www.europeanjournaloftaxonomy.eu

2015 • Leduc D.

ISSN 2118-9773

This work is licensed under a Creative Commons Attribution 3.0 License.

Research article

urn:lsid:zoobank.org:pub:16E64AF8-518C-47F0-B3CF-6BA707C222FA

New species of Thelonema , Metasphaerolaimus, and Monhystrella

(Nematoda, Monhysterida) from Kermadec Trench, Southwest Pacific

Daniel LEDUC

National Institute of Water and Atmospheric Research, Private Bag 14-901,

Wellington, New Zealand; +64 4 386 0379.

Email: [email protected]

um:lsid:zoobank.org: author: 9393949F-3426-4EE2-8BDE-DEFFACE3D9BC

Abstract. Three new species of the order Monhysterida are described based on specimens obtained at

depths of 8081 and 9177 m in the Kermadec Trench. Thelonema clarki sp. nov. is characterised by a

large body size (3230M461 pm), short cylindrical buccal cavity, gubernaculum without apophyses, and

long conico-cylindrical tail. This is the first record of the genus since its original description over two

decades ago from the Peru Basin. Metasphaerolaimus constrictus sp. nov. is characterised by a relatively

long body (1232-1623 pm), slightly arcuate spicules without gubernaculum, and conico-cylindrical tail

with inner cuticle conspicuously thickened immediately anterior to cylindrical portion. Monhystrella

kermadecensis sp. nov. is characterised by a circle of papillose outer labial sensillae slightly anterior to

the four short cephalic setae, gubernaculum with caudal apophyses, the presence of distinct cuticularised

piece along anterior vaginal wall, and a relatively short conical (males) or conico-cylindrical tail

(females) with conical, ventrally-curved spinneret. M. kermadecensis sp. nov. can be differentiated from

all other species of the genus, and, indeed, the entire family, based on the variable position of the anterior

gonad relative to the intestine. The new species is classified within the Monhysteridae, and not the

closely-related Xyalidae, based on the small body size, a smooth cuticle, and the presence of six outer

labial papillae and only one testis. Further work is required to clarify the placement of M. kermadecensis

sp. nov. relative to other monhysterid genera. A tabular key to all ten valid Metasphaerolaimus species

is presented.

Keywords. Linhomoeidae, Sphaerolaimidae, Monhysteridae, hadal. New Zealand.

Leduc D. 2015. New species of Thelonema , Metasphaerolaimus , and Monhystrella (Nematoda, Monhysterida)

from Kermadec Trench, Southwest Pacific. European Journal of Taxonomy 158: 1-19. http://dx.doi.org/10.5852/

eit.2015,158

Introduction

The Order Monhysterida is well represented in hadal trench samples (> 6000 m depth), with the family

Monhysteridae often dominant (Gambi et al. 2003; Vanhove et al. 2004). The families Sphaerolaimidae

and Linhomoeidae are also common in hadal trench samples, but are typically much less abundant (e.g.,

Tietjen 1989). Whilst species of the families Sphaerolaimidae and Linhomoeidae are relatively large (> 1

mm in length) and easy to identify, species of the family Monhysteridae are typically small (<0.8 mm in

1

European Journal of Taxonomy 158: 1-19 (2015)

length) and often represented by many morphologically similar species in deep-sea samples (D. Leduc

unpublished data). As a result, data on the distribution of Monhysteridae species in the deep sea are

scarce despite their high abundance (Tietjen 1989; Miljutin et al. 2010). In addition, there has been some

confusion related to the identification of the small and morphologically similar genera Monhystrella

Cobb, 1918 and Thalassomonhystera Jacobs, 1987. Vanhove et al. (2004) and Gambi et al. (2003)

recorded high densities of the genus Monhystera Bastian, 1865 in hadal trenches, despite a taxonomic

revision by Jacobs (1987) almost two decades earlier resulting in all marine Monhystera species being

transferred to either Thalassomonhystera or Monhystrella (Fonseca & Decraemer 2008). More work

is clearly needed on the taxonomy of this family, as well as other Monhysterida, in order to identify

patterns in species distribution within and among trenches.

Of the fifteen nematode species so far described / recorded from hadal trench environments worldwide

(Miljutin et al. 2010; Leduc 2015), three are known from Kermadec Trench: Synonchoides galathea

Wieser, 1956 (4570 m depth), Manganonema rowdeni Leduc, 2015 (8081 m) and Manganonema

majusculum Leduc, 2015 (8081 m). Here, three new species of the Order Monhysterida, one species each

of the families Linhomoeidae, Sphaerolaimidae and Monhysteridae, are described based on specimens

obtained at depths of 8081 and 9177 m.

