Fast drum strokes: Novel and convergent features of sonic muscle ultrastructure, innervation, and motor neuron organization in the pyramid butterflyfish (hemitaurichthys polylepis)

Transcriptome analysis reveals dysfunction of the endoplasmic reticulum protein processing in the sonic muscle of small yellow croaker (Larimichthys polyactis) following noise exposure

Noise pollution is increasingly prevalent in aquatic ecosystems, causing detrimental effects on growth and behavior of marine fishes. The physiological responses of fish to underwater noise are poorly understood. In this study, we used RNA-sequencing (RNA-seq) to study the transcriptome of the sonic muscle in small yellow croaker (Larimichthys polyactis) after exposure to a 120 dB noise for 30 min. The behavioral experiment revealed that noise exposure resulted in accelerated tail swimming behavior at the beginning of the exposure period, followed by loss of balance at the end of experiment. Transcriptomic analysis found that most highly expressed genes in the sonic muscle, including parvalbumin, slc25a4, and troponin C were related with energy metabolism and locomotor function. Further, a total of 1261 differentially expressed genes (DEGs) were identified, including 284 up-regulated and 977 down-regulated genes in the noise exposure group compared with the control group. Gene ontology (GO) analysis indicated that the most enriched categories of DEGs included protein folding and response to unfolding protein. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis found over-represented pathways including protein processing in the endoplasmic reticulum, chaperones and folding catalysts, as well as arginine and proline metabolism. Specifically, many genes related to fatty acid and collagen metabolism were up-regulated in the noise exposure group. Taken together, our results indicate that exposure to noise stressors alters the swimming behavior of croaker, inducing endoplasmic reticulum stress, disrupting lipid metabolism, and causing collagen degradation in the sonic muscle of L. polyactis.

Functional Adaptation of Vocalization Revealed by Morphological and Histochemical Characteristics of Sonic Muscles in Blackmouth Croaker (Atrobucca nibe)

Simple Summary

Sound production is common in numerous fish species. Some species can emit calls through the contraction of specialized muscles called sonic or drumming muscles. The sonic muscles of fish are among the fastest muscles in vertebrates. Although numerous studies have investigated the mechanism underlying sound production in fish, only the distinct features of the sonic muscles of a few species have been investigated. We demonstrated that the sonic muscles have functionally adapted for fast twitching and fatigue resistance, which support vocalization in the blackmouth croaker (Atrobucca nibe).

Abstract

Sound production in the blackmouth croaker (Atrobucca nibe) was characterized using acoustic, morphological, and histochemical methods. Their calls consisted of a train of two to seven pulses; the frequency ranged from 180 to 3000 Hz, with a dominant frequency of 326 ± 40 Hz. The duration of each call ranged from 80 to 360 ms. Male A. nibe possess a pair of bilaterally symmetric sonic muscles attached to the body wall adjacent to the swim bladder. The average diameter of the sonic muscle fibers was significantly shorter than that of the abdominal muscle fibers. Semithin sections of the sonic muscle fibers revealed a core-like structure (central core) and the radial arrangement of the sarcoplasmic reticulum and myofibrils. Numerous mitochondria were distributed within the central core and around the periphery of the fibers. Most of the fibers were identified as Type IIa on the basis of their myosin adenosine triphosphatase activities, but a few were identified as Type IIc fibers. All sonic muscle fibers exhibited strong oxidative enzyme activity and oxidative and anaerobic capabilities. The features suggest that the sonic muscles of A. nibe are morphologically and physiologically adapted for fast twitching and fatigue resistance, which support fish vocalization.

Acoustic Communication in Butterflyfishes: Anatomical Novelties, Physiology, Evolution, and Behavioral Ecology

Coral reef fishes live in noisy environments that may challenge their capacity for acoustic communication. Butterflyfishes (Family Chaetodontidae) are prominent and ecologically diverse members of coral reef communities worldwide. The discovery of a novel association of anterior swim bladder horns with the lateral line canal system in the genus Chaetodon (the laterophysic connection) revealed a putative adaptation for enhancement of sound reception by the lateral line system and/or the ear. Behavioral studies show that acoustic communication is an important component of butterflyfish social behavior. All bannerfish (Forcipiger, Heniochus, and Hemitaurichthys) and Chaetodon species studied thus far produce several sound types at frequencies of <1 to >1000 Hz. Ancestral character state analyses predict the existence of both shared (head bob) and divergent (tail slap) acoustic behaviors in these two clades. Experimental auditory physiology shows that butterflyfishes are primarily sensitive to stimuli associated with hydrodynamic particle accelerations of ≤500 Hz. In addition, the gas-filled swim bladder horns in Chaetodon are stimulated by sound pressure, which enhances and extends their auditory sensitivity to 1700-2000 Hz. The broadband spectrum of ambient noise present on coral reefs overlaps with the frequency characteristics of their sounds, thus both the close social affiliations common among butterflyfishes and the evolution of the swim bladder horns in Chaetodon facilitate their short-range acoustic communication. Butterflyfishes provide a unique and unexpected opportunity to carry out studies of fish bioacoustics in the lab and the field that integrate the study of sensory anatomy, physiology, evolution, and behavioral ecology.

