Halechiniscus janus, Bai & Wang & Gao & Li & Fontoura, 2022

Bai, Lifen, Wang, Xiaogu, Gao, Xiaohui, Li, Yujie & Fontoura, Paulo, 2022, First record of a deep-sea tardigrade from the South China Sea, Halechiniscus janus sp. nov. (Arthrotardigrada: Halechiniscidae), Zootaxa 5159 (3), pp. 425-439 : 428-433

Halechiniscus janus sp. nov.

( Figs 2–5 , Table 2 )

Diagnosis: Halechiniscus with a cylindrical body; cuticle finely punctuated; head with a complete set of cephalic cirri. Primary clava and cirrus A with a common cirrophore; cirrus A longer than the primary clava; semispherical secondary clava not fused with a flat, almost indistinct, tertiary clava. Sensory organs on legs I divided, undivided on legs II and III; papillate sensory organs on legs IV with short distal bipartite tips. All legs terminated with four wrinkled digits without peduncles or proximal pads; internal digits slightly longer than external digits; each digit terminates in a large simple crescent-shaped claw. Female gonopore rosette-like. Pair of elongated seminal receptacles and long S-shaped seminal receptacle ducts opening ventrally and latero-posteriorly in relation to the female gonopore. Male gonopore is a circular opening of a short tube.

Etymology: The specific epithet janus refers to Janus, the god of transitions in the Roman mythology, depicted as a man with two faces. The specific epithet alludes to the fact that some characters exhibited by the new species can be found in other halechiniscid species also, such as a semispherical secondary clavae and the morphology of seminal receptacles and ducts similar to those of the Euclavartinae, and cirri A and primary clavae sharing a common cirrophore as in the Halechiniscinae .

Material examined: Holotype, adult male (slide B6418400009) collected at station 2019MCA-MC04; allotype, adult female (slide B6418400028) collected at station 2019MA-MC02, both mounted in glycerine, and 25 paratypes (6 females, 4 males, 1 second-instar larva, 9 first-instar larvae and 5 specimens of undetermined gender and life stage). Eight paratypes were collected at station 2019MCA-MC01: two females (slide B6418400011 and B6418400017), five first instar larvae (one slide B6418400018 and four SEM stubs: B6418400012, B6418400013, B6418400014 and B6418400015) and one specimen of undetermined gender and life stage (SEM stub B6418400016). Eight paratypes were collected at station 2019MA-MC02: two females (slide B6418400027 and SEM stub B6418400030), one male (slide B6418400026), one second-instar larva (SEM stub B6418400031), three first-instar larvae (SEM stub B6418400034, slides B6418400032 and B6418400033) and one specimen of undetermined gender and life stage (SEM stub B6418400029). Three paratypes were collected at station 2019MA-MC03: two females (slides B6418400023 and B6418400025) and one male (slide B6418400024). One paratype, a specimen of undetermined gender and life stage (slide B6418400010), was collected at station 2019MCA-MC04. Four paratypes were collected at station 2019MA-MC05: two males (SEM stubs B6418400020 and B6418400022), one first-instar larva (SEM stub B6418400019) and one specimen of undetermined gender and life stage (SEM stub B6418400021). One paratype, a specimen of undetermined gender and life stage (slide B6418400035), was collected at station 2021MCA-MC01. Unfortunately, one specimen mounted on slide (B6418400023) and three specimens on SEM stubs (B6418400019, B6418400020 and B6418400034) were lost during the study.

Type locality: The South China Sea (Western Pacific Ocean, 21.1934°N, 118.0933°E), 1540 m bsl .

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

Description: Morphometric data are provided in Table 2 . The body is elongated and cylindrical. The conical head is separated from the trunk by a slight neck constriction ( Fig. 2 ). The dorsal cuticle, with dorsal metameric folds, is punctuated by fine epicuticular pillars. Ventrally the cuticle is almost smooth. The cuticular punctuation is present also in the coxal portions of legs. Eyes were not observed prior to mounting on slides.

A complete set of cephalic appendages is present (Fig. 3A), consisting of an unpaired median cirrus, and remaining paired organs: internal cirri, external cirri located latero-ventrally, cirri A, primary, secondary, and tertiary clavae. All cephalic cirri with cirrophores (very short in internal and external cirri). Each cephalic cirrus is divided into scapus and flagellum, which is subdivided into a tubular portion and thinner distal portion. Cirrus A and primary clava share a common cirrophore. Primary clavae much shorter than cirri A. Primary clava, inserted ventrally in relation to cirrus A, is sausage-shaped with van der Land’s body at its base. The semispherical secondary clava is located between the internal and external cirrus (Figs 3A–B). Flat tertiary clavae, almost indistinct and not fused with secondary clavae, can be hardly observed ventrally, between external cirri. Although their complete contours are difficult to recognize, tertiary clavae appear as a D-shaped structure (Figs 3A, C).

