Rethinking sound detection by fishes.

In this paper we reconsider the designation of fishes as being either "hearing specialists" or "hearing generalists," and recommend dropping the terms. We argue that this classification is only vaguely and variously defined in the literature, and that these terms often have unclear and different meaning to different investigators. Furthermore, we make the argument that the ancestral, and most common, mode of hearing in fishes involves sensitivity to acoustic particle motion via direct inertial stimulation of the otolith organ(s). Moreover, any possible pressure sensitivity is the result of the presence of an air bubble (e.g., the swim bladder), and that hearing sensitivity may be enhanced by the fish having a specific connection between the inner ear to a bubble of air. There are data showing that some fish species have a sensitivity to both pressure and motion that is frequency dependent. Thus such species could not possibly be termed as either hearing "generalists" or specialists," and many more species probably could be classified in this way as well. Furthermore, we propose that the term "specialization" be reserved for cases in which a species has some kind of morphological connection or close continuity between the inner ear and an air bubble that affects behavioral sensitivity to sound pressure (i.e., an otophysic connection).