Lobocapillaria austropacifica n. g., n. sp. (Nematoda: Capillariidae) from the obtuse barracuda Sphyraena obtusata Cuvier (Sphyraenidae, Perciformes) off eastern Australia.

Based on light and scanning electron microscopical studies, a new nematode parasite, Lobocapillaria austropacifica n. sp. (Capillariidae), is described from the gall-bladder of the marine fish (obtuse barracuda) Sphyraena obtusata Cuvier (Perciformes: Sphyraenidae) from off the eastern Pacific coast of Australia, for which a new genus Lobocapillaria n. g. is established. This new genus is mainly characterised by a single row of stichocytes, the presence of two large, conspicuously elongated lateral caudal lobes and a pair of subventral papillae at their base in males, a flat spicule distended laterally towards its proximal end and provided with superficial rough transverse grooves, a spicular canal and a very long, aspinose spicular sheath with a conspicuous expansion near its proximal end when evaginated. Capillaria sphyraeni Parukhin, 1971 is transferred to Lobocapillaria as L. sphyreni (Parukhin, 1971) n. comb. A key to capillariid genera containing species parasitic in fishes is provided.

Four new species of Epacanthion Wieser, 1953 (Nematoda: Thoracostomopsidae) in intertidal sediments of the Nanji Islands from the East China Sea.

Four new species of Epacanthion are described from intertidal sandy sediments in the Nanji Islands National Marine Natural Reserve from the East China Sea. Males of all four new species have short spicules (< 2 anal body diameter) and a characteristic cervical circle of distinctly short and densely arranged setae at the pharyngeal region: Epacanthion hirsutum sp. nov. with a cervical circle of a single row of setae posterior to the nerve ring; E. longicaudatum sp. nov. with a cervical circle of 16 bundles each composed of six setae in two longitudinal rows posterior to the nerve ring; E. fasciculatum sp. nov. with a cervical circle of 18 bundles each composed of about 10 setae posterior to the nerve ring; and E. sparsisetae sp. nov. with a cervical circle of eight bundles each composed of about 10 setae anterior to the nerve ring. Among the known species of Epacanthion, only two species possess these features: E. quadridisci and E. gorgonocephalum. Epacanthion quadridiscus has six bundles of setae situated at the same level of the nerve ring, while E. gorgonocephalum has distinctly dense setae forming a wide band at the pharyngeal region. Epacanthion hirsutum sp. nov. differs from all congeners by the cervical circle composed of a single row of setae. Epacanthion sparsisetae sp. nov. is unique in having the cervical circle of bundles anterior to the nerve ring. Epacanthion longicaudatum sp. nov. differs from E. fasciculatum sp. nov. by the body size and the structure of the cervical circle. An updated diagnostic key to 28 valid species of Epacanthion is proposed.

Five new species of the family Trischistomatidae (Nematoda: Enoplida) from North and Central America, with keys to the species of Trischistoma and Tripylina.

Three new species of Trischistoma and two new species of Tripylina (Trischistomatidae) are described. Trischistoma ripariana n. sp. was collected in the surface organic material and upper rhizosphere soil on a stream bank in Oakville, California, USA. It is characterized by a short, thin body, the vulva at 79-83%, a small index c (17-29) and a short tail, 34-57 µm. Trischistoma corticulensis n. sp. was found in moss on tree bark in a tropical forest at the La Mancha Ecological Institute, Veracruz State, México. The vulva is at 67-73% and the tail is elongate conoid (51-84 µm). Trischistoma helicoformis n. sp. was collected on lichen growing on tree bark near the Carretera Interamericana in Costa Rica. It is characterized by its spiral shape after fixation, the tail length (76-101 µm), a very small index c (10-14.5) and very small sclerotised pieces around the vagina. Tripylina rorkabanarum n. sp. was collected from moss on tree bark in a tropical forest at the La Mancha Ecological Institute. It is characterized by the presence of two cervical setae, the position of the subventral teeth posterior to the dorsal tooth, the absence of sclerotized pieces around the vagina and the distance of the dorsal tooth from the anterior, 10-15 µm. Tripylina iandrassyi n. sp. was collected from soil around a banana tree at the La Mancha Ecological Institute. It is characterized by the presence of a post-uterine sac, well-developed buccal lips, subventral teeth located posterior to the dorsal tooth, one cervical seta in females and two in males, and by spicules not completely surrounded by a muscular sheath.

