Moebjergarctus clarionclippertonensis, Bai & Wang & Zhou & Lin & Meng & Fontoura, 2020

Bai, Lifen, Wang, Xiaogu, Zhou, Yadong, Lin, Shiquan, Meng, Fanxu & Fontoura, Paulo, 2020, Moebjergarctus clarionclippertonensis, a new abyssal tardigrade (Arthrotardigrada, Halechiniscidae, Euclavarctinae) from the Clarion-Clipperton Fracture Zone, North-East Pacific, Zootaxa 4755 (3), pp. 561-575 : 564-572

Moebjergarctus clarionclippertonensis sp. nov.

( Figs 2–6 , Table 2 )

Diagnosis. Moebjergarctus with eleven cephalic sense organs present (paired internal cirri, paired external cirri, unpaired median cirrus, paired primary clavae, paired secondary clavae and paired cirri A). Primary clavae clubshaped, bent anteriorly. Secondary clavae spherical.Annulated spines on the first three pairs of legs I, II, III, divided on legs I, undivided on legs II and III. Small claviform papillae on legs IV. Long cirri E, each with cirrophore, annulated scapus and flagellum. Wrinkled digits terminated in simple crescent-shaped claws retractable into membranous sheaths. Internal digits longer than external digits. Cephalic cirri separated into three parts: short cirrophore, long annulated scapus and short flagellum. Scapi of cephalic cirri annulated only in the proximal portion followed by a smooth distal portion. Caudodorsally, a bulge covered by a crescent-shaped cuticular thickening is present. Female gonopore rosette-shaped. Male gonopore is circular with two slightly protruded internal folds, forming a longitudinal fissure. Adjacent to the male gonopore a posterior ovate thin and smooth cuticular platelet is present. Elongated seminal receptacles and long S-shaped seminal receptacle ducts opening laterally to the female gonopore.

Type locality. The Clarion-Clipperton Fracture Zone (9.60°N– 10.85°N, 154.16°W– 156.48°W), 5135–5281 m depth (North-East Pacific).

Material examined. Holotype, adult female (slide B 6418500008) collected at station KW1-S38, mounted in glycerine. Seventeen paratypes: One paratype of unknown gender (slide B 6418500004) collected at station KW1- S02; seven paratypes collected at station KW1-S05— three females (slides B 6418500019, SEM stubs B 6418500013 and B 6418500014), two males (slide B 6418500018 and SEM stub B 6418500016), a second instar juvenile (slide B 6418500017) and one specimen of unknown gender (slide B 6418500015); four paratypes collected at station KW1-S07— one female ( SEM stub B 6418500012), one male (slide B 6418500010) and two specimens of unknown gender ( SEM stub B 6418500009 and B 6418500011); three paratypes collected at station KW1-S38—one female (SEM stub B6418500005), one male (slide B6418500007) and a first instar juvenile (two-clawed larva, slide B6418500006); one paratype of unknown gender collected at KW1-S03 (slide B 6418500003); one paratype, male (SEM stub B6418500020) collected at station A5-S06. Unfortunately, slide B6418500004 and SEM stub B6418500011 were lost during the study. All specimens were collected from sediments outside of manganese nodules.

Type repository. The type material (slides and SEM stubs) is deposited in Lin’an Base Sample Bank of Key Laboratory of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources of the People’s Republic of China.

Etymology. The name clarionclippertonensis refers to the locus typicus of the new species, the Clarion-Clipperton Fracture Zone.

