Higginsarctus signeae, Hansen & Kristensen, 2021

Hansen, Jesper G. & Kristensen, Reinhardt M., 2021, A new genus and five new species of the subfamily Florarctinae (Tardigrada, Arthrotardigrada), European Journal of Taxonomy 762 (1), pp. 149-184 : 154-163

Higginsarctus signeae gen. et sp. nov.

urn:lsid:zoobank.org:act:4F3D0D8B-BC13-4B98-9DD5-A5DA3A3BD817

Figs 2–7

Florarctinae nov. gen. 1 et nov. sp. 1 – Hansen et al. 2001. — Hansen 2005.

Diagnosis (characters uniquely defining the taxon are written in bold)

Characterized by flat, rectangular secondary clavae with slightly protruding contours. Each clava inserted on a small elevation of the ventral cuticle. Unilobed, rectangular antero-lateral alae with weakly undulating distal margins without indentations. Unilobed, rectangular medio-lateral alae with weakly undulating distal margins without indentations. Antero-lateral alae and medio-lateral alae similar in size. Bilobed postero-lateral alae with a medial arched identation. Postero-lateral alae larger than antero-lateral alae and medio-lateral alae. Quadrilobed, rectangular caudal ala with 3 arched indentations, 1 medial and 2 lateral. Lateral lobes of the caudal ala nearly rectangular. Leg sense organs I–III with similar length. Genital stoup present. Anus inserted on ventral folium.

Etymology

The new species is dedicated to Signe G. Hansen, the daughter of the first author.

Material examined

Holotype FAROE ISLANDS • ♀; North Atlantic Ocean , Faroe Bank; 61°23.18′ N, 08°35.13′ W; depth 273 m; 1 Apr. 1992; R.M. Kristensen leg.; BIOFAR Station 788; coarse shell gravel; NHMD-293899 .

Allotype FAROE ISLANDS • ♂; same collection data as for holotype; 61°00′ N, 08°13′ W; depth 120 m; 19 Sep. 1998; R.M. Kristensen leg.; BIOFAR Station 2013; coarse shell gravel; NHMD-293900 .

Paratypes FAROE ISLANDS • 1 ♀; same collection data as for holotype; 61°12.33′ N, 08°28.23′ W; depth 148 m; 1 Apr. 1992; R.M. Kristensen leg.; BIOFAR Station 786; fine shell sand; NHMD-293898 • 1 ♀; same collection data as for holotype; 61°00′ N, 08°13′ W; depth 120 m; 19 Sep. 1998; R.M. Kristensen leg.; BIOFAR Station 2013; coarse shell gravel; NHMD-293901 • 1 ♀; same collection data as for preceding; used for SEM; NHMD-293902 .

Type locality

Faroe Bank, North Atlantic Ocean (between 61°00′ N, 08°13′ W and 61°23.18′ N, 08°35.13′ W; depth range, 120–273 m).

Description

HABITUS. The holotypic female ( Figs 2 , 3B–E ) is 147 µm long from the anterior margin of the head to the posterior margin of the body. The body is ovoid, being broadest (94 µm) at the level between the second and third pair of legs. It is highly convex dorsally, and flattened ventrally. The anterior margin of the head taper off, into a thin membrane-like structure (the ala anterior; van der Land 1968), which is different from the alae of the body. It is here referred to as the frontal ala ( Fig. 2 ). The dorsal cuticle has four transverse inter-segmental folds: one anterior to the first pair of legs, two between the first and second pair of legs and one between the second and third pair of legs. The SEM studies of a paratypic female shows two large pores mid-dorsally, at the level of the third pair of legs ( Fig. 5D ). A deep longitudinal furrow extends mid-dorsally from the insertion of the caudal ala and halfway to the dorsal pores. Also only visible by SEM (in LM the dorsal cuticle appears smooth with small pillars inside the epicuticle), the dorsal cuticle is remarkably sculptured with numerous small knob-like structures of two different types: one type which is organized in rosettes of five, and another type which is larger, with a more spongy appearance, occurring separately. The two types are evenly distributed in the cuticle ( Fig. 5E ). This kind of dorsal cuticular structure has never been observed in any tardigrade species before. The ventral cuticle is perfectly smooth with small pillars inside the epicuticle.

