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

Phylogenetic analysis of the highly enigmatic and aberrant acochlidian opisthobranch gastropods is mainly hindered by the lack of reliable anatomical data. Due to their small size, marine interstitial acochlidian species are not suitable for anatomical

The ultrastructure of sperm of an acochlidian opisthobranch is described for the ¢rst time in detail, in the tiny, gonochoristic Microhedyle remanei (Microhedylidae) from Bermuda. Transmission electron micro-scopy (TEM) shows the spermatozoa of M. remanei