Two new nematodes, Pseudelzalia longiseta gen. nov., sp. nov. and Paramonohystera sinica sp. nov. (Monhysterida: Xyalidae), from sediment in the East China Sea

Pseudelzalia longiseta gen. nov, sp. nov. and Paramonohystera sinica sp. nov. from subtidal sediment in the East China Sea are described. Pseudelzalia is characterized by 6 labial papillae and 10 cephalic setae, cylindrical buccal cavity, elongate (>2 anal body diameter) spicules, and conico-cylindrical tail devoid of terminal setae. It differs from Elzalia by the absence of terminal setae. Pseudelzalia longiseta sp. nov. is 647–853 μm long, has 7–8 μm long cervical setae, 11–14 μm long caudal setae, 25–41 μm long spicules about 2.1–2.7 anal diameter, and pointed tail-tip. Paramonohystera sinica possesses 12 cephalic setae, a character found in four congeners: Paramonohystera buetschlii (Bresslau and Schuurmans Stekhoven in Schuurmans Stekhoven, 1935, Paramonohystera pilosa Boucher, 1971, Paramonohystera concinna Lorenzen, 1977 and Paramonohystera halerba Fadeeva and Belogurov, 1987. It differs from P. buetschlii by shorter body (933–1023 μm versus 2000–2200 μm); from P. pilosa by the much shorter spicules (79–88 μm versus 167 μm) and narrower head (13–16 µm versus 32 µm); from P.concinna by smooth cephalic setae (versus segmented); and from P. halerba by the absence of two rows of setae on the ventral side of the tail (versus present). Based on the evaluation of nominal species, we recognize 14 valid species and provide an emended diagnosis and a tabular key for Paramonohystera. http://www.zoobank.org/urn:lsid:zoobank.org:pub:474B8F17-AED7-4078-8176-DFC499B78526


Introduction
Marine free-living nematodes are highly diverse and usually comprise 70-90% meiobenthic metazoans (Miljutin et al. 2010). Our preliminary investigation on nematode diversity in the East China Sea revealed about 40 taxa per 100 individuals in one sediment core. Data indicate that nematode diversity is high and most nematodes are rare species comprising only one or two individuals. Description of new nematode taxa is in progress but, because of taxonomic problems, the number of new species described from the seas of China so far represents only about 1/100 of known species in the world oceans (Tchesunov 2006;Zhang and Zhang 2006;Huang and Wu 2011;Huang and Xu 2013). In this paper we describe one new genus and two new species from the subtidal sediment in the East China Sea: Pseudelzalia longiseta gen. nov., sp. nov. and Paramonohystera sinica sp. nov.

Material and methods
Sediment samples were collected from the East China Sea in July 2012, using a 0.1-m 2 Gray-Ohara box corer, from which the samples used for meiofaunal analysis were taken using a modified syringe tube and preserved with formalin (5% final concentration) onboard. In the laboratory, the fixed samples were stained with 0.1% Rose Bengal for 12 h, washed on a 500-µm sieve to remove large particles and a 31-µm sieve to retain meiofauna. To avoid the loss of nematodes on the 500-µm sieve, we checked the sieve and retrieved the nematodes retained on it. Ludox HS 40 was used to extract meiofauna from the remaining sediments by centrifugation. The extracted samples were sorted under a dissecting microscope. Nematodes were transferred into a 9 : 1 (volume/ volume) solution of 50% alcohol-glycerol in a cavity block to slowly evaporate to pure glycerol, and then mounted onto permanent slides.
The descriptions were made from glycerine mounts (Platt and Warwick 1983) using a differential interference contrast microscope (Nikon E80i). Line drawings were made with the aid of a camera lucida. Morphological data are presented using the modification of Filipjev's standard formula described by Platt (1973). Type specimens have been deposited in the Marine Biological Museum, Institute of Oceanology at Qingdao, Chinese Academy of Sciences. All measurements are in µm, and all curved structures are measured along the arc.
Abbreviations are as follows: a, body length divided by maximum body diameter; b, body length divided by pharynx length; c, body length divided by tail length; a.b. d., anal body diameter; c.b.d., corresponding body diameter; V, distance of vulva from the anterior body end; V%, position of vulva from anterior end expressed as a percentage of total body length.

Etymology
Composition of the Greek prefix pseudo-(false) and the generic name Elzalia, referring to the similarity of the genus to Elzalia. Feminine gender.