Five new species of the genus Tripylella (Nematoda: Enoplida: Tripylidae).

Five new species of the genus Tripylella are described, two from México, one from Fátima, Portugal, one from Quito, Ecuador, and one from California, USA. Tripylella mexicana sp. n. is characterized by its short body (average 0.74 mm), short pharynx (average 161 µm), short tail (average 117 µm), presence of an excretory pore and small setae distributed sparsely along the body, the presence of body pores, the posterior position of the subventral teeth in relation to the small dorsal tooth with all teeth in contiguous stomal chambers, the finely-striated cuticle with many anastomoses, the non-protruding vulval lips, and the presence of sclerotized pieces in the vulval region. Tripylella muscusi sp. n. is characterized by its body length (average 0.94 mm), pharynx length (average 201 µm), tail length (average 140 µm), the anterior position of the subventral teeth in relation to the small dorsal tooth in a single stomal chamber, the presence of an excretory pore, the presence of body pores and sparse somatic setae, the finely-striated cuticle with sparse anastomoses, protruding vulval lips and sclerotized oval-shaped pieces present in the vulval region. Tripylella quitoensis sp. n. is characterized by the short body length (average 0.72 mm), the short outer labial setae, the short pharynx (average 175 µm), the location of the anterior subventral teeth and posterior dorsal tooth in the same stomal chamber, the short tail (average 98 µm), the apparent absence of an excretory pore, presence of body pores, presence of somatic setae, a finely-striated cuticle, non-protruding vulval lips, and very small oval sclerotized pieces in the vulva. Tripylella fatimaensis sp. n. is characterized by the short body, (average 0.74 mm) long, by the length of the pharynx (average 180 µm), the length of the tail (average 110 µm) and in the length of its reduced diameter portion, 45-58 µm, the presence of an excretory pore, body pores and three pairs of caudal setae (one pair each latero-ventral, latero-dorsal and ventral). Tripylella dentata sp. n. is characterized, and differs from all the species of the genus, by the presence of two adjacent stomal chambers, with two large teeth, one dorsal and one ventral, in the posterior stomal chamber and two subventral teeth in the anterior smaller chamber, short body (average 0.85 mm), pharynx length (average 209 µm), tail length (average 115 µm), the apparent absence of an excretory pore, the presence of two cervical setae in a lateral position, and by the presence of conspicuous pores along the body.

Paroctonchus nanjiensis gen. nov., sp. nov. (Nematoda, Enoplida, Oncholaimidae) from intertidal sediments in the East China Sea.

Paroctonchus nanjiensis gen. nov., sp. nov. is described from a sand beach in the China Nanji Islands Natural Reserve in the East China Sea. Paroctonchus nanjiensis is distinct in the family Oncholaimidae in having more than three teeth, a character found otherwise only in the monotypic genera Octonchus and Patagonema. The new genus and species differs from Octonchus and Patagonema in the number and pattern of teeth, bearing three large teeth with the left ventrosublateral one the largest plus about 27 small teeth distributed on the wall of buccal cavity. By contrast, Octonchus possesses one large tooth plus eight small teeth, while Patagonema has three large teeth with the right ventrosublateral one the largest plus 12 small teeth on three mandibular ridges. Moreover, Paroctonchus nanjiensis differs from Octonchus also in position of the amphidial aperture (located in the middle portion of buccal cavity vs. at the base of buccal cavity), and from Patagonema in the absence of a demanian system (vs. present) and the presence of two subventral rows of wart-like precloacal supplements (vs. a ventral row of papillate supplements). We provide an improved diagnosis for the subfamily Octonchinae, and discuss the evolution of teeth in the family Oncholaimidae.

