The ovary organization in the marine limnodriloidin Thalassodrilides cf. briani (Annelida: Clitellata: Naididae) resembles the ovary of freshwater tubificins

In Thalassodrilides cf. briani, the paired ovaries are inconspicuous and polarized structures with developmental gradient of germ cells along their long axis. The about 300 germ cells in the ovary are consolidated into one syncytial cyst and each cell is connected to a common and branched mass of cytoplasm via one stable cytoplasmic bridge. The germ cells differentiate their fate into nurse cells and oocytes. Only one oocyte grows in a given time; it gathers cell organelles and yolk and then it detaches from the gonad. Nurse cells appear to support oocytes development by providing with at least cell organelles. Such observations suggest ovary meroism. T. cf. briani, belonging to limnodriloidin naidids, has the same ovary organization as the representatives of tubificin naidids studied to date. This supports the concept of sister-group relations between Limnodriloidinae and Tubificinae. A similar ovary morphology is also known in several other groups of “microdrile” oligochaetes, which shows that this ovary type is the most widespread among this taxa. Moreover, living in marine or brackish-water sediments, T. cf. briani do not show any significant differences in their ovary structure and oogenesis with freshwater tubificins. This indicates its conservative character independent of the animal life environment.