WASHINGTON (Reuters) - Tiny freshwater organisms that have amazed scientists because of their sex-free lifestyle may have survived so well because they steal genes from other creatures, scientists reported on Thursday.

They found genes from bacteria, fungi and even plants incorporated into the DNA of bdelloid rotifers -- minuscule animals that appear to have given up sex 40 million years ago.

"Bdelloid rotifers are small freshwater invertebrates that apparently lack sexual reproduction and can withstand desiccation at any life stage," Irina Arkhipova and Matthew Meselson of Harvard University in Cambridge, Massachusetts and colleagues wrote in a report in the journal Science.

They spring back into action after being dried out and also resist radiation.

Their resilience is surprising, given that sex is used by an overwhelming majority of life forms to cope with changing circumstances, by allowing organisms to get useful new genes and ditch harmful, mutated ones.

The translucent, waterborne creatures, which range in size from 0.004 inch to 0.04 inch long (0.1 mm to 1 mm), lay eggs, but all their offspring are female. So Meselson's team looked at their DNA to see how they manage to survive.

Evidently, they steal.

"In bdelloid rotifers we found many genes that appear to have originated in bacteria, fungi, and plants," they wrote.

"These fascinating animals not only have relaxed the barriers to incorporation of foreign genetic material, but, more surprisingly, they even managed to keep some of these alien genes functional," Arkhipova said in a statement.

Understanding how the animals acquire and make use of these new genes could have implications for medicine. Genetic mutations, which occur constantly in any living organism, underlie cancer, heart disease and various other diseases.