Two new species of Parasaveljevia Wieser, 1953 (Thoracostomopsidae, Nematoda) from Argentinean coasts (Chubut, Argentina).

Two new Parasavaljevia species are described, one from Puerto Madryn, and one from El Límite beach San Jorge Gulf, Chubut Province, Argentina. The two species are characterised by a combination of characters. Parasavaljevia uncinoa n. sp. is characterised by having a long rectangular cirrus, positioned at the level of the inner labial setae, rectangular mandibles with prominent lateral hooks bearing denticles in three lines of nine each and six cephalic setae of the same length, inserted at the anterior end of a small head capsule. Parasavaljevia limitense n. sp. is characterized by having a cirrus posterior positioned, between labial and cephalic setae, rectangular-arched mandibles bearing denticles in five lines of ten each in a gradient of sizes; small buccal cavity, small cephalic capsule and abundant cervical setae (4+10). An emendation of the genus diagnosis and a new key are given and discussed.

A new species of Pontonema (Oncholaimidae, Nematoda) and a redescription of Pontonema incisum Wieser, 1953 from Santa Cruz and Chubut Provinces, Argentina.

Pontonema golfonuevensis sp. nov. from Chubut Province, Argentina is described and a description of the male of Pontonema incisum Wieser, 1953 from Chubut and Santa Cruz Provinces is provided. Pontonema golfonuevensis sp. nov. is characterized by having the slender sub-ventral teeth 25 µm long located at 24 % of stoma length, a short, broad dorsal tooth at 60 % of stoma length, excretory pore opening at level of base of the buccal cavity, and by having a ventral precloacal sensory field with four papillae and a glandular sub-ventral area with seven papilliform sensillae. The male of P. incisum has long slender sub-ventral teeth at 36 % of stoma length, a short broad dorsal tooth at 72 % of stoma length, excretory pore about two buccal cavity lengths from the anterior end, and a ventral precloacal sensory field without papillae and a glandular sub-ventral area with twelve to fourteen papilliform sensillae. A key for identification of males of Pontonema is presented.

Description of new species of Phanodermopsis (Enoplida, Phanodermatidae) with key to genera of family Phanodermatidae and pictorial key to Phanodermopsis species.

A new representative of the family Phanodermatidae, Phanodermopsis nana sp. n., is described. The new species is found in the Sea of Japan and characterized by its small body size, long anterior sensilla and short tail. The new species differs from P. ingrami in body length (~ 7000 µm in P. ingrami vs ~ 3000 µm in P. nana sp. n.). The De Man's ratios (a, b, c) are smaller in P. nana sp. n. and the outer labial setae are longer (1 corresponding body diameter) than in P. ingrami (1/2 corresponding body diameter). The vulva is more posterior in P. ingrami (V% = 58-61%) than in P. nana sp. n. (V% = 51-58%). A dichotomous key to the 9 valid genera of Phanodermatidae and a pictorial key to species of Phanodermopsis are provided. The length of the spicules is proposed as the main differentiating feature for distinguishing Phanodermopsis and Crenopharynx species. Phanodermopsis longisetae Chitwood, 1936 which has extremely long spicules (5.2 anal body diameters) is transferred to the genus Crenopharynx.

Three new and two known free-living marine nematode species of the family Ironidae from the East China Sea.

