Life beneath the sea is far from quiet. Apart from whales singing and dolphins clicking, there are families of fish that produce a repertoire of buzzes and whistles by vibrating their swim bladders. While toadfish and sea robins are well known for their vocal lifestyle, a few butterflyfish species have recently joined the ranks of croaking fish. Kelly Boyle and Timothy Tricas, from the University of Hawaii at Manoa, explain that vocal fish species mainly use two methods to make calls. Some vibrate their swim bladders with intrinsic muscles that insert onto the swim bladder and vibrate it directly while other species vibrate the swim bladder by contracting muscles that are adjacent to it. Curious to find out how one species of butterflyfish, the pyramid butterflyfish (Hemitaurichthys polylepis), makes its croaks, Boyle and Tricas went diving on the Puako coral reef to film and record the fish calling (p. 3881).
The duo found that the fish only used their voices in the late afternoon when they were close to the reef during courtship. Back in the lab, Boyle and Tricas decided to take a closer look at the sounds the fish made and the way they made them. Filming the fish at high speed as they croaked, they noticed a small (0.2 cm2) section of the skin on the fish's side – above the swim bladder and just behind the pectoral fin – buckling. Inserting minute EMG electrodes into individual muscles in the area, the team found that one muscle group, the anterior hypaxial muscles, were firing electrical signals in synch with the patch of skin. They also recorded the croaks' spectra and found that the fish produced rapid pulses of sound. Each pulse had a peak frequency of 97 Hz and the pulses were also in synch with the buckling movement on the fish's surface. Finally, Boyle and Tricas looked at the position of the anterior hypaxial muscles and found that the muscles were not attached directly to the swim bladder, but to adjacent structures. The duo suspects that when the muscles on both sides contract, they buckle the fish's sides forcing the swim bladder out in front to produce a sound. They also explain that the pyramid butterflyfish's indirect approach is similar to that of unrelated fish and the duo suspects that these butterflyfish have independently evolved the mechanism instead of inheriting it from close relatives.