Materials and methods

Kermadec Trench is situated in the Southwest Pacific Ocean and is formed by the subduction of the Pacific

Plate under the Indo-Australian Plate. It extends from approximately 26° to 36° S near the northeastern

tip of New Zealand’s North Island. Samples were collected from the axis of Kermadec Trench at

depths of 8081 and 9177 m during the Woods Hole Oceanographic Institute (WHOI) cruise TN309 (RV

Thomas G. Thompson) in May 2014. The sediment cores were obtained using the submersible ROV

Nereus (core internal diameter = 6.35 cm). Cores were sliced into 0-1, 1-2, 2-3, 3—4, 4-5 and 5-10 cm

layers and fixed in 10% buffered formalin. Samples were rinsed on a 20 pm sieve to retain nematodes.

Nematodes were extracted from the remaining sediments by Ludox flotation, stained with Rose Bengal,

and transferred to pure glycerol (Somerfield & Warwick 1996). Species descriptions were made from

glycerol mounts using differential interference contrast microscopy and drawings were made with the

aid of a camera lucida.

All measurements are in pm, and all curved structures are measured along the arc. Type specimens are

held in the NIWA Invertebrate Collection, Wellington, New Zealand. Abbreviations in the text are as

follows:

a = body length / maximum body diameter

abd = anal body diameter

b = body length / oesophagus length

c = body length / tail length

cbd = corresponding body diameter

%V = vulva distance from anterior end of body x 100 / total body length

2

LEDUC D., Kermadec Trench Monhysterida

Results

Class Chromadorea Inglis, 1983

Order Monhysterida Filipjev, 1929

Superfamily Siphonolaimoidea Filipjev, 1918

Family Linhomoeidae Filipjev, 1922

Diagnosis (From Fonseca & Bezerra 2014)

Cuticle often striated, sometimes smooth. Inner labial sensillae papillose or not observable. Amphideal

fovea usually circular, rarely unispiral. Inner side of labial region formed by an annular, soft-skinned pad

that narrows the buccal opening. Cardia long and conspicuous. Usually two outstretched and opposed

gonads, rarely single gonad present. Anterior gonad to the left or right of intestine, posterior gonad to

the opposite side.

Subfamily Thelonematinae Bussau, 1993

Type and only genus

Thelonema Bussau, 1993.

Diagnosis

See diagnosis of type and only genus Thelonema.

Thelonema Bussau, 1993

Diagnosis (modified from Bussau 1993)

Inner labial sensillae not observable with light microscopy. Six outer labial setae situated in separate

circle from the four sensillae of the third circle; the latter consists of papillae located in depressions.

Four sub-cephalic setae located immediately posterior to the cephalic papillae. Farge circular amphideal

fovea located far posteriorly, and with nerve process usually visible. Buccal cavity cylindrical or funnel-

shaped, cuticularised, without teeth. Pharynx with weak posterior bulb; cardia long and gradually

widening posteriorly. Female with two outstretched and opposed ovaries and male with two outstretched

and opposed testes. Tail conico-cylindrical.

Type species

Thelonema majum Bussau, 1993.

Thelonema clarki sp. nov.

urn:lsid:zoobank.org:act:F4ED998A-656F-48Dl-8FBC-B0031BA943D9

Figs 1-3, Table 1

Diagnosis

Thelonema clarki sp. nov. is characterised by large adult body size (3230M461 pm), short cylindrical

buccal cavity with cuticularised walls, sub-cephalic setae of similar length or shorter than outer labial

setae, secretory-excretory system with two or three renette cells, gubernaculum without apophyses, and

conico-cylindrical tail 6.4-10.4 abd long.

3

European Journal of Taxonomy 158: 1-19 (2015)

Etymology

This species is named after Malcolm R. Clark, principal investigator of the HADES project (HADal

Ecosystem Studies) who has made an outstanding contribution to the field of deep-sea ecology and who

kindly supported the involvement of the author on the Kermadec Trench voyage.

Material examined

Holotype

KERMADEC TRENCH: $ (NIWA 99760), collected 6 May 2014 (WHOI cruise TN309, Nereus dive

N074).

Fig. 1 . Thelonema clarki sp. nov. Holotype, S- A. Anterior body region. B. Head, showing buccal

cavity and cephalic sensillae. C. Posterior body region. D. Gonads, right-hand side view. E. Spicule and

gubernaculum. Scale bar: A, C = 40 pm; B = 12 pm; D = 45 pm; E = 20 pm.

