DESCRIPTION OF THE PHYLUM NEMATOMORPHA (VEJDOVSKY 1886)
| EUKARYA> UNIKONTA> OPISTHOKONTA> ANIMALIA> BILATERIA> PROTOSTOMATA> ECDYSOZOA> NEMATOIDA> NEMATOMORPHA |
PHYLUM NEMATOMORPHA LINKS
| Nematomorpha (ne-mat-o-MOR-fa) is formed from two Greek roots that mean “thread forms” [thread -nema (νήμα); and form- morphi (μορφή)]. This is a reference to the thread-like appearance of the animals, and to their obvious similarity to the Nematoda. |
| INTRODUCTION TO THE NEMATOMORPHA Horsehair worms look like nematodes, thus the name Nematomorpha, which means nematode form. Usually, the mature animals are found in water where they resemble wiry hairs like those of a horse’s tail or mane. While writhing about, the adults often tie themselves into knots (Figure 1). This feature gives them their other common name, gordian worms, after the fabled Gordian knot which Alexander the Great “untied” by cutting through it. Horsehair worms have larvae that are parasites of arthropods. The freshwater group (Gordoidea) specializes on insects, particularly Orthoptera (the insect order that includes grasshoppers and crickets). Nematomorphs, though parasitic, are not pathogenic to humans, their animals or plants. On the contrary, because the Orthoptera contain some of the most important insect pests, nematomorphs as well as nematodes may function in controlling them (e.g. Schmidt-Rhaesa et al. 2005). A mostly marine group of nematomorphs uses decapod Crustaceans as their hosts. Though the flamboyance of the emergence of adults from their insect hosts makes them quite well-known (Figure 2), surprisingly little is known about their parasitic life history. Schmidt-Rhaesa (2002) reviews what is known of the nematomorph life cycle. It seems that the eggs of gordians hatch to produce larvae that somehow infect and encapsulate themselves in a variety of hosts such as snails, aquatic insects, and even vertebrates (fishes and frogs). Schmidt-Rhaesa (1997) suggests that the larvae bore into the integuments of the primary hosts. Still, the problem remains as to how an intermediate stage encapsulated in the body of an aquatic animal ends up in the haemolymph cavity of an animal like a cricket. Schmidt-Rhaesa (2002) suggests that the orthopterous hosts are omniverous and become infected by feeding on the carcass of an infected intermediate host. Once inside the primary host, the larval gordian goes through several molts which can take months. Upon maturity, they induce the insect to seek out water (Thomas et al. 2005) where the worms emerge to form a brief free-living adult stage (Figure 2). Like nematodes, the nematomorphs have only longitudinal muscle and lack segmentation. Unlike nematodes, though, nematomorphs have a terminal cloaca and no functional gut. The similarities between the Nematoda and Nematomorpha are more than just superficial. The relationship has been confirmed by multiple molecular analyses in which Nematomorpha and Nematoda emerge as sister groups (e.g. Zrzavy 2001, Blaxter et al. 1998, and Giribet et al. 2007) in a clade called the Nematoida (a term coined by Schmidt-Rhaesa 1998). Furthermore, Telford et al. (2008) list 5 morphological synapomorphies, first identified by Nielsen (2001). The molecular support, though present, is weak (Telford et al. 2008, Petersen and Ernesse 2001, and Mallatt et al. 2003). Nematomorphs fall within a clade of protostome animals called the Ecdysozoa (Nicholas 2001b), a group that Nielsen (2001) does not recognize as valid. Nevertheless, much strong evidence for the monophyly of the Ecdysozoa has emerged (Telford et al. 2008, Philippe et al. 2005, and Webster et al. 2006). They are in a clade called Nematoida in which they are sisters to the Nematoda (Figure 3) |
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| FIGURE 1. A single horsehair worm beginning to tie itself into a knot. Image from http://www.inhs.uiuc.edu/chf/pub/surveyreports/mar-apr95/page4.html | FIGURE 2. A horsehair worm emerging from a cricket. Image from http://lancaster.unl.edu/enviro/pest/Nebline/HorseHairWorm.htm |
FIGURE 3. A cladogram that shows the relationship of Nematomorpha (within the shaded box) as a sister group to Nematoda within the clade, Nematoida (N). Phyla of the Ecdysozoa are in Clade E.
P = Protostomata
S = Scalidophora
N = Nematoida
PA = Panarthropoda
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| By Jack R. Holt and Carlos A. Iudica. Last revised: 04/10/2013 |
