An ultrastructural investigation of Hrabeiella Pizl & Chalupský, 1984 (Annelida). II. The spermatozoon

Abstract The mature spermatozoa of the terrestrial non-clitellate annelid Hrabeiella periglandulata Pizl & Chalupský, 1984 s.l. were examined using light and electron microscopy. They are about 150 μm long, filiform and extremely slender (maximum diameter, 450–475 nm). The acrosome is very elongate (about 25 μm), tapering and conical. Its transverse section is circular apically but shows an evident six-rayed symmetry in its basal region. The nucleus appears convex at both ends; apically, it extends laterally into the acrosome, and basally, it plugs into the centriolar region. The nucleus is about 23 μm long and has a rounded, tri- to pentalobed, slightly helical profile. The midpiece contains one elongate, free (paraxonemal) mitochondrion, 27 accessory tubules, which are slightly larger and more opaque than the axonemal microtubules; and seven electron-dense, non-membrane-bounded rods distributed around the axoneme. The flagellum tapers rapidly posteriorly. None of the observed similarities to the sperm (introsperm) of questids, protodrilids or other polychaetes seems to represent an immediate synapomorphy. None of the spermatozoal autapomorphies of the Euclitellata is shared by Hrabeiella .