Three new and two known species of free-living marine nematodes of the family Ironidae from the East China Sea are described and illustrated. Conilia sinensis sp. nov. is identified by the relatively large body size (1883-2399 µm); the well developed lips; the number, shape and length of spicule (single and striated, length 87-100 µm as arc); the shape of telamon; the number of supplements (1). Pheronous donghaiensis sp. nov. is characterized by its sharp tail point; caudal gland absent; buccal cavity armed with four big solid teeth and rows of minute denticles; spicules stout, with central septum at proximal end, male caudal region with two rows of small conical subventral papillae. Trissonchulus latispiculum sp. nov. can be distinguished by its head not set off from remaining body, tail short and blunt, buccal cavity with minute denticles, spinneret opening slightly ventrally, spicule broad and alate with central septum and head on proximal end. Trissonchulus benepapillosus (Schulz, 1935) and Trissonchulus oceanus Cobb 1920 which are first reported from China, are redescribed in detail with emphasis on new or hitherto poorly described morphological features. Types are deposited in the Institute of Oceanology, Chinese Academy of Sciences.

Description of Trischistoma abharensis n. sp. (Nematoda: Trischistomatidae) and first record of Tripylella intermedia (Bütschli, 1873) Brzeski & Winiszewska-Slipinska, 1993 (Nematoda: Tripylidae) from Iran.

Two populations of the genera Trischistoma and Tripylella were recovered from the rhizosphere of grapevines and mosses growing on alder trees in Zanjan and Guilan provinces, respectively, Iran. The nematodes were identified as Trischistoma abharensis n. sp. and Tripylella intermedia, respectively. Trischistoma abharensis n. sp. is characterized by having females with body length 1069-1322 µm, presence of sparse somatic setae on the sublateral body, absence of ventromedian cervical setae, a distinct dorsal tooth directed anteriorly, absence of post-vulval uterine sac, and tail with one pair of subdorsal caudal setae. Males were not found. Tripylella intermedia is characterised by having females with body length 905-990 µm, annulated cuticle, stoma with two chambers: with dorsal tooth lying in posterior buccal chamber, and one large subventral and one small subventral tooth, respectively lying in posterior and anterior buccal chambers, cardiac glands large, composed of six fused cells, and tail 121-155 µm long, ventrally bent, anterior half broad, then suddenly narrowing, with posterior half tapered narrowly and cylindrically. The phylogenetic relationships of both species were analysed using sequences of the partial small subunit (SSU) and D2/D3 expansion segments of large subunit (partial LSU) of ribosomal RNA genes and are discussed.

A new species and a new record of Trischistoma Cobb, 1913 (Nematoda: Enoplida: Trischistomatidae) from Shanxi Province, China.

This paper describes a new species and gives a new record and a re-description of a known species of Trischistoma Cobb, 1913. Trischistoma taiguensis sp. nov. is characterized by having one pair of caudal setae on the tail; female body length of 562-699 µm, a = 35-42, b = 4.1-4.7, c = 11-14, c' = 3.9-4.6 and V = 79-82%. For the first time, Trischistoma pellucidum Cobb, 1913 is recorded from Shanxi Province, China. The phylogenetic relationships among species in the genus Trischistoma were analyzed using data from the SSU (small subunit) and D2/D3 expansion segments of the LSU (large subunit) rDNA sequences. These analyses confirmed that T. taiguensis sp. nov. and T. pellucidum differ from other known members of the genus for which sequences are available in GenBank.

A new marine nematode genus Pseudoplatycoma with a new species from the Sulu Sea and revision of the subfamily Platycominae (Enoplida: Leptosomatidae).

The nematode Pseudoplatycoma malaysianis n. gen. n. sp. is described from the Sulu Sea (Malaysia). The new genus is classified in the subfamily Platycominae Platonova 1976. Revision of the new genus and four other genera in Platycominae, resulted in four species from the genus Platycomopsis being transferred to other genera: P. dimorphica and P. mazjatzavi to the genus Platycoma; P. effilata to the genus Micoletzkyia; and P. gibbonensis to the genus Anticoma. Pilosinema is regarded as a asynonym of Platycomopsis and Platycomopsis paracobbi is regarded as a synonym for P. cobbi. A key for identification of the genera and species of Platycominae is presented. 

