Scientists discovered a close cousin of the jellyfish evolved over time to become a microscopic parasite that lives inside of fish.

The new findings could redefine the long-held criteria used to categorize which type of organic life (such as animals, plants, and fungi) a species belongs to, the American Friends of Tel Aviv University reported.  The research also demonstrates the first case of extreme evolutionary degeneration of an animal body known to science. A team of researchers used genome sequencing to determine a diverse group of microscopic parasites called myxozoans are actually highly degenerated cnidarians, which is a category that includes jellyfish and corals.

"These micro-jellyfish expand our basic understanding of what makes up an animal," said Dorothée Huchon of TAU's Department of Zoology. "What's more, the confirmation that myxozoans are cnidarians demands the re-classification of myxozoa into the phylum cnidaria."

The tiny parasites have retained some basic characteristics of the jellyfish, including the genes necessary to produce jellyfish stingers, despite millions of years of changes in body structure and genetics.

"The myxozoa are microscopic -- only a few cells measuring 10 to 20 microns across -- and therefore biologists assumed that they were single-celled organisms," Huchon. said "But when we sequenced their DNA, we discovered the genome of an extremely strange macroscopic marine animal."

These new findings not only provide valuable insight into evolution and species categorization, but could also have important commercial applications. Myxozoans often infect commercial stock such as salmon and trout, causing a significant problem for the fish industry. 

"Some myxozoa cause a neurological problem in salmon called 'whirling disease,'" Huchon said. "These fish parasites cause tremendous damage to the fish industry, and unfortunately there is no general treatment against them. We hope that our data will lead to a better understanding of the biology of these organisms and the development of more effective drugs to fight against myxozoa."

The findings were published in a recent edition of the journal Proceedings of the National Academy of Sciences.