Hydrostatic skeletons in the Crustacea: support during molting in an aquatic and a terrestrial crab

Abstract

All animals require a skeletal support system for posture and locomotion. Arthropods, however, repeatedly shed their rigid exoskeleton in order to grow, yet they maintain shape and mobility during these periods. My research focuses on this apparent paradox and suggests that crabs, and possibly all arthropods, alternate between a rigid and a hydrostatic (fluid-based) skeleton in order to remain functional during molting. I tested for the use of hydrostatic skeletal support in blue crabs, Callinectes sapidus, by simultaneously measuring internal hydrostatic pressure and force of claw adduction. I found a strong correlation between force and hydrostatic pressure in soft-shell crabs, but not in hard-shell crabs, which is consistent with the use of hydrostatic support during molting. Switching skeletons requires a change in function of the cuticle, from resisting primarily bending, compression, and torsion, to resisting tension. This change in function implies correlated changes in the properties of the cuticle. I tested the mechanical properties of the cuticle throughout the molt cycle of C. sapidus and found that the flexural and tensile stiffness is greater in hard cuticle than soft cuticle, but the tensile strength is the same. The blackback land crab, Gecarcinus lateralis, does not molt in water and inflates its gut with air during molting, which may serve as a support mechanism. I simultaneously measured the force of claw flexion, hydrostatic pressure within the claw, and gas pressure within the gut. I obtained a strong correlation between all three measurements, which suggests that the gas helps maintain turgidity throughout the body, and thus acts as a critical component of the skeleton. Rigid and hydrostatic skeletons operate according to different principles and each is likely to be influenced by scale in distinct ways. Using morphological techniques, I found that cuticle thickness scales isometrically for rigid skeletons but allometrically for hydrostatic skeletons, suggesting that both play a role in determining growth to maximum size in crabs. This research provides novel insights into how skeletal support systems influence the way in which animals are built, develop, and function