Graphonema antarcticum Shimada & Tsujimoto & Watanabe 2019, sp. nov.

Graphonema antarcticum sp. nov. (Figs 1–4; Table 1)

Material examined. Holotype: adult male (ICHUM 5867), whole mount, 68°59′55″S, 39°35′28″E, Kita-noura, off Syowa Station in Lützow-Holm Bay, Antarctica, surface of macroalgae collected by means of bait traps at 27 m depth, 16 December 2005. Paratypes: six adult males (ICHUM 5868–5872, 5876) and five adult females (ICHUM 5873–5875, 5877, 5878), whole mounts, same collection data as with holotype.

Non-type: an adult male (ICHUM 5879), Au-coated SEM specimen, same collection data as with holotype.

Diagnosis. Graphonema antarcticum sp. nov. is characterized by large body size (equal to or more than 2.0 mm), truncated cephalic end, presence of lateral differentiation, long spicules (80–90 µm and 1.3–1.6 abd), without capitulum, well-developed gubernaculum with L-shaped lateral pieces bending at an obtuse angle with minute denticles at distal end, and long tail in both sexes (approximately 4–6 abd in males and 6–9 abd in females).

Measurements. See Table 1.

Description. Males. Body (Fig. 1A) almost cylindrical, tapering toward both ends. Epicuticle coarsely annulated (Fig. 2), except at anterior and posterior body ends (Figs. 1B, 2, 4A, G). Exocuticle with heterogeneous ornamentations (cf. Gourbault and Vincx 1994): tiny punctate at anterior half of cephalic region (Figs 1B, 4A); regularly hexagonal to posterior half of cephalic region to anterior 1/3 of pharynx (Figs 1B, C, 4A, B); longitudinally elongated hexagonal from anterior 1/3 of pharynx to cloacal region (Figs 1D, E, 4C, D), becoming longer toward posterior end; regularly hexagonal in caudal region (Figs 1F, 4F), becoming smaller toward posterior end; no ornamentation at tail end (Figs 3A, 4G). Lateral differentiation (Figs 1 C–E, 4B, C) present, beginning from anterior 1/3 of pharyngeal region to cloacal region, becoming wider toward posterior end. Cephalic region (Figs 1B, G, H, 2A, B) not set-off, truncated at anterior end. Cephalic diameter 25–30% of mbd. Inner labial sensilla inconspicuous. Six outer labial setae (2–3 µm) and four cephalic setae (4–6 µm) in single circle observed in the holotype and two paratype males (ICHUM 5869 and 5871), but inconspicuous in the other individuals including SEM specimen (Fig. 2A, B). Amphids not observed. Buccal cavity (Fig. 1G) divided into two sections: cheilo-stome—cup-shaped, with a circle of twelve tooth-like rugae; and esophastome—conical in shape, with a large dorsal onchium, two smaller ventrosublateral onchia, and four minute ventrosublateral teeth (Fig. 1G, H). Denticles absent. Pharynx (Fig. 1A, G) anteriorly surrounding esophastome, without posterior terminal bulb. Excretory pore and nerve ring indistinct. Ventral gland cell (Fig. 1A) located posterior to base of pharynx, posterior edge of gland at 1.5 times pharyngeal length from anterior body end. Testis (Fig. 1A) single, outstretched, beginning at anterior 20–25% of body length, located on right-hand side of intestine, posterior junction of vas deferens inconspicuous (complete testis observed only in the holotype and paratype ICHUM 5869, probably because of damage caused by freezing). Spicules (Fig. 3B, C) equal, arcuate, without capitulum, gradually tapering distally and 1.3–1.6 abd or 30–40% of tail length. Gubernaculum (Fig. 3C) well-developed: dorsal piece thin, almost straight and parallel to spicules; lateral pieces paired, L-shaped and bending at an obtuse angle, with one or two minute denticles at distal end; whole length (from proximal end of dorsal piece to distal end of lateral piece) 50–55% of spicule length. Precloacal supplement or cuticular elevation absent. A longitudinal row of cuticular wrinkles (Figs 3B, 4E) present at both subventral sides of precloacal region in all males, however it possibly artefact of preservation in ethanol and/or freezing. One or two short ventral setae just anterior to cloaca either present or absent. Tail (Fig. 3B) conico-cylindrical, 3.9–6.0 abd long, without ventral cuticular elevation. Three caudal glands just anterior to cloaca. Distinct spinneret at tip of tail.

Females. Body (Fig. 3D) similar to males but thicker (mbd approximately 100–120 µm). Cephalic diameter 20– 25% of mbd. In one specimen (ICHUM 5877), ventral gland cell reaches 1.9 times pharyngeal length from anterior body end. Female reproductive system didelphic and amphidelphic. Ovaries opposed and reflexed: anterior ovary beginning at 15–30% of body length, located on right-hand side of intestine; posterior ovary ending at 70% of body length, located on leπ-hand side of intestine. Eggs oval, 3–18 in uteri, 30–60 µm in diameter. Vulva situated slightly anterior to middle of body. Tail 6.1–8.4 abd long, longer and thinner than in males. Three caudal glands situated in anal region, one or two of them located preanally, and the others located postanally.

Etymology. The specific name antarcticum (Antarctic) is a Latin adjective taken from the type locality.

Remarks. Graphonema antarcticum sp. nov. is most similar to G. metuliferum described from Japan in respect of the longer spicules (approximately 1.5 abd), the shape of the gubernaculum (whole length approximately 1/2 of spicule length, and with L-shaped lateral pieces bending at an obtuse angle and equipped with minute denticles at the distal end), and tail length in both sexes (approximately 4–6 abd in males and 6–9 abd in females). However, G. antarcticum sp. nov. differs from G. metuliferum by larger body size (L=2.0– 2.5 mm in males, 2.2–2.7 mm in females of G. antarcticum sp. nov. vs. 1.0– 1.4 mm in males, 1.2–1.4 mm in females of G. metuliferum) and the presence of the lateral differentiation (absent in G. metuliferum) (Fig. 4 H–J). The presence or absence of lateral differentiation is a good diagnostic character used to distinguish species or genera in Chromadoridae (cf. Tchesunov 2014). The genus Graphonema contains five species reported to have lateral differentiation, viz., G. arcticum, G. northumbriae G. parafricanum, G. scampae, and G. antarcticum sp. nov., and only one species, G. metuliferum, reported to have no lateral differentiation (Filipjev 1946; Gerlach 1958; Coles 1965; Warwick and Coles 1975; Kito 1981). It is unknown that lateral differentiation is present or absent in the other four known species. In addition, following minor differences are found from the original description by Kito (1981) and our observation of the type series of G. metuliferum, but these may not be enough for the diagnostic characters: amphideal fovea (indistinct in G. antarcticum sp. nov. vs. distinct in G. metuliferum), ventral gland cell (rounded in G. antarcticum sp. nov. vs. elongated in G. metuliferum), and longitudinal rows of cuticular wrinkles in precloacal region (present in G. antarcticum sp. nov. vs. absent in G. metuliferum).The distinctions between the new species and all known congeners are shown in the following key.