Turning up the heat: Effects of temperature on agonistic acoustic communication in the two-spotted goby (Pomatoschistus flavescens)

Acoustic communication is linked to fitness traits in many animals, but under the current scenario of global warming, sound signals can be affected by rising temperatures, particularly in ectothermic organisms such as fishes. This study examines the effect of water temperature in acoustic communication in the two-spotted goby, Pomatoschistus flavescens. To address this, we looked at the effect of different temperatures on the acoustic features of drums produced by males during territorial defence and related it with their auditory sensitivity. We also analysed the differences in acoustic features between male agonistic drums and previously reported male courtship sounds, to better understand how acoustic communication may be affected by different temperature conditions. We recorded two-spotted goby males during territorial intrusions for 10 min at 16 °C, 19 °C, and 21 °C in the laboratory. We found that agonistic drums were shorter, had fewer pulses and shorter pulse periods at higher temperature, in contrast with the peak frequency that remained unaffected. Male agonistic and mating drums (recorded in a previous study) at 16 °C only differed in pulse period, which was higher in mating drums. Hearing thresholds obtained with Auditory Evoked Potentials at 16 °C, revealed higher sensitivity below 400 Hz, matching the main energy of agonistic and mating sounds. Our findings suggest that increasing temperature could potentially affect acoustic communication in this species by reducing the duration of agonistic drums, which might hinder effective communication. Nevertheless, the impact may not be significant as there was a good match between the best hearing sensitivity and the peak frequency range of their calls, which was not influenced by temperature. As fish and other organisms are increasingly threatened by multiple anthropogenic stressors, including warming, future research should address how changes in water temperature impact acoustic communication within a more realistic multi-stressor scenario.

Endemic fish calling: Acoustics and reproductive behaviour of the Neretva dwarf goby Orsinigobius croaticus

The Neretva dwarf goby Orsinigobius croaticus (Gobiiformes, Gobionellidae) is an endemic fish native to the freshwaters of the Adriatic Basin in Croatia and Bosnia and Herzegovina, a Mediterranean Biodiversity Hotspot. Due to its limited distribution range, specific karst habitat and endangered status, laboratory studies on reproductive biology are scarce but crucial. Herein, we investigated the sound production and acoustic behaviour of the endangered O. croaticus during reproductive intersexual laboratory encounters, utilising an interdisciplinary approach. We also performed dissections and micro-computed tomography (μCT) scanning of the pectoral girdle to explore its potential involvement in sound production. Finally, comparative acoustic analysis was conducted on sounds produced by previously recorded soniferous sand gobies to investigate whether acoustic features are species-specific. The endemic O. croaticus is a soniferous species. Males of this species emit pulsatile sounds composed of a variable number of short (~15 ms) consecutive pulses when interacting with females, usually during the pre-spawning phase in the nest, but also during courtship outside the nest. Pulsatile sounds were low-frequency and short pulse trains (~140 Hz, <1000 ms). Male visual behaviour rate was higher when co-occurring with sounds and females entered the male's nest significantly more frequently when sounds were present. Characteristic body movements accompanied male sound production, such as head thrust and fin spreading. Furthermore, μCT scans and dissections suggest that O. croaticus shares certain anatomical similarities of the pectoral girdle (i.e. osseous elements and arrangement of levator pectoralis muscles) to previously studied sand gobies that could be involved in sound production. Multivariate comparisons, using sounds produced by eight soniferous European sand gobies, effectively distinguished soniferous (and sympatric) species based on their acoustic properties. However, the discrimination success decreased when temperature-dependent features (sound duration and pulse repetition rate) were excluded from the analysis. Therefore, we suggest both spectral and temporal features are important for the acoustic differentiation of sand gobies.

Mating sounds in the two-spotted goby, Pomatoschistus flavescens: Effects of water temperature on acoustic featuresa)

Acoustic signals in teleost fishes play a fundamental role in reproduction. As fish are ectothermic animals, temperature has the potential to change their signal production and detection, with further implications for mating interactions. In this study, we describe the mating sounds made by the two-spotted goby, Pomatoschistus flavescens, for the first time and further investigate the effect of temperature on the acoustic features. Courtship sounds of 15 two-spotted goby males were recorded at three different temperatures: 16 °C, 19 °C, and 21 °C. As seen for other marine gobies, two-spotted goby produced two courtship sounds: drums and thumps. Drums showed similar acoustic features to other Pomatoschistus species already studied. Calling rates for both kinds of sound were not affected by the increases in temperature. However, pulse rate increased from 16 °C to 19 °C and stabilised between 19 °C and 21 °C, suggesting that two-spotted gobies reached their physiological limits at 19 °C. Spectral features were also affected by temperature, presenting higher values at 19 °C. Whether or not the observed changes in acoustic features with temperature lead to changes in mating remains to be addressed. Studies like the present one are fundamental to better comprehend how reproduction will be affected by global warming in soniferous fishes.

