Bdelloid rotifers - An entire class of animals with no males

We have previously looked at the microscopic animals called Rotifers in general, but today we are going to take a closer look at a specific class of rotifers called Bdelloidea with quite a unique way of living. Bdelloid rotifers are extremely common all over the world where they can be found in different freshwater habitats like ponds, streams, moss, and garden birdbaths where they can be found in enormous amounts from time to time. These animals are incredibly tiny with a size of only 150 - 700 microns when fully stretched. Like other rotifers they feed on microalgae, bacteria, and single celled organisms. So far they sound just like other rotifers, however they are very different in a few ways.

All Bdelloid rotifers in the entire world are females. And despite the fact that we have known about them and searched for more than 300 years, we have never found a single male. This would under normal circumstances be a very huge problem, as most animals mix the genes from a male and a female to produce offspring. But these animals reproduce asexually by a process known as parthenogenesis (obligate parthenogenesis). This means that when the mother develops an egg it will never be fertilized by another individual but instead be filled with a genetic clone of the mother.

Usually a group of animals with such identical genetic material would quickly cease to exist because all of them would be vulnerable to the same diseases. And a single pathogen could therefore potentially wipe out the entire population. But fossil records have found Bdelloids to be at least 35 million years old, and genetic studies suggest that they could have been around for more than twice that long, maybe even more than 100 million years. Which means that these ladies have been producing clones since the dinosaurs still ruled the world.

But how have they been able to survive for this long without the help of sexual reproduction to secure genetic variation?

Bdelloid rotifers have the capability to completely dry out and stay that way for several years, just like tardigrades. So when the rotifer once again rehydrates, the pathogen could have died due to desiccation. Or the rotifer and pathogen could have been separated by the wind. A trick some have described as a microscopic Houdini.

A narrated video showing multiple Bdelloid rotifers.

20x plan APO objective (one clip with 60x EC plan objective), 

brightfield illumination.