Mechanisms of sound production in the ariid catfishes Galeichthys and Bagre. Bulletin of the AMNH ; v. 124, article 1

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New York : [American Museum of Natural History]
"The skeletal basis of the sound producing mechanism in the ariid catfishes Galeichthys felis and Bagre marinus consists of a thin shelf of bone which is firmly attached to the anterior dorsal wall of the swim bladder. This so-called 'elastic spring,' or 'Springfederapparat,' is formed from the parapophysis of the fourth vertebra. The anterior ramus of this parapophysis, herein named the 'Müllerian ramus' (after Johannes Müller who, in 1842, first described the structure), is the main vibrating element in sound production. The skulls and anterior vertebral complexes of Galeichthys and Bagre are figured and described, with comparisons with a non-sonic silurid form, Wallago. The 'protractor muscle' (so named by Bridge and Haddon in 1893) which activates the Müllerian ramus is a highly vascularized conical muscle, the origin of which is on the under side of the epiotic lamina and insertion on the Müllerian ramus. It is innervated by the dorsal branch of the occipital nerve. The relationship of this nerve to the cranial nerves of higher vertebrates is controversial, but it is probably homologous with the hypoglossal (XII). Stimulation of the protractor muscle or its nerve supply with repetitive spike-form potentials results in an audible sound output from the Springfederapparat. This response can be recorded and analyzed, and its fundamental pitch is equivalent to the pulse frequency of the stimulus. The protractor muscle is remarkable in that it can withstand stimulations of 300 or more pulses per second without going into immediate tetany. In Bagre, the frequency response of the sonic system is about an octave higher than that in Galeichthys. Spontaneous sounds from these species consist of low-pitched grunts (fundamental pitch of about 150 cycles per second) and, in Bagre, higher, sob-like sounds (400 cycles per second or over). The harmonic components of both artificially induced and spontaneous sounds can be influenced by the amount of sound reflectance in the external environment. Even under partially anechoic conditions, the sound output is virtually a pure tone. Damage to the swim bladder reduces the amplitude of the sound but not its timbre (i.e., harmonic content). It was concluded that the swim bladder does not serve as a resonating chamber for these sounds, nor is it a true amplifier. Rather, it transfers the energy from the small area of the vibrating Müllerian ramus to the larger area of its entire outer surface, thus making the propagation of the sound from its source to the water more efficient. In acoustical and electronic terms, the swim bladder is an impedance matching device"--P. 27.
30 p., 28 p. of plates : ill. ; 27 cm.
Includes bibliographical references (p. 27-30).