Phylum Nematomorpha, commonly known as horsehair worm or gordiid, is now generally accepted as a monophyletic taxa (Hanelt et al., 2005; Schmidt-Rhaesa, 2012;
Bolek et al., 2015). Before applying the molecular data in restructuring the phylogeny,
Nematomorpha was considered as a subtaxon of (phylum) Nemathelminthes which is composed of Gastrotricha, Nematoda, Nematomorpha, Rotifera, Acanthocephala, Kinorhyncha, Loricifera and Priapulida (Ehlers et al., 1996). Now these pseudocoelomate groups in Nemathelminthes are thought to be less phylogenetically related and Nematomorpha is now re-grouped by the data of 18s rDNA in the taxon Ecdysozoa, which is composed of the molting animals including Arthropoda, Tactopoda, Onychophora, Nematoda, Nematomorpha, Kinorhyncha, and Priapulida(Aguinaldo et al., 1997).
Nematoda is the most closed taxon of Nematomorpha. Their sister group relationship is supported by both morphological (Ehlers et al., 1996) and molecular data (Auinaldo et al., 1997; Bleidorn et al., 2002; Petrov and Vladychenskaya, 2005, but see Sørensen et al., 2008). They are similar in the body shape, the presence of longitudinal dorsal and ventral epidermal cords with unpaired nerve cords, the absence of ring musculature, the absence of protonephridia and the cloaca in males
(Schmidt-Rhaesa, 2012). This similar morphologies make the horsehair worms to be
originally put together especially with its ecologically similar Mermithida, but several considerable differences still indicate the monophyly of Nematomorpha. These autapomorphic characteristics, including the degenerated intestine(Schmidt-Rhaesa,
2005),
the subpharyngeal brain, the reduction of pharyngeal musculature, thegenerally dark cuticle, the round or bilobed end (pointed in mermithids), the smaller egg size in the adult females (Schmidt-Rhaesa, 2012) suggest Nematomorpha and Nematoda are sister groups united in a taxon Nematoida
(Schmidt-Rhaesa, 1998
reviewed in Schmidt-Rhaesa, 2012). Such debate has been raised since mid 19th century(Meissner, 1856 reviewed in Schmidt-Rhaesa, 2012), and then
further confirmed and generally accepted by the modern molecular studies(Auinaldo et al., 1997; Bleidorn et al., 2002; Petrov and Vladychenskaya, 2005).
Among Nematomorpha, more than 350 species belonged to 21 genera (including two extinct genera, Cretachordodes and Paleochordodes) have been decribed (Poinar,
2008; Bolek et al., 2015). Two main taxa among the 19 extant genera are freshwater
hairworms (Gordiaceae) and marine hairworms (Nectonematoidea)(Bleidorn et al., 2002; Poinar, 2008). The marine hairworms are composed of 5 known species of
genus Nectonema which mainly parasitize with marine crustaceans such as hermit crabs or crabs (Oku et al., 1983; Poinar and Brockerhoff, 2001; McDermott et al.,2010). Their life cycle is hypothesized to be different from the typical life cycle of
freshwater hairworms since the larval horsehair worms might directly parasitize the definitive hosts instead of via the paratenic hosts (McDermott et al., 2010).The freshwater hairworm contains most species of Nematomorpha. Most of them adopt the typical complex life cycle with the aquatic paratenic host and the terrestrial definitive host, while few species might directly parasitize their aquatic definitive hosts (Schmidt-Rhaesa and Kristensen, 2006) as that in the marine hairworm. These 18 freshwater genera were once classified as four families: Gordiidae, Spinochordodidae, Lanochordodidae, and Chordodidae
(Bleidorn et al., 2002), but
only two (Gordiidae, Chordodidae) are accepted in the recent studies (Poinar, 2008).The taxon Gordiidae contains only two genera (Gordius and Acutogordius) which
share the postcloacal crescent on the male tail as the autapomorphy. The left 16 genera belong to Chordodidae were morphologically classified into three subfamilies Chorclodinae (Chordodes,
Euchordodes, Lanochordodes, Neochordodes, Noteochordodes, Pseudochordodes, Spinochordodes), Paragordiinae (Paragordius, Digordius, Progordius, Pseudogordius), Parachordodinae (Parachordodes, Cordionus, Beatogordius, Paragordiunus, and Semigordionus) (Bleidorn et al., 2002;
Schmidt-Rhaesa, 2002, 2012), but few studies has conducted to further examine this
suggestion.After the last genus (Noteochordodes) was suggested in 2000 (Schmidt-Rhaesa,
2002a), the 19 genera of Nematomorpha are generally accepted and not changed in
the recent years. Whereas, more than 50 new species have been newly described in recent decades since the new method applied, especially the sequencing data. The monotonous morphology of the horsehair worms causes two main challenges in the morphological identification: 1) finding a stable diagnosis characters and 2) setting a cutting point at the boundary of intra- and inter-species variation. Body surface of the horsehair worm is usually smooth which lacks diagnostic characteristics in many horsehair worm species. The male tail is relatively complex and frequently used in the classification(Schmidt-Rhaesa, 2002a), but it is still hard to distinguish all the 350
species by the few cuticlar characteristics. Application of scanning electronic microscope (SEM) since 1970s(Chandler and Wells, 1989) solved part of this
problem and currently becomes the standard method (Schmidt-Rhaesa, 2001). In addition, the recent molecular studies further challenge the current morphological identification by exposing the cryptic species among the samples with similar diagnostic characteristics (Bolek et al., 2015; Hanelt et al., 2015).Despite the new methods solve the problem of finding diagnostic characteristics,
lack of information in the morphological variation makes the difficulty in considering a morphological difference as intra- or inter-species variation. This problem mainly comes from the lack of reliable ways of collecting adult horsehair worms and their relatively short life span in the free-living adult stage (Bolek et al., 2015). In addition, damages to the cuticle from the environment and different developmental condition in the definitive host are two possible factors to cause morphological variation among conspecific individuals. In our previous study (Chiu et al., 2011), the 39 adult horsehair worms collected inside Hierodula mantids showed the slightly difference in their COi sequences which indicates the conspecific status among these samples.
However, the characteristics are various among these examined individuals. The flat ornamentations or smooth cuticle on the head might be resulted from the damage from the environment or covering of the larval cuticle (Chiu et al., 2011;
Schmidt-Rhaesa, 2012). Other morphological differences including the short,
unbranched or thin bristles in the bristlefields, the lack of thorn areole(Chiu et al., 2011), and the lack of bristlefields and extreme small crowned areoles in the samples
from novel hosts (Appendix 4), might respectively separate the samples into several different species.The use of molecular data, as the application of SEM, is now improving the understanding of horsehair worm's phylogenetic relationship, despite more sequence data from each described and unknown species are necessary and it has not yet seen the technique to amplify the DNA sequence from the cysts inside the paratenic hosts.
It also influences my strategy in the taxonomic study that in the following context, I basically judge the conspecific status of the samples collected from Taiwan, and describe their morphologies to compare their phylogenetic relationship with the described species and the possible morphological variation.