From soundscape to species: identifying Brotula multibarbata as the source of a reef sound with conservation implications

Passive acoustic monitoring (PAM) is a powerful tool for collecting different kinds of data on species biology and ecology. A distinctive frequency-modulated sound, referred to as the “whoot”, has been recurrently recorded in coral reef soundscapes of French Polynesia during the night. However, the identity of the emitter remained unknown. In this study, we combine tank and field acoustic recordings with morphological analyses to highlight that the whoot is produced by the goatsbeard brotula, Brotula multibarbata, a cryptic, nocturnal ophidiiform fish. Some individuals were recorded in captivity, and their calls matched field recordings in structure, duration, frequency, and modulation pattern. The sound producing apparatus consists of three main components: (1) a thin, flexible region of the anterior swimbladder (swimbladder fenestra), (2) a bony dome formed by enlarged epineurals (swimbladder plates) located dorsally above the fenestra, and (3) a pair of fast-contracting sonic muscles. Muscle contractions induce rapid movements of the fenestra and vibration of the overlying plates, producing sound. Literature indicates whoot calls have been recorded across the Indo-Pacific, from Mozambique to French Polynesia, confirming both the wide geographic distribution of B. multibarbata and the potential of PAM to provide key ecological insights, all information being valuable for conservation strategies.