THE DIMORPHIC CLAWS OF THE HERMIT CRAB, PAGURUS POLLICARIS: PROPERTIES OF THE CLOSER MUSCLE

¹ Villanova University, Department of Biology, Villanova, Pennsylvania 19085

The first pair of chelipeds of the flat clawed hermit crab (Pagurus pollicaris) are dimorphic. The crusher claw is always on the right side and is larger than the cutter claw on the left. The closer muscle in the crusher claw has a wet weight that is 3.4 times greater than that in the smaller cutter claw.

The closer muscle fiber types are different in the two claws. The crusher closer muscle has fibers with long (9-14 µm) sarcomeres, a high thin to thick filament ratio, and low myofibrillar ATPase activity; these fibers are presumably slow. The cutter closer, by contrast, has two types of fibers that are regionally distributed within the muscle. Fibers located on the dorsal and ventral margins of the muscle have long (8-13 µm) sarcomeres, a high thin to thick filament ratio, low myofibrillar ATPase activity, and are presumably slow fibers. In the central portion of the cutter closer muscle, however, there is a band of fibers with short sarcomeres and high myofibrillar ATPase activity. These features suggest that these fibers are fast. However, the high thin to thick filament ratio found in these "fast" muscle fibers would argue against this presumption. Finally, sarcomere length measurements taken from closer muscle fibers in other thoracic appendages revealed a bimodal distribution of fiber types similar to that observed in the cutter claw.

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