Abstract
Based on histochemical detection of the specific activities of myofibrillar ATPase in fast and slow mucle, the fiber composition of the paired clawโcloser muscles in the lobster Homarus americanus was examined during development. The cutter muscle of juvenile lobsters, 1 year (13th stage) and older, has a majority of darkly staining, fast fibers and a minority of lightly staining, slow fibers which are restricted to the ventral area. The crusher muscle, on the other hand, stained lightly over its entire area, showing that it is composed only of slow fibers. In the 3rd larval stage the paired muscles are similar, each having a central band of intenseโstaining, fast fibers sandwiched by lightโstaining, slow fibers. From the 4th stage onwards, the differentiation to the dimorphic condition occurs with gradual expansion, in the cutter claw of the darkโstaining, fast fibers, and in the crusher claw of lightโstaining, slow fibers. The cutter closer muscle is differentiated completely and consistently by the 9thโ10th stage, whereas the crusher differentiated more slowly between the 13thโ22nd stages. During this process the transformation of slow muscle to fast in the cutter claw may be via an intermediate type.