Life where you least expect it: Biodiversity, abundance and prevalence of kleptoparasitic nematodes living inside the gastrointestinal tract of North American diplopods

Millipede-parasitic nematodes belong to the infraorders Oxyuridomorpha and Rhigonematomorpha. Oxyuridomorpha contains two millipede-parasitic superfamilies (Thelastomatoidea and Coronostomatoidea). Rhigonematomorpha is exclusively parasitic in millipedes and also has two superfamilies (Rhigonematoidea and Ransomnematoidea). An 1853 monograph by Joseph Leidy is still the best reference to these nematodes in North America; currently, only 16 species have been recognized from temperate North American millipede fauna. Most are poorly characterized by today’s standards and difficult to place. The primary goal of this research is a comprehensive taxonomic analysis of these nematodes and their specific host-parasite relationships with millipedes. Extensive redescription of nematodes within the millipede digestive tract was conducted utilizing both morphology and molecular analysis. Nematodes were dissected from the intestines of millipedes and studied with several different approaches. Species-level taxa from each millipede were sorted by live microscopic examination of various characters. Some nematodes were fixed in formalin and processed to glycerin for permanent mounts, while others were prepared for SEM and molecular analysis. In dissections undertaken so far, 972 millipedes have yielded 0‒1,752 nematodes per specimen. Two families of nematodes appear to favor different regions of the intestine; thelastomatids are often encountered in the posterior gut, while rhigonematids are mostly observed in the midgut. Spirobolid millipedes harbor the greatest abundance and largest nematodes. Rhigonematids typically are more numerous but thelastomatids are more diverse, with at least 20 species found so far. The width of the body is a determining factor for nematode infestation; smaller millipedes, such as some parajulids and platydesmids, are devoid of nematodes. The intestinal nematode fauna is primarily adult in July, with a rapid shift to almost completely juvenile nematodes by late summer and fall, suggesting these nematodes have one generation per year