A review of the genus Litinium Cobb, 1920 (Nematoda: Enoplida: Oxystominidae) with descriptions of four new species from two contrasting habitats.

Four new species of the genus Litinium are described from mangroves and from deep sea. The genus Litinium now includes ten valid species. An emended generic diagnosis and a pictorial guide for species identification are given. Litinium quangi sp. n. and L. curticauda sp. n., both found in mangroves of South Vietnam, are morphologically similar and differ from other congeneric species in body size, having short anterior setae, ovoid amphideal fovea and a short rounded tail. Litinium quangi differs from L. curticauda in the number of midventral preanal supplementary setae (one in L. quangi and two in L. curticauda) and relative tail length (c' 1.12-1.63 in L. quangi and 0.83-0.94 in L. curticauda). Litinium abyssorum sp. n. and L. profundorum sp. n., both collected from the Angola Basin, South-East Atlantic Ocean, at depth 5400 m, are also morphologically similar and differ from other species of the genus by having a smaller body and relatively large amphideal fovea with deep invagination of the anterior edge. Litinium abyssorum differs from L. profundorum in the number of midventral preanal setae (two in L. abyssorum, one in L. profundorum), relative tail length (c' 3.61 in L. abyssorum and 1.17 in L. profundorum) and L. abyssorum has unequal spicules.

Pseudocapillaria (Pseudocapillaria) moraveci sp. n. (Nematoda: Capillariidae) from the stomach of Gobius paganellus (Perciformes: Gobiidae) from Vigo estuary (NW Spain).

Pseudocapillaria moraveci sp. n. (Nematoda, Capillariidae) is described from the stomach of Gobius paganellus Linnaeus (Perciformes, Gobiidae) from rocky shores of the Vigo estuary (Northwest Atlantic coast of Spain). Scanning electron microscopy reveals the absence of a dorsal cuticular membrane interconnecting the caudal lobes of male, thus confirming the assignment of this new species to the nominotypical subgenus Pseudocapillaria Freitas, 1959. Pseudocapillaria moraveci can be easily differentiated from other marine congeneric species belonging to subgenera Pseudocapillaria and Ichthyocapillaria Moravec, 1982 by its muscular oesophagus, which is distinctly thickened at the distal third, the terminal or almost terminal anus in female, the presence of a basal internal papilla on each ventrolateral caudal lobe in male, and its exclusive site preference for the stomach. In addition, the spicule, which measures 0.180-0.292 mm in length (4.9-7.9% of body length) and possesses a proximal end expanded and oblique in lateral view, a narrowed middle part, and a distal end almost pointed, also distinguishes this species. Pseudocapillaria moraveci is, together with P. tomentosa (Dujardin, 1843) Moravec, 1987, the only species in the genus Pseudocapillaria that has been reported to infect members of the family Gobiidae.

A new species of the genus Tripylina Brzeski, 1963 (Nematoda: Enoplida: Trischistomatidae) from Shanxi province, China.

Tripylina puxianensis n. sp. is described and illustrated from Shanxi province, China. It is characterized by having a triangular dorsal tooth, pointing towards the ventral side, anterior to two sub-ventral teeth, with dorsal and sub-ventral teeth similar in shape and size; one ventromedian seta in the cervical region; vulva with protruding lips, absence of post-uterine sac; and a pair of setae on the tail; male not found; female body length 1048-1331 µm, a = 25.5-33.8, b = 5.5-6.2, c = 14.4-23.1, c' = 2.1-3.5 and V = 56.1-68.5%. The phylogenetic relationships among species in the genus Tripylina were analyzed using data from the SSU (small subunit) and D2/D3 expansion segments of the LSU (large subunit) rDNA. These analyses confirmed that T. puxianensis n. sp. is different from other members of the genus for which sequences are available.

