Spermatozoa in the reproductive system of a hermaphroditic marine tardigrade, Orzeliscus belopus (Tardigrada: Arthrotardigrada)

Orzeliscus belopus has long been regarded as the only hermaphroditic marine tardigrade, yet there has been no published detailed information on its internal anatomy. Our study elucidates the ultrastructure of the ovotestis and the spermatozoa of Orzeliscus cf. belopus from Bermuda. The ovotestis had no septum to separate male and female germ cells and, while early stages of spermatogenesis were not observed, many spermatozoa were found at the periphery in both anterior and posterior areas of the gonad. The nucleus and the mitochondria of the spermatozoa in the ovotestis extend backward from the centriole region, forming a half-headed arrow-shape with the nucleus on the outer side of the 'arrowhead'. The cross section of a long vesicular body (the paranuclear vesicle) is dumbbell- or horseshoe-shaped and attached to almost the entire length of the nucleus. In the seminal receptacle, we found both a complete spermatozoon and some which started to degrade. There is an indication that sperm is further modified after discharge as within the receptacle duct the sperm is no longer half-headed arrow-shaped but has a straight nucleus. This modification might be correlated with the degeneration of the paranuclear vesicle. Our observations clearly show that O. belopus is a simultaneous hermaphrodite, and suggests that the reproductive mode includes copulation and cross-fertilization.