim bladder, otic bladders) by eliminating these morphological structures in various ways are dealt with. Furthermore, studies on the ontogenetic development of hearing are summarized. The AEP-technique was frequently used to study the effects of high sound/noise levels on hearing in particular by measuring the temporary threshold shifts after exposure to various noise types (white noise, pure tones and anthropogenic noises). In addition, the hearing thresholds were determined in the presence of noise (white, ambient, ship noise) in several studies, a phenomenon termed masking. Various ecological (e.g., temperature, cave dwelling), genetic (e.g., albinism), methodical (e.g., ototoxic drugs, threshold criteria, speaker choice) and behavioral (e.g., dominance, reproductive status) factors potentially influencing hearing were investigated. Finally, the technique was successfully utilized to study acoustic communication by comparing hearing curves with sound spectra either under quiet conditions or in the presence of noise, by analyzing the temporal resolution ability of the auditory system and the detection of temporal, spectral and amplitude characteristics of conspecific vocalizations.
In the transition from an aquatic to a terrestrial lifestyle, vertebrate auditory systems have undergone major changes while adapting to aerial hearing. Lungfish are the closest living relatives of tetrapods and their auditory system may therefore be a suitable model of the auditory systems of early tetrapods such as Acanthostega. Therefore, experimental studies on the hearing capabilities of lungfish may shed light on the possible hearing capabilities of early tetrapods and broaden our understanding of hearing across the water-to-land transition. Here, we tested the hypotheses that (i) lungfish are sensitive to underwater pressure using their lungs as pressure-to-particle motion transducers and (ii) lungfish can detect airborne sound. To do so, we used neurophysiological recordings to estimate the vibration and pressure sensitivity of African lungfish (Protopterus annectens) in both water and air. We show that lungfish detect underwater sound pressure via pressure-to-particle motion transduction by air volumes in their lungs. The morphology of lungfish shows no specialized connection between these air volumes and the inner ears, and so our results imply that air breathing may have enabled rudimentary pressure detection as early as the Devonian era. Additionally, we demonstrate that lungfish in spite of their atympanic middle ear can detect airborne sound through detection of sound-induced head vibrations. This strongly suggests that even vertebrates with no middle ear adaptations for aerial hearing, such as the first tetrapods, had rudimentary aerial hearing that may have led to the evolution of tympanic middle ears in recent tetrapods.
© 2015. Published by The Company of Biologists Ltd.
This review highlights significant gaps in our knowledge of the effects of seismic air gun noise on marine mammals. Although the characteristics of the seismic signal at different ranges and depths and at higher frequencies are poorly understood, and there are often insufficient data to identify the appropriate acoustic propagation models to apply in particular conditions, these uncertainties are modest compared with those associated with biological factors. Potential biological effects of air gun noise include physical/physiological effects, behavioral disruption, and indirect effects associated with altered prey availability. Physical/physiological effects could include hearing threshold shifts and auditory damage as well as non-auditory disruption, and can be directly caused by sound exposure or the result of behavioral changes in response to sounds, e.g. recent observations suggesting that exposure to loud noise may result in decompression sickness. Direct information on the extent to which seismic pulses could damage hearing are difficult to obtain and as a consequence the impacts on hearing remain poorly known. Behavioral data have been collected for a few species in a limited range of conditions. Responses, including startle and fright, avoidance, and changes in behavior and vocalization patterns, have been observed in baleen whales, odontocetes, and pinnipeds and in some case these have occurred at ranges of tens or hundreds of kilometers. However, behavioral observations are typically variable, some findings are contradictory, and the biological significance of these effects has not been measured. Where feeding, orientation, hazard avoidance, migration or social behavior are altered, it is possible that populations could be adversely affected. There may also be serious long-term consequences due to chronic exposure, and sound could affect marine mammals indirectly by changing the accessibility of their prey species. A precautionary approach to management and regulation must be recommended. While such large degrees of uncertainty remain, this may result in restrictions to operational practices but these could be relaxed if key uncertainties are clarified by appropriate research.
