Scanning electron micrographs show physical variations among bdelloid rotifers which suggest the aquatic animals have split into different species. ((Diego Fontaneto/PLoS Biology))

A group of microscopic aquatic animals has managed to evolve into many separate species over the past 40 million years without sexual reproduction, according to a study published on Tuesday.

The curious case of the organisms challenges the assumption that sexual reproduction is necessary for species to diverge, the authors contend.

The research, published in PLoS Biology, focused on bdelloid rotifers, a group of microscopic aquatic animals that live in ponds, rivers, and occasionally wet habitats like soils, mosses and lichens. Bdelloid rotifers are asexual and multiply by producing eggs that are genetic clones of the mother. There are no males, just females.

Asexual animals and plants can evolve and mutate over time, but usually do not diversify any further anddo not usually last long. But fossil records of bdelloid rotifers show the creatures have been around formore than 40 million years.

And like creatures that sexually reproduce, the study found the bdelloid rotifers have managed to evolve into hundreds of distinct species uniquely adapted to their environments: allowing some to prosper in hot springs while other swim in Antarctic waters.

Biologists Diego Fontaneto from the University of Milan, Timothy Barraclough from Imperial College London and a team of international scientists found evidence of distinct species of the bdelloid rotifers by comparing DNA sequencing and jaw measurements of animals living across the U.K., Italy and other parts of the world.

"We found evidence that different populations of these creatures have diverged into distinct species, not just because they become isolated in different places, but because of the differing selection pressures in different environments," said Barraclough in a statement.

"One remarkable example is of two species living in close proximity on the body of another animal, a water louse," he said. "One lives around its legs, the other on its chest, yet they have diverged in body size and jaw shape to occupy these distinct ecological niches.

"Our results show that, over millions of years, natural selection has caused divergence into distinct entities equivalent to the species found in sexual organisms."

Sexual reproduction was long thought to be necessary for species to split into divergent species because interbreeding introduced genetic variety to the offspring, the authors said. But they argue the case of bdelloid rotifers disproves this theory.

The conclusion, however, still leaves the scientists with another question: how do these other species diverge without new genetic material added to the mix? It's a question they are still puzzling over.

"These really are amazing creatures, whose very existence calls into question scientific understanding, because it is generally thought that asexual creatures die out quickly, but these have been around for millions of years," said Barraclough.