Marta Bolgan, Justine Soulard, Lucia Di Iorio, Cedric Gervaise, Pierre Lejeune, Sylvie Gobert, and Eric Parmentier

The /Kwa/ dominates the soundscape of Posidonia oceanica meadows but the identity of the species emitting this peculiar fish sound remains a mystery. Information from sounds recorded in the wild indicate that the emitting candidates should be abundant, nocturnal and benthic. Scorpaena spp. combine all these characteristics. This study used an inter-disciplinary approach to investigate the vocal abilities of Scorpaena spp.; morphological, histological and electrophysiological examinations have been interpreted together with visual and acoustic recordings conducted in semi-natural conditions. All observed Scorpaena spp. (S. porcus, S. scrofa and S. notata) share the same sonic apparatus at the level of the abdominal region. This apparatus, present in both males and females, consists of 3 bilaterally symmetrical muscular bundles, having 3 to 5 long tendons, which insert on ventral bony apophyses of the vertebral bodies. In all chordophones (stringed instruments), the frequency of the vibration is dependent on the string properties and not on rates at which the strings are plucked. Similarly, we suggest that each of the 3 to 5 tendons found in the sonic mechanism of Scorpaena spp. acts as a frequency multiplier of the muscular bundles contractions, where the resonant properties of the tendons determine the peak frequency of the /Kwa/, its frequency spectra and pseudo-harmonic profile. The variability in the length and number of tendons found between and within species could explain the high variability of /Kwa/ acoustic features recorded in the wild. Finally, acoustic and behavioural experiments confirmed that Scorpaena spp. emit the /Kwa/ sound.

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