Spermatozoan morphology as a character for tardigrade systematics: Comparison with sclerified parts of animals and eggs in eutardigrades

The male gamete, a cell widely used for evaluating phylogeny in different animal groups, remains relatively unknown in tardigrades. In this paper the spermatozoa of thirteen species of eutardigrades among four genera and three families is evaluated in order to determine whether sperm morphology can be used as a taxonomic character. Spermatozoa of Amphibolus volubilis and A. weglarskae are very similar and this resemblance is congruent with the remarkable similarity of sclerified parts of the species. In addition, the spermatozoa of eight species of the genus Macrobiotus were examined yielding two groups showing strong intragroup similarities. The first group includes M. pseudohufelandi, M. sandrae, M. macrocalix, M. terminalis and M. joannae, and the second M. richtersi, M. areolatus and M. harmsworthi. Again, these groupings were congruent with those determined by analysis of sclerified structures. In contrast, a marked similarity was found between the spermatozoa of Diphascon (Adropion) scoticum and Platicrista angustata, whereas Diphascon (Diphascon) humicus was appreciably different from both species. Resemblances found in this study between spermatozoa and sclerified body parts suggests that the spermatozoa are suitable characters for use in systematic studies.