Quisarctus armatus, Trokhymchuk & Arbizu & Kristensen & Kieneke, 2025

Trokhymchuk, Roman Ruslanovych, Arbizu, Pedro Martínez, Kristensen, Reinhardt Møbjerg & Kieneke, Alexander, 2025, Marine tardigrades of Princess Alice Bank (Azores, Northeast Atlantic) with a description of a new species of Quisarctus Fujimoto, 2015, Zootaxa 5711 (1), pp. 125-139 : 128-132

Quisarctus armatus sp. nov.

( Figs 2–8A, B , Table 2)

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Diagnosis

Quisarctus with sexual dimorphism in the shape of cirri E: female has flagellum-shaped cirri, male has lance-shaped cirri. Body cuticle with punctation and dorsal transverse folds. Buccal apparatus consists of buccal tube, stylets and a pharyngeal bulb with three placoids. Legs I–III sensory organs as tripartite spines. Leg IV sensory organ developed as an elongated papilla with tubular tip. Digits may have proximal folds. Seminal receptacles open ventrally.

Type material

The holotype ( Figs 2A , 3 ) is an adult female (specimen ID M150-123-1-SG_2v2, SMF 1313 ) with seminal receptacles, and rosette gonopore. The allotype ( Figs 2B , 4 ) is an adult male (specimen ID M150-161-1-2_2v48, SMF 1323 ) with one seminal vesicle filled with spermatozoa and oval gonopore. Sixteen further specimens are paratypes ( Figs 5–7 ). Two of them, M150-226-1-6_5v6, SMF 1384 (female, Fig. 6 ) and M150-226-1-6_3v6, SMF 1382 (adult male exuvium, Fig. 7 ) were prepared for the SEM investigation. The other 14 specimens are three males, four females, two juveniles and five specimens with undetermined sex. The whole type series is deposited at the Senckenberg Natural History Museum as part of the Tardigrada collection under inventory numbers as specified in Table 2 .

Type locality

Princess Alice Bank, Azores; holotype: 37°59.968'N, 029°17.054'W; 151 m depth, bioclastic and volcanogenic sediment ( Fig. 1C ), collected on 5 th September 2018; allotype: 38°01.061'N, 029°19.691'W; 306 m depth, bioclastic sediment; collected on 6 th September 2018.

Etymology

The name armatus (from Latin; means armed) refers to lance-shaped cirrus E in males and dorsal transverse folds forming a pattern of dorsal armour plating in females.

Description of holotype (SMF 1313)

( Figs 2A , 3 , Table 2)

Adult female tardigrade with cylindrical body (95 µm long × 30.8 µm wide) and loose, punctated cuticle. A number of transverse folds with less punctated margins on the dorsal side ( Fig. 3A ). The punctation of the dorsal side is more prominent; the ventral surface, distal portions of legs, head cuticle and the margins of dorsal transverse folds have less marked or no cuticle punctation. Long primary clavae have a mid-portion of folded cuticle ( Fig. 3A ) and an apical pore. Primary clava and the cirrus A arise from a common cirrophore. Cirri A situated antero-dorsally to primary clavae. Cephalic cirri present as paired internal and external cirri plus an unpaired median cirrus; all cirri, including cirri A, consist of three parts: scapus, tubular portion and short flagellum. The internal cirri are dorsolateral; the external cirri are ventrolateral. Secondary clavae are absent. Lipoid eyespots present about at the position of the median cirrus ( Fig. 3C ). Mouth cone is retracted ( Fig. 3C ). Pharyngeal bulb is macerated. Leg I–III sensory organs present as tripartite spines with scapus, tubular portion and short flagellum ( Fig. 3D, E ). Sensory organ of the leg IV as an elongated papilla with a short distal tubular tip ( Figs 2A , 3E , 8A ). Cirri E are flagellum-shaped with a proximal portion, distal portion and spine, arising from a cirrophore ( Figs 2 , 3A ). All legs have two internal digits longer than two external digits. Each digit terminates in a simple crescent-shaped claw with calcar. Leg IV digits are larger than legs I–III digits. Proximal part of digits sometimes has folds ( Fig. 2A ). Rosette-shaped female gonopore present ( Fig. 3G ). A pair of seminal receptacle openings ventrally at the bases of legs IV is connected to slender, slightly S-shaped ducts and terminates as vesicles ( Figs 2A , 3F ). Anus is 4.7 µm posterior to gonopore and appears as a zigzag cuticle fold ( Fig. 3G ).

