Bdelloidea (
Greek βδελλα,
bdella, "leech-like") is a
class of
rotifers found in
freshwater habitats all over the world. There are over 450 described
species of bdelloid rotifers (or 'bdelloids'), distinguished from each other mainly on the basis of
morphology. The main characteristics that distinguish bdelloids from related groups of rotifers are exclusively
parthenogenetic reproduction and the ability to survive in dry, harsh environments by entering a state of desiccation-induced
dormancy (
anhydrobiosis) at any life stage. They are often referred to as "ancient asexuals" due to their unique asexual history that spans back to over 25 million years ago through fossil evidence. Bdelloid rotifers are microscopic organisms, typically between 150 and 700 µm in length. Most are slightly too small to be seen with the naked eye, but appear as tiny white dots through even a weak
hand lens, especially in bright light.