In rainbow trout, infection by a parasite called a myxozoan literally drives the fish crazy, causing a “whirling disease” that robs the fish of any control of its behavior, posing a serious threat to fish farms in the process. Now, researchers have proof that these tiny pests, consisting of just a few cells, are actually stripped-down jellyfish. Myxozoans were once considered protists—falling into the same group as amoebas, paramecia, and slime molds. But a few researchers questioned this categorization, noting that myxozoans contain a complex structure called a polar capsule that includes a barbed filament used to latch onto the host. This capsule looks a lot like the stinging cells of jellyfish. And with good reason, researchers report online today in the Proceedings of the National Academy of Sciences: Genetically speaking, myxozoans belong to the cnidarian family tree, a group that includes jellyfish, hydra, and corals. The team compared the genomes and surveys of active genes of two myxozoans with those of true jellyfish, as well as those of Polypodium hydriforme, a parasite with a jellyfishlike life stage. Myxozoans, the team reports, are most closely related to Polypodium and share a common ancestor with jellyfish. As they became parasitic and pared their bodies to less than a dozen cells, myxozoans let go of a lot of their DNA, with one species shrinking its genome to 22.5 million bases, one-fortieth the size of Polypodium and other jellyfish genomes. Today myxozoans have 30% fewer genes than these relatives, and they lack the genes needed for multicellular development and differentiation, as well as those for cell-to-cell communication. Next, the scientists want to try to figure out just how a complex multicellular free-living jellyfish evolved into a parasite organism with just a few cells.