Multiple exaptations leading to fish sound production

The term exaptation introduced by Stephen J. Gould and Elizabeth Vrba has been used infrequently. The high diversity of sound-producing mechanisms in fishes highlights a recurrent use of this process in unrelated taxa. We propose that sonic evolution typically involves exaptations: in many fish taxa, sound production was acquired by the independent modification of existing structures with other functions. These structures were modified into complex effectors for courtship and agonistic sound production without major changes to their gnathostome Bauplan. Existing anatomical structures (teeth, bones, etc.) were likely first used in non-voluntary sound production, which incidentally provided advantages and could then be selected specifically for signal production leading to the refinement of more sophisticated sonic organs. We postulate that in many if not most cases, sound-production specializations originated in fish taxa that took advantage of incidental non-voluntary sounds. We use different case-studies to show that exaptation may be a key, albeit largely unrecognized, agent of major morphological and behavioural changes.