Correlation between acoustic divergence and phylogenetic distance in soniferous European gobiids (Gobiidae; Gobius lineage)

In fish, species identity can be encoded by sounds, which have been thoroughly investigated in European gobiids (Gobiidae, Gobius lineage). Recent evolutionary studies suggest that deterministic and/or stochastic forces could generate acoustic differences among related animal species, though this has not been investigated in any teleost group to date. In the present comparative study, we analysed the sounds from nine soniferous gobiids and quantitatively assessed their acoustic variability. Our interspecific acoustic study, incorporating for the first time the representative acoustic signals from the majority of soniferous gobiids, suggested that their sounds are truly species-specific (92% of sounds correctly classified into exact species) and each taxon possesses a unique set of spectro-temporal variables. In addition, we reconstructed phylogenetic relationships from a concatenated molecular dataset consisting of multiple molecular markers to track the evolution of acoustic signals in soniferous gobiids. The results of this study indicated that the genus Padogobius is polyphyletic, since P. nigricans was nested within the Ponto-Caspian clade, while the congeneric P. bonelli turned out to be a sister taxon to the remaining investigated soniferous species. Lastly, by extracting the acoustic and genetic distance matrices, sound variability and genetic distance were correlated for the first time to assess whether sound evolution follows a similar phylogenetic pattern. The positive correlation between the sound variability and genetic distance obtained here emphasizes that certain acoustic features from representative sounds could carry the phylogenetic signal in soniferous gobiids. Our study was the first attempt to evaluate the mutual relationship between acoustic variation and genetic divergence in any teleost fish.

Sound production in piranhas is associated with modifications of the spinal locomotor pattern

In piranhas, sounds are produced through the vibration of the swim bladder wall caused by the contraction of bilateral sonic muscles. Because they are solely innervated by spinal nerves, these muscles likely evolved from the locomotor hypaxial musculature. The transition from a neuromuscular system initially shaped for slow movements (locomotion) to a system that requires a high contraction rate (sound production) was accompanied with major peripheral structural modifications, yet the associated neural adjustments remain to this date unclear. To close this gap, we investigated the activity of both the locomotor and the sonic musculature using electromyography. The comparison between the activation patterns of both systems highlighted modifications of the neural motor pathway: (1) a transition from a bilateral alternating pattern to a synchronous activation pattern, (2) a switch from a slow- to a high-frequency regime, and (3) an increase in the synchrony of motor neuron activation. Furthermore, our results demonstrate that sound features correspond to the activity of the sonic muscles, as both the variation patterns of periods and amplitudes of sounds highly correspond to those seen in the sonic muscle electromyograms (EMGsonic). Assuming that the premotor network for sound production in piranhas is of spinal origin, our results show that the neural circuit associated with spinal motor neurons transitioned from the slow alternating pattern originally used for locomotion to a much faster simultaneous activation pattern to generate vocal signals.

Characterization of the acoustic community of vocal fishes in the Azores

Sounds produced by teleost fishes are an important component of marine soundscapes, making passive acoustic monitoring (PAM) an effective way to map the presence of vocal fishes with a minimal impact on ecosystems. Based on a literature review, we list the known soniferous fish species occurring in Azorean waters and compile their sounds. We also describe new fish sounds recorded in Azores seamounts. From the literature, we identified 20 vocal fish species present in Azores. We analysed long-term acoustic recordings carried out since 2008 in Condor and Princesa Alice seamounts and describe 20 new putative fish sound sequences. Although we propose candidates as the source of some vocalizations, this study puts into evidence the myriad of fish sounds lacking species identification. In addition to identifying new sound sequences, we provide the first marine fish sound library for Azores. Our acoustic library will allow to monitor soniferous fish species for conservation and management purposes.

