Characterisation of the primary sonic muscles in Carapus acus (Carapidae):
a multidisciplinary approach
Parmentier E, Gennotte V, Focant B, Goffinet G &Vandewalle P
Proceedings of the Royal Society of London part B, 270, 2301-2308. 2003
Sound production in carapid fish results from the action of extrinsic muscles that insert on the swim bladder. Biochemical, histochemical and morphological techniques were used to examine the sonic muscles and compared them with epaxial muscles in Carapus acus. Sonic fibres are thicker than red and thinner than white epaxial fibres, and sonic fibres and myofibrils exhibit an unusual helicoidal organization : the myofibrils of the centre are in a straight line whereas they are more and more twisted towards the periphery. Sonic muscles have both features of red (numerous mitochondria, high glycogen content) and white (alkali-stable ATPase) fibres. They differ also in the isoforms of the light-chain (LC3) and heavy-chain (HC), in having T-tubules at both the Z-line and the A-I junction and in a unique parvalbumin isoform (PA I) that may aid relaxation. All these features allow to express two assumptions on sound generation : the sonic muscle should be able to realise fast and powerful contractions that provoke the frontward movement of the forepart of the swim bladder and the stretching and "flapping" of the swim bladder fenestra; the helicoidal organisation allows to progressively draw the swim bladder fenestra which emits sound when rapidly released in a spring-like manner.
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