EVALUATION OF A SECONDARY SEX CHARACTER IN EUTARDIGRADES

Secondary sex characters appear in many heterotardigrades and a limite d number of species of eutardigrades. In the eutardigrades Milnesium t ardigradum and Pseudobiotus megalonyx, males are considered to always have modified claws on the first pair of legs; the basal branch of bot h claws in Milnesium and of the inner claw in Pseudobiotus is shaped l ike a robust hook. We examined one gonochoristic population of both Mi lnesium tardigradum and of Pseudobiotus megalonyx to evaluate the asso ciation 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 norm al claws on the front legs of the old cuticle and modified claws on th e front legs of the new cuticle. Examination of the gonad revealed tha t 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 copulat ion. Since this secondary sex character is not always present during t he life of the tardigrade, but appears after a molt, a sex ratio deter mined only by the presence of the modified claw is not valid.