Familial assignment and comparison with related genera and species
The new species Pseudelzalia longiseta sp. nov. described below is obviously a member of the family Xyalidae characterized by transversely striated cuticle, usually 10 cephalic setae, and a single anteriorly outstretched ovary to the left of the intestine. Within the family Xyalidae, Pseudelzalia longiseta sp. nov. is very similar to members of Elzalia Gerlach, 1957 in having the labial papillae, large cylindrical buccal cavity and elongate spicules. However, the new species possesses a character clearly different from all known species of Elzalia, namely, the tail devoid of terminal setae versus with three terminal setae in Elzalia ( Figure 1A, E). The structure of the tail is a significant character at the genus level within the family Xyalidae, in which the two largest genera, Daptonema Cobb, 1920 andTheristus Bastian, 1865, are separated only by the tail morphology (conico-cylindrical with terminal setae versus conical without terminal setae) (Warwick et al. 1998). Pseudelzalia longiseta sp. nov. is also similar to the monotypic genus Parelzalia Tchesunov, 1990 which, however, has a conical buccal cavity with domed anterior end, shorter spicules of about 1 a.b.d. and in particular the presence of terminal setae (Tchesunov 1990). Accordingly, we propose Pseudelzalia as a new genus. Except for the tail morphology, Pseudelzalia differs from Daptonema and Theristus also by the cylindrical buccal cavity (versus conical) and elongate spicules (>2 a.b.d. versus <2 a.b.d.). The presence of long caudal setae in the new species Pseudelzalia longiseta is another striking character that is absent in the genera Elzalia, Daptonema and Theristus. However, only one species is described for the new genus and such a character has never been regarded as a generic character within the family Xyalidae. Hence, we consider it a specific character for Pseudelzalia at the current state of knowledge.  Table 1)

Type material
Four males and two females were available for measurement and description.

Etymology
Composition of the Latin adjective longus (long) and the Latin noun seta (bristle), referring to the long somatic setae in the cervical and tail region of the species.
Excretory pore and ventral gland not observed. Tail devoid of terminal setae, conico-cylindrical and tapered at terminal end, 103-121 μm long and 7.7-9.7 a.b.d.; cylindrical part occupying about one-third of tail length. Three caudal glands ( Figure 1A).
Single outstretched testis to the right of intestine. Paired, slender and arcuate spicules, 25-41 μm long and 2.1-2.7 a.b.d. Gubernaculum composed of four parts: the ventral thin piece about 11 μm long, the longer dorsal thin piece about 18 μm long, the shorter dorsal thin piece about 14 μm long, and the ventral main part with several conical projections ( Figure 1C). Precloacal supplements not seen.

Type material
Five males and four females were measured and studied.

