The Nematomorpha is a monophyletic taxon of parasitic worms that develop within their hosts (primarily terrestrial insects and other arthropods) but reproduce in aquatic environments. Around 300 species of nematomorphs (or "hairworms") have been described. Nematomorphs are dioecious (i.e., have separate sexes). Tiny larvae (about 100 µm long) hatch from eggs. These larvae are equipped with two or three rings of cuticular hooks and terminal stylets with which they are able to penetrate soft epithelia of their hosts. Terrestrial hosts may become infected by drinking water containing larvae or they may prey on a paratenic host (see below). Although adults of most nematomorph species live in freshwater, a few species live in damp soil and the five known species in the distinctive genus Nectonema are pelagic in coastal marine environments. For Nectonema, only hosts containing juveniles are known and these are all decapod crustaceans—either pelagic shrimps such as Pandalus or benthic crabs such as Cancer, Munida or Pagurus. Because nematomorph copulation takes place close to the surface, nematomorph larvae may infect crustaceans while the latter are still planktonic larvae. Within the host, nematomorphs increase dramatically in size from about 100 µm to several centimeters (some may exceed two meters in length). The size of the mature worm substantially exceeds the length of the host. By the time the worm is mature, it fills most of the host cavity with the exception of the head and the legs. Worms are ready to emerge only once they reach this stage. (Schmidt-Rhaesa 2002 and references therein; Thomas et al. 2002; Brusca and Brusca 2003)
The life cycle of nematomorphs has 4 stages: the egg, the pre-parasitic larva that hatches from the egg, the parasitic larva that develops within an invertebrate host (the "definitive" or developmental host), and the free-living aquatic adult. Within the definitive host, worms complete development, but they do not mate and oviposit until they are free-living in aquatic environments. Many nematomorph species have another type of host as well, a "paratenic" or transport host. The pre-parasitic larva enters a paratenic host but does not develop further until the paratenic host is eaten by a scavenger or predator in which it can develop. Within paratenic hosts, larvae penetrate through the gut, secrete a cyst wall, fold up, and become cysts. For at least some nematomorph species, the nematomorphs may make the transition from water to land by forming cysts in aquatic insect larvae, with the cysts surviving the host's metamorphosis to an adult which can then convey the nematomorphs to land. The great majority of freshwater nematomorphs have been collected from beetles or orthopterans. Poinar (2001) includes a table listing the the approximately three dozen invertebrate families that have been recorded as definitive hosts for developing nematomorphs. Paratenic hosts run the gamut from trematode flatworms to vertebrates. (Poinar 2001; Hanelt and Janovy 2004)
Originally, nematomorphs were classified as nematodes, but they have been recognized as a separate taxon since the name Nematomorpha was introduced in 1886. The traditional view, based on both morphological and molecular characters, considers Nematoda and Nematomorpha to be sister taxa, together forming a clade that has sometimes been termed Nematoida. (Bleidorn et al. 2002 and references therein; Bourlat et al. 2008 and references therein). However, Sørensen et al. (2008) argue that based on their analyses of 18S rRNA and histone 3 sequences, and consistent with some key morphological features, the Nematomorpha are actually sister to the Loricifera, not the Nematoda.
Many types of parasites are known to modify the behavior of their host in ways that benefit the parasite. Based on anecdotal observations, it has long been suspected that at least some mature nematomorphs, which must reach water to mate and reproduce, manipulate the behavior of their terrestrial insect hosts, causing them to seek water and jump into it. Thomas et al. (2002) found clear evidence of this phenomenon in 8 tettigoniid orthopterans infected by the nematomorph Spinochordodes tellinii, as well as in the gryllid cricket Nemobius sylvestris infected by the nematomorph Paragordius tricuspidatus. In experiments, however, they found no evidence that hosts actively seek out water; rather, they suggested, infected hosts seem to display erratic behavior that eventually brings them close to water, which they then enter. This behavior was investigated experimentally with the woodland cricket N. sylvestris infected by P. tricuspidatus nematomorphs. Consistent with the findings of Thomas et al. (2002), Sanchez et al. (2008) found that this behavioral manipulation has two phases, first causing the cricket to wander into atypical habitats and next causing it to commit suicide by entering water. The erratic behavior was induced before the worms were sexually mature, timing which the authors suggested may be an adaptation to increase the likelihood that the host would be near water when it was an optimal time (from the parasite's perspective) for the cricket host to enter water and drown. Biron et al. (2006) have taken intriguing first steps at identifying, at a molecular level, the mechanisms by which this parasite manipulates its host's behavior.
Based on several species that have been investigated, nematomorphs appear not to have an associated symbiotic or non-symbiotic bacterial flora, in striking contrast to groups such as the arthropods (Hudson and Floate 2009 and references therein).
Poinar (2008) reviewed the global diversity of freshwater nematomorphs, which he estimated includes over 300 described species and an actual species richness of around 2000 species. Poinar (2001) provides a dichotomous key to the known genera of nematomorphs; Smith (2001) includes a dichotomous key to genera known from the United States. Schmidt-Rhaesa et al. (2003) review the 17 species of freshwater nematomorphs they recognize from North America, including their known geographic occurrences and a dichotomous identification key. Poinar and Chandler (2004, cited in Hudson and Floate 2009) also provide a synopsis of North American nematomorphs.
The biology of the Nematomorpha has been reviewed by Poinar (2001) and Hanelt et al. (2005). Poinar (2001) also includes a discussion of the freshwater Mermithidae, a family of nematodes that parasitize invertebrates and are superficially similar to nematomorphs (a key to North American mermithid genera is included as well).
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