Temperature and neuromuscular development in the tambaqui

The development of muscle innervation pattern was investigated in larvae of the Amazonian fish, the tambaqui Colossoma macropomum. The time to hatching decreased from 28–29 h at 23.5° C to 11–12 h at 31° C. The larvae hatched after the completion of somitogenesis (38-somite stage) at 23.5° C but only at the 33-somite stage at 28–31° C. Embryos were stained for acetylcholinesterase activity and with an acetylated tubulin antibody in order to visualize neural processes. All muscle fibre types were initially innervated at their myoseptal ends. The development of motor innervation to the trunk muscle was delayed with respect to hatching at higher temperatures. At hatching, muscle fibres were innervated only to somites 16–17 at 28–31° C and somite 23–26 at 23.5–25° C (counting from the head), although the larvae swam vigorously to avoid sinking. In contrast, in newly hatched larvae myofibrils were present right along the trunk at all temperatures in both the superficial and inner muscle fibres. At hatching numerous multi-layered membrane contacts with the ultrastructural characteristics of gap junctions, were found between muscle fibres and at the inter-somite junctions, suggesting the somites were initially electrically coupled. These structures disappeared concomitant with the development of muscle endplates right down the trunk. The larvae started feeding 5 days post-hatch at 28° C. First feeding was associated with a dramatic decrease in the volume density of mitochondria and an increase in the volume density of myofibrils in the inner muscle fibres. The polyneuronal and multi-terminal pattern of innervation characteristic of adult slow-muscle fibres also developed around the time of first feeding.