The purpose of this study was to compare underwater behavioral and auditory evoked potential (AEP) audiograms in a single captive adult loggerhead sea turtle (Caretta caretta). The behavioral audiogram was collected using a go/no-go response procedure and a modified staircase method of threshold determination. AEP thresholds were measured using subdermal electrodes placed beneath the frontoparietal scale, dorsal to the midbrain. Both methods showed the loggerhead sea turtle to have low frequency hearing with best sensitivity between 100 and 400 Hz. AEP testing yielded thresholds from 100 to 1131 Hz with best sensitivity at 200 and 400 Hz (110 dB re. 1 μPa). Behavioral testing using 2 s tonal stimuli yielded underwater thresholds from 50 to 800 Hz with best sensitivity at 100 Hz (98 dB re. 1 μPa). Behavioral thresholds averaged 8 dB lower than AEP thresholds from 100 to 400 Hz and 5 dB higher at 800 Hz. The results suggest that AEP testing can be a good alternative to measuring a behavioral audiogram with wild or untrained marine turtles and when time is a crucial factor.
1.1. Lung morphology and its dual function as a hydrostatic and gas exchange organ are described for the sea snake, Pelamis platurus.2.2. The lung volume is large and surface-resting snakes have a high positive buoyancy.3.3. Prior to diving, buoyancy is reduced although snakes remain positively buoyant.4.4. With prolonged submergence, neutral or negative buoyancy is attained due to compression of lung volume with depth and the removal of O2 from the lung without equivalent replacement of CO2.5.5. The respiratory and hydrostatic functions of the lung are highly compatible and relate well to the swimming and feeding behaviour of this snake.
The hearing thresholds of the nurse shark, Ginglymostoma cirratum, and the yellow stingray, Urobatis jamaicensis, were measured using auditory evoked potentials (AEP). Stimuli were calibrated using a pressure-velocity probe so that the acoustic field could be completely characterized. The results show similar hearing thresholds for both species and similar hearing thresholds to previously measured audiograms for the lemon shark, Negaprion brevirostris, and the horn shark, Heterodontis francisi. All of these audiograms suggest poor hearing abilities, raising questions about field studies showing attraction of sharks to acoustic signals. By extrapolating the particle acceleration thresholds into estimates of their equivalent far-field sound pressure levels, it appears that these sharks cannot likely detect most of the sounds that have attracted sharks in the field.
Snakes lack both an outer ear and a tympanic middle ear, which in most tetrapods provide impedance matching between the air and inner ear fluids and hence improve pressure hearing in air. Snakes would therefore be expected to have very poor pressure hearing and generally be insensitive to airborne sound, whereas the connection of the middle ear bone to the jaw bones in snakes should confer acute sensitivity to substrate vibrations. Some studies have nevertheless claimed that snakes are quite sensitive to both vibration and sound pressure. Here we test the two hypotheses that: (1) snakes are sensitive to sound pressure and (2) snakes are sensitive to vibrations, but cannot hear the sound pressure per se. Vibration and sound-pressure sensitivities were quantified by measuring brainstem evoked potentials in 11 royal pythons, Python regius. Vibrograms and audiograms showed greatest sensitivity at low frequencies of 80-160 Hz, with sensitivities of -54 dB re. 1 m s(-2) and 78 dB re. 20 μPa, respectively. To investigate whether pythons detect sound pressure or sound-induced head vibrations, we measured the sound-induced head vibrations in three dimensions when snakes were exposed to sound pressure at threshold levels. In general, head vibrations induced by threshold-level sound pressure were equal to or greater than those induced by threshold-level vibrations, and therefore sound-pressure sensitivity can be explained by sound-induced head vibration. From this we conclude that pythons, and possibly all snakes, lost effective pressure hearing with the complete reduction of a functional outer and middle ear, but have an acute vibration sensitivity that may be used for communication and detection of predators and prey.
