Undersized and underestimated: 3D visualization of the Mediterranean interstitial acochlidian gastropod Pontohedyle milaschewitchii (Kowalevsky, 1901)

Pontohedyle milaschewitchii (Kowalevsky, 1901) is one of the most common mesopsammic opisthobranchs in the Mediterranean and Black Seas and has been considered as a comparably well-described acochlidian species. However, data on its complex internal anatomy were fragmentary and little detailed due to inadequate methodology available, and contradictory between different sources. The present study redescribes all major organ systems of P. milaschewitchii in full detail by three-dimensional reconstruction from serial semithin sections using AMIRA software. The prepharyngeal central nervous system (cns) of P. milaschewitchii is highly concentrated and shows a euthyneurous and epiathroid condition. Contrary to earlier reports, the cerebral and pleural ganglia are not fused. Aggregations of precerebral accessory ganglia can be grouped into three complexes supplied by distinct cerebral nerves. Rhinophoral ganglia with thin, double cerebro-rhinophoral connectives are described for the first time in acochlidians. A Hancock's organ is present in the form of a conspicuous, curved fold in the epidermis posterior to the oral tentacles. Cerebral nervous features and sensory structures are discussed comparatively. Our study confirms P. milaschewitchii as having the male genital opening in an unusual position above the mouth. Homology of the ciliated vas deferens of the gonochoristic and aphallic P. milaschewitchii with that of hermaphroditic acochlidian species with cephalic male genitals is discussed. The radula formula of P. milaschewitchii is 41-54×1-1-1, i.e. the single lateral teeth are broad and, contrary to previous descriptions, undivided. SEM examination of the body wall of entire specimens revealed a special and constant ciliary pattern. Providing a novel additional set of characters for taxonomic and phylogenetic purposes, external SEM examination is suggested as the standard method for describing acochlidian species in the future.