Fig. 1.

Danionella cerebrum vocalizations. (A) Photo of a D. cerebrum male. Note the distinct separation between the anterior and posterior swim bladder chambers. (B) Example of vocalizations (Audio S2). D. cerebrum produce pulses that are arranged into bursts composed of up to several hundred pulses (Top). Zoom-in of the audio trace (Bottom) shows that the large pulse (blue arrow head) is followed by a quieter after-pulse (red arrow head). (C) Spectrogram of the audio trace shown at the Bottom of (B). The pulses are broadband and have a dominant frequency component around 5 kHz. (D) A histogram of the inter-pulse interval has a bimodal distribution with peaks at 9.25 ± 0.97 and 17.98 ± 0.96 ms, estimated from the mean and SD of a Gaussian fit. This corresponds to rates of around 120 and 60 Hz, respectively. (E) Waveform of a single pulse. The main pulse (blue arrow head) begins with a rarefaction and completely decays before the after-pulse (red arrow head) starts. The amplitude of a single pulse can reach 7 Pa, corresponding to 137 dB (re 1 μP). The oscillations after the initial pulse are likely due to reflections (SI Appendix, Fig. S1). (F) Relative to their body weight, Danionella (blue dot) sounds are particularly loud compared with those of communication calls in other animals. All dB values are given relative to the standard reference pressures of 1 µPa in water and 20 µPa in air.