Morphological and histochemical characterization of the pectoral fin muscle of the stripped weakfish,Cynoscion guatucupa

The fibres of superficial and deep abductor muscles of the pectoral fins of the stripped weakfish, Cynoscion guatucupa have been studied using histochemical techniques: succinic dehydrogenase (SDH) for mitochondria, periodic acid–Schiff (PAS) for glycogen, myosin‐adenosintriphosphatase (mATPase) to identify different fibre types based on the contraction speed and modified ATPase to identify capillaries. The fibre diameters were measured, and the capillaries of the main fibre types – red, pink and white— were counted. The two muscles showed both macroscopically and microscopically two well‐differentiated zones with predominant white fibres. The area of insertion of muscles into the fin rays had red, pink and white fibres. The origin zone of the muscle into the bone was composed by white fibres only. Both zones of white muscle evidenced a mosaic of small, medium and large polygonal white fibres. Red, pink and white muscles showed a wide histochemical diversity of fibre subtypes. The area per peripheral capillary increased from the red to the white muscles. Due to the predominance of white fibres, the pectoral fins of C. guatucupa were mainly involved in rapid movements to stop/discontinue and stabilize the body during swimming.