4

LEDUC D., Kermadec Trench Monhysterida

Paratypes

KERMADEC TRENCH: 2 $$ (NIWA 99761) and 1 juvenile (NIWA 99762), collected 7 May 2014

(WHOI cruise TN309, Nereus dive N075).

Type habitat

KERMADEC TRENCH: water depth: 8081 m (178.17571° W, 34.34030° S), sediment depth: 2-3 cm

(holotype); water depth: 9177 m (177.65414° W, 32.85037° S), sediment depth: 0-2 cm (paratype).

Fig. 2. Thelonema clarki sp. nov. Paratypes, $ and juvenile. A. Female head. B. Female anterior body

region. C. Juvenile head. D. Juvenile anterior body region. E. Female posterior body region. F. Juvenile

posterior body region. G. Female reproductive system. Arrow shows position of vulva. Scale bar: A-C

= 40 pm; B-D = 100 pm; E-F = 90 pm; G = 210 pm.

5

European Journal of Taxonomy 158: 1-19 (2015)

Description

Male

Body cylindrical, tapering slightly towards both extremities. Cuticle striated from level of buccal cavity

to near tail tip. Somatic setae absent except for a few 1 pm long setae in pharyngeal region. Lip region

slightly concave; head region otherwise rounded. Internal labial sensillae not observed. Six outer labial

setae, 5 pm long, situated in separate circle from the four sensillae of third circle; the latter consists

of papillae located in depressions (Fig. 3B). Small, granular glands sometimes observed, apparently

connected to base of outer labial setae (Fig. IB); larger unicellular glands also sometimes observed

and apparently connected to cephalic sensillae (Fig. 2C). Four sub-cephalic setae located immediately

posterior to the cephalic papillae, 3M pm long, similar in length or slightly shorter than outer labial setae.

Large circular amphideal fovea with cuticularised outline situated 1.7 cbd from anterior body extremity.

Mouth opening narrow, surrounded by bulge of inner portion of lip region; buccal cavity cylindrical,

7 pm deep, 9 pm wide, with cuticularised walls, not surrounded by pharyngeal tissue. Pharynx narrow,

muscular, with oval posterior bulb; pharyngeal glands and their orifices indistinct. Nerve ring slightly

posterior to middle of pharynx length. Secretory-excretory system with two or three renette cells (third

cell may sometimes be obscured) all situated just posterior to pharyngeal bulb; ampulla and pore between

nerve ring and posterior pharyngeal bulb. Cardia conspicuous, 90 pm long, widening posteriorly, not

surrounded by intestine walls.

Reproductive system diorchic with two opposed and outstretched testes, anterior testis to the

left of intestine and posterior testis to the right of intestine. Mature sperm globular, nucleated, 4-5 x

6-7 pm. Spicules paired, 1.2 abd long, strongly arcuate, with well-developed capitulum and pointed

distal end. Gubernaculum thin, pointed at both ends, without apophyses but with rounded glandular

tissue extending dorsally. Rectal glands not observed; one ejaculatory gland situated between spicules.

Pre-cloacal supplements absent. Tail long, conico-cylindrical, with rounded tip. Caudal glands not

observed; short and sparse caudal setae present, no terminal setae. Intestine with numerous transparent

crystalline structures, square to rhomboid-shaped, up to 14 x 14 pm (Fig. 3C-D).

Female

Similar to males but with lower values of a, b, and c, slightly smaller amphids (0.42-0.44 vs 0.65 cbd)

and longer tail (9.0-10.4 vs 6.4 abd). Buccal cavity 5 pm wide and 4-5 pm deep. Reproductive system

didelphic; anterior branch outstretched, to the right of intestine, posteror branch poorly developed, to the

left of intestine. Spermatheca present in anterior branch only, simple, not cuticularised. Vulva transverse,

situated slightly posterior to mid-body; small vaginal glands present on either side of vagina; muscular

pars proximalis vaginae.

Juvenile

Similar to females, but with shorter body and lower values of a and b.

Remarks

Thelonema clarki sp. nov. can be differentiated from the only other species of the genus, T. majum , by

the larger body size (323CM1461 vs 1000-1460 pm), short cylindrical buccal cavity (vs long and funnel-

shaped in T. majum), sub-cephalic setae of similar length or shorter than outer labial setae (sub-cephalic

setae longer than outer labial setae in T. majum), secretory-excretory system with two or three renette

cells (secretory-excretory system not observed in T. majum), absence of gubernacular apophyses (vs

dorso-caudal apophyses present in T. majum), and longer tail (6.4-10.4 vs 2.7 abd).

