Directional sensitivity of saccular microphonic potentials in the haddock.

The function of the swimbladder in fish hearing is well established for the ostariophysine species, which possess a chain of ossicles connecting the swimbladder mechanically to the perilymph of the labyrinth (cf. von Frisch, 1936; Poggendorf, 1952). However, the auditory function of the swimbladder in fish lacking a mechanical connexion between the swimbladder and the ear is less clear. Van Bergeijk (1964) and Alexander (1966), from hypothetical considerations, have pointed out the possible widespread use of the swimbladder in sound reception. They suggested that the swimbladder might function as a pressure/displacement transformer, even in the absence of a mechanical linkage, acting as a secondary sound-source re-radiating near-field particle displacements which stimulate the auditory receptors. However, experimental evidence supporting this theory has been sparse. Several authors have compared the hearing of species with and without swimbladders under the same acoustic conditions and concluded that the swimbladder is involved in sound reception (Enger & Andersen, 1967; Iversen, 1969; Chapman & Sand, 1973). Recently a more direct approach has been employed by Sand & Enger (1973), who showed that in cod kept at 6 m depth the saccular microphonic potentials were drastically reduced when the swimbladder was emptied by sucking the gas out of it through a hypodermic syringe. The advantage of utilizing the swimbladder pulsations in hearing is a lowering of the auditory threshold. A concomitant disadvantage, however, is a possible loss of ability to determine the direction of the sound source (van Bergeijk, 1964). This follows from the assumption that the re-radiated sound from the swimbladder will mask any differences in arrival time, phase and intensity of the incident sound at the two ears. Earlier behavioural studies supported this view by indicating that fish are unable to determine the direction of a sound source except at close range, where the lateral line organs probably are involved (von Frisch & Dijkgraf, 1935). Sharks are known to have directional hearing at long range (Nelson & Gruber, 1963; Myrberg, Banner & Richard, 1969), but recent investigations by Olsen (1969) on herring and by Schuijf, Baretta & Wildschut (1972) on wrasse (Labrus berggylta), undertaken under much better acoustic conditions than earlier studies, have shown that fish with swimbladders can also distinguish between different sound-source directions. Herring were reported to be able to determine direction within at least 45 °, probably