Spinonema gracilispiculum, Jacob & Rayaroth & Useph & Philip & Damodaran, 2020

Jacob, Jini, Rayaroth, Anilkumar Padinjare, Useph, Abdul Jaleel Koovapurath, Philip, Rosamma & Damodaran, Rayaroth, 2020, A novel species Spinonema gracilispiculum sp. n. (Nematoda: Desmodoridae) from the oxygen minimum zone of Eastern Arabian Sea margin, Zootaxa 4869 (4), pp. 587-598 : 589-592

Spinonema gracilispiculum sp.n. Jacob

( Table 1 ; Figures. 1–4 ; Plate 2 )

Materials examined: Holotype male and four paratypes, three males and one female, slide No. IO /SS/NEM/00024; deposited at FORV Referral Centre , Centre for Marine Living Resources and Ecology, Cochin, Kerala, India.

Type locality: Trivandrum, 08° 27’ 768” N, 76° 23’ 637” E, 216 m collected on 19.05.2007 ( FORVSS 254). The sediments consisted of 94% sandy sediments with 1.07% organic matter content, bottom temperature 15.45°C, bottom salinity 35.05, bottom dissolved oxygen concentration 0.35 ml/l.

Etymology: The species is named for the slender spicules.

Description: Holotype and paratype males ( Figure 1–3 ; Plate 2 ), Short (1000˗1100 μm) cylindrical body, widest at level of pharyngeal bulb, tapering towards both extremities. Cuticle annulated posterior to head capsule. Coarse body annuli ornamented with a row of vacuoles; interannual spaces distinct ( Figure 1D & E ; Plate 2E ). Ornamentation of body annuli (vacuoles) along anterior third part of body large and distinct but small and less conspicuous further on ( Plate 2E ). Larger annuli and interannual space in pharyngeal region, 10 annuli in pharyngeal region measure 33.5 µm wheareas 10 annuli further onwards measure 14 µm. The 29th cuticular annule in pharyngeal region bears a single, thornlike 14 μm long cuticularised spine ( Figure 1D & 2A ) on its dorsal side. Sexual dimor-phism in presence of thorn-like spine and the direction of winding in amphids. Males bears dorsal spine and dorsally wound amphid whereas in females dorsal spine absent and amphids ventrally wound. Wide lateral alae (17˗18 μm) present in mid body, originates 200 µm posterior ( Figure.1F ) to base of pharyngeal bulb, appears as two raised longitudinal rows of cuticular protrusion with a flat middle groove (‘valley’) in between ( Figure 1A & G ; Plate 2G & H ) resembling the letter ‘C’. Body annuli do not interdigitate at level of lateral alae.

Blunt, rounded head. Cephalic capsule smooth in general but ornamentation with small vacuoles are found at posterior edge of main part of head capsule. Head capsule of some specimens wider (21 µm) than long (19 µm) with very fine groove (sutura) in cuticle, separating lip region from main head region ( Figure 1B ; Plate 2A ). Main head region wider than lip region. Lips often folded inwards, sometimes extended. Six outer labial papillae. Four thick, stout cephalic setae located at anterior edge of main part of head capsule (just posterior to sutura between lip region and main part), and at mid-level of fovea amphidialis. Amphids anteriorly placed, anterior edge of amphid touches anterior edge of cephalic capsule ( Figure 1B ). Cryptospiral fovea amphidialis and apertura amphidialis. Buccal cavity with single dorsal and two ventrolateral teeths and without denticles. Muscular pharynx, pyriform with bipartite terminal end bulb. Cuticle of the lumen thickened ( Figure 2A ).

Reproductive system monorchic with one anterior outstretched testis located on right or left of intestine. Mature sperm cells globular ( Figure 1C ). Spicules narrow, long and arcuate (75˗80 µm) with short ventral hook-like projection in rounded capitulum; Barely visible velum, originates from base of rounded capitulum. Gubernaculum simple, short and parallel to spicule. There are 6 setiform precloacal papillae. Conical tail with slender spinneret. Non-annulated tail tip vacuolated ( Figure 2B ; Plate 2I ); it makes out 50% of total tail length.

Paratype female ( Figure. 4 ): Similar to males except for a ventrally wound amphid and lack of dorsal spine. Reproductive system didelphic, amphidelphic with short reflexed ovaries. Small vulva located posteriorly along the body (V =75%), Vagina vera cuticular and vagina uterina surrounded by constrictor muscle. Some crystalloid structures are observed in ovary ( Plate 2J ).

Juvenile: Juveniles are similar to adults in all respects except for the presence of slender long spines along the mid body. Cuticular spines are also observed along the lateral alae of the genus Desmodorella Cobb, 1933 . In Desmodorella , the spines are short, conspicuous and are arranged in two rows. In the present species, they look more like somatic setae appearing only in the mid body region of juveniles and are lost in mature adults. The spines are present throughout the life history of genus Desmodorella . Similar kind of observation was made in the species Spinonema spirale Larrazábal-Filho et al., 2019 while this characteristic is not observed in the other species of Spinonema . Genital primordium consists of a string of cells.

Diagnosis: Cuticle annulated with distinct inter annual spaces and vacuoles. Sexual dimorphism in the presence of a strongly cuticularised single, thick, dorsal thornlike spine on the 29th cuticular annuli in the pharyngeal region of males. Wide lateral alae ( Figure 1E ; Plate 2 ) on mid body, starting 200 μm posterior to pharyngeal bulb and dis-appearing before reaching the level of cloaca. Posterior edge of cephalic capsule ornamented with small vacuoles, main part of capsule is smooth. Six smaller outer labial papillae in lip region and four cephalic setae at mid-level with unispiral amphid. Buccal cavity with dorsal and a pair of ventrosublateral teeth, no denticles. Muscular pharynx, bipartite. Thickened lumen cuticle. Male reproductive system monorchic with one anterior outstretched testis. Spicules thin, about 3 times as long as corresponding body diameter, arcuate with beak-shaped capitulum and a velum. Gubernaculum simple without apophysis. Short conical tail ornamented with vacuoles. Spinneret short.

Figures

PLATE 2. Spinonema gracilispiculum sp. n. (A) Head region showing amphid and cephalic setae, (B) & (C) Pharyngeal thorn, (D) Pharyngeal thorn dorsal view, (E) Cuticular ornamentation, (F) Somatic setae, (G) & (H) Lateral alae, (I) Caudal region of male showing spicule, (J) Vulval region of female. (Scale bar 20 µm).

FIGURE. 1 Spinonema gracilispiculum sp. n. Holotype male (A) Entire male, (B) Head region, (C) Spicular apparatus, (D) Pharyngeal horn, (E) Lateral alae, (F) Beginning of lateral alae, (G) Ending of lateral alae. (Scale bar 20 µm).

FIGURE. 2 Spinonema gracilispiculum sp. n. Holotype male (A) Pharyngeal region, (B) Caudal region showing spicule. (Scale bar 20 µm).

FIGURE. 3 Spinonema gracilispiculum sp. n. Paratype entire male. (Scale bar 20 µm).

FIGURE. 4 Spinonema gracilispiculum sp. n. Paratype entire female. (Scale bar 50 µm).