A microstructural study of the pleon-holding mechanism in Carcinus maenas (Decapoda, Brachyura, Portunoidea, Carcinidae) of different sizes.

Within the Brachyura there are a variety of specialized holding mechanisms, which facilitate the close attachment of the highly reduced pleon underneath the cephalothorax. The most common mechanism in eubrachyurans, known as the press-button, consists of a sternal protrusion and a corresponding pleonal socket. Reports on the microstructural properties of the surface of these holding structures are scarce and patchy. In this study, the European Green Crab Carcinus maenas, is used as model to describe the microstructure of a typical press-button mechanism with the use of scanning electron microscopy and light microscopic histology. A highly tuberculate cuticle and an apical ridge on the sternal knob are found in juveniles of both sexes. The microstructures are lost in adult males. In adult females, the holding structures themselves are reduced, but never completely lost. These findings show that C. maenas does not undergo a single final puberty moult, after which all juvenile characteristics are lost, as previously assumed. Further comparison of the morphology of juveniles to another species indicates a close resemblance of the holding structures at this stage. Therefore, the use of the microstructure of the pleon-holding mechanism for phylogenetic analysis is restricted to adult specimens.