Of the more than 12,000 species and subspecies of extant reptiles, about 100 have re-entered the ocean. Among them are seven species of sea turtles and about 80 species and subspecies of sea snakes, as well as a few other species that are occasionally or regularly found in brackish waters, including various other snakes, the saltwater crocodile, and the marine iguana of the Galapagos Islands. The largest group of marine reptiles, the sea snakes, occur in the tropical and subtropical waters of the Indian and Pacific Oceans from the east coast of Africa to the Gulf of Panama. They inhabit shallow waters along coasts, around islands and coral reefs, river mouths and travel into rivers more than 150 km away from the open ocean. A single species has been found more than 1000 km up rivers. Some have also been found in lakes. The taxonomic status of the sea snakes is still under review and no general agreement exists at the moment. The effects of the exploitation on sea snakes have been investigated in the Philippines and Australia but are almost unknown from other areas. Investigations indicate that some populations are already extinct and others are in danger of extinction in various parts of Asia. All sea turtles are endangered except one. The marine iguana of the Galapagos Islands remains vulnerable due to its limited range. Brackish water snakes are closely associated with mangrove forests and as such are subject to deforestation and coastal development schemes that result in habitat loss. In addition, some are collected for their skins. While none of the coastal species are considered in danger of extinction at the present time, many are data deficient.
Auditory brainstem response (ABR) techniques, an electrophysiological far-field recording method widely used in clinical evaluation of human hearing, were adapted for fishes to overcome the major limitations of traditional behavioral and electrophysiological methods (e.g., invasive surgery, lengthy training of fishes, etc.) used for fish hearing research. Responses to clicks and tone bursts of different frequencies and amplitudes were recorded with cutaneous electrodes. To evaluate the effectiveness of this method, the auditory sensitivity of a hearing specialist (goldfish, Carassius auratus) and a hearing generalist (oscar, Astronotus ocellatus) was investigated and compared to audiograms obtained through psychophysical methods. The ABRs could be obtained between 100 Hz and 2000 Hz (oscar), and up to 5000 Hz (goldfish). The ABR audiograms are similar to those obtained by behavioral methods in both species. The ABR audiogram of curarized (i.e., Flaxedil-treated) goldfish did not differ significantly from two previously published behavioral curves but was lower than that obtained from uncurarized fish. In the oscar, ABR audiometry resulted in lower thresholds and a larger bandwidth than observed in behavioral tests. Comparison between methods revealed the advantages of this technique: rapid evaluation of hearing in untrained fishes, and no limitations on repeated testing of animals.
It has previously been shown that at least one species of fish (the American shad) in the order clupeiforms (herrings, shads, and relatives) is able to detectsounds up to 180 kHz. However, it has not been clear whether other members of this order are also able to detectultrasound. It is now demonstrated, using auditory brainstem response (ABR), that at least one additional species, the gulf menhaden (Brevoortia patronus), is able to detectultrasound, while several other species including the bay anchovy (Anchoa mitchilli), scaled sardine (Harengula jaguana), and Spanish sardine (Sardinella aurita) only detectsounds to about 4 kHz. ABR is used to confirm ultrasonichearing in the American shad. The results suggest that ultrasounddetection may be limited to one subfamily of clupeiforms, the Alosinae. It is suggested that ultrasounddetection involves the utricle of the inner ear and speculate as to why, despite having similar ear structures, only one group may detectultrasound.
A standing wave tube apparatus was used to determine the biophysical basis of underwater hearing sensitivity in 3 species of Rana and in Xenopus laevis. A speaker inside the base of a vertical, water-filled 3 m steel pipe produced standing waves. Pressure and particle motion were measured with a hydrophone and geophone respectively and were spatially 90 degrees out of phase along the length of the tube. Microphonic responses were recorded from the inner ear of frogs lowered through pressure and particle motion maxima and minima. The air-filled lungs of whole frogs produced distortions of the sound field. Preparations of heads with only an air-filled middle ear produced little distortion and showed clear pressure tracking at sound intensities 10-20 dB above hearing thresholds from 200-3000 Hz. Filling the middle ear with water decreased or abolished microphonic responses. Severing the stapes reduced responses except at certain frequencies below about 1000 Hz which varied with body size and likely represent resonant frequencies of the middle ear cavity. We conclude that the frog species examined respond to underwater sound pressure from about 200-3000 Hz with the middle ear cavity responsible for pressure transduction.
