Sound pressure enhances the hearing sensitivity of Chaetodon butterflyfishes on noisy coral reefs.

Butterflyfishes are conspicuous members of coral reefs that communicate with acoustic signals during social interactions with mates and other conspecifics. Members of the genus Chaetodon have a laterophysic connection (LC) - a unique association of anterior swim bladder horns and the cranial lateral line - but the action of the LC system on auditory sensitivity is unexplored. Here, we show in baseline auditory evoked potential threshold experiments that Forcipiger flavissimus (which lacks swim bladder horns and LC) is sensitive to sound tones from 100 Hz up to 1000 Hz, and that thresholds for three species of Chaetodon are 10-15 dB lower, with extended hearing ranges up to 1700-2000 Hz. The relatively high thresholds to sound pressure and low pass response near 500 Hz for all four species are consistent with a primary sensitivity to hydrodynamic particle acceleration rather than sound pressure. Deflation of the swim bladder in F. flavissimus had no measurable effect on auditory sensitivity. In contrast, displacement of gas from the swim bladder horns in Chaetodon multicinctus and Chaetodon auriga increased thresholds (decreased sensitivity) by 5-20 dB, with the greatest effect at 600 Hz. The evolution of swim bladder horns associated with the LC system in Chaetodon species has increased hearing sensitivity through sound pressure transduction in the frequency bands used for social acoustic communication. The close affiliative behaviors that are common in Chaetodon species and other butterflyfish facilitate sound perception and acoustic communication at close distances relative to the high background noise levels found in their natural reef environment.