Halalaimus shinkai Shimada, 2020

Shimada, Daisuke, Takeda, Naoya, Tsune, Akira & Murakami, Chisato, 2020, Three new species of free-living marine nematodes (Nematoda: Enoplida) from the Clarion-Clipperton Fracture Zone (CCFZ), North Pacific, Zootaxa 4859 (4), pp. 507-526 : 513-514

Halalaimus shinkai Shimada , sp. nov.

Figures 3–4 , Table 4 .

Type material. Holotype. Adult male (NSMT-As 4618), formalin-fixed, permanent whole mount.

Type locality. CCFZ (10° 25.9376′ N, 147° 50.0029′ W, 5321 m depth) with a spade corer by the third author on 1 Oct. 2016 .

Diagnosis. Halalaimus shinkai sp. nov. is characterized by the presence of the somatic and ornamented caudal alae, the absence of the inner labial sensilla, the papilliform outer labial and cephalic sensilla, short (10 cloacal body diameters) tail with a longer conical portion than cylindrical portion, the tail tip without bifurcation, the longer (2.0 cloacal body diameters) spicules, the gubernacula with lateral pieces at both sides, and the absence of the precloacal sensillum and pore.

Etymology. The specific name shinkai is a noun in apposition, derived from the Japanese word shinkai (deepsea).

Description. Male. Body almost cylindrical, tapering in cervical and caudal regions. Cuticle 5–6 μm thick and finely striated throughout whole body, except cylindrical portion of tail. Somatic alae present in both lateral sides, from just anterior to end of pharynx to 7.0 cloacal body diameters anterior to cloaca, not ornamented and looking like longitudinal grooves. Ornamented (with scale-like structures) caudal alae also present, from just anterior to cloaca to end of conical portion of tail: ornaments transversely elongated, posteriorly unclear. Somatic setae sparse, 1–2 μm long, arranged in eight longitudinal rows. Head rounded, not set off from cervical region, diameter at cephalic sensilla bases equal to 0.14 maximum body diameters. Inner labial sensilla indistinct. Six outer labial and four cephalic sensilla papilliform, arranged in two well-separated circles: outer labial sensilla located at 0.45 cephalic diameters from anterior end; cephalic sensilla at 1.0 cephalic diameter from anterior end. Amphids longitudinally elongated: anterior end at 1.7 cephalic diameters from anterior body end; posterior end at 9.1 cephalic diameters from anterior body end; 0.18 corresponding body diameters wide at anterior end; 0.12 corresponding body diameters wide at the middle; and 0.09 corresponding body diameters wide at posterior end. Buccal cavity minute. Pharynx enlarged at posterior end. Nerve ring and secretory-excretory system not observed. Cardia short, 1/5 of corresponding body diameters long. Tail conico-cylindrical, 10.1 cloacal body diameters long: anterior conical portion 65% of tail length; posterior cylindrical portion filiform, ca. 5% of cloacal body diameter wide; tip very slightly expanded, with spinneret and no terminal setae. Caudal glands located postcloacally. Caudal sensilla papilliform or very short setiform, difficult to observe. Reproductive system diorchic. Testes opposed and outstretched: anterior testis located on right side of intestine, situated from 45% to 52% of body length; posterior testis located on left side of intestine, situated from 56% to 67% of body length. Sperms globular or spheroid in shape, 5–10 μm in diameter. Vas deferens conspicuous. Spicules equal, arcuate with ventral velum, 1.8–1.9 cloacal body diameters or 0.2 tail lengths. Gubernacula paired, consisting of dorsal and lateral pieces: dorsal piece parallel to spicules, 0.7 cloacal body diameters or 0.4 spicule lengths, with a large ventral apophysis between both spicules; lateral piece as long as dorsal piece, constricted at both ends. Precloacal sensillum or pore absent.

Female. Not found.

Remarks. A male was identified in the genus Halalaimus based on the longitudinally elongated amphids. Keppner (1992) reviewed Halalaimus with descriptions of 11 new species divided into four groups using male characteristics, viz. the presence or absence of caudal alae and the presence or absence of precloacal supplements (sensillum and/or pore). Since the publication of Keppner’s work (1992), 12 new species have been described: H. aciculus Gagarin & Nguyen, 2014 , H. dimorphus Turpeenniemi, 1997 , H. minimus Gagarin, 2016 , H. orientalis Gagarin, 2016 , and H. vietnamicus Gagarin, 2016 , which are in group 1; H. durus Gagarin & Nguyen, 2004 , H. gidanensis Nasira & Turpeenniemi, 2002 , and H. minor Gagarin & Nguyen, 2004 which are in group 2; H. dolgovi Alexeev & Linnik, 1994 which is in group 3; H. longipharynx Gagarin & Nguyen, 2018 and H. parvulus Gagarin & Nguyen, 2018 which are in group 4; and H. deseadensis Pastor de Ward, 1998 which is only known from female specimens. In addition, H. longistriatus Timm, 1961 was categorized in group 4 by Keppner (1992), but should be removed, as a male of this species has not been described. Currently, Halalaimus consists of 87 valid species: each of the four groups contain 15, 12, 13, and 31 species, respectively; the other 16 species were established based only on females.

