Analysis of the mero-carpopodite joint of the American lobster and snow crab. I. Apodeme surface area and muscle fibre pinnation angle

Abstract

The patterns and variability of pinnation angle along the length of apodemes, and the scaling of apodeme surface area, were investigated in the American lobster (Homarus americanus) and the snow crab (Chionoecetes opilio) to assess the morphological variation between legs and species, and how these may affect force generation. Muscle fibre pinnation angle shows general patterns of decline along the disto-proximal axis of the apodeme, but the specific patterns of each apodeme or leg are highly variable. There is only small change in pinnation angle along the length of the apodeme or between flexed and extended positions. It appears that changes in pinnation angle are not of sufficient magnitude to contribute significantly to force generation in these pinnated muscle systems. The apodeme surface area as a function of carapace size shows positive allometry for all legs of the snow crab, and for some of the apodemes of legs 2, 3, and 5 of the lobster. The flexor apodeme within the snow crab is larger than the extensor in legs 2 to 4 implying a greater force generating ability in flexion than extension, and legs 2 to 4 all possess much larger apodemes than leg 5. The lobster indicates some specialization of apodeme surface area between the legs along the length of the body, with the anterior legs having larger flexor apodemes than extensor. This pattern is reversed for the last pair of legs. This suggests that each of the four walking legs of the lobster may generate approximately equal forces in extension, while leg 4 has the ability to generate greater forces in flexion than legs 2 or 3, which in turn likely produce larger forces than leg 5. The apodeme surface area is suggested to play a much greater role in determining force generation in crustacean walking legs.