ALAE. Eight alae, which are all clearly separated from each other, are present ( Fig. 2 ): frontal ala, a pair of antero-lateral alae, a pair of medio-lateral alae, a pair of postero-lateral alae and a single caudal ala (see also paratypes Figs 4A , 5A ). The antero-lateral alae and medio-lateral alae are all similar in size and shape being unilobed and rectangular with weakly undulating distal margins without indentations. The postero-lateral alae each have a medial arched indentation in the distal margin, dividing the ala into two lobes of equal size. The caudal ala is narrowed at the insertion on the body and has an overall rectangular shape with a deep medial, arched indentation and a pair of lateral, arched indentations dividing the ala into four lobes. The medial lobes are small and rounded whereas the lateral lobes are larger and more rectangular. The pillars in the alae are visible as closely spaced dots. The proximal halve of the lateral and caudal alae is internally supported by continuous procuticle which sends out branching processes (ramuli) into the distal halve of the alae. The branching processes all start at the same level, thus creating a clear dividing line between continuous procuticle and branching procuticle, giving the alae a very characteristic appearance ( Figs 2 , 3D ).

SENSORY ORGANS. The head is well defined from the body by a constriction and a complete set of sense organs is present. All the cephalic cirri consist of an hourglass-shaped scapus, a long tubular portion and a protruding flagellum. As in most other species of Florarctinae the scapus of each cirrus appears somewhat outsized, enveloping the internal sensory structures rather than lining them. The internal cirri (35 µm) emerge from the frontal ala at the anterior margin of the head. The external cirri (29 µm) are inserted ventrally and the median cirrus mid-dorsally. The primary clava is relatively long (43 µm), slightly curved and non-flexible ( Fig. 2 and paratypes Figs 3A , 7A ). A van der Land’s body is visible inside its base. Primary clava and lateral cirrus (34 µm) arise on the same cirrophore, and a common membrane (extended margin of cirrophore) surrounds the base of primary clava and lateral cirrus. A very large and thick cuticular ring supports the cirrophore internally, and is probably functioning as an anchor of the primary clava (see paratype Fig. 7C ). The secondary clavae are large, rectangular flat sacs (8 µm × 15 µm) flanking the mouth cone ( Fig. 3B ). Each clava is inserted on a small elevation of the ventral cuticle. In SEM of a paratypic female, the contours of the secondary clavae are recognizable, protruding slightly from the ventral cuticle ( Fig. 4B ). The leg I sense organ (11 µm) is an unsegmented spine with a slightly swollen base and a terminal tube. The sense organs of leg II (8 µm) and III (8 µm) are unsegmented tapering spines. The fourth leg sense organ (12 µm) is an elongate papilla with a basal van der Land’s body and a terminal pore. The cirrus E (46 µm) has a prominent cirrophorus, scapus and a long tapering flagellum. The scapus has a highly complex composition. The outer lining is composed of small pellets organized in regular rings. At least 33 rings are recognisable by LM and SEM, however the pellets are recognizable as separate units, only by using SEM-technique ( Fig. 4F–G ).

LEGS, DIGITS AND CLAWS. The legs consist of coxa, femur, tibia and tarsus as found in all species of Florarctinae . A rectangular folium-like structure is weakly evident ventrally on each coxa of leg pair IV ( Fig. 2 ). Whether or not this is a true folium awaits the analysis of additional material. The external digits are supported by internal hook-shaped peduncles. The external claw is simple and with a calcar. The internal claw has an accessory spine, but no calcar. All the claws are of the same size, however the external claws are thicker basally and the internal claws have an almost straight portion dorsally ( Fig. 3D and paratype Fig. 5B–C ). An internal partition is evident as a small notch in each claw, dividing the claw in a basal portion and a distal portion ( Fig. 3D and paratype Fig. 7D ).

BUCCO- PHARYNGEAL APPARATUS. The mouth cone is large and consists of three parts; a large basal cone, a middle telescopic cylinder and a terminal, very refractive cupola, through which the distal part of the stylet sheaths, protrude ( Fig. 3B and paratype Fig. 7B ). The buccal tube is 41 µm long and thin and has a small refractive bulb anterior to the placoids. The stylets are 44 µm long and very thin, each with a small furca ( Fig. 2 and paratype Fig. 7C ). The placoids are short, thick and strongly curved. Each placoid has a droplet-shaped terminal swelling ( Fig. 2 and paratype Fig. 7C ).

