Maturative pattern of the ovotestis in two hermaphrodite species of eutardigrades

We studied the maturative pattern of the ovotestis and the reproductive cycle of two hermaphrodite species of eutardigrades belonging to different families and collected independently from a chestnut litter. A long-term study on Macrobiotus joannae (Macrobiotidae), collected monthly, and a short-term study on Amphibolus weglarskae (Eohypsibiidae) were carried out. In both species the reproductive apparatus is represented by an ovotestis and by a short gonoduct. A caudal sack-like evagination of the ovotestis was found only in M. joannae. The size of the gonad varied in relation to the body size and degree of maturation of the germinal elements. In both species the maturative pattern of the ovotestis can be subdivided into four stages. The first stage includes three phases (undifferentiated, male and previtellogenic phases), whereas stages 2-4 correspond to the stages of vitellogenesis. Groups of 3-8 oocytes mature in strict relation to the molting cycle. In both species there are at least two cycles of egg production, indicating that hermaphroditic tardigrades are iteroparous. Apart from the first two phases of stage 1, all stages simultaneously possess male and female germinal elements which are not separated by barriers. The gonad is exclusively male only in relatively small specimens of both species, indicating that the male phase (m) is present only in the first reproductive cycle. The contemporary presence of mature oocytes and spermatozoa in close contact with each other is evidence that tardigrades are simultaneous hermaphrodites, and it suggests the existence of self-fertilization. This conclusion allows us to view our results in the light of predictions from current theories on sex energy allocation in hermaphrodite and gonochoristic tardigrades. Lastly, we compared the reproductive cycle of these two hermaphroditic species and those of a gonochoristic species belonging to the same genera.