This is the first time that Thelonema is recorded outside the type locality in the Peru Basin (~ 4150 m

depth) since the original description of the genus by Bussau (1993). The genus was not included in

6

LEDUC D, Kermadec Trench Monhysterida

Fig. 3. Thelonema clarki sp. nov. Light micrographs. A. Holotype, entire biody. B. Cross-section

view of sunken papilla, second circle of cephalic sensillae. C. Female gut content, showing crystalline

structures and other unidentified contents. D. Juvenile gut showing crystalline structures. Arrow shows

position of sunken papilla of third circle of cephalic sensillae. Scale bar: A = 520 pm; B = 5 pm; C-D =

20 pm.

7

Table 1. Morphometries (pm) of Thelonema clarki sp. nov., Metasphaerolaimus constrictus sp. nov., and Monhystrella kermadecensis sp. nov. (n,

number of specimens; L, body length; V, vulva distance from anterior end of body).

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8

LEDUC D., Kermadec Trench Monhysterida

the recent overview of all Monhysterida by Fonseca & Bezerra (2014), but the discovery of a new

Thelonema species in the Southwest Pacific confirms the validity of the genus.

The presence of crystalline structures in the intestine is intriguing, as many appear too large to have

been ingested (the structures are up to 14 pm wide, and the width of the cuticularised buccal cavity is

4-9 pm). It seems unlikely that the buccal cavity can stretch to accommodate such large particles given

the presence of cuticularised walls. The crystalline structures may have grown through accretion whilst

in the intestine, but this process would presumably require a relatively long period of time.

Superfamily Sphaerolaimoidea Filipj ev, 1918

Family Sphaerolaimidae Filipjev, 1918

Diagnosis (from Fonseca & Bezerra 2014)

Cuticle striated or annulated. Six outer labial setae and four cephalic setae in one circle, with cephalic setae

longer than outer labial setae. Eight groups of sub-cephalic setae present (four groups in Megalamphis).

Buccal cavity wide, barrel-shaped (except for Subsphaerolaimus Forenzen, 1978, which is conical),

with longitudinal ribs, surrounded only at the base by pharyngeal tissue. Pharyngeal lumen with thick

cuticle. Renette cell usually present, opening behind nerve ring. Females with one outstretched anterior

ovary at left or right of intestine. Males usually with two testes; anterior one either to the left or right of

intestine, posterior one at opposite side.

Subfamily Metasphaerolaiminae Gourbault & Boucher, 1981

Type genus

Metasphaerolaimus Gourbault & Boucher, 1981.

Diagnosis

See diagnosis of type and only genus Metasphaerolaimus.

Genus Metasphaerolaimus Gourbault & Boucher, 1981

Ceratosphaerolaimus Fadeeva, 1983: 1329, fig. 5. (see Jensen 1992: 8)

Diagnosis (modified from Fonseca & Bezerra 2014)

Cuticle finely striated. Six inner labial papillae; six outer labial setae at same level as four longer cephalic

setae; eight groups of sub-cephalic setae. Amphideal fovea circular, situated posteriorly to buccal

cavity. Buccal cavity strongly cuticularised with six H- or X-shaped mandibles, hooked anteriorly and

articulating on cuticularised rim posteriorly. Pharynx cylindrical with strongly cuticularised lumen.

Type species

M. cancellatus Gourbault & Boucher, 1981.

Metasphaerolaimus constrictus sp. nov.

um:lsid:zoobank.org:act:C2DDlCB2-2B9B-4C31-BF13-C0ElA143A275

Figs 4-5, Table 1

Diagnosis

Metasphaerolaimus constrictus sp. nov. is characterised by a relatively long body (1232-1623 pm),

amphids located 1.0-1.4 cbd from anterior extremity, slightly arcuate spicules 1.3 abd long without

9

European Journal of Taxonomy 158: 1-19 (2015)

Fig. 4. Metasphaerolaimus constrictus sp. nov. A. Holotype, S, anterior body region. B. Holotype,

head. C. Female head. D. Female anterior body region. Scale bar: A = 50 pm; B-C = 25 pm; D = 55 pm.