Halalaimus shinkai sp. nov. belongs to group 2, members of which have the caudal alae and lack the precloacal sensillum or pore. Halalaimus shinkai sp. nov. can be distinguished from all of the congeners in group 2 by the presence of the lateral pieces of gubernacula (vs. absence in the congeners). In addition, it differs from H. brimi Keppner, 1992 due to the tail tip which does not have bifurcation (vs. with bifurcation in H. brimi ); from H. alatus Timm, 1952 , H. durus , H. gidanensis , and H. sarsi Gerlach, 1967 by the presence of the somatic alae (vs. absence in the latter); from H. filum Gerlach, 1962 , H. lineatoides Timm, 1961 , H. lineatus Timm, 1961 , and H. relatus Gerlach, 1967 by the ornamented caudal alae (vs. unornamented in the latter); from H. gerlachi Keppner, 1992 by the shorter tail (10 cloacal body diameters long in H. shinkai sp. nov. vs. longer than 20 cloacal body diameters in H. gerlachi ); from H. gracilis de Man, 1888 by the papilliform outer labial and cephalic sensilla (vs. setiform in the latter); and from H. minor by the larger body size (ca. 4 mm in H. shinkai sp. nov. vs. ca. 0.5 mm in H. minor ).

Halalaimus shinkai sp. nov. resembles H. americanus Keppner, 1992 , H. dolgovi , and H. paracomatus Keppner, 1992 in the presence of the lateral pieces of gubernacula. However, it can be distinguished from these species since it belongs to group 2 (vs. belonging to group 1, 1, and 3, respectively). It also differs in the papilliform outer labial and cephalic sensilla (vs. setiform in the latter three species). Halalaimus shinkai sp. nov. also resembles H. leptoderma Platonova, 1971 and H. pachyderma Filipjev, 1927 in the papilliform outer labial and cephalic sensilla, but differs from them by belonging group 2 (vs. both belonging to group 4). It also differs due to larger gubernacula with lateral pieces (vs. small, without lateral pieces).

The taxonomic key for the species of Halalaimus group 2 amended from Keppner (1992) is as follows:

1 Caudal alae unornamented.............................................................................. 2

- Caudal alae ornamented................................................................................ 8

2 Gubernaculum with dorso-caudally directed apophysis........................................................ 3

- Gubernaculum without dorso-caudally directed apophysis..................................................... 5

3 Somatic alae present.............................................................................. H. filum

- Somatic alae absent................................................................................... 4

4 Outer labial setae shorter than cephalic diameter....................................................... H. alatus

- Outer labial setae twice as long as cephalic diameter.................................................... H. sarsi

5 Somatic alae absent......................................................................... H. gidanensis

- Somatic alae present................................................................................... 6

6 Tail length ca. 30 cloacal body diameters........................................................... H. relatus

- Tail length ca. 10–15 cloacal body diameters............................................................... 7

7 Outer labial sensilla setiform, equal to cephalic diameter.............................................. H. lineatus

- Outer labial sensilla papilliform................................................................ H. lineatoides

8 Tail tip bifurcated............................................................................... H. brimi

- Tail tip not bifurcated.................................................................................. 9

9 Gubernaculum with dorso-caudally directed apophysis....................................................... 10

- Gubernaculum without dorso-caudally directed apophysis.................................................... 11

10 Tail longer than 20 cloacal body diameters......................................................... H. gerlachi

- Tail length ca. 10 cloacal body diameters............................................................ H. minor

11 Gubernaculum with lateral piece........................................................... H. shinkai sp. nov.

- Gubernaculum without lateral piece...................................................................... 12

12 Inner labial sensilla setiform..................................................................... H. gracilis

- Inner labial sensilla papilliform.................................................................... H. durus

Figures

FIGURE 3. Halalaimus shinkai sp. nov. Holotype male: A, body; B, body with somatic and caudal alae; C, head; D, cervical region; E, tail; F, right spicule and gubernaculum. Scale bars: A, B, 1 mm; C, F, 10 μm; D, E, 100 μm. Abbreviations: a.t., ante- rior testis; c.s., cephalic sensillum; o.l.s., outer labial sensillum; p.t., posterior testis; v.d., vas deferens; vel., velum.

FIGURE 4. Halalaimus shinkai sp. nov. Holotype male:A, head, inner view; B, head, outer view with cephalic sensilla; C, amphid; D, somatic ala at midbody; E, anterior part of caudal ala; F, posterior part of caudal ala; G, right spicule and gubernaculum; H–J, right gubernaculum; K, sperms in testis. Scale bars:A–K, 10 μm.Abbreviations: c.s., cephalic sensillum; d.e., distal end of gubernaculum; d.p., dorsal piece; l.p., lateral piece; v.a., ventral apophysis.