Fig. 6. Sound production by multiband butterflyfish, Chaetodon multicinctus, intruders during encounters with conspecific territory holders. (A) The pelvic flick behavior by an intruder involves the extension of the pelvic fins that produces an acoustic stimulus during this motion (waveform on lower trace). This sound has a frequency spectrum just below 200 Hz and is also produced during displays by resident fish towards bottled intruders. Arrows show start and end of pelvic fin extension. (B) The grunt train sound is produced by bottled intruder pairs when confronted by territory residents. Unlike all other acoustic signals observed, the grunt train was not associated with any visible body movements, thus most likely is caused by internal muscle movements. This sound included a series of regular spaced pulses (waveform on lower trace) that have a strong frequency component at 400-500 Hz (spectrogram). Note the harmonics associated with each pulse that indicate that this internally generated sound may result in part from resonance of the swim bladder.