Sexual reproduction by diploid populations from the Lumbriculus complex (Clitellata: Lumbriculidae) is extensive and seasonal

While collecting Lumbriculus from natural habitats for karyotyping, several sexually mature specimens were observed. This was unexpected since the scientific literature describes sexual reproduction in Lumbriculus as rare. To document this sexual reproduction, worms determined by genetic and morphological analysis to be diploid Lumbriculus were collected weekly in 2010, 2011, and 2018 from early spring through late fall. Worms were visually examined for reproductive structures and maintained in the laboratory where cocoon production was monitored. Worms collected from mid-April through mid-August produced cocoons for about one week, indicating that sexual reproduction was seasonal. Histological analysis revealed reproductive structures including atria, spermathecae containing sperm, testes, ovaries, sperm sacs, and eggs. The paradigm-shifting finding was that, rather than rarely being found, over 60% of Lumbriculus collected from the habitats throughout the summer were sexually mature. Mating experiments suggested that for cocoon production to occur, sperm was transferred between worms. Laboratory-maintained Lumbriculus showed degeneration of reproductive structures except for gonads. Following amputation, regeneration of reproductive structures was limited to gonads. Access to sexually reproducing populations of Lumbriculus in several natural habitats opens new avenues of research, ranging from comparison of gene expression in asexually and sexually reproducing worms to studies on embryonic development.