Remarks on the allotype (SMF 1323)

( Figs 2B , 4 , Table 2)

Male with cylindrical body (118.4 µm long × 29.0 µm wide) and loose, thin, punctated cuticle, which forms transversal folds on the dorsal side. Long primary clavae with folded cuticle ( Fig. 4A, D ). Secondary clavae absent. Lipoid eyespots present ( Fig. 4D ). Mouth cone is rather narrow (diameter: 3.8 µm) and directed anteriorly ( Fig. 4A ). About 25 µm long stylets without supports are present in the head section and inside the mouth cone. Three placoids ( Fig. 4E ) are visible in the oval pharyngeal bulb (10 × 8.5 µm). The pharyngeal bulb is followed by a globular oesophagus ( Fig. 4A ). Leg I–III sensory organs present as tripartite spines with scapus, tubular portion and short flagellum ( Fig. 4F–H ). Sensory organ of leg IV is an elongated papilla with a short distal tubular tip ( Figs 2B , 4I , 8B ). Cirri E arise from a cirrophore and have a lance-shaped distal portion ( Fig. 4J ). All legs have two internal digits longer than two external digits ( Fig. 4K ). Each digit terminates in a simple crescent-shaped claw with calcar. The claw sheath completely covers the claw. Leg IV digits are larger than legs I–III digits. Proximal part of digits sometimes has folds. One seminal vesicle with spermatozoa present ( Fig. 4L ). Oval male gonopore present a short distance anterior to leg IV insertions ( Fig. 4M ). Anus slightly posterior to gonopore and appears as a zigzag cuticle fold ( Fig. 4M ).

Remarks on paratypes

( Figs 5–7 , Table 2)

A number of 16 paratype specimens were either investigated with light microscopy, or with SEM in order to study further structural details and character variability. All paratypes exhibit morphology coherent with that of the holotype and the allotype ( Fig. 5 ). Males are 96–123 µm in body length (n=2) and the primary clavae are 27–30 µm (n=2) long. Females are 92–118 µm in body length (n=3) and the primary clavae are 32–35 µm (n=3) long. Juvenile specimens are 75–93 µm in body length (n=2) and the primary clavae are 24 µm (n=1) long. The primary clavae of the investigated females appeared significantly longer than that of the males (Mann-Whitney test, z = 2.653, p = 0.002). The juveniles have cirri E in the shape of a flagellum, which may indicate either the potential late development of the lance-shaped cirri E specific to males, or that the juvenile specimens are immature females. During SEM investigation, we were able to better examine and confirm the cuticular structures, especially dorsal transverse folds forming a pattern of segmentation or dorsal armour plating ( Fig. 6B ) and the epicuticular pillars ( Fig. 7 ). Furthermore, SEM imaging helped to investigate the differences in cirri E between sexes ( Figs 6G , 7D, E ), and the fine structure of the sensory organs and digits folds ( Figs 6F , 7C ).

The sexual dimorphism in Quisarctus armatus sp. nov. is observable in the difference between the length of primary clavae; the shape of leg IV sensory organ, and the shape of cirri E: males have spine-like cirri with a lance-shaped distal end, while females have flagellum-shaped tripartite cirri. Males have oval gonopores situated close to the anus, and females have rosette-shaped gonopores opened further from the anus.

Figures

FIGURE 1. Study area (A, B) and sediment sample in the Shipek Grab (C, D). Blue rectangle (A) delineates sampling area. White dots (B) indicate sampling sites. Station code (B) shortened to the station number (e.g. “M150-123-1-SG” becomes “123”). Bathymetry data from GEBCO, map created in QGIS 3.34.12. Photos (C, D) by H. Arndt.

