These aquatic invertebrates are few tenths of a millimeter long. Characterised by their ciliated head structure and bilateral ovaries, they have:

  • ganglia
  • muscles
  • digestive and secretory organs
  • photosensitive and tactile sensory organs
  • structures for crawling, feeding, and swimming


Cytological and molecular genetic studies have provided evidence that bdelloids evolved from a common ancestor that lost sexual recombination (meiosis and syngamy) about 80 million years ago.


Humans and most other organisms reproduce sexually. That results in two nearly identical copies of each gene in each cell, and therefore two nearly identical proteins. The "re-shuffling" of genetic material over many generations allows sexual animals to adapt to changes in their natural environment.

In contrast, many asexual organisms have died out because their rigid genetic make-up means they are unable to adapt in this way. So how have bdelloids escaped this fate?

Molecular biology studies provide an insight into the paradox of the bdelloids unprecedented evolutionary success.

If a bdelloid is deprived of water, it survives in the dry state until water becomes available again.

The secret to this survival mechanism might lie in asexual reproduction, whereby the animal is able to make two separate proteins from two different copies of a key gene with complementary functions.

Two copies of a particular gene, which encodes a LEA protein, in the asexual pond-dweller Adineta ricciae were found to be different - giving rise to proteins with separate functions that protect the animal during dehydration.

One copy is thought to stop other essential protein molecules from clumping together as the animal dries out, while the second copy might help to protect the fragile membranes that surround the creature's cells.

The study reported in Science magazine (N.N.Pouchkina-Stantcheva et al., 2007).