The ultrastructure of sperm of an acochlidian opisthobranch is described for the first time in detail, in
the tiny, gonochoristic Microhedyle remanei (Microhedylidae) from Bermuda. Transmission electron microscopy
(TEM) shows the spermatozoa of M. remanei sharing many plesiomorphic features with
opisthobranch gastropods, such as having an apical spiral nucleus with a basal invagination filled with a
bell-shaped centriolar derivative; there is a single glycogen helix embedded into matrix and paracrystalline
material. Sperm of M. remanei are characterized by possessing a strongly helically coiled nucleus with
up to five keels terminating into one very prominent and intertwined keel. The sperm midpiece shows the
glycogen helix which is very densely arranged. This clearly differs from Hedylopsis ballantinei, a member of
the related acochlidian family Hedylopsidae, where three glycogen helices with different lengths and a
probably much shorter nucleus are present. This variation among acochlidian sperm may be phylogenetically
relevant and may be related to special acochlidian reproductive features such as sperm transfer via
hypodermic injection or dermal fertilization via spermatophores.