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

While collecting <i>Lumbriculus</i> from natural habitats for karyotyping, several sexually mature specimens were observed. This was unexpected since the scientific literature describes sexual reproduction in <i>Lumbriculus</i> as rare. To document this sexual reproduction, worms determined by genetic and morphological analysis to be diploid <i>Lumbriculus</i> 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 <i>Lumbriculus</i> 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 <i>Lumbriculus</i> showed degeneration of reproductive structures except for gonads. Following amputation, regeneration of reproductive structures was limited to gonads. Access to sexually reproducing populations of <i>Lumbriculus</i> 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.