Microsporidia are unicellular fungi that are obligate endoparasites. Although 
nematodes are one of the most abundant and diverse animal groups, the only 
confirmed report of microsporidian infection was that of the "nematode killer" 
(Nematocida parisii). N. parisii was isolated from a wild Caenorhabditis sp. and 
causes an acute and lethal intestinal infection in a lab strain of 
Caenorhabditis elegans. We set out to characterize a microsporidian infection in 
a wild nematode to determine whether the infection pattern of N. parisii in the 
lab is typical of microsporidian infections in nematodes. We describe a novel 
microsporidian species named Sporanauta perivermis (marine spore of roundworms) 
and characterize its infection in its natural host, the free-living marine 
nematode Odontophora rectangula. S. perivermis is not closely related to N. 
parisii and differs strikingly in all aspects of infection. Examination by 
transmission electron microscopy (TEM) revealed that the infection was localized 
in the hypodermal and muscle tissues only and did not involve the intestines. 
Fluorescent in situ hybridization (FISH) confirmed infection in the muscle and 
hypodermis, and surprisingly, it also revealed that the parasite infects O. 
rectangula eggs, suggesting a vertical mode of transmission. Our observations 
highlight the importance of studying parasites in their natural hosts and 
indicate that not all nematode-infecting microsporidia are "nematode killers"; 
instead, microsporidiosis can be more versatile and chronic in the wild.