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Lungworms of the cosmopolitan genus Rhabdias (Nematoda: Rhabdiasidae) are among the most common parasites of amphibians and squamate reptiles. The life cycles, life histories, host specificities, and evolutionary relationships of Rhabdias spp. were studied through examination of their free-living and parasitic stages in amphibians and reptiles. This study found free-living development of anuran lungworms was primarily limited to heterogonic reproduction, whereas snake lungworms primarily reproduced homogonically. Infective anuran lungworms penetrated the skin of frogs and toads; in contrast, snake lungworms penetrated snake esophageal tissue during per os infections. Our molecular phylogeny strongly supported separate clades for anuran and snake lungworms, and supported our species identifications. Field studies and experimental infections indicated that snake lungworms were generalist parasites of snakes and maybe lizards, whereas lungworms from anurans ranged from strict host specificity to relative generalist. Host specificity in nature appeared to be limited by both ecological and physiological factors, which varied among worm species. Field studies found that the majority of lungworm infections appeared to occur within 2 m of wetland shorelines in Nebraska. Field and laboratory experiments suggested that the predominantly sandy soils of Nebraska’s Sandhills limit R. joaquinensis from infecting anurans in western Nebraska. This work is an important addition to the small number of previous studies that have used a multifaceted, comparative approach that includes field and laboratory experiments to investigate host-parasite relationships. Ultimately, this work provides much needed basic life history and ecological data for a fascinating group of parasites.