Investigations into the life histories of horsehair worms (Nematomorpha: Gordiida)
The life cycles and life histories of species of gordiids (Nematomorpha: Gordiida) were studied through the examination of the role and fate of their cysts. This study was based on three species of common American gordiids, Gordius robustus Leidy, 1851, Paragordius varius (Leidy, 1851), and Chordodes morgani Montgomery, 1898. Non-adult characters of these species were described. Morphometric measurements as well as morphological characters can be used to distinguish between egg strings, eggs, larvae and cysts of the three species. In addition, differences were found in oviposition behaviors of adult worms. Comparative experimental examinations of paratenic host specificity revealed that the three species had identical host specificities. Gordiids were able to encyst within annelids, molluscs, crustaceans, insects, and a vertebrate. Three species of putative hosts failed to get infected with any of the three species. Internal defense reactions and feeding strategies are implicated as preventing infection in these three species. Hosts involved in the natural life cycle were identified as aquatic insects. This proposed natural life cycle was tested by studying the survival of cysts within aquatic insects. All three species were able to survive and remain viable following the metamorphosis of midges, thus providing a critical link between the free-living aquatic adult and terrestrially based hosts. Furthermore, serial cyst passage between paratenic hosts was documented. Cysts in paratenic snail hosts were used to determine the geographic distribution and commonness of gordiid species in nature. Gordiid cysts were found at 70 percent of sites examined, including 8,073 cysts from 1,000 snails. Paragordius varius was established and maintained in the laboratory. This is the first time the life cycle of a member of the phylum Nematomorpha has been completed experimentally. This work provides evidence that cysts are a critically important stage within the life cycle of gordiids and suggests that the role of the paratenic host extends beyond bridging trophic levels. Ultimately, this work provides much of the basic life cycle biology of this phylum, which has been sought for over a century.