Bdelloid rotifer, light microscopy. Rotifers are microscopic aquatic animals that are related to roundworms. Bdelloids are of interest because males are not present within the species, and females reproduce exclusively by parthenogenesis (without fertilisation). Each individual has paired gonads. Despite the fact that they have been asexual for millions of years, they have diversified into more than 300 species and are fairly similar to other sexually-reproducing rotifer species. Darkfield illumination.