Description of the holotype. Adult female ( Figs 2A, B ) with an unarmoured ovoid body (288 µm long, 105 µm wide between legs III and IV). Cuticle finely punctated, exhibiting a caudodorsal bulge, protruding 7.6 µm above the body surface, and covered by a crescent-shaped cuticular thickening ( Figs 3 A–D). Conical head (30.9 µm wide between the bases of the primary clavae) with a long apical retractable buccal cone, bearing eleven cephalic sensory organs ( Figs 2B , 4A, B ). Primary clavae ( Figs 4A, B ) club-shaped, bent anteriorly (15.7 µm long and 4.9 µm wide). Secondary clavae spherical (diameter 5.9 µm) ( Figs 4B, E ). Tertiary clavae absent. All cephalic cirri ( Figs 4 B–F) with a short elongated cirrophore with annular enlarged distal margin, long scapus with the proximal portion annulated followed by a smooth distal portion terminated by a blunt tip and a very short tubular flagellum with an opened tip (the opened tip is visible only in paratypes observed under SEM, Fig. 4E ). Paired cirri A ( Fig. 4C ) 14.8 µm long (cirrophore 1.9 µm long, scapus 11.7 µm long comprising of the basal annulated portion (6.4 µm), apical smooth portion (5.3 µm), flagellum 1.2 µm long). Cirri A and primary clavae are clearly separated ( Figs 2A, B , 4A, B ). Paired external cirri, inserted ventrally to secondary clavae ( Figs 4B, E ), 12.3 µm long (cirrophore 2.9 µm long, scapus 7.6 µm long comprising of the basal annulated portion (4.8 µm), apical smooth portion (2.8 µm), flagellum 1.8 µm long). Paired dorsal internal cirri ( Fig. 4F ) 9.1 µm long (cirrophore 2.9 µm long, scapus 4.8 µm long comprising of the basal annulated portion (2.0 µm), apical smooth portion (2.8 µm), flagellum 1.4 µm long). The unpaired median cirrus ( Fig. 4D ) 12.6 µm long (cirrophore 1.4 µm long, scapus 10.2 µm long comprising of the basal annulated portion (5.8 µm), apical smooth portion (4.4 µm), flagellum 1.0 µm long).

Cirri E, each 62.6 µm long, are inserted on a short cirrophore and have long annulated scapus and long and thin flagellum ( Figs 2 A–C, 3A, D). Legs I–III with spine-shaped sensory organs. Leg I sensory organs (8.7 µm long) have an inconspicuous cirrophore and are divided into a scapus, with the proximal annulated portion and the distal smooth portion, and a short tubular tip ( Fig. 5A ). On legs II and III sensory organs are similar in size and shape (6.8 and 6.9 µm long, respectively). They are undivided, with annulated proximal portions and smooth distal portions ( Figs 5B, C ). Sensory organ on legs IV is a papilla (8.2 µm long) with a very short protruded tip ( Fig. 5D ). Each telescopic leg with coxa, femur, tibia and tarsus. Coxa and femur clearly punctated, tibia finely punctated and tarsus smooth. Each foot has four helically wrinkled digits. Digits are terminated by simple crescent-shaped claws ( Figs 5E, F ) retractable into membranous sheath. Digits are longer on legs IV than on legs I–III and, on each leg, internal digits are longer than external ones (30.9 and 24.0 µm on legs IV, 19.1 and 16.5 µm on legs I–III for internal and external digits, respectively).

Long and thin buccal tube (76.6 µm long and ca. 2 µm wide) ending with an almost spherical pharyngeal bulb (15.8 × 15.4 µm), containing three thin, rod-shaped placoids (about 7.5 µm). Stylets 74.3 µm long, with stylet sheaths (43.2 µm long). Stylet supports were not observed.

Female gonopore consists of a rosette (about 10.3 µm in diameter) with six cells ( Figs 6A, C, E ). Seminal receptacles are elongated vesicles, continued by long S-shaped seminal receptacle ducts ( Figs 6A, E ). Each seminal receptacle duct leaves the seminal receptacle posteriorly, then loops frontwards and after a second loop it turns backwards again to open laterally into the gonopore, after a small internal triangular receptacle duct pouch ( Figs 6A, E ). The anal opening is 12 µm distant from the gonopore.

Remarks. Males seem to have slightly longer cirri E than females, but this character needs to be confirmed with the examination of a larger number of specimens; regarding other morphological and morphometric characters, males are similar to females ( Table 2 ). The male gonopore is peculiarly formed, circular, with two slightly protruded internal folds, forming a longitudinal fissure. Adjacent to the gonopore a posterior oval thin and smooth cuticular platelet (9.3 µm wide in male B6418500018) is present. This platelet, sloping towards the gonopore, forming a funnel-like depression ( Figs 6B, D, F ).

The two-clawed first instar juvenile (= larva, B6418500006) was the smallest specimen examined with a body length of 101 µm (measurements in Table 2 ). With the exception of missing external claws, anus and gonopore, all other characteristics as in the adults. Specimen B6418500017, the second instar four-clawed juvenile (gonopore was not observed), although smaller, is also similar to the adults ( Table 2 ). The largest specimen examined was a 352 µm long female (B6418500014).

