Tanarctus hirsutospinosus

Jørgensen, Aslak, Boesgaard, Tom M., Møbjerg, Nadja & Kristensen, Reinhardt M., 2014, The tardigrade fauna of Australian marine caves: With descriptions of nine new species of Arthrotardigrada, Zootaxa 3802 (4), pp. 401-443 : 434-438

Description of Tanarctus hirsutospinosus nov. sp.

Diagnosis. A small tanarctid, with primary clavae longer than the total body length. The primary clavae with cuticular hairs. Teardrop-shaped secondary clava with a posterior arched furrow. Two cephalic projections are situated laterally on the head. A small cephalic vesicle filled with associated symbiontic bacteria is embedded in the head. Typical telescopic tanarctid leg construction with coxa, femur, lanceolate tibia and a conical tarsus with long internal and smaller external digits. Large simple clava-like fourth leg appendages with small cuticular hairs and a corrugated section on the flagellum.

Type material. The holotype (Fig. 18) is an adult female [ ZMUC TAR 1309] with openings for seminal receptacles and a fully developed gonopore; collected on 13 January 1999 from Jim’s Cave. A total of 16 specimens of Tanarctus were retrieved from the sediment samples; of which 12 were T. hirsutospinosus nov. sp., and the rest assigned to Tanarctus nov. sp. A (2 undescribed specimens), Tanarctus nov. sp. B (a single undescribed specimen) and a single undetermined specimen ( Table 1 ). The type material is deposited in The Natural History Museum of Denmark, University of Copenhagen, Denmark.

Etymology. The name refers to the Latin hirsutus meaning “hairy” and spinosus meaning “spiny”.

Description of the holotype. The holotypic female of Tanarctus hirsutospinosus nov. sp. measures 103 µm long and 59 µm wide between second and third leg pair (Figs 18 and 19A). The head is clearly distinct from the body, which appears slender, and ovoid with a complete set of Halechiniscus -like cephalic appendages (Fig. 20A; SEM of paratype).

Ten amoebocytes are embedded in the surface of the brain, five anterior to the mouth cone and five posterior. On both sides of the head two cephalic projections protrude in between the large cirrophorus of the primary clava and the first leg pair. A small cephalic vesicle filled with associated symbiontic bacteria is situated anteriorly in between the internal cirri and ventrally to the median cirrus.

The 12 µm long median cirrus consists of a simple but strong flagellum extending from the area above the vesicle. The 27 µm internal cirrus consists of a prominent scapus and a long thin flagellum. The scapi of the internal cirri are dorsally connected by a cuticular sheet. The 25 µm external cirrus consists of a large scapus and a composite flagellum divided into proximal and distal sections. The 15 µm short lateral cirrus is a well developed flagellum which shares a prominent cirrophorus with the primary clava. The “van der Land’s organ” is clearly visible within the cirrophorus. The 153 µm primary clava has proximal small thin hairs. The teardrop-shaped secondary clavae are situated ventrally above and around the mouth cone.

The mouth opening is located in the center of a prominent six-lobed mouth cone. The 23 µm long buccal tube stretches from the mouth to the three-lobed pharyngeal bulb. The two long needle-shaped stylets are 23 µm long and supported by 9 µm long stylet supports, which extend from the stylet furcae (Fig. 19B). The hooked-shaped tanarctid placoids are 7 µm long with a bulbous outer branch imbedded in the three-lobed pharyngeal bulb. From the short esophagus the pharyngeal bulb is connected to the large apparently empty intestine which ends at a posterior anus. The anus is covered with two overlapping folds and a minor caudal fold giving it a characteristic wave-like appearance. The gonopore, the outer character of the reproductive system, is located on a large ovoid papilla surrounded by a rosette comprised of six cells. The distance between the gonopore and anus is 12 µm. The reproductive system consists of a single ovary with a single ovum or oocyte. Nurse cells were not visible in the upper part of the ovary. Seminal receptacles are visible laterally between leg pair III and IV. Each receptacle consists of a globe-like vesicle connected to a tube-shaped genital duct that opens lateral to the female gonopore. A few spermatozoa are visible in the seminal receptacles.

Each telescopic leg pair consists of a coxa, femur, tibia and tarsus. Each coxa, indistinctly separated from the femur, bears a simple leg sense organ. The first leg spine is 10 µm long. The second and third spines are of both 12 µm. The sense organ on leg pair IV consists of a large 15 µm long ampulla-like cirrophorus and a very long and slender clava-like appendage (flagellum) with a combined length of 352 µm. Small thin cuticular hairs are placed in the first (proximal) half of the appendage (Figs 20B and 20C). The extensions of the hair-like structures end in a 5 µm long corrugated section. The rest of the appendage is smooth.

