Brachyuran decapod crustaceans are characterized by a reduced abdomen, folded beneath the cephalothorax, and inserted between the pereiopods or in a special cavity, which prevents it from impeding movements. The acquisition of a retaining/locking mechanism may be regarded as a synapomorphy for the Brachyura. In primitive podotrematous forms (Dromiidae, Homolidae), the structures involve the coxae of thoracopods. In higher Brachyura, the structures located on limbs have moved onto the sternal plate. The most common holding system is the press-button, consisting of a prominence always on the fifth sternite and a socket always positioned on the sixth abdominal segment. The innovation with a sternal differentiation appears to have occurred independently several times: on sternite 4 (Homoloidea: homolid press-button), on sternite 5 (Dynomenidae, Lyreidinae), on sternite 6 (Phyllotymolinidae, Cyclodorippidae), and on sternite 5 (Eubrachyura: typical press-button). In the Heterotremata, only the Leucosiidae display a retaining system that differs from the typical press-button. A true socket evolved within the Podotremata (Homoloidea, Lyreidinae), and in almost all Eubrachyura. The abdominal socket is hypothesized to be homologous with the uropod. The absence of any locking structures in the most highly carcinized thoracotrematous crabs (for instance Ocypodinae pro parte ) is considered to be the result of a secondary loss, occurring at about the time when terrestrial habitats replace the exclusively marine life. The microstructures, discovered on both homolid and typical press-buttons, might be regarded as an indicator of sex, puberty moult and age of individuals, and are probably reliable indicators of phylogenetic affinities at different levels.
Abdomen, Brachyura, coaptation, holding system, homology, locking mechanism, microstructures, press-button, phylogeny, uropod.