A new species of the genus Tripylina Brzeski, 1963 (Nematoda: Enoplida: Trischistomatidae) from Zhejiang Province, eastern China.

Tripylina zhejiangensis n. sp. is described and illustrated from Zhejiang province, eastern China. It has a robust body, smooth cuticle with numerous pores, two small subventral teeth in the posterior part of the large triangular dorsal tooth, one ventral-median seta in the cervical region and a pair of setae on the tail. Females are characterized by a body length of 1325-1573 µm, a = 23.3-31.2, b = 5.5-6.6, c = 13.6-19.4, c'= 2.7-3.6 and V = 61.1-68.1%. Small subunit (SSU) and D2/D3 expansion segments of large subunit (LSU) of ribosomal DNA were used to analyze the phylogenetic relationships of sequenced species in the genus Tripylina. Tripylina zhejiangensis n. sp. varied significantly from other related nematodes both in morphological characterizations and phylogenetic analyses.

Biased sex ratio and niche restriction in Baruscapillaria obsignata (Madsen 1945) (Nematoda, Capillariidae) from Columba livia (Aves, Columbidae).

In the present study populations of the avian nematode species Baruscapillaria obsignata are described from Columba livia. Male and female individuals were obtained from 27 birds, fixed in alcohol/formalin/acetic acid (AFA) and preserved in 70% ethanol. Nematodes were identified and then counted under a stereoscopic microscope. Baruscapillaria obsignata were much more frequent in the anterior third of the small intestine, and females were more abundant than males in all infra populations. The prevalence was 55.6%, mean intensity was 11.8 (median 11.0; range 1-31) and abundance 6.56. In the present study, we observed an aggregated distribution of parasite infrapopulations, as demonstrated by the value of the exponent of the negative binomial distribution, K = 0.2773; by the discrepancy index, D = 0.656 and by the variance/mean ratio, 12.44. The female/male sex ratios found in all infrapopulations were always greater than 1, showing a bias in favour of female abundance. This tendency was especially marked in infrapopulations containing fewer individuals. The sizes of infrapopulations ranged from 5 to 31 individuals. The mean sex ratio observed was 2.69 ± 3.28 (median 1.83; range 0-11). In infrapopulations with 5-15 individuals, the sex ratios observed varied from 2.6 to 11, while in those with 17-31 individuals, the sex ratios were lower, ranging from 1.7 to 2.4. There was a negative correlation between the intensity of infection and the sex ratio of infrapopulations. Results are discussed in terms of possible factors influencing the processes that lead to niche restriction and biased sex ratios in parasite infrapopulations.

Longidorus poessneckensis Altherr, 1974 and L. piceicola Lisková, Robbins & Brown, 1997 (Nematoda: Longidoridae): new records from Poland and the first description of the L. poessneckensis male and a bivulval female.

Longidorus poessneckensis Altherr, 1974 and L. piceicola Lisková, Robbins & Brown, 1997 (Nematoda: Longidoridae) represent new records from Poland. These two species are described and illustrated together with a male and bivulval female of L. poessneckensis. In its general morphology and morphometrics, the male of L. poessneckensis is similar to the females, but has a spicule 100 µm long and one adanal pair, two double and a row of six single ventromedian supplements. Comments on the differential diagnosis of L. poessneckensis and two morphologically related species, L. uroshis Krnjaic, Lamberti, Krnjaic, Agostinelli & Radicci, 2000 and L. macrosoma Hooper, 1961 are given.

Low endemism, continued deep-shallow interchanges, and evidence for cosmopolitan distributions in free-living marine nematodes (order Enoplida).

