Evaluation of a secondary sex character in eutardigrades

Secondary sex characters appear in many heterotardigrades and a limited number of species of eutardigrades. In the eutardigrades Milnesium tardigradum and Pseudobiotus megalonyx, males are considered to always have modified claws on the first pair of legs; the basal branch of both claws in Milnesium and of the inner claw in Pseudobiotus is shaped like a robust hook. We examined one gonochoristic population of both Milnesium tardigradum and of Pseudobiotus megalonyx to evaluate the association of the modified claws with the presence of male germ cells in the testis. Three additional populations of Milnesium tardigradum were examined to provide data on the frequency of the modified claw among all individuals. In both species, molting animals were found with normal claws on the front legs of the old cuticle and modified claws on the front legs of the new cuticle. Examination of the gonad revealed that both species may have males with and without the modified claws. The appearance of the modified claw probably occurs at the last molt, and is used in mating when the male attaches to the female during copulation. Since this secondary sex character is not always present during the life of the tardigrade, but appears after a molt, a sex ratio determined only by the presence of the modified claw is not valid.