Humans are a superficial bunch. For us, beauty tends to be skin deep. But for many creatures, it goes much deeper. Weakfish males prepare for courtship by building up a pair of swim bladder-vibrating muscles, which contract simultaneously to serenade the ladies with a booming low croak. But weakfish aren't the only fish blessed with an unusual swim bladder-voice; toadfish and searobins also call when the need arises. Fascinated by these fish and their extraordinary sonic muscles, Connaughton was puzzled when he read about the searobin. According to recordings made by Andy Bass from the neural centre that governs the fish's call, searobins appeared to alternate the muscles'contractions, rather than vibrating them simultaneously. Puzzled by the searobin's alternative sonic approach, Connaughton began recording the fish's call and sonic muscle activity, to find out how the fish project their voice(p. 1643).
Unfortunately for Connaughton, searobins don't reside in the waters around the Mount Desert Island Biological Lab in Maine. They had to be caught in the warmer waters off Woods Hole and transported north before Connaughton could begin recording croaks and electromyographs (EMGs) from the fish's contracting sonic muscles, to see whether the muscles worked together or alternately.
At first Connaughton made recordings from the sonic muscle on the fish's right side. For every two vibrations produced by the oscillating swim bladder,the muscle only twitched once. And when he recorded from the left side's muscle, he found the same 2:1 pattern again. But when he recorded EMGs from both muscles simultaneously, each swim bladder oscillation corresponded to a contraction from one, or other, of the muscles. The muscles were contracting alternately, at a relatively low frequency of 100 Hz to produce the fish's 200 Hz call.
But had the fish traded off their voice's volume in favour of their low frequency solution to a high frequency problem? Surely the amplitude of searobin vibrations produced by single muscular contractions would be half the amplitude if both muscle's contracted simultaneously. Connaughton recorded the amplitude of the acoustic waveforms generated by each searobin muscular contraction to see whether the fish had softer voices.
As Connaughton suspected, the amplitude of the vibrations was less than if both muscles contracted simultaneously; but it wasn't cut by half. He realised that each individual vibration was being amplified somehow, so decided to take a closer look at the fish's acoustic waveforms.
Watching the vibration's traces, Connaughton noticed that both the contraction and relaxation phases of the sonic muscle's twitch generated a vibration in the swim bladder. During the first few cycles of a croak, the fish's voice was weak, until the relaxation vibration began interfering constructively with the contraction vibration from the same muscle. Suddenly the fish's voice gained strength as the vibrations produced by that muscle interfered constructively to boost the amplitude produced by a single muscle. So these resourceful fish have come up with a constructive solution to pump up a weak whisper's volume.
