Computer-based 3D anatomical reconstruction and systematic placement of the mesopsammic gastropod Platyhedyle denudata Salvini-Plawen, 1973 (Opisthobranchia, Sacoglossa)

The small marine interstitial gastropod species Platyhedyle denudata Salvini-Plawen, 1973 has a dorsoventrally flattened, worm-like body without a shell and head tentacles; its systematic position was enigmatic. Wawra's [1979. Zur systematischen Stellung von Platyhedyle denudata Salvini-Plawen, 1973 (Opisthobranchia, Gastropoda). Z. Zool. Syst. Evolutionsforsch. 17, 221–225] anatomical redescriptions based on paraffin-embedded histology preparations indicated the original description of P. denudata to be erroneous in many essential features. The present study uses newly collected specimens from the Mediterranean type locality, Secche della Meloria, Italy, for serial semi-thin histological sectioning. All major organ systems of P. denudata are reconstructed three-dimensionally with AMIRA software. Platyhedyle denudata is a simultaneous hermaphrodite with an androdiaulic, complex genital system lacking a receptaculum seminis. A heart could not be detected; the nephric duct opens into the intestine. The central nervous system is postpharyngeal and the cerebropleural ganglia are completely fused. The visceral loop is short, showing three distinct ganglia which connect to an additional, presumably osphradial ganglion, and to a posteriorly situated genital ganglion. Aggregations of precerebral accessory ganglia are present. The originally described jaws could not be detected. An ascus for storage of worn radula teeth clearly indicates that P. denudata is a sacoglossan. The digestive gland of the mesopsammic P. denudata resembles that of the depressed, tentacle-less semi-terrestrial sacoglossan Gascoignella aprica Jensen, 1985 by forming two long branches that are connected at the rear end. In G. aprica, the digestive-gland rami are separated by a unique median septum, which is described here, too, as part of the muscle system in P. denudata. The shared, most likely synapomorphic presence of this septum strongly supports the monophyly of a Platyhedyle+Gascoignella clade in a sacoglossan family Platyhedylidae.