10

LEDUC D., Kermadec Trench Monhysterida

gubernaculum, tail conico-cylindrical with inner cuticle conspicuously thickened immediately anterior

to cylindrical portion and with three short terminal setae.

Etymology

The species name is derived from the latin constrictus (= drawn together or contracted), and refers to the

distinctive shape of the tail with thickening of the inner cuticle.

Material examined

Holotype

KERMADEC TRENCH: $ (NIWA 99763), collected 6 May 2014 (WHOI cruise TN309, Nereus dive

N074).

Paratypes

KERMADEC TRENCH: 3 ? ? (NIWA 99764-6), collected 7 May 2014 (WHOI cruise TN309, Nereus

dive N075).

Type habitat

KERMADEC TRENCH: water depth: 8081 m (178.17571° W, 34.34030° S), sediment depth: 2-3 cm

(holotype); water depth: 9177 m (177.65414° W, 32.85037° S), sediment depth: 0-2 cm (paratype).

Description

Male

Body cylindrical, tapering slightly towards both extremities. Cuticle faintly striated along entire body.

Eight rows of somatic setae, relatively long (4-7 pm) and numerous in pharyngeal region, short and

sparse elsewhere. Head rounded, with well-developed lip region. Inner labial sensillae not observed; six

outer labial setae, 1-2 pm long, and four cephalic setae, 2-3 pm long, in one circle. Eight groups of three

to four sub-cephalic setae, 3-8 pm long. Large circular amphideal fovea with strongly cuticularised

outline, 1.4 cbd from anterior body extremity. Buccal cavity large, 24 pm deep and 13 pm wide; six

H-shaped mandibles hooked anteriorly and with wide base articulating onto cuticularised rim posteriorly.

Posterior portion of buccal cavity surrounded by pharyngeal tissue. Pharynx muscular, cylindrical,

widening very slightly towards posterior extremity, with strongly cuticularised lumen. Cardia extend

into intestine lumen. Nerve ring situated at middle of pharynx length. Secretory-excretory system not

observed.

Reproductive system diorchic with two outstretched testes; anterior testis to the left and posterior testis

to the right of intestine. Mature sperm cells nucleated, spherical to globular, 9 x 9-13 pm. Spicules

1.3 abd long, slightly arcuate, with swollen proximal ends and pointed distal ends. Gubernaculum and

pre-cloacal supplements absent. Tail conico-cylindrical with conspicuous thickening of inner cuticle

(maximum thickness ~7 pm) immediately anterior to cylindrical portion (Fig. 5D); cylindrical portion

shorter than conical portion. A few short, sparse caudal setae present sub-ventrally and sub-dorsally;

three short terminal setae, 2 pm long. Caudal glands not observed.

Female

Similar to male, but with longer body and substantially smaller amphids (0.11-0.14 vs 0.48 cbd)

situated 1.0-1.4 cbd from anterior body extremity. Reproductive system monodelphic with outstretched

anterior branch to the left or right of intestine. Spermatheca present, simple and not cuticularised. Vulva

transverse, situated at almost two thirds of body length from anterior. Vaginal glands not observed.

Three caudal glands observed in one specimen. Cuticle immediately anterior to cylindrical portion of

tail up to 9 pm thick.

11

European Journal of Taxonomy 158: 1-19 (2015)

Fig. 5. Metasphaerolaimus constrictus sp. nov. A. Entire J. B-C. Female posterior body region. D. Male

posterior body region. E. Male gonads. Scale bar: A = 300 pm; B-C = 55 pm; D = 45 mm; E = 80 pm.

12

LEDUC D., Kermadec Trench Monhysterida

Remarks

Metasphaerolaimus constrictus sp. nov. can be differentiated from all other species of the genus by

the distinctive tail shape with conspicuous thickening of cuticle (up to 7-9 pm thick) immediately

anterior to the cylindrical portion. M. constrictus sp. nov. is most similar to M. gerlachi Jensen, 1992,

M. hadalis (Freudenhammer, 1975), and M. inglisi Gourbault & Boucher, 1981. The new species can be

differentiated from M. gerlachi based on larger body size (1232-1623 vs 893-971 pm), larger amphids

in males (0.48 vs 0.30-0.40 cbd), and presence of short terminal setae (vs long terminal setae in M.

gerlachi ); from M. hadalis by higher values of a (30-38 vs 26-27), larger amphids in males (0.48 vs

0.35 cbd) and shorter tail (2.9-3.1 vs 3.4-3.8 abd); from M. inglisi by the absence of setae immediately

anterior to amphids in females (vs three long setae in M. inglisi ), and presence of terminal setae (absent

in M. inglisi).