FIGURE 2. Drawing of the holotypic female, dorsal view (A) and allotypic male, ventral view (B) of Quisarctus armatus sp. nov. Abbreviations: an—anus, cA—cirrus A, cE—cirrus E, ec—external cirrus, ey—eye, go—gonopore (in A it is delineated in the ventral side), ic—internal cirrusmc—median cirrus, mo—mouth cone, pb—pharyngeal bulb, pc—primary clava, pl— placoid, seI–IV—leg I–IV sensory organ, sr—seminal receptacle, sro—seminal receptacle opening, st—stylet, sv—seminal vesicle.

FIGURE 3. DIC microphotographs of Quisarctus armatus sp. nov. (holotypic female, SMF 1313). A habitus, dorsal view. B habitus, ventral view. C–G details in close-up. C cephalic region. D sensory organ I. E sensory organs II–IV. F caudal region, ventral view, note seminal duct (arrowheads). G caudal region, ventral view. Abbreviations: an—anus, cE—cirrus E, ec—external cirrus, ey—eye, go—gonopore, ic—internal cirrus, mc—median cirrus, mo—mouth cone, pc—primary clava, seI–IV—leg I–IV sensory organ, sro—seminal receptacle opening.

FIGURE 4. DIC microphotographs of Quisarctus armatus sp. nov. (allotypic male, SMF 1323). A habitus. B–M details in close-up. B cirrus A (arrowhead). C median cirrus. D cephalic region, note folded median portion of primary clava (arrowhead). E pharyngeal bulb, note three placoids (arrowheads). F sensory organ I. G sensory organ II. H sensory organ III. I sensory organ IV. J cirrus E. K leg I digits. L seminal vesicle with spermatozoa. M caudal region, ventral view.Abbreviations: an—anus, cE— cirrus E, ec—external cirrus, ed—external digit, ey—eye, go—gonopore, ic—internal cirrus, id—internal digit, mo—mouth cone, oe—eosophagus, pb—pharyngeal bulb, pc—primary clava.

FIGURE 5. DIC microphotographs of Quisarctus armatus sp. nov. (paratypes). A–D males; E–J females. A leg I digits, note folds (arrowhead). B sensory organ IV. C cirrus E. D caudal region, ventral view. E caudal region, ventral view. F leg IV digits, note folds (arrowhead). G caudal region, ventral view, note gonopore (arrowhead). H cirrus E. I caudal region, ventral view, note seminal receptacle vesicle (arrowhead). J caudal region, ventral view, note seminal receptacle vesicle (arrowhead). Abbreviations: an—anus, cr—calcar, go—gonopore, seI–IV—leg I–IV sensory organ, sro—seminal receptacle opening.

FIGURE 6. SEM micrographs of Quisarctus armatus sp. nov. (paratypic female). A habitus, ventrolateral view, note folded portion of primary clava (arrowheads). B habitus, dorsolateral view. C cephalic region, note protruded stylets (arrowhead). D cephalic region, dorsal view, note less punctated zone (arrowhead). E cephalic region, note primary clava pore (arrowhead). F leg sensory organs, note digit folds (arrowheads). G caudal region, ventrolateral view.Abbreviations: cA—cirrus A, cE—cirrus E, ec—external cirrus, ic—internal cirrus, mc—median cirrus, pc—primary clava, seI–IV—leg I–IV sensory organ.

FIGURE 7. SEM micrographs of Quisarctus armatus sp. nov. (paratypic male exuvium). A habitus, ventral view. B cephalic region, ventral view. C legs II in close-up, note digit folds (arrowhead). D caudal region, dorsal view. E caudal region, ventral view.Abbreviations: an—anus, cE—cirrus E, ec—external cirrus, go—gonopore, ic—internal cirrus, seI–IV—leg I–IV sensory organ.

FIGURE 8. Comparison of leg IV sensory organs of Quisarctus armatus sp. nov. (A holotypic female, B allotypic male) and Q. yasumurai (C holotypic female, redrawn from Fujimoto 2015).