Scientific and diagnostic imaging allow visual representations of invertebrate sensory structures, organs or tissues for various purposes such as the study of normal anatomy and function, or the diagnosis of the effects of sound on these structures. Imaging techniques include Electron Microscopy and 3D imaging techniques ( Figure 1 ).

Electron microscopes have a higher resolution than light microscopes and are capable of a higher magnification (up to 2 million times) (Rudenberg and Rudenberg, 2010), allowing the visualization of structures that would not normally be visible by optical microscopy. There are two major types of electron microscopes used in invertebrate bioacoustics: Transmission Electron Microscopes and Scanning Electron Microscopes. Scanning Electron Microscopy produces images of a sample by scanning it with a focused beam of electrons that interact with atoms in the sample, providing information about its surface topography and composition (Butterfield et al., n.d.) and achieving resolution better than 1 nanometre (Suzuki, 2002). In invertebrates, this technique allows description of the surface of sensory epithelium and effects of noise upon it ( Figures 1A–E ) (Solé et al., 2013a; Solé et al., 2013b; Day et al., 2016; Solé et al., 2016; Solé et al., 2018; Day et al., 2019). In Transmission Electron Microscopy, a beam of electrons is passed through an ultrathin specimen and an image is formed from the interaction of the electrons transmitted through it. This technique is used in the description of invertebrate ultrastructural sensory epithelia, allowing the inner cellular organelles to be visualised and analysis of the effects of sound on them. ( Figure 1F ) (Solé et al., 2013b)

Magnetic Resonance Imaging (MRI) is a non-invasive imaging technique that allows creation of a 3D image of a body’s internal organs using powerful magnetic fields and radio waves. This technique has been used to construct models of the morphological structure of invertebrate sensory systems (Ziegler et al., 2018). Computer tomography (CT) relies on differences in X-ray attenuation of biological tissues to do a 3D reconstruction of them. Major molluscan organs have been visualized using CT techniques (Ziegler et al., 2018).

Auditory evoked potential recordings have been used in a variety of invertebrate taxa as a measurement of sound sensitivity ( Figure 2A ). The evoked potential technique for hearing was popularized by Hong Yan’s work on fishes before to spreading it among invertebrates (Yan, 2002). This method involves measuring responses from neurons associated with sound detection and the resulting conduction of responses toward a brain or central set of ganglia (Hall, 2007). Recording may be thus from nearby sensory organs, such as the statocyst, or if sound detection comes from more peripheral hair cells or organs, it may occur nearby the brain/central ganglia area (Jezequel et al., 2021). While evoked potential methods have been widely applied to measure hearing abilities in many aquatic vertebrates e.g., (Supin et al., 2001; Kastak et al., 2005; Nachtigall et al., 2007; Mooney et al., 2012; Piniak et al., 2016; Jones et al., 2021), it has only been sparingly applied to invertebrates, including squid (Mooney et al., 2010), prawns (Lovell et al., 2005), snapping shrimp (Dinh and Radford, 2021), lobsters (Jezequel et al., 2021) and other crustaceans (Hughes et al., 2014; Radford et al., 2016). Some of its advantages include that it can be applied to a variety of taxa, including wild caught animals, and it can be non-invasive. Although often times it is a more invasive method involving sedation, needle electrodes and surgery to access nerve structures. Evoked potential methods are generally cost-effective and permit to reach a relatively high animal sample size of (i.e. > 10), that is higher than psychophysical methods, and whole audiograms can be measured quickly (tens of minutes to a few hrs).

There are a number of techniques used to assess the effects of a stimulus on the metabolic rate of an organism. One such method, respirometry, provides an indirect calorimetric approach to the measurement of metabolic heat changes through monitoring and measurement of variations in oxygen uptake ( Figures 2B, C ). For marine invertebrates, changes in respiration rate are observed indirectly through changes in the dissolved oxygen of the surrounding water. Animals are encapsulated in a sealed, water-filled chamber and dissolved oxygen is measured either at the start and end points of the exposure using an oxygen probe, or continuously throughout the exposure using an oxygen sensor. During long exposures, intermittent flow respirometry may be used (Steffensen et al., 1984; Steffensen, 1989) when periodic flushing of the respirometry chamber is performed to maintain sufficient oxygen saturation. In both static and intermittent-flow respirometry, oxygen consumption is calculated accounting for bacterial respiration, water volume, exposure time and environmental conditions, and calibrated against the animal’s mass to allow comparability between individuals and across species. Respirometry has been used to investigate the effects of anthropogenic noise on decapods (Regnault and Lagardere, 1983; Wale et al., 2013b; Ruiz-Ruiz et al., 2020), bivalves (Shi et al., 2019; Wale et al., 2019) and cephalopods (Woodcock et al., 2014).