Temperature affects sound production in fish with two sets of sonic organs: The Pictus cat

Sound communication is affected by ambient temperature in ectothermic animals including fishes. The present study examines the effects of temperature on acoustic signaling in a fish species possessing two different sound-generating mechanisms. The Amazonian Pictus catfish Pimelodus pictus produces low-frequency harmonic sounds (swimbladder drumming muscles) and high-frequency stridulation sounds (rubbing pectoral fin spines in the pectoral girdle). Sounds of 15 juveniles were recorded when hand-held after three weeks of acclimation at 30 °C, 22 °C and again 30 °C. The following sound characteristics were investigated: calling activity, sound duration, fundamental frequency of drumming sounds and dominant frequency of stridulation sounds. The number of both sound types produced within the first minute of experiments did not change with temperature. In contrast, sound duration was significantly shorter at 30 °C than at 22 °C (drumming: 78-560 ms; stridulation: 23-96 ms). The fundamental frequency of drumming sounds and thus the drumming muscle contraction rate varied from 127 Hz to 242 Hz and increased with temperature. The dominant frequency of broadband stridulation sounds ranged from 1.67 kHz to 3.39 kHz and was unaffected by temperature changes. Our data demonstrate that temperature affects acoustic signaling in P. pictus, although the changes differed between sound characteristics and sound type. The effects vary from no change in calling activity and dominant frequency, to an increase in fundamental frequency and shortened duration of both sound types. Together with the known effects of temperature on hearing in the Pictus cat, the present results indicate that global warming may affect acoustic communication in fishes.

Identifying sagittal otoliths of Mediterranean Sea gobies: variability among phylogenetic lineages

In this study, we describe and analyse the morphology of the sagitta, the largest otolith, of 25 species of Gobiidae inhabiting the Adriatic and north-western Mediterranean seas. Our goal was to test the usefulness and efficiency of sagittal otoliths for species identification. Our analysis of otolith contours was based on mathematical descriptors called wavelets, which are related to multi-scale decompositions of contours. Two methods of classification were used: an iterative system based on 10 wavelets that searches the Anàlisi de Formes d'Otòlits (AFORO) database and a discriminant method based only on the fifth wavelet. With the exception of paedomorphic species, the results showed that otolith anatomy and morphometry can be used as diagnostic characters distinguishing the three Mediterranean phylogenetic goby lineages (Pomatoschistus or sand-goby lineage, Aphia lineage and Gobius lineage). The main anatomical differences were related to overall shape (square to rhomboid), the development and shape of the postero-dorsal and antero-ventral lobes and the degree of convexity of dorsal and ventral margins. Iterative classifications and discriminant analysis of otolith contour provided very similar results. In both cases, more than 70% of specimens were correctly classified to species and more than 80% to genus. Iterations in the larger AFORO database (including 216 families of teleosts) attained a 100% correct classification at the family level.

Sound-production mechanism in Pomatoschistus pictus

Fish acoustic signals play a major role during agonistic and reproductive interactions. Among the sound-generating fish, Gobiidae, a large fish family with 1866 valid species, is one of the most studied groups of acoustic fishes, with sound production being documented in a number of species. Paradoxically, the sound-producing mechanism remains poorly studied in this group. The painted goby, Pomatoschistus pictus, produces two distinct sounds called drums and thumps. A combination of morphological and experimental analyses involving high-speed videos synchronized with sound recordings supports that drums are produced during lateral head movements involving at least the alternate contractions of the levator pectoralis muscles originating on the skull and inserting on the pectoral girdle. These movements are reported in many Gobiidae species, suggesting the pectoral-girdle-based mechanism is common in the family and could have evolved from locomotory movements.

Vocalisation Repertoire of Female Bluefin Gurnard (Chelidonichthys kumu) in Captivity: Sound Structure, Context and Vocal Activity

Fish vocalisation is often a major component of underwater soundscapes. Therefore, interpretation of these soundscapes requires an understanding of the vocalisation characteristics of common soniferous fish species. This study of captive female bluefin gurnard, Chelidonichthys kumu, aims to formally characterise their vocalisation sounds and daily pattern of sound production. Four types of sound were produced and characterised, twice as many as previously reported in this species. These sounds fit two aural categories; grunt and growl, the mean peak frequencies for which ranged between 129 to 215 Hz. This species vocalized throughout the 24 hour period at an average rate of (18.5 ± 2.0 sounds fish^-1 h^-1) with an increase in vocalization rate at dawn and dusk. Competitive feeding did not elevate vocalisation as has been found in other gurnard species. Bluefin gurnard are common in coastal waters of New Zealand, Australia and Japan and, given their vocalization rate, are likely to be significant contributors to ambient underwater soundscape in these areas.