The auditory brainstem response (ABR) was recorded in adult budgerigars (Melopsittacus undulatus) in response to clicks and tones. The typical budgerigar ABR waveform showed two prominent peaks occurring within 4 ms of the stimulus onset. As sound-pressure levels increased, ABR peak latency decreased, and peak amplitude increased for all waves while interwave interval remained relatively constant. While ABR thresholds were about 30 dB higher than behavioral thresholds, the shape of the budgerigar audiogram derived from the ABR closely paralleled that of the behavioral audiogram. Based on the ABR, budgerigars hear best between 1000 and 5700 Hz with best sensitivity at 2860 Hz-the frequency corresponding to the peak frequency in budgerigar vocalizations. The latency of ABR peaks increased and amplitude decreased with increasing repetition rate. This rate-dependent latency increase is greater for wave 2 as indicated by the latency increase in the interwave interval. Generally, changes in the ABR to stimulation intensity, frequency, and repetition rate are comparable to what has been found in other vertebrates.
It has recently been shown that a few fish species, including American shad (Alosa sapidissima; Clupeiformes), are able to detect sound up to 180 kHz, an ability not found in most other fishes. Initially, it was proposed that ultrasound detection in shad involves the auditory bullae, swim bladder extensions found in all members of the Clupeiformes. However, while all clupeiformes have bullae, not all can detect ultrasound. Thus, the bullae alone are not sufficient to explain ultrasound detection. In this study, we used a developmental approach to determine when ultrasound detection begins and how the ability to detect ultrasound changes with ontogeny in American shad. We then compared changes in auditory function with morphological development to identify structures that are potentially responsible for ultrasound detection. We found that the auditory bullae and all three auditory end organs are present well before fish show ultrasound detection behaviourally and we suggest that an additional specialization in the utricle (one of the auditory end organs) forms coincident with the onset of ultrasound detection. We further show that this utricular specialization is found in two clupeiform species that can detect ultrasound but not in two clupeiform species not capable of ultrasound detection. Thus, it appears that ultrasound-detecting clupeiformes have undergone structural modification of the utricle that allows detection of ultrasonic stimulation.
The sea snake Lapemis curtus is a piscivorous predator that hunts at dusk. Like land snakes, sea snakes have scale sensillae that may be mechanoreceptive, i.e. that may be useful for the detection of water motions produced by prey fish. In addition, inner ear hair cells of sea snakes may also be involved in the detection of hydrodynamic stimuli. We generated water motions and pressure fluctuations with a vibrating sphere. In the test range 50-200 Hz evoked potentials were recorded from the midbrain of L. curtus in response to vibrating sphere stimuli. In terms of water displacement the lowest threshold amplitudes were in the frequency range 100-150 Hz. In this range peak-to-peak water displacement amplitudes of 1.8 microm (at 100 Hz) and 2.0 microm (150 Hz) generated a neural response in the most sensitive animal. Although this low sensitivity may be sufficient for the detection of fish-generated water motions, it makes it unlikely that L. curtus has a special hydrodynamic sense.
Sciaenid fishes are important models of fish sound production, but investigations into their auditory abilities are limited to acoustic pressure measurements on five species. In this study, we used auditory brainstem response (ABR) to assess the pressure and particle acceleration thresholds of six sciaenid fishes commonly found in Chesapeake Bay, eastern USA: weakfish (Cynoscion regalis), spotted seatrout (Cynoscion nebulosus), Atlantic croaker (Micropogonias undulatus), red drum (Sciaenops ocellatus), spot (Leiostomus xanthurus) and northern kingfish (Menticirrhus saxatilis). Experimental subjects were presented with pure 10 ms tone bursts in 100 Hz steps from 100 Hz to 1.2 kHz using an airborne speaker. Sound stimuli, monitored with a hydrophone and geophone, contained both pressure and particle motion components. Sound pressure and particle acceleration thresholds varied significantly among species and between frequencies; audiograms were notably flatter for acceleration than pressure at low frequencies. Thresholds of species with diverticulae projecting anteriorly from their swim bladders (weakfish, spotted seatrout, and Atlantic croaker) were typically but not significantly lower than those of species lacking such projections (red drum, spot, northern kingfish). Sciaenids were most sensitive at low frequencies that overlap the peak frequencies of their vocalizations. Auditory thresholds of these species were used to estimate idealized propagation distances of sciaenid vocalizations in coastal and estuarine environments.