Several techniques for the assessment of invertebrate stress are based on cellular, biochemical and molecular aspects. It is possible to determine the physiological state of an animal using stress analysis after sound exposure. Stress bioindicators can be measured in invertebrate haemolymph. Total haemocyte count (THC), heat shock protein 27 (Hsp27) expression in haemocyte lysate, total protein concentration (PT) and phenoloxidase activity (PO) in cell-free haemolymph, were considered potential biomarkers of stress (Filiciotto et al., 2014; Celi et al., 2015).

In aquatic invertebrates, the homeostasis of total haemocyte density and composition may be considered an important well-being predictive parameter. Decreases of total haemocyte count (THC) under stressful conditions, usually carried out with cell counter chambers, have been reported for several aquatic crustacean species (Le Moullac et al., 1998; Sánchez et al., 2001; Mercier et al., 2006), suggesting the possibility of immune depletion as well as an increased risk of infection (Filiciotto et al., 2014; Celi et al., 2015). Although the variation in differential haemocyte count in the presence of different stressors is not well understood, it has been used as a stress indicator in crustaceans (Jussila et al., 1997; Johansson et al., 2000; Filiciotto et al., 2014) ( Figure 3 ). The measurement of this parameter is easily feasible under the microscope after on slide cell fixation and stain.

Another parameter useful to evaluate the disturbance of the homeostatic balance of animals is the measurement of glucose haemolymphatic. Hyperglycemia is a primary response typical of many aquatic animals to different stressors (Lorenzon, 2005; Fazio et al., 2013; Faggio, 2014). Glucose haemolymphatic, which can be measured in haemolymph using commercial kits, increases in marine invertebrates under exposure to acoustic stimulu (Filiciotto et al., 2014; Vazzana et al., 2016). In the haemolymph, it is possible to measure the total protein concentration. This parameter is non-destructive, easy, cheap and measurable through fluorimetric methods. It can be used as a “warning” of poor environmental conditions such as noise (Filiciotto et al., 2014; Vazzana et al., 2016). A further indicator of the negative effect of altered conditions on invertebrates is a change in enzyme activities. There are still few studies on the variations of enzymes in stressed invertebrates, but some have shown a modulation of peroxidase, alkaline phosphatase and esterase activity measured through rapid colorimetric methods (Vazzana et al., 2016; Vazzana et al., 2020a; Vazzana et al., 2020b) after acoustic stimulus. Among bioindicators of stressful conditions in crustaceans is also included expression of heat shock proteins (Snyder and Mulder, 2001; Liberge and Barthelemy, 2007). Some authors showed, through the use of western blot analysis and Real-Time PCR (RT-PCR), that, in marine invertebrates exposed to acoustic stimuli, occurs a protein and gene overexpression of the Hsp70 (Filiciotto et al., 2014; 2016; Vazzana et al., 2016; 2020a). The latter aspect is useful to understand better the variations of the complex cellular–biochemical–molecular network of organism in stress condition.

In a sound wave, particles of the medium (e.g., water) oscillate around a point of origin (‘particle motion’) causing local compressions and expansions (‘sound pressure’) that transfer the sound energy to neighbouring particles (ISO 18405:2017; Gray et al., 2016). Thus, all sound involves both pressure and particle motion fluctuations. The number of oscillations per second is the frequency in Hertz (Hz). Sound pressure fluctuations are omnidirectional and are measured as force per unit area in Pascals (Pa), typically using piezoelectric hydrophones, which have been readily available for many years (ISO 18405:2017, Robinson et al., 2014). Sound particle vibrations are directional and are described by displacement (m), velocity (ms^-1) or acceleration (ms^-2); three metrics that have a frequency-dependent relationship to one another (Nedelec et al., 2016, ISO 18405:2017). The directional information is described by angles relative to references such as magnetic north and gravity. 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). Underwater sound is often reported in decibel units (dB), which are represented on a logarithmic scale relative to 1 µPa for pressure, 1 pm for displacement, 1 nm s^-1 for velocity and 1 um s^-2 for acceleration (ISO 18405:2017).