FFIGURE 3. Halechiniscus janus sp. nov. — head structures: A. The head of a paratype B6418400012 (SEM photo, scale bar=4 µm); B. Lateral view of the secondary clava of holotype B6418400009 (DIC photo, scale bar=10 µm); C. Ventral view of the tertiary clava (paratype B6418400035, DIC photo, scale bar=10 µm), insert shows the edge of tertiary clava (dashed line); D. pharyngeal bulb (paratype B6418400028, DIC photo, scale bar=10 µm). Abbreviations: bt—buccal tube; ca—cirrus A; ec—external cirrus; ic—internal cirrus; mc—median cirrus; pb—pharyngeal bulb; pc—primary clava; sc—secondary clava; st—stylet; tc—tertiary clava.

Bucco-pharyngeal apparatus (Fig. 3D) consists of the antero-ventral mouth opening surrounded by a thick mouth ring, buccal tube (18–25 µm long), paired stylets (21–38 µm long), three long simple placoids and a spherical muscle pharynx (diameter 14–19 µm). Stylet supports seem to be present but this character needs to be confirmed.

Cirri E are inserted on a short cirrophore and have an annulated scapus and long, thin flagellum ( Figs 2C , 4B ). Four pairs of legs. Each leg consists of coxa, femur, tibia and tarsus, which terminates with four wrinkled digits. Each digit terminated by a large (5–6 µm long), simple crescent-shaped claw, entirely retractable into a membranous sheath. Digits are longer on legs IV than on legs I–III and; on each leg, internal digits are slightly longer than the external ones. Digits without basal pads or peduncles ( Figs 4A, E–F ).

Sensory organs present on all legs. Sensory organs on legs I divided into scapus and flagellum. The flagellum is subdivided into a tubular portion and a distal, shorter and thinner portion ( Fig. 4A ). Sensory organs on legs II–III are undivided spines ( Fig. 2 ). On legs IV, each sensory organ consists of a proximal portion (cirrophore) followed by a short papilla with van der Land’s body at base and terminated by a short tip (1–2 µm long). A careful observation of the tip reveals that it is bipartite ( Figs 4C–D ).

The female gonopore has the typical six-lobed rosette-like appearance, its diameter is about 6 µm and it is positioned at a 10 µm distance from the anus ( Figs 5A–B ). The male gonopore (4–5 µm in diameter) is a circular opening of a short tube ( Figs 5C–D ), close to the anus (distance anus–gonopore about 5 µm). The anus is a sinuous slit between two longitudinally elongated lateral lobes ( Fig. 5D ). Seminal receptacles are elongated vesicles, continued by long S-shaped seminal receptacle ducts. Each seminal receptacle duct leaves the seminal receptacle posteriorly, then loops immediately frontwards and after a second loop it turns backwards again to open ventrally after a small internal receptacle duct pouch, located latero-posteriorly, but far away from the gonopore ( Figs 5A–B ).

Males are very similar to females in qualitative characters, except for their circular gonopore, which is also located closer to the anus. Two-clawed larvae (first-instar), with the exception of missing external digits, anus and gonopore, and the second-instar larva, with four digits but without visible gonopore, exhibit all other characteristics as in the adults. The sample size is too small and does not allow to draw any conclusions about morphometric differences between the sexes and life stages ( Table 2 ).

Figures

FIGURE 2. Halechiniscus janus sp. nov.—general morphology: A. Drawing based on the holotype B6418400009 (scale bar=20 µm); B. Dorsal view (SEM photo of a paratype, B6418400021, scale bar=10 µm); C. Lateral view (SEM photo of a paratype, B6418400012, scale bar=10 µm). Abbreviations: bt—buccal tube; ca—cirrus A; ce—cirrus E; ec—external cirrus; go—gonopore; ic—internal cirrus; mc—median cirrus; p1–p4—sensory organs on legs I–IV; pb—pharyngeal bulb; pc—primary clava; sc—secondary clava; st—stylet.

FIGURE 4. Halechiniscus janus sp. nov.— leg sensory organs, legs and digits: A. Sensory organ on leg I, showing the divided terminal portion (paratype B6418400012, SEM photo, scale bar=4 µm); B. cirrus E (paratype B6418400012, SEM photo, scale bar=3 µm); C. Sensory organ on leg IV (paratype B6418400021, SEM photo, scale bar=4 µm); D. Sensory organ on leg IV, showing van der Land’s body and bipartite tip (DIC photo of holotype B6418400009, scale bar=10 µm); E. Digits and claws on leg IV of a paratype (B6418400012, SEM photo, scale bar=3 µm); F. Digits and claws on leg IV of a paratype B6418400028 (DIC photo, scale bar=10 µm). Abbreviations: ce—cirrus E; ed—external digit; id—internal digit; p1—sensory organ on leg I; p4—sensory organ on leg IV; van—van der Land’s body; tip—tip of sensory organ on leg IV.

FIGURE 5. Halechiniscus janus sp. nov.— genital structures: A. Schematic drawing of the female genital structures (scale bar=10 µm); B. Female genital structures in paratype B6418400028 (DIC photo, scale bar=10 µm) and paratype B6418400027 (upper left small insert); C. Male gonopore in a paratype B6418500026 (DIC photo, scale bar=10 µm); D. Male gonopore in a paratype (B6418400022, SEM photo, scale bar=4 µm); Abbreviations: an—anus; go—gonopore; sdo—seminal receptacle duct opening; sdp—seminal receptacle duct pouch; sdu—seminal receptacle duct; sr—seminal receptacle.