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.