The statolith organs of many invertebrates measure the relative motion of the body of the animal to the dense statocyst, which moves with a lag due to its greater mass and inertia, creating a biological analogue of a proof-mass instrument (Packard et al., 1990; Kaifu et al., 2011). Therefore, measuring the whole-body vibration of animals is of interest because it links acoustic stimulus and sound detection. Piezoresistive accelerometers that measure acoustic vibrations of solid objects they are fixed to exist, however their scale relative to the bodies of aquatic invertebrates means that the accelerometers themselves would alter the vibration of the whole body. Recently, the availability of non-contact laser Doppler vibrometer techniques, that have already been applied to research on hearing in several amphibian, reptile and crustacean species (Hetherington and Lindquist, 1999; Hetherington, 2001), has opened the possibility of measuring whole-body vibration of aquatic animals. Whole-body vibrations of cephalopods and scallops that were exposed to air borne sound (<360 Hz) were successfully measured using a laser Doppler vibrometer, confirming the hypothesis that particle motion can vibrate the whole body of invertebrates (André et al., 2016). However, to report the particle motion levels measured by an instrument, it is necessary to calibrate the instrument for its coupling to the medium in which the sound is to be measured. The coupling of animal bodies to the water column remains poorly understood, thus measuring whole-body motion gives us a limited understanding of responses to particle motion levels in the water. Further advancement of measurement techniques on whole-body vibration of aquatic animals elicited by propagating acoustic waves will improve understanding of particle motion reception in invertebrates. This will involve calibrating the animals themselves as well as any accelerometers that are attached to them.

The motion of the ‘particles’ that make the medium (e.g., air, water, or solid substrate) is an intrinsic aspect of sound. Sound pressure can be described by its magnitude and its temporal and frequency characteristics, but at a single point, sound pressure does not contain directional information. Particle motion can be described by its magnitude, temporal and frequency characteristics, but additionally it always contains directional information because of its inherent ‘back and forth’ action (Hawkins and Popper, 2017). Many aquatic invertebrates sense and use particle motion, including to detect the direction of the source, (André et al., 2016; Nedelec et al., 2016). Particle motion and sound pressure are proportional in ‘plane wave’ conditions (far from the source and from any boundaries that may cause reflections relative to the wavelength). Close to the source in the ‘near field’, particle motion is higher than would be expected from equivalent pressure in plane wave conditions in the ‘far field’ due to interactions between the wavelength, frequency and distance from the source. This interaction, which causes additional particle motion near to the source decreases with inverse proportion to the distance from the source until it can be treated as negligible after approximately one wavelength. A good rule of thumb is therefore that the boundary of the near field region with additional particle motion is one wavelength from the source. Therefore, particle motion is present wherever there is sound and a good rule of thumb is that the boundary of the near field region with additional particle motion is one wavelength from the source. Sensory hair cells in the sensory systems (see below) are stimulated by mechanisms that respond to particle motion and convert these motions to electrical signals that stimulate the nervous system. Because aquatic invertebrates lack gas-filled cavities, it seems that they mostly perceive the particle motion of the sound. But recent experiments put this statement in question: particle motion may not be the sole component implied in sound lesions in invertebrates (Solé et al., 2017).

Using a broad definition – the reception of vibratory stimuli of any kind and nature, provided that the sound source is not in direct contact with the animal’s body (Budelmann, 1992b) – hearing is widespread among invertebrates. Although the research on invertebrate acoustic sensitivity is scarce, some studies on bivalves, cephalopods and crustaceans have determined some important aspects about the invertebrate threshold sensitivities.

Early studies on sound detection by bivalves reported induced burrowing behaviour in clam species (Mosher, 1972; Ellers, 1995). Recent work has quantified sensitivity of marine bivalves to substrate-borne vibration (Zhadan, 2005; Kastelein, 2008; Roberts et al., 2015). By exposure to vibration under controlled conditions using valve closure as the behavioural indicator of reception and response (Roberts et al., 2015), the thresholds were shown to be within the range of vibrations measured in the vicinity of anthropogenic operations such as pile-driving and blasting. Using pure-tone exposures and an accelerometer fixed to the shell to detect valve closure, Japanese oysters (Crassostrea gigas) were shown to have maximum sensitivity from 10 to 200 Hz (Charifi et al., 2017). The bivalve abdominal sense organ (ASO) is highly sensitive to water-born vibration in the range 20–1500 Hz (Zhadan and Semen’kov, 1984; Zhadan et al., 2004).

