DIFFERENTIAL MOLT-INDUCED ATROPHY IN THE DIMORPHIC CLAWS OF MALE FIDDLER-CRABS, UCA-PUGNAX

Molt-induced atrophy was examined in the closer muscles of the dimorphic claws of the fiddler crab, Uca pugnax. In adult males, the major claw, which is about 30 times larger than the minor claw, contains primarily S1 fibers and some S2 fibers, while the minor claw contains exclusively S2 fibers. Ultrastructurally, both S1 and S2 fibers resembled slow fibers of other crustacean species: high ratios of thin:thick myofilaments, thickened Z-lines, and myofibrils with long sarcomeres. However, the S1 fibers had higher thin:thick myofilament ratios (9:1 vs. 6:1), longer sarcomeres (6-mu-m vs. 4.5-5-mu-m) and fewer mitochondria than the S2 fibers. During proecdysis, the muscle of the major claw underwent a greater atrophy than that of the minor claw. Furthermore, atrophy of the major claw muscle was enhanced by the number of walking legs being regenerated. The weight of the major claw muscle in animals regenerating one walking leg decreased about 50%, while that in animals regenerating eight walking legs decreased about 67%. The weight of the minor claw muscle decreased about 25% in animals regenerating either one or eight walking legs. Changes in myofibrillar cross-sectional areas paralleled the reductions in muscle mass. In S1 fibers, cross-sectional area decreased 43% in animals regenerating one walking leg and 78% in animals regenerating eight walking legs. In S2 fibers, cross-sectional area decreased 27% and 32%, respectively. Increases in the thick myofilament packing density and decreases in the thin:thick myofilament and actin:myosin heavy chain ratios suggest that this atrophy involves a preferential hydrolysis of thin myofilaments. The ultrastructural changes in S2 fibers from major and minor claws were similar, suggesting that the preferential atrophy of the major claw muscle results, at least in part, from a differential sensitivity of S1 and S2 fibers to factor(s) that trigger protein degradation.