The spermatozoon in tardigrades: Evolution and relationships with the environment

The spermatozoon ultrastructure of seven species of tardigrades, one heterotardigrade echiniscid and six eutardigrades, was analysed and described. Tardigrade species were collected from freshwater sediments, moss and leaf litter. A phylogenetic evaluation was made and relationships of spermatozoon shape and habitat were discussed. The spermatozoon morphology of the moss-dwelling echiniscid (heterotardigrade) was very similar to that found in marine species of the same order. This indicates that in Echiniscoididae change of habitat had little influence on sperm structural organisation. In contrast, eutardigrade spermatozoa. have developed several different structural organisations that may be related to family and/or habitat.