Members of the phylum Nematomorpha are commonly known as horsehair worms or hairworms because of their resemblance to, and the common myth arising from horse tail hairs that fell into water and became worms. Hairworms are often found entangled in masses of two to many individuals and are therefore known as Gordian worms or gordiids because of a Phrygian tale in which Alexander the Great disentangled an impossible knot (Hanelt et al., 2005). The phylum Nematomorpha is one of only three completely parasitic metazoan animal phyla. As the sister group to the phylum Nematoda, hairworms consist of the freshwater Gordiida and the marine Nectonematida. There are approximately 350 described freshwater species within 18 extant and 2 extinct genera (Poinar and Buckley, 2006; Bolek, et al., 2015). During their parasitic phase, freshwater gordiids infect terrestrial arthropods such as orthopterans (grasshoppers, crickets, etc.), mantises, roaches, beetles, and millipedes (Poinar and Weissman, 2004; Bolek, et al., 2015). Inside their arthropod host and depending on the gordiid species, hairworms can grow to 40 centimeters in length for some North American species and up to 2 meters for some species in tropical regions (Schmid-Rhaesa, 2012). During development, hairworms manipulate the behavior of their arthropod host to seek water, where worms emerge as free-living adults (Bolek, et al., 2015). In the aquatic habitat, hairworms do not feed but instead find mates and reproduce during their short two to eight week, free-living phase of the life cycle (Fig. 1). Adult free-living hairworms are often difficult to observe in their natural habitats due to their ephemeral life span and ability to blend into their environment (Bolek et al., 2015).
The hairworm-cricket system is an appropriate model to study life history traits and co-evolutionary theory. To date, one species of hairworm has been domesticated in the laboratory and no studies have examined how the reproductive output of the cricket host post-infection. Hairworms are an understudied invertebrate phylum of worms which parasitize crickets and other arthropods. They demonstrate a fascinating behavior in which they manipulate their cricket host to find water, where the hairworm will live out the last stage of its life cycle as an adult. Anecdotal observations suggest that when crickets are infected with hairworms, reproduction in crickets is inhibited. With more than 350 species of hairworms described, there may be different nutrient acquisition, manipulation, and other adaptive behaviors utilized within this phylum. Using comparative analyses to examine parasite-induced changes in the cricket host can contribute to the evolution of host exploitation strategies.
To address these issues my research will: (1) Evaluate host use and distribution of cyst and juvenile stages of Paragordius varius and Gordius cf. robustus in various aquatic and terrestrial invertebrates; (2) Evaluation of modes of transmission of the semi-terrestrial life cycle of Gordius cf. robustus compared to the aquatic life cycle of Paragordius varius; (3) For my third objective, I will evaluate the morphological and physiological consequences of hairworm infection in the definitive arthropod host after hairworms emerge from the host.
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