Sound production by dusky grouper Epinephelus marginatus at spawning aggregation sites

Sound production by the dusky grouper Epinephelus marginatus was monitored both in captivity and at two Mediterranean spawning sites during the summers of 2012 and 2013. The results of long-term passive acoustic recordings provide for the first time a description of the sounds produced by E. marginatus. Two types of sounds were mainly recorded and consisted of low-frequency booms that can be produced singly or in series with dominant frequencies below 100 Hz. Recordings in captivity validated these sounds as belonging to E. marginatus and suggested that they may be associated with reproductive displays usually performed during early stages of courtship behaviour. This study also allowed the identification of a third, low-frequency growl-like type of sound typically found in other grouper species. These growls were, however, not recorded in tanks and it is cautiously proposed that they are produced by E. marginatus. Acoustic signals attributed to E. marginatus were produced throughout the spawning season, with a diel pattern showing an increase before dusk, i.e., from 1900 to 2200 hours, before decreasing until the morning. The occurrence of sounds during the spawning season of this species suggests that they are probably involved in social activity occurring close to aggregation sites. Passive acoustics offer a helpful tool to monitor aggregation sites of this emblematic species in order to improve conservation efforts.

Environmental constraints drive the partitioning of the soundscape in fishes

The underwater environment is more and more being depicted as particularly noisy, and the inventory of calling fishes is continuously increasing. However, it currently remains unknown how species share the soundscape and are able to communicate without misinterpreting the messages. Different mechanisms of interference avoidance have been documented in birds, mammals, and frogs, but little is known about interference avoidance in fishes. How fish thus partition the soundscape underwater remains unknown, as acoustic communication and its organization have never been studied at the level of fish communities. In this study, passive acoustic recordings were used to inventory sounds produced in a fish community (120 m depth) in an attempt to understand how different species partition the acoustic environment. We uncovered an important diversity of fish sounds, and 16 of the 37 different sounds recorded were sufficiently abundant to use in a quantitative analysis. We show that sonic activity allows a clear distinction between a diurnal and a nocturnal group of fishes. Moreover, frequencies of signals made during the day overlap, whereas there is a clear distinction between the different representatives of the nocturnal callers because of a lack of overlap in sound frequency. This first demonstration, to our knowledge, of interference avoidance in a fish community can be understood by the way sounds are used. In diurnal species, sounds are mostly used to support visual display, whereas nocturnal species are generally deprived of visual cues, resulting in acoustic constraints being more important.

Variation in swim bladder drumming sounds from three doradid catfish species with similar sonic morphologies

A variety of teleost fishes produce sounds for communication by vibrating the swim bladder with fast contracting muscles. Doradid catfishes have an elastic spring apparatus (ESA) for sound production. Contractions of the ESA protractor muscle pull the anterior transverse process of the 4th vertebra or Müllerian ramus (MR) to expand the swim bladder and elasticity of the MR returns the swim bladder to the resting state. In this study, we examined the sound characteristics and associated fine structure of the protractor drumming muscles of three doradid species: Acanthodoras cataphractus, Platydoras hancockii, and Agamyxis pectinifrons. Despite important variations in sizes, sounds from all three species had similar mean dominant rates ranging from 91-131 Hz and showed frequencies related to muscle contraction speed rather than fish size. Sounds differed among species in terms of waveform shape and their rate of amplitude modulation. In addition, multiple distinguishable sound types were observed from each species: three sound types from A. cataphractus and P. hancockii, and two sound types from A. pectinifrons. Though sounds differed among species, no differences in muscle fiber fine structure were observed at the species level. Drumming muscles from each species bear features associated with fast contractions, including sarcoplasmic cores, thin radial myofibrils, abundant mitochondria, and an elaborated sarcoplasmic reticulum. These results indicate that sound differences between doradids are not due to swimbladder size, muscle anatomy, muscle length, or Müllerian ramus shape, but instead result from differences in neural activation of sonic muscles.

Sound production in Onuxodon fowleri (Carapidae) and its amplification by the host shell

Onuxodon species are well known for living inside pearl oysters. As in other carapids, their anatomy highlights their ability to make sounds but sound production has never been documented in Onuxodon. This paper describes sound production in Onuxodon fowleri as well as the anatomy of the sound production apparatus. Single-pulsed sounds and multiple-pulsed sounds that sometimes last more than 3 s were recorded in the field and in captivity (Makemo Island, French Polynesia). These pulses are characterized by a broadband frequency spectrum from 100 to 1000 Hz. Onuxodon fowleri is mainly characterized by its ability to modulate the pulse period, meaning that this species can produce pulsed sounds and tonal-like sounds using the same mechanism. In addition, the sound can be remarkably amplified by the shell cavity (peak gain can exceed 10 dB for some frequencies). The sonic apparatus of O. fowleri is characterized by a rocker bone in front of the swimbladder, modified vertebrae and epineurals, and two pairs of sonic muscles, one of which (primary sonic muscle) inserts on the rocker bone. The latter structure, which is absent in other carapid genera, appears to be sexually dimorphic suggesting differences in sound production in males and females. Sound production in O. fowleri could be an example of adaptation where an animal exploits features of its environment to enhance communication.