Metasphaerolaimus was initially described by Gourbault & Boucher (1981) to accommodate

species similar to Sphaerolaimus Bastian, 1865, but with a buccal cavity with six mandibles instead

of a solid, heavily cuticularised capsule. Gourbault & Boucher (1981) described three new species

(.Metasphaerolaimus cancellatus Gourbault & Boucher, 1981, M. hamatus Gourbault & Boucher, 1981

and M. inglisi Gourbault & Boucher, 1981), and transferred the species Sphaerolaimus campbelli Allgen,

1927, Sphaerolaimus hadalis Freudenhammer, 1975, and Sphaerolaimus crassicauda Freudenhammer,

1975 to Metasphaerolaimus. Fadeeva (1983) later described the identical genus Ceratosphaerolaimus

Fadeeva, 1893, including one new species ( Ceratosphaerolaimus japonicus Fadeeva, 1983); the

latter author also transferred Sphaerolaimus horrendus Sergeeva, 1981 to Ceratosphaerolaimus.

Ceratosphaerolaimus was synonymised with Metasphaerolaimus by Jensen (1992), who also described

Metasphaerolaimus gerlachi Jensen, 1992. There are currently ten valid Metasphaerolaimus species

including Metasphaerolaimus constrictus sp. nov. (Table 2). Species ofthis genus are mainly differentiated

based on amphideal fovea size, spicule length, and length and shape of the tail. The majority of species

occur in the deep sea below 1000 m depth, except for Metasphaerolaimus campbelli , M. horrendus , and

M. japonicus , which were described from coastal waters. These three shallow water species are also the

only species of the genus that possess a gubemaculum with dorso-caudal apophyses.

Superfamily MonhysteroideaFilipjev, 1929

Family Monhysteridae de Man, 1876

Diagnosis (modified from Fonseca & Decraemer 2008)

Small, slender nematodes usually less than 2.5 mm long. Cuticle smooth or striated. Anterior sensillae

in two circles: anterior circle with six inner labial sensillae (usually papilliform) and posterior circle

with six outer labial sensillae and four cephalic sensillae (both usually setiform). Amphideal fovea

circular or cryptocircular. Buccal cavity small to medium-sized, bipartite or single, with or without

denticles. Pharynx with or without posterior bulb. Secretory-excretory system present or absent. Female

reproductive system monodelphic with outstretched gonad to the right of intestine; male monorchic

with anterior testis to the right of intestine (except Monhystrella kermadecensis sp. nov. where position

of anterior gonad relative to intestine is variable). Spicules usually simple and arcuate. Tail conical to

conico-cylindrical; terminal setae absent.

Genus Monhystrella Cobb, 1918

Diagnosis (modified from Fonseca & Decraemer 2008)

Short body length (< 0.8 mm). Small somatic setae often present. Six outer labial and four cephalic

sensillae usually setiform. Buccal cavity tubiform, funnel-shaped or conoid, often with cuticularised

walls; denticles usually present, but difficult to observe in small species. Circular amphideal fovea

13

Table 2. Key morphological characteristics of all valid Metasphaerolaimus species, and their bathymetric distribution. A: dorso-caudal apophyses

present; Co = conical; CoC = conico-cylindrical; G = gurbernaculum present; nd = no data; - = absent. Depth range data based on type locality and

additional distribution data summarised by Jensen (1992).

European Journal of Taxonomy 158: 1-19 (2015)

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LEDUC D., Kermadec Trench Monhysterida

usually 1.5-2.0 or more head diameters from anterior end. Posterior part of pharynx enlarged forming a

single or double bulb without valves. Secretory-excretory system usually absent. Female reproductive

system short to medium sized; vulva near mid-body. Spicules mostly arcuate and short (< 2 abd). Tail

usually with ventrally curved conical anterior portion and dorsally curved cylindrical filiform posterior

portion. Spinneret a long and slender cone or cylinder.

Remarks

The taxonomy of all marine Monhystrella species was revised by Fonseca & Decraemer (2008), who

provided a detailed diagnosis of the genus and a key to all thirteen valid marine species; no new marine

species have been described since.