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.

Vocal behavior and vocal central pattern generator organization diverge among toadfishes

Among fishes, acoustic communication is best studied in toadfishes, a single order and family that includes species commonly known as toadfish and midshipman. However, there is a lack of comparative anatomical and physiological studies, making it difficult to identify both shared and derived mechanisms of vocalization among toadfishes. Here, vocal nerve labeling and intracellular in vivo recording and staining delineated the hindbrain vocal network of the Gulf toadfish Opsanus beta. Dextran-biotin labeling of the vocal nerve or intracellular neurobiotin fills of motoneurons delineated a midline vocal motor nucleus (VMN). Motoneurons showed bilaterally extensive dendritic arbors both within and lateral to the paired motor nuclei. The motoneuron activity matched that of the spike-like vocal nerve motor volley that determines the natural call duration and frequency. Ipsilateral vocal nerve labeling with biocytin or neurobiotin yielded dense bilateral transneuronal filling of motoneurons and coextensive columns of premotor neurons. These premotor neurons generated pacemaker-like action potentials matched 1:1 with vocal nerve and motoneuron firing. Transneuronal transport further revealed connectivity within and between the pacemaker-motor circuit and a rostral prepacemaker nucleus. Unlike the pacemaker-motor circuit, prepacemaker firing did not match the frequency of vocal nerve activity but instead was predictive of the duration of the vocal nerve volley that codes for call duration. Transneuronally labeled terminal-like boutons also occurred in auditory-recipient hindbrain nuclei, including neurons innervating the inner ear and lateral line organs. Together with studies of midshipman, we propose that separate premotor populations coding vocal frequency and duration with direct premotor coupling to auditory-lateral line nuclei are plesiomorphic characters for toadfishes. Unlike in midshipman, transneuronal labeling in toadfishes reveals an expansive column of pacemaker neurons that is weakly coupled to prepacemaker neurons, a character that likely depends on the extent of gap junction coupling. We propose that these and other anatomical characters contribute to neurophysiological properties that, in turn, sculpt the species-typical patterning of frequency and amplitude-modulated vocalizations.

Expression evolution facilitated the convergent neofunctionalization of a sodium channel gene

Ion channels have played a substantial role in the evolution of novel traits across all of the domains of life. A fascinating example of a novel adaptation is the convergent evolution of electric organs in the Mormyroid and Gymnotiform electric fishes. The regulated currents that flow through ion channels directly generate the electrical signals which have evolved in these fish. Here, we investigated how the expression evolution of two sodium channel paralogs (Scn4aa and Scn4ab) influenced their convergent molecular evolution following the teleost-specific whole-genome duplication. We developed a reliable assay to accurately measure the expression stoichiometry of these genes and used this technique to analyze relative expression of the duplicate genes in a phylogenetic context. We found that before a major shift in expression from skeletal muscle and neofunctionalization in the muscle-derived electric organ, Scn4aa was first downregulated in the ancestors of both electric lineages. This indicates that underlying the convergent evolution of this gene, there was a greater propensity toward neofunctionalization due to its decreased expression relative to its paralog Scn4ab. We investigated another derived muscle tissue, the sonic organ of Porichthys notatus, and show that, as in the electric fishes, Scn4aa again shows a radical shift in expression away from the ancestral muscle cells into the evolutionarily novel muscle-derived tissue. This study presents evidence that expression downregulation facilitates neofunctionalization after gene duplication, a pattern that may often set the stage for novel trait evolution after gene duplication.

Sound production mechanism in Gobius paganellus (Gobiidae)

Gobiidae, the largest fish family (&gt;1500 species), has species from at least 10 genera that produce sounds for communication. Studies focused on goby sound production mechanisms have suggested that sounds are produced by the forcible ejection of water through small apertures in the opercles (hydrodynamic mechanism). The present study was a multidisciplinary investigation (morphology, muscle histology, high-speed video, sound analysis and electromyography) of the sound emission mechanism in Gobius paganellus, which produces both pulsed and tonal calls. Two populations were used, from Brittany and Venice. In the French population, sounds were accompanied by a suite of coordinated movements of the buccal, branchial and opercular regions. This was not the case in the Venetian population, and thus the direct role of head movements in sound production was rejected. The hydrodynamic mechanism hypothesis was also rejected in G. paganellus on the basis of sound oscillogram shape and because sounds are still produced after the opercles and hyohyoid muscles are cut. The use of both electromyography and electron microscopy showed that the levator pectoralis muscle, which originates on the skull and inserts on the dorsal tip of the cleithrum, is involved in sound production. We propose that the contraction of this muscle and associated vibration of the large radials is used to make sounds. In addition, we propose that different sound types (pulsed sounds and tonal calls) could occur because of differences in fish size.