The number of annuli on scapi of cephalic cirri, described in Moebjergarctus manganis as an important trait ( Bussau 1992), is difficult to evaluate in the specimens examined for the purpose of this study because some annuli exhibit an asymmetric subdivision, a single on one side and a double on the other side (see Fig. 4F ). In the studied specimens and mostly based on SEM images, the number of annuli on scapi of the cephalic cirri was evaluated as follows: median cirrus: 4–6, internal cirri: 3–5, external cirri: 4–6, cirri A: 7–10. Concerning leg sensory organs, it was not possible to estimate the number of annuli due to the abovementioned reason, and also to the accumulation of debris at the bases of these structures, what prevented a confident observation in SEM (see Figs 5B, C ).

Figures

FIGURE 2. Moebjergarctus clarionclippertonensis sp. nov. A. Habitus (LM photo of the holotype, B6418500008, scale bar=50 µm); B. Schematic drawing of the holotype (scale bar=50 µm); C. Dorsal view (SEM photo of a paratype, B6418500005, scale bar=20 µm). Abbreviations: an—anus; bt—buccal tube; bu—caudodorsal bulge; ce—cirrus E; ec—external cirrus; go—gonopore; ic—internal cirrus; ca—cirrus A; mc—median cirrus; pb—pharyngeal bulb; pc—primary clava; p1–p4—sensory organs on legs I–IV; sc—secondary clava; sr—seminal receptacle; st—stylet.

FIGURE 3. Moebjergarctus clarionclippertonensis sp. nov.—the caudodorsal bulge. A. In lateral view (arrow, LM photo of a paratype, B6418500014, scale bar=50 µm); B. The cuticular thickening covering the caudodorsal bulge (arrow, DIC photo of the holotype B6418500008, scale bar=10 µm); C. The cuticular thickening covering the caudodorsal bulge (arrow, SEM photo of a paratype B6418500014, scale bar=10 µm); D. Caudodorsal bulge, showing the cuticular thickening (arrow, SEM photo of a paratype B6418500005, scale bar=10 µm).

FIGURE 4. Moebjergarctus clarionclippertonensis sp. nov.—the head and cephalic appendages. A. The head of a paratype (B6418500012), showing the separation of the cirrus A and primary clava; B. Ventral view of the head, showing the ventral location of external cirri (paratype B6418500016); C. Cirrus A (paratype B6418500005); D. Median cirrus (paratype B6418500013); E. External cirrus (paratype B6418500005); F. Internal cirrus (paratype B6418500013). SEM photos; scale bars A–B 6 µm; C–F 1 µm. Abbreviations: cr—cirrophore; ec—external cirrus; fl—flagellum; ic—internal cirrus; ca—cirrus A; mc—median cirrus; pc—primary clava; sa—annulated portion of the scapus; sc—secondary clava; ss—smooth portion of the scapus.

FIGURE 5. Moebjergarctus clarionclippertonensis sp. nov.—leg sensory organs, legs and digits. A. Sensory organ on leg I, showing the divided terminal portion (paratype B6418500013); B. Sensory organ on leg II (paratype B6418500013); C. Sensory organ on leg III (paratype B6418500013); D. Sensory organ on leg IV (paratype B6418500005); E. Leg II of a paratype (B6418500012); F. Digits and claws on leg I of a paratype (B6418500005). SEM photos, scale bars A–D, F—1 µm; E—10 µm. Abbreviations: ed—external digit; fl—terminal portion of the sensory organ; id—internal digit; p4—sensory organ on leg IV.

FIGURE 6. Moebjergarctus clarionclippertonensis sp. nov.—genital structures.A. Female gonopore, seminal receptacle, seminal receptacle duct opening and seminal receptacle pouch in the holotype (B6418500008, scale bar=10 µm); B. Male gonopore and adjacent posterior platelet in a paratype (B6418500018, DIC photo, scale bar=10 µm); C. Female gonopore and seminal receptacle duct opening in a paratype (B6418500012, SEM photo, scale bar=5 µm); D. Close-up of the male gonopore and adjacent posterior platelet in a paratype (B6418500020, SEM photo, scale bar=5 µm); E. Schematic drawing of the female genital structures (scale bar=10 µm); F. Schematic drawing of the male genital structures (scale bar=10 µm). Abbreviations: an—anus; go—gonopore; pl—platelet; sdo—seminal receptacle duct opening; sdp—seminal receptacle duct pouch; sdu—seminal receptacle duct; sr—seminal receptacle.