Monhystrella kermadecensis sp. nov.

um:lsid:zoobank.org:act:AA638C6A-E7A8-4636-A03D-8CF851B70498

Fig. 6, Table 1

Diagnosis

Monhystrella kermadecensis sp. nov. is characterised by circle of papillose outer labial sensillae slightly

anterior to circle of four short cephalic setae, amphideal fovea 1.9 cbd from anterior extremity and 0.33-

0.43 cbd (females) or 0.43-0.50 cbd (males), gubernaculum with caudal apophyses, presence of distinct

cuticularised piece along anterior vaginal wall, relatively short conical (males) or conico-cylindrical tail

(females) with conical, ventrally-curved spinneret.

Etymology

This species is named after the type locality.

Material examined

Holotype

KERMADEC TRENCH: $ (NIWA 99767), collected 6 May 2014 (WHOI cruise TN309, Nereus dive

N074).

Paratypes

KERMADEC TRENCH: 2 $$ (NIWA 99768) and 5 ? ? (NIWA 99769) same data as holotype.

Type habitat

KERMADEC TRENCH: water depth: 8081 m (178.17571° W, 34.34030° S), sediment depth: 0-1 cm.

Description

Male

Body cylindrical, tapering slightly towards both extremities. Cuticle smooth. Anterior end rounded or

slightly truncated. Six minute inner labial papillae, difficult to observe; circle of six small outer labial

papillae slightly anterior to circle of four short cephalic setae, 0.23 cbd long. Amphideal fovea medium¬

sized, circular, situated 1.9 cbd from anterior end, outline not cuticularised. Two short setae situated

in sublateral row posterior to amphideal fovea. Buccal cavity small, funnel-shaped, surrounded by

pharyngeal tissue. Pharynx gradually widening posteriorly, apparently forming a weak posterior bulb in

some cases. Nerve ring situated slightly posterior to middle of pharynx. Secretory-excretory system not

observed. Cardia small, partly surrounded by intestinal tissue. Progaster not observed. Distinct layer of

glycocalyx present in intestinal lumen.

15

European Journal of Taxonomy 158: 1-19 (2015)

Fig. 6. Monhystrella kermadecensis sp. nov. A. Male anterior body region. B. Male head region.

C. Female anterior body region. D. Male posterior body region. E. Female reproductive system. F. Entire

§. G. Entire S, holotype. H. Male copulatory apparatus. I. Female posterior body region. Arrow shows

position of cuticularised piece anterior to vulva. Scale bar: A, C-E, I = 25 pm; B = 7 pm; F-G = 70 pm;

H = 12 pm.

16

LEDUC D., Kermadec Trench Monhysterida

Reproductive system monorchic with anterior outstretched testis situated to left or right of intestine.

Mature sperm nucleated, globular, 1.3-2.0 x 1.5-2.4 pm. Spicules paired, arcuate, without capitulum,

widest at proximal end and with pointed distal tip. Gubernaculum with caudal apophyses and wide

lateral crurae. Pre-cloacal supplements or setae not observed. Tail conical, with two pairs of sub-ventral

setae and one pair of sub-dorsal setae. Well-developed, ventrally curved, conical spinneret; caudal

glands not observed.

Female

Similar to males but body widest immediately anterior to vulva, then markedly smaller immediately

posterior to vulva; body width decreases gradually towards both extremities. Amphideal fovea slightly

smaller than in males (0.33-0.43 vs 0.43-0.50 cbd), only one seta present posterior to amphideal fovea;

tail longer than in males (7.8-11.0 vs 6.2-6.7 abd), conico-cylindrical, without setae. Reproductive

system monodelphic with outstretched anterior branch to the left (two specimens) or right of intestine

(three specimens). Mature egg dimensions up to 12 x 39 pm. Vulva located at almost two thirds of

body length from anterior. Vagina transverse or oblique, with distinct cuticularised piece situated along

anterior wall (Fig. 6D). No vaginal glands visible. Post-vulvar sac not observed.

Remarks

Monhystrella kermadecensis sp. nov. differs from most other Monhystrella species (except M. marina

T imm , 1964) by the relatively short tail lacking a cylindrical filiform posterior portion. This trait may

suggest affinities with Thalassomonhystera Jacobs, 1987, but species of this genus always possess a

well-developed secretory-excretory system and lack a posterior pharyngeal bulb (Fonseca & Decraemer

2008). M. kermadecensis sp. nov. is also unusual in the position of the vulva at almost two thirds of

body length from the anterior extremity instead of near mid-body, a trait which suggests affinities with

Halomonhystera Andrassy, 2006. The vulva in M. kermadecensis sp. nov. is, however, situated more

anteriorly than in Halomonhystera (62-64 vs 76-92% of body length from anterior extremity); the

new species also differs from Halomonhystera in the absence of secretory-excretory system and shape

of the buccal cavity (Fonseca & Decraemer 2008). Elucidating the taxonomic relationships between

M. kermadecensis sp. nov. and other Monhysteridae will require detailed molecular analyses which are

not possible at present based on available specimens.

