A new type of brachyuran seminal receptacle in the masked crab Ethusa mascarone (Brachyura, Ethusidae).

The reproductive system of the female Ethusa mascarone was studied with a combination of histological and MRI-techniques. The study reveals a completely new type of eubrachyuran seminal receptacle. This receptacle consists of two largely separate chambers that engage with each other in a manner similar to shaking hands. One chamber facing the medial axis is lined by cuticle while the second chamber consists of a thick holocrine epithelium. Both chambers are connected by two openings of a unique structure. First, the glandular chamber opens ventro-laterally to the cuticle chamber via a laterally flattened connective duct that is lined by a highly folded cuticle. A second opening connects both chambers dorsally with the oviduct orifice. A distinct character is the cuticular hook-like projection that is situated in between the connection of oviduct opening, the glandular chamber and the cuticle chamber of the seminal receptacle. The complete seminal receptacle exhibits a combination of plesiomorphic and apomorphic characters. The arrangement of the receptacles featured in two separate chambers, including the ventro-lateral connection of the glandular chamber to the cuticle chamber, presumably reflects an early evolutionary stage of an eubrachyuran receptacle. In contrast, the dorso-lateral opening between both chambers, including the hook-like projection, appears to be an apomorphic character of at least E. mascarone. J. Morphol. 277:1497-1508, 2016. © 2016 Wiley Periodicals, Inc.