Redescription and 3D morphology of Williamia gussonii (Gastropoda: Siphonariidae)

Abstract

Since it has never been described adequately, the taxonomic status of the type species of the siphonariid genus Williamia, W. gussonii (da Costa, 1829), is unclear. A redescription with the designation of a neotype is provided in the present study. Shell, radula and soft-part anatomy are investigated in detail by light microscopy, SEM and 3D-computer reconstruction and visualization of serial sections. The most significant features of the hard parts are the freely extending periostracum at the shell edge and the radula organization, with a tiny central tooth, bicuspid lateral teeth and scale-like trapezoid lateral-marginal teeth. These characters perfectly correspond to other Williamia species. In contrast, soft-part anatomy reveals features which differ strongly from those of other Williamia species. In particular the genital system lacks a bursa copulatrix but has other accessory structures, the pallial cavity that does not extend to the left side of the body, and there is only a single pulmonary vessel. Characters are discussed with regard to their usefulness for phylogenetic consideration, with a focus on siphonariid relationships. The hard-part characters appear to be more significant than the soft -part anatomy for the phylogeny of Williamia, and it is suggested that the genus is monophyletic.