These organisms were long included in taxonomic classifications with members of phylum Nematoda, which were themselves subjected to a number of changes in name and taxonomic rank throughout the 19th and early 20th century. The first formal description of a nematomorph species was provided by Linnaeus in 1758, who erected the genus Gordius for the species Gordius aquaticus. ( Brusca and Brusca, 2003 Hanelt, et al., 2013 Shapiro, 2012) They are also found as parasitic larvae in their hosts, which are typically terrestrial arthropods and insects. Gordioids may be found in nearly any freshwater environment, including not only rivers, lakes, and streams, but even puddles or grasses after a heavy rain. They are most often collected as larvae from their decapod crustacean hosts.
Adult Nectonema species are free-swimming and pelagic, and are sometimes found near the coast during high tides. Generally speaking, nematomorphans are found in aquatic or occasionally terrestrial (either as semi-aquatic individuals themselves or within terrestrial hosts) environments throughout the world.
They are known from every continent, with the exception of Antarctica. Gordioids are found in freshwater streams and ponds as adults (a few are found in damp soil) and most typically in terrestrial insects, as parasitic larvae. They are known from the waters of Indonesia, Japan, New Zealand, the Northern Atlantic, and the Mediterranean. Nectonema species are found in coastal, marine, and pelagic environments as adults, and are found as parasites in decapod crustaceans as larvae. ( Brusca and Brusca, 2003 Hanelt, et al., 2013 Poinar Jr., 2008 Schmidt-Rhaesa, 2012 Shapiro, 2012) Geographic Range Nematomorphans are parasitic as larvae ( Nectonema species parasitize marine invertebrates, while gordioids utilize terrestrial arthropods) and are free-living and aquatic as adults. While Nectonema species are marine, planktonic worms, gordioids are found in freshwater, most commonly along the banks of ponds and streams, and some are semi-aquatic and live in damp soil. There are 19 currently recognized genera within Nematomorpha, and estimates of global species diversity for this phylum are as high as 2,000. The phylum Nematomorpha (also known as horsehair worms) is comprised of two orders: Nectonematoidea (monogeneric ( Nectonema), with four known species) and Gordioidea, in which the remainder of the over 300 described nematomorphan species are placed.
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