While there is uncertainty regarding the biological importance of particle motion sensitivity versus acoustic pressure, recent behavioural (including changes in ventilation rhythm) and electrophysiological studies confirmed cepaholopd sensitivity to frequencies under 400 Hz (Sepia officinalis, (Packard et al., 1990); Sepioteuthis lessoniana, (Hu et al., 2009); Octopus vulgaris (Packard et al., 1990; Kaifu et al., 2007; Kaifu et al., 2008; Hu et al., 2009; Kaifu et al., 2011), Loligo vulgaris, (Packard et al., 1990), Loligo pealeii, (Mooney et al., 2010). Whole body vibrations due to particle motion were detected in cuttlefish Sepia officinalis (André et al., 2016) through an experimental set-up based on laser Doppler vibrometer techniques (frequencies 60, 120 and 320 Hz). This work confirmed the hypothesis that particle motion can encompass the whole body of cephalopods and cause it to move with a similar phase and amplitude. Mantle movement (lengthened ventilation or jetting) has been used as an indicator of the sound perception to understand the perceptionmechanism (Kaifu et al., 2007; 2008 Packard et al., 1990) or to understand the biological significance of their acoustical perception (Wilson et al., 2007; Samson et al., 2014; Mooney et al., 2016; Jones et al., 2021). In most cases, unconditioned animals were used to observe their baseline behavior. Mantle muscle movements were recorded using an electromyograph (Kaifu et al., 2007; Kaifu et al., 2008) or measurement of the changes of mantle muscle thickness based on impedance between two electrodes inside and outside the mantle (Packard et al., 1990). Cephalopod behavioural responses were then categorized to response type (e.g., inking, jetting, startle, colour change, fin movement, no response).

Among crustaceans, Lovell and colleagues studied the mechanism of the reception of sound and hearing abilities of the prawn Palaemon serratus using a combination of anatomical techniques, electron microscopy and electrophysiology (Lovell et al., 2005). They concluded that P. serratus is sensitive to sounds with frequencies ranging between 100 and 3000 Hz. The same authors (Lovell et al., 2006) demonstrated that all P. serratus individuals were able to hear sound with a frequency of 500 Hz, regardless of their size. Although data are not available on frequency-specific hearing/particle motion detection capability, preliminary experiments demonstrated Nephrops norvegicus postural responses to water vibrations (Goodall et al., 1990). The hermit crab (Pagurus berhnardus) showed antenna/maxilliped movement and forward locomotion in response to particle motion (Roberts et al., 2016). Auditory evoked potential (AEP) analyses of Panopeus sp. crabs evidenced their sensitivity to particle motion (Hughes et al., 2014). This response range overlaps with peak frequencies associated with airgun, pile-driving, sonar activities and biologically sources of underwater noise (Jeffs et al., 2003; Radford et al., 2007). Marine crustaceans present sensory hairs covering their bodies, which, when stimulated by water or substrate-borne vibrations associated with changes in acceleration hydrodynamic flow or sound, help animals sense nearby biological movements (Tautz and Sandeman, 1980; Radford et al., 2016). The American lobster Homarus americanus shows sensory hairs sensitives to low frequency (Derby, 1982) and ontogenic variations in AEP response up to 5 kHz (Pye and Watson, 2004; Jezequel et al., 2021). Crustacean chordotonal organs are stimulated by vibrations. One specialised organ, present on fiddler and ghost crabs, Barth’s myochordotonal organ (Barth’s MCO), is sensitive to frequencies above 300 Hz. All walking legs contain the sensory organ and if an individual loses a walking leg, it would still be able to detect vibrations through its other walking legs (Derby, 1982). Pelagic crab larva with capacity to detect specific underwater sounds/vibrations are able to use sound as an orientation cue to settle (Montgomery et al., 2006; Stanley et al., 2010; Stanley et al., 2012) (Jeffs et al., 2003; Radford et al., 2007).

Relevant studies on marine invertebrate acoustic sensitivity are detailed in Table 2 .

In bivalves, field studies of airgun exposure found no evidence of increased mortality in adult scallops and clams (La Bella et al., 1996; Parry et al., 2002; Harrington et al., 2010). In another field study, a dose-dependent increase scallop mortality was found four months after exposure to an airgun (Day et al., 2016). In addition, scallops exhibited abnormal reflexes that may indicate damage to mechanosensory organs (Day et al., 2017). The opposite results of these works could be explained by the time of monitoring. Harrington et al. (2010) only monitored scallops for two months, whereas Day et al. (2016) showed that significantly higher mortality rates only occurred towards the end of the 4-month period. Parry and Gason (2006) also stated that to detect mortality in such studies, very significant mortality level would be needed.

