A close cousin of the jellyfish evolved into a microscopic parasite that lives in fish, according to researchers in Israel, France and the US. Kansas. The finding represents the first case of extreme evolutionary degeneration of an animal body.The discovery of the dramatic change from macroscopic marine animal to microscopic parasite is interesting on its own, but it may also have commercial applications, as the parasites commonly plague commercial fish stock such as trout and salmon.There is a popular belief instilled since childhood that all living organisms – from animals, plants and fungi to bacteria and single- celled organisms – belong to specific categories of organic life. A new discovery by Tel Aviv University researchers and international collaborators is poised to redefine the very criteria used to define and classify these animals.The research was led by Prof. Dorothée Huchon of TAU’s zoology and Prof. Paulyn Cartwright of the University of Kansas, in collaboration with Prof. Arik Diamant of Israel’s National Center for Mariculture and Prof. Hervé Philippe of the French National Center for Scientific Research. It was recently published in the journal Proceedings of the National Academy of Scientists.They used genome sequencing and found that myxozoans – a diverse group of microscopic parasites that infect invertebrate and vertebrate hosts – are actually are highly degenerated cnidarians, the phylum (category) that includes jellyfish, corals and sea anemones.“These micro-jellyfish expand our basic understanding of what makes up an animal,” said Huchon. “What’s more, the confirmation that myxozoans are cnidarians demands the re-classification of myxozoa into the phylum Cnidaria.”Despite the radical changes in its body structure and genome over millions of years, the myxozoa have retained some of the basic characteristics of the jellyfish, including the essential genes to produce the jellyfish stinger.“The myxozoa are microscopic – only a few cells, measuring 10 to 20 microns across – and therefore biologists assumed that they were single-celled organisms,” said Huchon. “But when we sequenced their DNA, we discovered the genome of an extremely strange macroscopic marine animal.”Some myxozoa cause a neurological problem in salmon called “whirling disease.”said Huchon. “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 researchers are currently studying the evolution in myxozoa of genes that form the stinging organ of jellyfish.