An experimental program was run by the Centre for Marine Science and Technology of Curtin University between March 1996 and October 1999 to study the environmental implications of offshore seismic survey noise. This work was initiated and sponsored by the Australian Petroleum Production and Exploration Association. The program:characterised air gun signal measurements; modelled air gun array sources and horizontal air gun signal propagation;developed an 'exposure model' to predict the scale of potential biological effects for a given seismic survey over its duration;made observations of humpback whales traversing a 3D seismic survey;carried out experiments of approaching humpback whales with a single operating air gun;carried out trials with an air gun approaching a cage containing sea turtles, fishes or squid; andmodelled the response of fish hearing systems to airgun signals.The generalised response of migrating humpback whales to a 3D seismic vessel was to take some avoidance manoeuvre at >4 km then to allow the seismic vessel to pass no closer than 3 km. Humpback pods containing cows which were involved in resting behaviour in key habitat types, as opposed to migrating animals, were more sensitive and showed an avoidance response estimated at 7−12 km from a large seismic source. Male humpbacks were attracted to a single operating air gun due to what was believed the similarity of an air gun signal and a whale breaching event (leaping clear of the water and slamming back in). Based on the response of captive animals to an approaching single air gun and scaling these results, indicated sea turtles displayed a general 'alarm' response at an estimated 2 km range from an operating seismic vessel and behaviour indicative of avoidance estimated at 1 km. Similar trials with captive fishes showed a generic fish 'alarm' response of swimming faster, swimming to the bottom, tightening school structure, or all three, at an estimated 2−5 km from a seismic source. Modelling the fish ear predicted that at ranges
This chapter discusses the structure and function of hearing in aquatic amphibians, reptiles, and birds. It examines patterns of evolutionary change in the auditory systems of amphibians, reptiles, and birds that have returned to an aquatic lifestyle. It begins with the basic features of the ancestral terrestrial auditory system, followed by the terrestrial auditory system underwater and the mechanisms used for underwater hearing. These mechanisms include sound transmission (bone conduction and pressure transduction) and sound localization. It also compares the auditory systems of aquatic amphibians, reptiles, and bird. Finally, it examines the evolutionary transformations of the ears of secondarily aquatic tetrapods.
Because of the history of sonar and sonar engineering, the concept of "source level" is widely used to characterize anthropogenic sound sources, but is it useful for sources other than sonar transmitters? The concept and applicability of source level are reviewed for sonar, air guns, explosions, ships, and pile drivers. International efforts toward the harmonization of the terminology for underwater sound and measurement procedures for underwater sound sources are summarized, with particular attention to the initiatives of the International Organization for Standardization.
In this article the theoretical concept of sound intensity (a quantity describing the net flow of acoustic energy) is described, and shows how it makes a distinction between the acive and reactive parts of sound fields. Furthermore, it is shown how sound intensity can be measured over a wide frequency range by the use of a specially designed probe, consisting of a pair of closely spaced pressure microphones. The different principles of signal processing are discussed, such as the digital filtering employed in the Bruel & Kjaer Sound Intensity Analyzing System Type 3360, and the use of the Fast Fourier Transform (FFT) technique. The digital filter sound analyzing system operates in real time and calculates third octave spectra more than 100 times faster than an analyzer based upon FFT-technique. The high and low frequency limitations as well as the near field limitations with the two microphone technique are outlined and it is shown how they are minimized. Calibration procedure is also described.
