The morphology of the reproductive system in the crab Percnon gibbesi (Decapoda: Brachyura: Grapsoidea) reveals a new combination of characters in Thoracotremata.

Recent studies revealed a high diversity of reproductive structures in heterotreme brachyurans, while those of Thoracotremata seem rather uniform. Yet, there still is a huge lack of data in this group as only few species have been studied with respect to their reproductive system. The phylogenetic position of Percnidae is ambiguous. Recent molecular studies place it within polyphyletic grapsoids. We herein study the reproductive morphology of Percnon gibbesi using histology, scanning electron microscopy, micro-computed tomography and 3D-reconstructions to test whether this species shows the characteristic thoracotreme pattern. Our results reveal that the male copulatory system conforms to other thoracotremes. It is composed of two pairs of pleopods (gonopods) and likewise paired penes. The first gonopod is relatively long. It possesses a bent terminal process with a distal opening of the ejaculatory canal, a character also present in other grapsoids. The second gonopod is short and terminates in an apical girdle. The female reproductive system reveals a combination of characters, so far unknown for thoracotremes. The paired oviducts do not lead into the seminal receptacles, but run into separate cuticular ducts joined with the vaginae. Accessory sperm storage organs, the bursae, are also connected to the vaginae. Bursae have previously only been described in heterotreme crabs. The data presented in this study reveals a higher diversity of thoracotreme reproductive systems than anticipated.