Morphology of Xenodasys (Gastrotricha): the first species from the Mediterranean Sea and the establishment of Chordodasiopsis gen. nov. and Xenodasyidae fam. nov.

During a survey of the Italian marine meiofauna, several specimens of the rare gastrotrich genus Xenodasys were found in a submarine cave along the Ionian coast of Apulia. The finding represents the first record of the genus of the Mediterranean Sea and reinforces the consideration of marine caves as habitats of high naturalistic value. The specimens, analysed using different microscopy techniques, showed a new species, named Xenodasys eknomios. Scanning electron microscopy, unveiling the astonishing morphology of this unusual gastrotrich, indicates that, due to technical artefacts, light microscopy may generate unreal features, which in the past may have led to misinterpretation of the anatomical traits of these creatures. Transmission electron microscopy indicated that ‘Seitenfüsschen’, are genuine elements of the adhesive apparatus, in contrast with previous investigation, which attributed an exclusive sensorial function to these organs. Confocal laser scanning microscopy, combined with actin-binding fluorochromes, revealed muscular elements in a region where originally the muscular chordoid organ was reported for gastrotrich species belonging to the genus Chordodays. A taxonomic revision of the species currently allocated to the genus Xenodasys led to establishment of Chordodasiopsis gen. nov. to integrate the former Xenodasys (=Chordodasys) antennatus and to drafting of emended diagnosis of the genus Xenodasys. An overview of the high-rank systematization of these genera is also provided, with the establishment of Xenodasyidae fam. nov. to allocate both Xenodasys and Chordodasiopsis.