BACKGROUND: Nematodes represent the most abundant benthic metazoa in one of the largest habitats on earth, the deep sea. Characterizing major patterns of biodiversity within this dominant group is a critical step towards understanding evolutionary patterns across this vast ecosystem. The present study has aimed to place deep-sea nematode species into a phylogenetic framework, investigate relationships between shallow water and deep-sea taxa, and elucidate phylogeographic patterns amongst the deep-sea fauna. RESULTS: Molecular data (18 S and 28 S rRNA) confirms a high diversity amongst deep-sea Enoplids. There is no evidence for endemic deep-sea lineages in Maximum Likelihood or Bayesian phylogenies, and Enoplids do not cluster according to depth or geographic location. Tree topologies suggest frequent interchanges between deep-sea and shallow water habitats, as well as a mixture of early radiations and more recently derived lineages amongst deep-sea taxa. This study also provides convincing evidence of cosmopolitan marine species, recovering a subset of Oncholaimid nematodes with identical gene sequences (18 S, 28 S and cox1) at trans-Atlantic sample sites. CONCLUSIONS: The complex clade structures recovered within the Enoplida support a high global species richness for marine nematodes, with phylogeographic patterns suggesting the existence of closely related, globally distributed species complexes in the deep sea. True cosmopolitan species may additionally exist within this group, potentially driven by specific life history traits of Enoplids. Although this investigation aimed to intensively sample nematodes from the order Enoplida, specimens were only identified down to genus (at best) and our sampling regime focused on an infinitesimal small fraction of the deep-sea floor. Future nematode studies should incorporate an extended sample set covering a wide depth range (shelf, bathyal, and abyssal sites), utilize additional genetic loci (e.g. mtDNA) that are informative at the species level, and apply high-throughput sequencing methods to fully assay community diversity. Finally, further molecular studies are needed to determine whether phylogeographic patterns observed in Enoplids are common across other ubiquitous marine groups (e.g. Chromadorida, Monhysterida).

Moving towards a complete molecular framework of the Nematoda: a focus on the Enoplida and early-branching clades.

BACKGROUND: The subclass Enoplia (Phylum Nematoda) is purported to be the earliest branching clade amongst all nematode taxa, yet the deep phylogeny of this important lineage remains elusive. Free-living marine species within the order Enoplida play prominent roles in marine ecosystems, but previous molecular phylogenies have provided only the briefest evolutionary insights; this study aimed to firmly resolve internal relationships within the hyper-diverse but poorly understood Enoplida. In addition, we revisited the molecular framework of the Nematoda using a rigorous phylogenetic approach in order to investigate patterns of early splits amongst the oldest lineages (Dorylaimia and Enoplia). RESULTS: Morphological identifications, nuclear gene sequences (18S and 28S rRNA), and mitochondrial gene sequences (cox1) were obtained from marine Enoplid specimens representing 37 genera. The 18S gene was used to resolve deep splits within the Enoplia and evaluate the branching order of major clades in the nematode tree; multiple phylogenetic methods and rigorous empirical tests were carried out to assess tree topologies under different parameters and combinations of taxa. Significantly increased taxon sampling within the Enoplida resulted in a well-supported, robust phylogenetic topology of this group, although the placement of certain clades was not fully resolved. Our analysis could not unequivocally confirm the earliest splits in the nematode tree, and outgroup choice significantly affected the observed branching order of the Dorylaimia and Enoplia. Both 28S and cox1 were too variable to infer deep phylogeny, but provided additional insight at lower taxonomic levels. CONCLUSIONS: Analysis of internal relationships reveals that the Enoplia is split into two main clades, with groups consisting of terrestrial (Triplonchida) and primarily marine fauna (Enoplida). Five independent lineages were recovered within the Enoplida, containing a mixture of marine and terrestrial species; clade structure suggests that habitat transitions have occurred at least four times within this group. Unfortunately, we were unable to obtain a consistent or well-supported topology amongst early-branching nematode lineages. It appears unlikely that single-gene phylogenies using the conserved 18S gene will be useful for confirming the branching order at the base of the nematode tree-future efforts will require multi-gene analyses or phylogenomic methods.