SUMMARY Diameters of some white locomotor muscle fibers in the adult blue crab, Callinectes sapidus, exceed 500 μm whereas juvenile white fibers are <100 μm. It was hypothesized that aerobically dependent processes,such as metabolic recovery following burst contractions, will be significantly impeded in the large white fibers. In addition, dark aerobic fibers of adults,which rely on aerobic metabolism for both contraction and recovery, grow as large as the white fibers. These large aerobic fibers are subdivided, however,thus decreasing the effective diameter of each metabolic functional unit and enabling aerobic contraction. The two goals of this study were: (1) to characterize the development of subdivisions in the dark levator muscle fibers and (2) to monitor post-contractile metabolism as a function of fiber size in aerobic and anaerobic levator muscles. Dark levator muscle fibers from crabs ranging from <0.1 g to >190 g were examined with transmission electron microscopy to determine the density of mitochondria and subdivision diameters. Across all size classes, there was a constant mitochondrial fractional area(25% of the total subdivision area) and subdivision size (mean diameter of 36.5±2.7 μm). Thus, blue crab dark levator fibers are unusual in having metabolic functional units (subdivisions) that do not increase in size during development while the contractile functional units (fibers) grow hypertrophically. The body mass scaling of post-contractile lactate dynamics was monitored during recovery from anaerobic, burst exercise in white and dark muscle, and in hemolymph. There were no differences among size classes in lactate accumulation during exercise in either muscle. However, in white fibers from large crabs, lactate continued to increase after exercise, and lactate removal from tissues required a much longer period of time relative to smaller crabs. Differences in lactate removal among size classes were less pronounced in dark fibers, and post-contractile lactate accumulation was significantly higher in white than in dark fibers from large animals. These data suggest that the large white fibers invoke anaerobic metabolism following contraction to accelerate certain phases of metabolic recovery that otherwise would be overly slow. This implies that, in addition to the typical mass-specific decrease in oxidative capacity that accompanies increases in animal mass, aerobic metabolic processes become increasingly limited by surface area to volume and intracellular diffusion constraints in developing white muscle fibers.

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The ecdysial suture is the predetermined region on an arthropod exoskeleton that splits open to allow the animal to escape from its old carapace [1]. In order to understand why this region preferentially splits, we examined the morphology and composition of the ecdysial suture of Callinectes sapidus using scanning electron microscopy (SEM) and light microscope histology and histochemistry.No structural or compositional differences could be detected between the suture and the adjacent cuticle when stained with acridine orange (Fig. a) or hematoxylin and eosin. The suture region (in the center of the micrograph) possesses the same cuticular layers, epicuticle (arrowhead), exocuticle (ex), endocuticle (en) and membranous layer (arrow), as the adjacent cuticle with no apparent differences in thickness or affinity for these dyes. Staining with a battery of FITC-labeled lectins [2] revealed that only three were able to differentiate the suture. Lens culinaris agglutinin (Fig. b), Vicia faba agglutinin, and Pisum sativum agglutinin all bound intensely to the exocuticle in the region of the suture (arrowhead) and bound less to a wedge-shaped region of the endocutilcle (arrow). All three of these lectins have an affinity for fucosylated αN-acetylglucosamine with mannose dendrimers [3,4].

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The ecdysial suture is the region of the arthropod exoskeleton that splits to allow the animal to emerge during ecdysis. We examined the morphology and composition of the intermolt and premolt suture of the blue crab using light microscopy and scanning electron microscopy. The suture could not be identified by routine histological techniques; however 3 of 22 fluorescein isothiocyanate-labeled lectins tested (Lens culinaris agglutinin, Vicia faba agglutinin, and Pisum sativum agglutinin) differentiated the suture, binding more intensely to the suture exocuticle and less intensely to the suture endocuticle. Back-scattered electron (BSE) and secondary electron observations of fracture surfaces of intermolt cuticle showed less mineralized regions in the wedge-shaped suture as did BSE analysis of premolt and intermolt resin-embedded cuticle. The prism regions of the suture exocuticle were not calcified. X-ray microanalysis of both the endocuticle and exocuticle demonstrated that the suture was less calcified than the surrounding cuticle with significantly lower magnesium and phosphorus concentrations, potentially making its mineral more soluble. The presence or absence of a glycoprotein in the organic matrix, the extent and composition of the mineral deposited, and the thickness of the cuticle all likely contribute to the suture being removed by molting fluid, thereby ensuring successful ecdysis.