Not Only For Males: The Pleon-Holding Mechanism In Female Calappids (Crustacea: Decapoda: Brachyura)

Adult female Calappidae were examined with light microscopy and microcomputed tomography for the presence of a pleon-holding mechanism. A typical press-button consisting of a protrusion on the fifth thoracic sternite and a socket on the sixth pleon segment was found in species of the genera Calappa, Calappula, Cycloes, Mursia, and Paracyclois. Within the genus Calappa, 15 of 17 species showed a complete press-button mechanism, whereas in two species only a sternal button or none of the holding structures could be observed. The functionality of the mechanism was assessed by manually flexing and opening the pleon. This revealed a lack of function in species with fully developed holding structures, while others exhibited full function. Additionally, signs of abrasive wear could be identified with scanning electron microscopy on the surface of the holding structures of Calappa hepatica. The documentation of pleon-holding structures of varying degrees of functionality in adult female eubrachyurans provides new insight into a morphological characteristic known exclusively from Brachyura.