Etymology
The New Latin adjective sinicus (of China) refers to the country where the species was discovered.
Three terminal setae, about 7 μm long. Three caudal glands in tail region ( Figure 2E). Two opposite and outstretched testes, the anterior one to the left of the intestine, and the posterior one to the right. Spicules paired, slender and arcuate, 79-88 μm long and 4.0-4.4 anal body diameter. Gubernaculum complex, paired large parts slender and proximally pointed, 20-24 μm long; paired small parts stout, distally hook-shaped, pointed in the proximal end, with a ventral apophysis in the middle part, 8-11 μm long. No precloacal supplements ( Figure 2D).
Comparison with related species and genera and overview of Paramonohystera species Paramonohystera was first established by Steiner (1916) as a subgenus of Monhystera Bastian, 1865 being characterized by retractable head, bubble amphids and simple conical buccal cavity, with Monhystera (Paramonohystera) megacephala Steiner, 1916 as the type species. Soon afterward, it was raised to genus level by Filipjev (1918). It is worthy of note that the similar name Paramonhystera as used by Filipjev (1918) and many subsequent authors is an invalid emendation, as clearly stated by Gerlach and Riemann (1973). Two subgenera have previously been proposed for Paramonohystera: Paramonohystera and Leptogastrella (Wieser 1956;Gerlach and Riemann 1973;Lorenzen 1994). Wieser (1956) defined the two subgenera mainly by the arrangement of cephalic (10 cephalic setae, without additional cephalic setae versus 10-12 cephalic setae, plus additional cephalic setae) and cervical setae (of equal length versus one circle much longer). However, these differences are not distinct and the characters may overlap in a single species (e.g. P. riemanni; Table 4). On the other hand, all species assigned to the subgenus Leptogastrella have either been synonymized or considered as unreliable reports. Hence, the two subgenera have rarely been employed. The redescription of P. (Leptogastrella) pellucida (Cobb, 1920) by Wieser (1956) is probably involved in a species complex composed of specimens with about 20 cephalic setae and specimens with a normal circle of 10-12 setae. These specimens might be classified if other typical specimens are available, as stated by Wieser (1956).
Paramonohystera sinica sp. nov. possesses 12 cephalic setae, whereas most known species of Paramonohystera have 10 cephalic setae (Table 4). As the number of cephalic setae is very stable within species, it seems reasonable to split Paramonohystera and erect a new genus with 12 cephalic setae. However, variability in the number of cephalic setae is usual in Xyalidae, in which Xyala and Cobbia have 10 or 12 cephalic setae, and Theristus and Daptonema have even 10, 12 or 14 cephalic setae. Wieser (1956) also emphasized the variability in the number of the cephalic setae in Paramonohystera. Hence, it is too early to create a new genus before further materials and molecular proofs are available. Within the genus Paramonohystera, only P. buetschlii (Bresslau and Schuurmans Stekhoven in Schuurmans Stekhoven, 1935), P. concinna Lorenzen, 1977, P. halerba Fadeeva and Belogurov, 1987and P. pilosa Boucher, 1971 possess 12 cephalic setae ( Table 4). The new species P. sinica differs from P. buetschlii by body length (933-1023 μm versus 2000-2200 μm) and the ratio of spicule length to a.b.d. (4.0-4.4 versus 2.7). Paramonohystera concinna has segmented cephalic setae which are smooth in P.sinica. Paramonohystera pilosa has much longer spicules (167 µm versus 79-88 µm), broader head (32 µm versus 13-16 µm) and smaller ratio of spicule length to anal body diameter (2.7 versus 4.0-4.4). In addition P. halerba has two rows of setae on the ventral side of the tail (versus absent in P. sinica), shorter cervical setae (4-6 μm versus up to 14 μm), longer spicules (105-113 μm versus 79-88 μm) and smaller ratio of spicule length to anal body diameter (2.5 versus 4.0-4.4). All other species of Paramonohystera are easily distinguished from the new species by the number of cephalic setae.
Within the family Xyalidae, Paramonohystera Steiner, 1916 is similar to Daptonema, differentiated by the elongate (>2 a.b.d.) and slender spicules (Lorenzen 1977;Warwick et al. 1998). Chen and Vincx (2000) recognized nine species of Paramonohystera and provided a key of species including P. breviseta Juario, 1974, P. longicaudata Timm, 1963and P. wieseri Ott, 1977. Among these, P. breviseta has been regarded as a member of the genus Retrotheristus Lorenzen, 1977. Paramonohystera longicaudata and P. wieseri have relatively short spicules (<1.5 a. b.d.) and probably belong to the genus Daptonema (Table 3). Chen and Vincx (2000) did not include P. buetschlii (Bresslau and Schuurmans Stekhoven in Schuurmans Stekhoven, 1935), P. parabutschlii (Timm, 1961), P. riemanni (Platt, 1973) and P. zizichi Pastor de Ward, 1985 etc., without any comments. All of these species have spicules >2 a.b.d., match Paramonohystera well and should be considered as valid species of the genus (Table 3). Venekey et al. (2014) recognized 18 species of Paramonohystera as valid, and regarded P. micramphis as species inquirenda and P. mystacoderma as nomen nudum. Among the valid species of Venekey et al. (2014), P. longicaudata is probably a member of Daptonema; P. paranormandica has already been transferred to Daptonema (Ansari et al. 2013); P. pellucida is likely to be a species complex, as mentioned above; P. stricta is probably a member of Promonhystera possessing distinct labial setae; and P. tschilenkoi might be a species inquirenda since the number of cephalic setae that distinguishes Paramonohystera from Retrotheristus was not included in the original description of Platonova (1971), Venekey et al. (2014) also overlooked P. buetschlii, which has been considered as a valid species (Warwick et al. 1998). Pastor de Ward (1985) previously described a population under the name of Paramonohystera (P.) parabutschlii Timm, 1961. It is likely a misidentification because the specimens are distinctly longer (2100 μm vs. 918 μm in male) and has a higher number of cephalic setae (12 vs. 10) than the type specimen described by Timm (1961).
Based on the evaluation of 28 nominal species of Paramonohystera, we recognize 14 valid species and provide a tabular key to the genus (Tables 3, 4). Additionally, we provide an emended generic diagnosis for Paramonohystera: Xyalidae with 6 labial papillae and 10 or 12 cephalic setae usually in six groups, unarmed conical buccal cavity with domed anterior end, circular (mostly) or elliptical amphids, elongate (≥2 a.b.d.) spicules, and conico-cylindrical tail with terminal setae. Paramonohystera is most similar to Daptonema and Promonhystera, but differs from Daptonema by the elongate (≥2 a.b.d. versus ca.1 a.b.d) and slender spicules and from Promonhystera by the lack of long and distinct labial setae (Warwick et al. 1998;Coomans and Abebe 2006).