Protist joins metazoan family tree.

They may look a bit like single-celled organisms such as a euglena or paramecium, but the fish and worm parasites called myxozoans are not protozoans, says Richard D. Spall, a pathologist from Idaho State University in Pocatello.

For years, Spall has spent much of his spare time collecting and dissecting fish and annelids -- segmented worms such as earthworms -- that often carry myxozoans and studying the microscopic features of these mysterious creatures.

Typically, myxozoan spores consist of just 6 to 12 cells, not enough for taxonomists to promote these organisms from the kingdom Protista to the status of multicellular animals. Yet genetic material taken from the myxozoan's protein production unit, called the ribosome, indicates that this group's closest cousins are actually nematode worms, not paramecia, Spall and his colleagues report in the Sept. 16 SCIENCE.

Spall had suspected as much. The myxozoan spore, 15 to 18 millimeters in diameter, contains highly differentiated cells: two valve cells that cover four end, or polar capsule, cells, which contain ejectable filaments. These polar cells and the cell-to-cell junctions remind him of the stinging cells and junctions seen in jellyfish and hydroids -- both radially symmetrical invertebrates. But the myxozoan's ribosomal RNA shows closer ties to bilateral animals, nematodes in particular, he notes.

Little else is known about these organisms. So far, scientists have described 1,100 species of myxozoans, but Spall finds new ones everywhere he looks and suspects many more kinds exist. Though they do not make their natural hosts sick, myxozoans introduced by stocking U.S. waters with nonnative fish have resulted in serious epidemics among native rainbow trout and Pacific salmon. Young native fish develop a whirling swimming behavior and often die, he points out.