Acoustics of fish shelters: background noise and signal-to-noise ratio

Fish shelters (flat stones, shells, artificial covers, etc., with a hollow beneath) increase the sound pressure levels of low frequency sounds (<150 Hz) outside the nest cavity, see Lugli [(2012). J. Acoust. Soc. Am. 132, 3512-3524]. Since some calling males only produce sound when a female is inside the shelter, this study examines the effect of sound amplification by the shelter on signal-to-noise ratio (SNR) in the nest. Background noise amplification by the shelter was examined under both laboratory (stones and shells) and field (stones) conditions, and the SNR of tones inside the nest cavity was measured by performing acoustic tests on stones in the stream. Stone and shell shelters amplify the background noise pressure levels inside the cavity with comparable gains and at similar frequencies of an active sound source. Inside the cavity of stream stones, the mean SNR of tones increased significantly below 125 Hz and peaked at 65 Hz (+10 dB). Implications for fish acoustic communication inside nest enclosures are discussed.

A superfast muscle in the complex sonic apparatus of Ophidion rochei (Ophidiiformes): histological and physiological approaches

In teleosts, superfast muscles are generally associated with the swimbladder wall whose vibrations result in sound production. In Ophidion rochei, three pairs of muscles were named 'sonic' because their contractions affect swimbladder position: the dorsal sonic muscle (DSM), the intermediate sonic muscle (ISM), and the ventral sonic muscle (VSM). These muscles were investigated thanks to electron microscopy and electromyography in order to determine their function in sound production. Fibers of the VSM and DSM were much thinner than the fibers of the ISM and epaxial musculature. However, only VSM fibers had the typical ultrastructure of superfast muscles: low proportion of myofibrils, and high proportions of sarcoplasmic reticulum and mitochondria. In females, each sound onset was preceded by the onset of electrical activity in the VSM and the DSM (ISM was not tested). The electromyograms of the VSM were very similar to the waveforms of the sounds: means for the pulse period were 3.6±0.5 ms and 3.6±0.7 ms, respectively. This shows that the fast VSM (ca. 280 Hz) is responsible for the pulse period and fundamental frequency of female sounds. DSM electromyograms were generally characterized by one or two main peaks followed by periods of lower electrical activity which suggests a sustained contraction over the course of the sound. The fiber morphology of the ISM and its antagonistic position relative to the DSM are not indicative of a muscle capable of superfast contractions. Overall, this study experimentally shows the complexity of the sound production mechanism in the nocturnal fish O. rochei.

Contribution to the study of acoustic communication in two Belgian river bullheads (Cottus rhenanus and C. perifretum) with further insight into the sound-producing mechanism

BACKGROUND

The freshwater sculpins (genus Cottus) are small, bottom-living fishes widely distributed in North America and Europe. The taxonomy of European species has remained unresolved for a long time due to the overlap of morphological characters. Sound production has already been documented in some cottid representatives, with sounds being involved in courtship and agonistic interactions. Although the movements associated with sound production have been observed, the underlying mechanism remains incomplete. Here, we focus on two closely related species from Belgium: C. rhenanus and C. perifretum. This study aims 1) to record and to compare acoustic communication in both species, 2) to give further insight into the sound-producing mechanism and 3) to look for new morphological traits allowing species differentiation.

RESULTS

Both Cottus species produce multiple-pulsed agonistic sounds using a similar acoustic pattern: the first interpulse duration is always longer, making the first pulse unit distinct from the others. Recording sound production and hearing abilities showed a clear relationship between the sound spectra and auditory thresholds in both species: the peak frequencies of calls are around 150 Hz, which corresponds to their best hearing sensitivity. However, it appears that these fishes could not hear acoustic signals produced by conspecifics in their noisy habitat considering their hearing threshold expressed as sound pressure (~ 125 dB re 1 μPa). High-speed video recordings highlighted that each sound is produced during a complete back and forth movement of the pectoral girdle.

CONCLUSIONS

Both Cottus species use an acoustic pattern that remained conserved during species diversification. Surprisingly, calls do not seem to have a communicative function. On the other hand, fish could detect substrate vibrations resulting from movements carried out during sound production. Similarities in temporal and spectral characteristics also suggest that both species share a common sound-producing mechanism, likely based on pectoral girdle vibrations. From a morphological point of view, only the shape of the spinelike scales covering the body allows species differentiation.