Bdelloid Rotifers

Short answer: Bdelloid rotifers (class Bdelloidea) are microscopic animals, usually well under a millimeter long. They live in fresh water and also in damp moss, lichen and soil films that often dry out.

In a 2000 paper in Science, they were called the largest animal group in which males, hermaphrodites and meiosis are unknown. Females produce daughters from unfertilized eggs (parthenogenesis). Genetic studies suggest the group may have done without conventional sex for many millions of years, though 'ancient asexuality' is still debated. Bdelloids carry an unusually high share of genes picked up from bacteria, fungi and plants by horizontal gene transfer. A 2015 study estimated about 13 new foreign genes per lineage per million years, with more in species from habitats that often dry out. A 2018 reanalysis, however, found that claims of DNA swaps between bdelloid individuals came from sample contamination. When their habitat dries, bdelloids contract into a 'tun' and enter anhydrobiosis, a dried, dormant state, and revive when water returns. The best-documented dry survival is about 9 years in stored moss. In 2021, researchers revived an Adineta rotifer from Siberian permafrost dated to about 24,000 years old, though it had been frozen rather than dried, and it went on to clone itself in the lab.