Low-frequency noise exposure causes anatomical damage in cephalopods. After an increase in the frequency of strandings in North Spain (Guerra et al., 2004), recent findings showed that exposure to artificial noise had a direct consequence on the functionality and physiology of cephalopod statocysts, which are the sensory organs responsible for equilibrium and movements in the water column (André et al., 2011; Solé et al., 2013a; Solé et al., 2013b; Solé et al., 2017). Exposure to noise was challenging the life of exposed individuals in laboratory and offshore conditions (feeding and mating cancellation and irregular swimming). Lesions present on the exposed animals were consistent with a manifestation of a massive acoustic trauma observed in vertebrate species.

Cnidarians and ctenophores, both in the polyp and the medusa stage, possess sensory organs located in their tentacles, able to detect vibration in water associated to prey movement and changes in their surrounding environment. A study described morphological effects (severe damages to the statocyst sensory epithelia) after noise exposure on two species of Mediterranean Scyphozoan medusa, Cotylorhiza tuberculata and Rhizostoma pulmo (Solé et al., 2016).

Among crustaceans, blue crabs (Callinectes sapidus) suffer mortality as a result of underwater explosions (Moriyasu et al., 2004). Although no lethal effects of underwater noise have been described for C. pagurus, Homarus gammarus or Nephrops norvegicus, sub-lethal effects of continuous, low-frequency anthropogenic noise have been reported among the Decapoda (Edmonds et al., 2016).

Although no significant effects were detected in snow crabs after exposure (Christian et al., 2003), airgun exposure caused ultrastructural statocyst damages in rock lobsters up to a year later (Day et al., 2016). In a recent study, lobsters showed impaired righting and significant damage to the sensory hairs of the statocyst after exposure equivalent to a full-scale commercial assay passing within 100–500 m (Day et al., 2019). Reflex impairment and statocyst damage persisted over the course of the experiment – up to 365 days post-exposure – and did not improve following moulting.

Behavioural responses, not necessarily associated with startle responses, has been observed in bivalves (e.g., valve closure and recessing reflex behaviour). These responses were used to establish thresholds of sound detection (Roberts et al., 2015). In addition to classic behavioural patterns (i.e., persistent alterations in recessing reflex behaviour), a novel flinching behaviour (a rapid retraction of the velum and then returned to position) was observed on commercial scallops (Pecten fumatus) after exposure to a seismic survey. This behaviour was observed before the acoustic wave reached the animal, suggesting that it was a response to the faster traveling ground roll wave (Day et al., 2016). Changes in scallop behaviour and reflex responses disruption were observed at least 120 days after seismic survey exposure (Day et al., 2017).

Among cephalopods, behavioural startling responses (jetting and inking) were observed in squids during seismic surveys (Fewtrell & McCauley, 2012) and in response to noise in laboratory conditionss (Samson et al., 2014). Squid show fewer alarm responses with subsequent exposure to noise from seismic surveys (Fewtrell & McCauley, 2012). This process of habituation has been observed in different species of cephalopods (McCauley et al., 2000; Samson et al., 2014; Mooney et al., 2016). While other studies also reported behavioural response to acoustic stimuli in a context of anti-predator defence (Hanlon and Budelmann, 1987; Kaifu et al., 2007); the capture of Todarodes pacificus reportedly increased in the presence of underwater sound (Maniwa, 1976). Feeding and foraging behaviour has been shown to be altered in response to different noise stimuli in cephalopods (Jones et al., 2021).

Decapod crustaceans exposed to seismic sound exhibited alarm behaviour (startle responses) when they were very near from the sound source (Goodall et al., 1990; Christian et al., 2003). Carcinus maenas subjected to boat noise were more likely to suspend their search for food, although their ability to find food was not affected (Wale et al., 2013a). Crabs subjected to boat noise took longer to find refuge than when subjected to ambient noise (Wale et al., 2013a). Increased respiration, decreasing escape responses and reduction on foraging activity in the presence of sound from its predatory species suggests that crustaceans use sound as a sensory cue for the presence of fish (Regnault and Lagardere, 1983; Hughes et al., 2014). Nephrops norvegicus showed a reduced activity, bury less deeply and flush their burrows less regularly under impulsive anthropogenic noise (Solan et al., 2016). Anthropogenic noise can modify foraging interactions, reducing food aggregation in crabs (C. maenas) and thereby release competition for shrimps (C. crangon) (Hubert et al., 2018).