The clinical use of anesthetic agents in reptiles presents a number of unique challenges because of the diversity of the class Reptilia with respect to natural history, size, anatomy, and physiology. Reptiles are commonly maintained as companion animals, widely displayed in zoological institutions, and many species serve as subjects in laboratory facilities. Therefore, to become a skillful clinician, developing an understanding of anesthetic efficacy across reptile species is important. The objective of this review is to provide a current perspective on the practical application of anesthetic agents in commonly maintained pet reptile species.
Ocean ambient noise results from both anthropogenic and natural sources. Different noise sources are dominant in each of 3 frequency bands: low (10 to 500 Hz), medium (500 Hz to 25 kHz) and high (>25 kHz). The low-frequency band is dominated by anthropogenic sources: primarily, commercial shipping and, secondarily, seismic exploration. Shipping and seismic sources contribute to ambient noise across ocean basins, since low-frequency sound experiences little attenuation, allowing for long-range propagation. Over the past few decades the shipping contribution to ambient noise has increased by as much as 12 dB, coincident with a significant increase in the number and size of vessels comprising the world's commercial shipping fleet. During this time, oil exploration and construction activities along continental margins have moved into deeper water, and the long-range propagation of seismic signals has increased. Medium frequency sound cannot propagate over long ranges, owing to greater attenuation, and only local or regional (10s of km distant) sound sources contribute to the ambient noise field. Ambient noise in the mid-frequency band is primarily due to sea-surface agitation: breaking waves, spray, bubble formation and collapse, and rainfall. Various sonars (e.g. military and mapping), as well as small vessels, contribute anthropogenic noise at mid-frequencies. At high frequencies, acoustic attenuation becomes extreme so that all noise sources are confined to an area close to the receiver. Thermal noise, the result of Brownian motion of water molecules near the hydrophone, is the dominant noise source above about 60 kHz.
The viviparous sea snakes (Hydrophiinae) are a young radiation of at least 62 species that display spectacular morphological diversity and high levels of local sympatry. To shed light on the mechanisms underlying sea snake diversification, we investigated recent speciation and eco-morphological differentiation in a clade of four nominal species with overlapping ranges in Southeast Asia and Australia. Analyses of morphology and stomach contents identified the presence of two distinct ecomorphs: a 'macrocephalic' ecomorph that reaches >2 m in length, has a large head and feeds on crevice-dwelling eels and gobies; and a 'microcephalic' ecomorph that rarely exceeds 1 m in length, has a small head and narrow fore-body and hunts snake eels in burrows. Mitochondrial sequences show a lack of reciprocal monophyly between ecomorphs and among putative species. However, individual assignment based on newly developed microsatellites separated co-distributed specimens into four significantly differentiated clusters corresponding to morphological species designations, indicating limited recent gene flow and progress towards speciation. A coalescent species tree (based on mitochondrial and nuclear sequences) and isolation-migration model (mitochondrial and microsatellite markers) suggest between one and three transitions between ecomorphs within the last approximately 1.2 million to approximately 840 000 years. In particular, the macrocephalic 'eastern' population of Hydrophis cyanocinctus and microcephalic H. melanocephalus appear to have diverged very recently and rapidly, resulting in major phenotypic differences and restriction of gene flow in sympatry. These results highlight the viviparous sea snakes as a promising system for speciation studies in the marine environment.
Visual observations of the behavior of over 150 loggerhead turtles (Caretta caretta) were collected over a period of 2 weeks during a seismic survey in the Mediterranean Sea off Algeria in September and October 2009. All turtles were observed during active operation of an airgun array that had a peak source level of 252 dB re 1 muPa. Recordings from three hull-mounted hydrophones allowed concurrent estimation of near-surface airgun array source signatures and sound exposure levels. Of the 53% of turtles that were successfully visually tracked until they had passed more than 100 m behind the seismic vessel, 51% dived at or before their closest point of approach to the airgun array. Among animals that dived, approximately 20% did so immediately following an airgun shot, often showing a startle response that was clearly distinguishable from the predominant basking behavior. Turtle dive probability as a function of distance from the airgun array and sound exposure level will be considered in detail. The observed diving behavior may be interpreted as an avoidance response and may have negative consequences for turtles if it interferes with thermoregulation (basking) or results in inhabitual energy expenditures.