Each conical tarsus extends from a lanceolate tibia and consists of four digits, two shorter (8 µm long) external and two larger 12 µm internal digits with a basal fold. A small cushion-like structure is seen ventrally in the Vshaped part of each tarsus (Figs 18 and 19C).

The simple claw of internal digits has a well-defined calcar and a small anterior spur (Fig. 20E; SEM of paratype). Claw glands are visible in each coxa extending into the femur. Cirrus E, situated between leg pair III and IV, consists of a short (4 µm) cirrophorus and a long (40 µm) smooth thread-like flagellum with several small proximal punctuations (Figs 19A and 20D).

Morphometric data of the holotype of Tanarctus hirsutospinosus nov. sp. is presented in Table 2.

FIGURE 18. Drawing of the holotypic female of Tanarctus hirsutospinosus nov. sp. (ventral view). Abbreviations: am—amoebocyte; an—anus; bt—buccal tube; bv—bacteria filled vesicle; cE—cirrus E; ch—cuticular hair; cg—claw gland; ec—external cirrus; fe—femur; go—gonopore; ic—internal cirrus; lc—lateral cirrus; mc—median cirrus; mo—mouth cone; ov—ovary; pl—placoid; pc—primary clava; sc—secondary clava; se1–4—leg sense organs 1–4;; sr—seminal receptacle; st—stylet; vb1—“van der Land’s organ” of primary clava; vc—ventral cushion.

FIGURE 19. LM of Tanarctus hirsutospinosus nov. sp. A. Ventral view of holotypic female. B. Close-up of buccal apparactus of holotype. C. Close-up of fourth pair of legs (third paratype). Abbreviations: bt—buccal tube; cE—cirrus E; ec—external cirrus; ed—external digit; ic—internal cirrus; id—internal digit; lc—lateral cirrus; pl—placoid; pc—primary clava; se2–4—leg sense organs 2–4; ss—stylet support; st—stylet; ta—tarsus; ti—tibia; vc—ventral cushion.

Remarks. A unique feature of Tanarctus hirsutospinosus nov. sp. is the small vesicle in the head filled with symbiotic bacteria, a character usually associated with the subfamily Florarctinae . The cave dwelling T. hirsutospinosus nov. sp. has the longest (352 µm) fourth leg appendages known from any arthrotardigrade, which are more than three times the length of the body, and small cuticular hair-like structures (Figs 18–20). Small cuticular hairs are also present on the primary clavae; a character shared with T. arbospinosus . The recently described Tanarctus diplocerus , from a Japanese marine cave, has almost as long but simple sense organs (Fujimoto et al. 2013). When compared to T. arborspinosus , T. dendriticus and T. longisetosus , which do not possess vesicles with symbiotic bacteria and have more branched fourth leg appendages, the slender and spiny clava-like fourth leg appendage of T. hirsutospinosus nov. sp. appears to be a plesiomorphic character (see discussion in Jørgensen & Kristensen, 2001).

In most tanarctid literature the leg appendages are called papilla I–IV (Gallo D’Addabbo et al. 1999). The description of Tanarctus bubulubus added a new species to the group of tanarctids with strongly modified leg appendages and this clade of four tanarctid species with balloon- or leaf-like structures terminating the sense organ of the fourth leg was suggested by Jørgensen & Kristensen (2001). T. hirsutospinosus nov. sp. joins a second clade of tanarctids with hair-like structures on simple or branched, clavae-like fourth leg sense organs, which includes: T. arborspinosus , T. dendriticus and T. longisetosus (Jørgensen & Kristensen 2001) .

Among the many undescribed tanarctid species present at the Faroe Bank ( Hansen et al. 2001) at least one shares many similarities with T. hirsutospinosus nov. sp., including the long and hairy clava-like fourth leg appendages.

FIGURE 20. SEM of Tanarctus hirsutospinosus nov. sp. A. Dorsal view. B. Close up of primary clavae with hair-like structure. C. Close up of sense organ of fourth leg. D. Close up of Cirrus E. E. Close up of external and internal digits of first leg. Abbreviations: ac—accessory hook; cE—cirrus E; ch—cuticular hair; cl—claw; ec—external cirrus; ed—external digit; ic—internal cirrus; id—internal digit; lc—lateral cirrus; mc—median cirrus; pc—primary clava; se4—leg sense organ 4; ta—tarsus.