Abstract
Sperm ultrastructure is described for Pleurehdera haraldi Er. & Ev. Marcus, 1970 and contrasted with data from previously investigated members of the Notaspidea. The acrosomal vesicle is small (length 0.13 pm, breadth 0.09 pm) and supported basally by a short (0.37 pm) pedestal which overlaps extensively with the tapered nuclear apex. Nuclei are approxi-mately 15 pm long and sometimes exhibit a single helical keel. Absence of the keel in many of the observed spermatozoa suggests possible dimorphism with regard to this feature. A short basal invagination of the nucleus forms the point of attachment for the centriolar derivative and axoneme/coarse fibre com-plex. A single glycogen-filled helix is present within the mitochondrial derivative of the midpiece. Distally, an annulus occurs at the junction of the elongate (at least 160 pm) midpiece and short (1.5 pm) glycogen piece. Axonemal microtubules persist to the posterior tip of the spermatozoon. Comparison with other Notaspidea confirms that Pleurehdera is closest to Berthella and Berthellina within the Pleurobranchinae (of the family Pleurobranchidae). Sperm morphology supports the unity of the pleurobranchine tribe Berth- ellini.
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