Systematics and redescription of the European meiofaunal slug Microhedyle glandulifera (Kowalevsky, 1901) (Heterobranchia: Acochlidia): evidence from molecules and morphology

Despite the long history of meiofaunal research in Europe our knowledge of its Acochlidia—the most diverse, abundant and widespread group of interstitial slugs—is still fragmentary. Distribution ranges and modes of dispersal are unknown and taxonomic hypotheses based on traditional light-microscopical examination have never been tested against a modern integrative approach combining microanatomical techniques with molecular analyses. This study redescribes Microhedyle glandulifera (Kowalevsky, 1901), a key species for microhedylid taxonomy and focus of taxonomic disorder. Three-dimensional reconstructions from histological semithin serial sections reveal several previously unknown characters, in particular concerning the nervous system (e.g. presence of gastro-oesophageal ganglia). There are no jaws, but a ‘cuticular element’ is attached anteriorly to the radula cushion. Scanning electron microscopic examination shows a radula with the formula 34–38 x 1.1.1. Microhedyle glandulifera can be distinguished from other Microhedylidae by a combination of external and radular features, and the unique presence of triaxonic spicules. Population genetic analyses based on mitochondrial markers support M. glandulifera as a widespread European species known to range from the North Sea to the Sea of Marmara (eastern Mediterranean). Accordingly, northern Atlantic ‘M. lactea’ and Mediterranean ‘M. glomerans’ are confirmed as junior synonyms of M. glandulifera. Molecular data indicate a recent radiation of M. glandulifera in European waters and potential means of dispersal in meiofaunal slugs with low reproductive output and no pelagic larval stages are discussed. Based on our molecular phylogeny and revision of distinguishing morphological characters, four valid Microhedylidae species occur in European waters: Pontohedyle milaschewitchii, Parhedyle tyrtowii, Parhedyle cryptophthalma and Microhedyle glandulifera. Morphological and molecular evidence indicate that Microhedyle odhneri is a member of the genus Parhedyle, and possibly a junior synonym of Parhedyle tyrtowii.