REPRODUCTIVE SYSTEM. Consists of a single ovary bearing numerous small oocytes and three larger ova. The ovary is 58 µm long and is attached dorsally, at the level of the second pair of legs. The gonopore consists of a rosette with six large cells. Posterior to the rosette, the cuticle forms what appears in LM to be a broad fold. The SEM studies of a paratypic female shows that the cuticle is in fact forming a kind of basin or cup-like structure ( Fig. 4E ). It is here referred to as a ‘genital stoup’. The two cuticular seminal receptacles each consist of a spheroid vesicle and an S-shaped genital duct ( Figs 2 , 3C, E ). The cuticle is slightly elevated at each duct opening but does not form a true papilla. The vesicles are filled with spermatozoa. The anus is situated on a large rectangular folium ( Figs 2 , 3C ), and is closed by a threelobed cuticular system consisting of two large lateral lobes and a smaller posterior lobe.

Allotypic male ( Figs 6–7 )

No strong secondary sexual dimorphism is observed in Higginsarctus signeae gen. et sp. nov. The male is a little smaller (134 µm) than the female, and the primary clavae a little longer (46 µm). Also the secondary clavae have a slightly different shape, being more oval than rectangular ( Fig. 7B ). Bacterial vesicles are present just beneath the secondary clavae. The male gonopore is a small triangular papilla with two large pores (openings of the gonoducts), and is situated very close to the anus. The testis is large (87 µm), extending to the pharyngeal bulb, at the level of the first pair of legs. Two small lateral seminal vesicles are present.

Paratypes The paratype NHMD-293898 is a juvenile female without a fully developed rosette gonopore. It measures 113 µm in body length, 66 µm in width and the primary clavae are 42 µm. The other paratype NHMD-29902 is a fully mature female with a body length of 142 µm, body width of 90 µm and the

primary clavae are 49 µm.

Ecology and distribution

Known only from the type locality.

Figures

Fig 5. SEM-micrographs of Higginsarctus signeae gen. et sp. nov., paratype, ♀ (NHMD-293902). A. Dorso-lateral overview. B. Digits and claws of leg I. C. Digits and claws of leg II and leg III. D. Caudal end, dorsal view. Arrows indicate the large dorsal pores. E. Close up on the sculpture of the dorsal cuticle. The small knob-like structures arranged in rosettes are indicated by white polygons. The larger knob-like structures are indicated by white circles. Abbreviations: see Material and methods.

Fig. 2. Higginsarctus signeae gen. et sp. nov. Ventral overview. Holotype, ♀ (NHMD-293899). Abbreviations: see Material and methods. Drawing by Stine B. Elle.

Fig. 3. DIC-micrographs of Higginsarctus signeae gen. et sp. nov. A. Paratype, ♀ (NHMD-293898), overview. B–E. Holotype, ♀ (NHMD-293899). B. Close up on the head, ventral view showing the secondary clavae and the very refractive cupola of the mouth cone. White solid arrows indicate elevation of the cuticle associated with the secondary clavae. C, E. Optical sections of the caudal end, showing gonopore, genital stoup, duct openings (white hollow arrows), ducts of the seminal receptacles (white solid arrows) and the ventral folium with anus (C), and ovum, duct of the seminal receptacles (white solid arrow) and vesicle of seminal receptacles (white hollow arrow) (E).D. Close up on the procuticular branches (ramuli) in the ala, the internal and external claw of leg II and the sense organ of leg III. Abbreviations: see Material and methods.

Fig. 4. SEM-micrographs of Higginsarctus signeae gen. et sp. nov., paratype, ♀ (NHMD-293902). A. Ventro-lateral overview. B. Close up on the head, lateral view showing cephalic cirri and secondary clava. C. Internal cirrus. D. External cirrus. E. Gonopore and genital stoup. F–G. Scapus of cirrus E broken in two (note the unique small papillae of scapus). H. Primary clavae (broken) and lateral cirrus. Abbreviations: see Material and methods.

Fig. 6. Higginsarctus signeae gen. et sp. nov., allotype, ♂ (NHMD-293900). Dorsal overview. Abbreviations: see Material and methods. Drawing by Stine B. Elle.

Fig. 7. DIC-micrographs of Higginsarctus signeae gen. et sp. nov., allotype, ♂ (NHMD-293900). A. Overview. B. Close up on the head, ventral view showing the external cirri, secondary clavae and the very refractive cupola of the mouth cone. Note the presence of bacteria underneath the secondary clavae. C. Close up on the bucco-pharyngeal apparatus and the insertion of the primary clava with large refractive structures inside the base (white solid arrows) and van der Land’s body. D. Close up on the external claw of leg I, with evident internal notch (white solid arrow). Abbreviations: see Material and methods.