ABSTRACT The adductor pectoral fin muscle in post-spawning females of the white croaker Micropogonias furnieri (Desmarest, 1823) is described from a morphological, histochemical and morphometric perspectives. A description of the morphological characteristics was conducted after dissections in different layers, down to the deep layer. Five muscles were found: superficial, medial, radial and deep adductors, and dorsal arrector. Their fibers were studied after applying histochemical techniques: succinic dehydrogenase (SDH) to reveal mitochondria, periodic acid-Schiff (PAS) for glycogen, Sudan black for lipids, myosin adenosinetriphosphatase (mATPase) to reveal the types of fibers, and modified mATPase to evidence the capillaries. Fiber diameters were measured and the number of capillaries was counted. Fiber subtypes named small, medium and large were found within red, pink and white fibers, the latter prevailing. Staining homogeneity was observed in white fibers after alkaline pre-incubations. The number of capillaries decreased from red to white fibers. Due to the prevalence of white fibers, the adductor muscle of the pectoral fins appears to be capable of rapid and discontinuous movements, which are important to body stabilization during subcarangiform swimming. The homogenous staining of white fibers observed in this research corresponds to the post-spawning gonad stage studied.

The present study highlights the morphology of the Micropogonias furnieri sonic muscle by means of histochemical techniques, relating it to previous histochemical studies of the pectoral fin and myotomal muscles. In order to classify the muscle fibres, succinic dehydrogenase (SDH) for mitochondria, periodic acid Schiff (PAS) for glycogen, Sudan Black and Red for lipids and myosin adenosine triphosphatase (m-ATPase) pre-incubated at alkaline and acid pHs to visualize the contraction velocity, were used. The sonic fibres were smaller than the white myotomal and pectoral fibres, showed homogenous size and distribution and had features common to white fibres: negative to SDH and lipids, weakly positive to PAS and m-ATPase following acid pre-incubation, and positive to m-ATPase at alkaline pre-incubation. The morphology of the sonic muscle of M. furnieri showed no differences between sexes, probably due to the fact that the individuals were at the post spawning maturity stage. This similarity would indicate a similar sound production in both sexes, related to the "disturbance calls" of this species.

El presente trabajo estudia la morfología del músculo del sonido de Micropogonias furnieri por medio de técnicas histoquímicas y lo compara con trabajos previos de la histoquímica del músculo pectoral y miotomal. Con el fin de clasificar las fibras musculares se utilizaron distintas técnicas: succinato deshidrogenasa (SDH) para mitocondrias, ácido periódico Schiff (PAS) para evidenciar glucógeno, Sudan Black y Red para lípidos y miosin adenosina trifosfatasa (m-ATPasa) preincubada a pH alcalinos y ácidos para determinar la velocidad de contracción muscular. Las fibras del músculo del sonido son más pequeñas que las fibras blancas del músculo miotomal y pectoral, presentan un tamaño y distribución homogéneos, y tienen características histoquímicas de fibras blancas: son negativas para SDH y lípidos, débilmente positivas al PAS y a la técnica de m-ATPasa luego de preincubación ácida, y positivas a esta última técnica luego de preincubaciones alcalinas. La morfología del músculo del sonido de M. furnieri no mostró diferencias entre sexos, probablemente debido a que los individuos utilizados se encontraban en el estadio de post-desove. Esta similitud estaría relacionada con el tipo de sonido de "disturbio" que es emitido tanto por machos como por hembras durante todo el año.