Muscle development in larvae of a fast growing tropical freshwater fish, the curimatã-pacú

The distribution and ultrastructure of myotomal muscle fibres was studied in larvae and early juveniles of the curimatã-pacú Prochilodus marggravii , a tropical freshwater fish endemic to the São Francisco River system, Brazil. At 26°C, larvae hatched 15 h post-fertilization at a relatively early stage of development with the head still curved around the yolk-sac (head-trunk angle greater than 45°), and prior to pigmentation of the eyes and formation of the jaws, gut and pectoral fins. Although motile the swimming muscles of newly-hatched larvae were largely undifferentiated. The myotomes were made up of a single layer of superficial muscle fibres containing six to eight myofibrils and abundant mitochondria, surrounding an inner core of myoblasts, myotubes and immature muscle fibres. The volume densities of mitochondria and myofibrils in the immature inner muscle fibres of 1-day-old lavae were 14.5 and 6.4% respectively. The body axis straightened within 24 h of hatching and the yolk sac was completely absorbed by 72 h. Larval development was rapid with gill filaments, a muscular stomach, liver and swimbladder present after 7 days. The inner muscle fibres were well differentiated in 7-day-old larvae; the volume density of myofibrils had increased to 63.1% whereas the volume density of mitochondria had decreased to 3.5%. In 14-day-old juveniles the superficial muscle had thickened to a layer two to three fibres thick in the region of the lateral line nerve and capillaries were present in the inner muscle. Muscle growth until 14 days was largely due to the hypertrophy of the fibres present at hatching.