Ultrastructural and contractile activation properties of crustacean muscle fibres over the moult cycle

Crustacean muscles are composed of muscle fibres with greatly different ultrastructure. At the time of ecdysis the large chelae muscle undergoes an atrophy triggered by the moult to aid in the withdrawal of the muscle mass. There appear to be distinct differences between the signs of atrophy observed in long- and short-sarcomere fibres while erosion within the myofibrillar bundles is more extensive in fibres with long sarcomeres. Immediately after ecdysis the carapace expands stretching the internal muscles. This review also describes the mechanisms that fibres with different ultrastructure use to elongate as the new exoskeleton expands. Fibres with short sarcomeres increase their length by inserting new sarcomeres along the length of the myofibril by transverse sarcomere splitting and Z line splitting. Long- sarcomere fibres, however, may utilize mechanisms other than sarcomere insertion. Large electron dense structures located around the Z lines, in the A and I bands and between myofibrils are observed in long-sarcomere fibres thought to be undergoing lengthening. These structures may be involved in myofibrillar elongation. As elongation and atrophy involve changes to the myofibrillar lattice, the functional properties at certain stages of the moult cycle are discussed. Long-sarcomere fibres maintain their function over the moult cycle developing forces of similar magnitude in the intermoult, premoult and postmoult stages. The contractile properties of fibres with short sarcomeres were modified over the moult cycle as these fibres could not withstand maximal activation during the premoult stage suggesting that sarcomeric proteins have been disrupted.