The holophyly of the Monhysteridae was established based on the anterior gonad always positioned to

the right of the intestine (Forenzen 1981). Monhystrella kermadecensis sp. nov. can be differentiated

from all other species of the genus, and indeed the entire family, based on the variable position of the

anterior gonad relative to the intestine (to the right or left of intestine in both sexes). The holophyly of

the closely related family Xyalidae was established based on the anterior gonad always positioned to the

left of the intestine and the posterior gonad (when present) always to the right of it (Forenzen 1981). The

new species therefore shows affinities with both the Mohysteridae and Xyalidae; within the Xyalidae

it resembles the genus Theristus Bastian, 1965 most due to the conical shape of the tail and lack of

terminal setae. The following traits, however, suggest closer affinities with the Monhysteridae: small

body size (< 500 pm in length), smooth cuticle (always striated in Xyalidae; Forenzen 1981), presence

of only four setae in second circle (always ten setae in Theristus ), and presence of only one testis (two

testes often present in Xyalidae).

Discussion

Monhystrella kermadecensis sp. nov. was among the most common species at the 8081 m site in

Kermadec Trench and represented 8% of total nematode abundance (D. Feduc unpublished data). This

species, however, was not present at the 9177 m site. Thelonema clarki sp. nov. and Metasphaerolaimus

constrictus sp. nov. were relatively common at the 9177 m site, representing 3 and 5%, respectively, of

17

European Journal of Taxonomy 158: 1-19 (2015)

total nematode abundance. These two species were also present at the 8081 m site but in lower numbers

(< 1% of total nematode abundance). The present study increases the number of nematode species known

from hadal trench enviro nm ents from fifteen to eighteen. Many more species are yet to be discovered

and described, and future sampling in trench environments should ensure that preservation and fixation

methods allow for both morphological and molecular analyses to be conducted (Yoder et al. 2006).

Acknowledgments

Funding was provided by NIWA’s Coasts and Oceans Centre Research Programme 2 (2013/14 SCI)

and the programme "Impact of resource use on vulnerable deep-sea communities” (CO 1X0906). I am

grateful to Tim Shank, principal investigator of the HADES project (HADal Ecosystem Studies, funded

by the National Science Foundation (NSF-OCE1130712, 1130494 and 1131620), and to expedition

leader Casey Machado, the officers, crew and scientific staff of RV Thomas G. Thompson (voyage

TN309), and ROVNereus engineers and technicians. I thank three anonymous reviewers for providing

constructive criticisms on the manuscript.

References

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Fadeeva N.P 1983. A contribution to the family Sphaerolaimidae Filipjev, 1918 (Nematoda,

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logy Gastrotricha, Cyclioneura and Gnathifera Volume 2, Nematoda: 435-465. De Gruyter, Hamburg.

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there? Marine Biodiversity 40: 143-159. http://dx.doi.org/10.1007/s 12526-010-0041 -4

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Somerfield P.J. & Warwick R.M. 1996. Meiofauna in Marine Pollution Monitoring Programmes: a

Laboratory Manual. Ministry of Agriculture, Fisheries and Food, Fowestoft.

Tietjen J.H. 1989. Ecology of deep-sea nematodes from the Puerto Rico Trench area and hatteras Abyssal

Plain. Deep-Sea Research 36: 1579-1594. http://dx.doi.org/10.1016/0198-0149(89)90059-9

Vanhove S., Vermeeren H. & Vanreusel A. 2004. Meiofauna towards the South Sandwich Trench (750-

6300 m), focus on nematodes. Deep-Sea Research II 51: 1665-1687.

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Manuscript received. 9 September 2015

Manuscript accepted. 14 October 2015

Published on: 3 December 2015

Topic editor: Rudy Jocque

Desk editor: Kristiaan Hoedemakers

Printed versions of all papers are also deposited in the libraries of the institutes that are members of the

EJT consortium: Museum national d’Histoire naturelle, Paris, France; Botanic Garden Meise, Belgium;

Royal Museum for Central Africa, Tervuren, Belgium; Natural History Museum, Fondon, United

Kingdom; Royal Belgian Institute of Natural Sciences, Brussels, Belgium; Natural History Museum of

Denmark, Copenhagen, De nm ark.

19