Fishes show great variability in hearing sensitivity, bandwidth, and the appropriate stimulus component for the inner ear (particle motion or pressure). Here, hearing sensitivities in three vocal marine species belonging to different families were described in terms of sound pressure and particle acceleration. In particular, hearing sensitivity to tone bursts of varying frequencies were measured in the red-mouthed goby Gobius cruentatus, the Mediterranean damselfish Chromis chromis, and the brown meagre Sciaena umbra using the non-invasive auditory evoked potential-recording technique. Hearing thresholds were measured in terms of sound pressure level and particle acceleration level in the three Cartesian directions using a newly developed miniature pressure-acceleration sensor. The brown meagre showed the broadest hearing range (up to 3000 Hz) and the best hearing sensitivity, both in terms of sound pressure and particle acceleration. The red-mouthed goby and the damselfish were less sensitive, with upper frequency limits of 700 and 600 Hz, respectively. The low auditory thresholds and the large hearing bandwidth of S. umbra indicate that sound pressure may play a role in S. umbra's hearing, even though pronounced connections between the swim bladder and the inner ears are lacking.
Evoked potentials are usually analyzed in the time domain (voltage versus time). The most familiar frequency-domain measure, the power spectral density function, displays power as a function of frequency but doesn't distinguish signal power from noise power. The coherence function estimates, for each frequency, the ratio of signal power to total (signal plus noise) power and, thus, indicates the degree to which system output (scalp potential) is determined by input (acoustic stimulus). Coherence ranges from 0 to 1; values above specified critical values can be accepted as demonstrating statistically significant system response. In this paper, we present coherence analysis of human scalp responses to clicks and amplitude-modulated tones. In both cases, this analytic method provides insight into the spectral character of the response (for example, assisting in specifying desirable filter characteristics). Threshold sensitivity is also improved: statistically significant responses can be detected at lower intensity by coherence analysis than by inspection of time-domain waveforms.
Airborne sound and substrate vibration each elicit electrical responses below the surface of the tectum in species of three families of snakes. Tones of 50 to 1000 hertz evoke responses independently of substrate vibration. Sensitivity to locally applied sound is present over much of the body surface. This sensitivity is attributed to the auditory nerve, because it is not altered by spinal section but is eliminated by destruction of the inner ear.
1. A standing wave tube apparatus was used to determine the biophysical basis of underwater hearing in Ambystoma tigrinum. 2. A. tigrinum responds to the pressure component of underwater sound, and the mouth cavity appears responsible for transduction of sound pressure. 3. Near-field displacements produced by pulsations of the air-filled mouth cavity apparently stimulate the inner ear. 4. Salamander head preparations with no air-filled mouth cavity respond to the particle motion component of underwater sound, but only at sound pressure levels 40 dB or more above levels producing clear pressure sensitivity in intact salamanders or head preparations including an air-filled mouth cavity.
Sinusoids in background noise can conveniently be detected using unsegmented power spectra, comparing power at the signal frequency to average power at several neighbor frequencies. In this case, the F test is preferable to t tests based on rms or dB values, because of the skewed distributions of rms and dB when signal-to-noise ratio (SNR) = 0. F-test performance improves as the number of frequencies increases, to about 15, but can be degraded if the background noise is not white, with a slope exceeding about 10 dB for the range of frequencies sampled. Segment analysis, using magnitude-squared coherence (MSC) or related statistics, has equivalent statistical power; MSC and F each yield unbiased SNR estimates that have identical distributions when SNR = 0. Selection of F or MSC for detection of sinusoids will usually be a matter of convenience.
Assessing the impact of marine seismic surveys on southeast Australian scallop and lobster fisheries
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Assessing the impact of marine seismic surveys on southeast Australian scallop and lobster fisheries (FRDC Report 2012/008)
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