Novel vocal repertoire and paired swimbladders of the three-spined toadfish, Batrachomoeus trispinosus: insights into the diversity of the Batrachoididae

Toadfishes (Teleostei: Batrachoididae) are one of the best-studied groups for understanding vocal communication in fishes. However, sounds have only been recorded from a low proportion of taxa within the family. Here, we used quantitative bioacoustic, morphological and phylogenetic methods to characterize vocal behavior and mechanisms in the three-spined toadfish, Batrachomoeus trispinosus. B. trispinosus produced two types of sound: long-duration 'hoots' and short-duration 'grunts' that were multiharmonic, amplitude and frequency modulated, with a dominant frequency below 1 kHz. Grunts and hoots formed four major classes of calls. Hoots were typically produced in succession as trains, while grunts occurred either singly or as grunt trains. Aside from hoot trains, grunts and grunt trains, a fourth class of calls consisted of single grunts with acoustic beats, apparently not previously reported for individuals from any teleost taxon. Beats typically had a predominant frequency around 2kHz with a beat frequency around 300Hz. Vocalizations also exhibited diel and lunar periodicities. Spectrographic cross-correlation and principal coordinates analysis of hoots from five other toadfish species revealed that B. trispinosus hoots were distinct. Unlike any other reported fish, B. trispinosus had a bilaterally divided swimbladder, forming two separate swimbladders. Phylogenetic analysis suggested B. trispinosus was a relatively basal batrachoidid, and the swimbladder and acoustic beats were independently derived. The swimbladder in B. trispinosus demonstrates that toadfishes have undergone a diversification of peripheral sonic mechanisms, which may be responsible for the concomitant innovations in vocal communication, namely the individual production of acoustic beats as reported in some tetrapods.