Variables related to locomotion such as distance travelled, linear and angular velocity, or single events such alarm responses, intraspecific aggressive encounters and sheltering behaviour were found in crustacean species exposed to underwater noise (Celi et al., 2013; Filiciotto et al., 2014; De Vincenzi et al., 2015). Lobsters and common prawn exposed to boat noises modified their locomotor activities (distance moved, velocity, proximity with conspecific) when exposed to ship noise (Filiciotto et al., 2014; Filiciotto et al., 2016). Roberts et al. showed modification on the hermit crab (Pagurus bernardus) antennae movement under sound exposure (Roberts et al., 2016). Righting reflex (time to right itself) of the rock lobster (Jasus edwardsii) was delayed after exposure to airguns (Day et al., 2016). Shrimp Procrambarus clarkii showed decreased agonistic behaviour under frequencies between 100 and 25,000 Hz (Celi et al., 2013).

Behavioural effects on movement of snow crabs (Chionoecetes opilio) after 2D seismic noise exposure, analysed by positioning telemetry, were similar to natural vibrations, and smaller than the responses of crabs to handling, temperature and time of day (Morris et al., 2020a). Habituation to vibrations in crabs has been shown and crabs maintained in captivity for short periods of time presented greatest sensitivity to particle motion (Roberts et al., 2016).

Hermit crabs (Pagurus bernhardus show interaction of ship noise exposure with predator presence reaction, shell size and the mean duration to accept or reject the optimal empty shell (Tidau and Briffa, 2019b). Ship noise, but not loud natural ambient noise, causes adverse effects on the shore crabs (C.maenas) capacity to change the carapace colour to improve camouflage and predator escape responses (Carter et al., 2019). Bioturbation may affect intra and inter-specific behaviour on lobster (Nephrops norvegicus) and after exposure to continuous and impulsive low-frequency noise (Solan et al., 2016).

A few studies conducted on marine bivalves exposed to sound have highlighted its effects on physiological and molecular mechanisms. Increased sound intensity result in an alteration in metabolism related genes (Peng et al., 2016) or increases in the levels of biochemical stress parameters measured in their plasma and tissues (La Bella et al., 1996; Vazzana et al., 2016; Vazzana et al., 2020a). The long-term capability of scallops to maintain homeostasis was reduced after airgun exposure (Day et al., 2016).

Among cephalopods, analysis of statocyst endolymph of the Mediterranean common cuttlefish (Sepia officinalis) showed changes in the protein content immediately and 24 h after sound exposure (Solé et al., 2019). The affected proteins were mostly related to stress and cytoskeletal structure. Hemocyanin isoforms, tubulin alpha chain and intermediate filament protein were down-regulated after exposure.

Among crustaceans sub-lethal physiological changes (serum biochemistry and hepatopancreatic cells) were observed in American lobsters (H. americanus) after one month of sound exposure (Payne et al., 2007). Permanent high-level exposure to sound caused a significant reduction in the rate of growth and reproduction, an increase in the level of aggressiveness (cannibalism) and the mortality rate, and a reduction in feed intake of shrimp Crangon crangon (Lagardère, 1982; Regnault and Lagardere, 1983). Reduced growth and reproductive rates are known tertiary effects of stress response (Barton, 2002). Some crustaceans show alterations on respiration (increase on metabolic rate) in high ambient noise conditions (Regnault and Lagardere, 1983; Wale et al., 2013b). European spiny lobsters are affected by noise in both cellular and biochemical parameters. Filiciotto et al. (Filiciotto et al., 2016) found in laboratory experiments that the common prawn Palaemon serratus exhibits stress responses to playback of boat noise. In particular, noise exposure produced alterations in total protein concentrations in the haemolymph and brain, in DNA integrity, in the expression protein levels of HSP 27 and 70 in brain tissues.

Respiratory responses to noise exposure are often species-specific with some animals, such as the shore crab Carcinus maenas (Wale et al., 2013b), displaying an increased oxygen consumption in response to noise exposure, whilst others, such as the blue mussel Mytilus edulis (Wale et al., 2019) and the blood clam Tegillarca granosa (Shi et al., 2019), showing decreased respiration during noise exposure. Among the echinoderms, brittle stars (Amphiura filiformis) showed signs of physiological stress after low-frequency noise exposure (Solan et al., 2016) and in the sea urchin Arbacia lixula significant change was found in enzyme activity and in gene and protein expression of the HSP70 ( Vazzana et al., 2020b).