Retention of embryonated eggs in parthenogenetic Macrobiotus richtersi J. Murray, 1911 (Eutardigrada)

According to the current opinion, bisexual tardigrades lay their eggs before pronuclear fusion and partenogenetic tardigrades before cleavage and differentiation. Meiosis (or a corresponding ameiotic oocyte division in many parthenogenetic strains) stops at the first metaphase and starts again after oviposition. Tardigrades, therefore, are considered as being exclusively oviparous (for review see Marcus 1929; Bertolani and Rebecchi 1999; see also Suzuki 2003). When checking a series of permanent preparations enclosed in Faure’s medium made by one of us (K.H.) Dr. H. Dastych, Hamburg, called our attention to a female of Macrobiotus richtersi that obviously carried two embryonated eggs and, thus, differed remarkably from other gravid females of the same population. The specimens of the series came from a long-term culture on agar and were fed with nematodes (see Hohberg, in prep.). During April 1999–December 2003 no males were detected. Juvenile females isolated from others continuously produced young without males when they reached maturity. Therefore, we assume that the population is thelitokous parthenogenetic. In the wild and in culture gravid females reach the remarkable length of up to 860 mm (smallest gravid females measured 450 mm) and carry up to 27 mature oocytes in their ovaries. More than once we observed