Raiarctus jesperi

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 : 424-427

Description of Raiarctus jesperi nov. sp.

Diagnosis. A slender body form with a cuticular wing supported by more than 210 minor pillars giving an Actinarctus -like body form. Buccal tube with a small cuticular droplet situated midways between the three-lobed pharyngeal bulb and a bulbous mouth cone. The mouth opening is surrounded by six triangular mouth papillae. Crescent-shaped to roundish spermatids and rod-shaped spermatozoa can be seen in the seminal vesicles on each side of the crescent-shaped male gonopore located on a small papilla. The relatively small testis, which extends to the third pair of legs, is filled with (anterior) spermatocytes and (posterior) round-headed spermatids. The digits on all four pairs of legs (I–IV) are of similar shape. The two shorter external and two larger internal digits have basal folds. Each digit has a 3-pointed crescent-shaped claw consisting of an accessory hook and a secondary hook. A membranous sheath may cover each claw. Strong Styraconyx -like horizontal peduncles are present at the base of the external digits in all four leg pairs. The primary clavae are like those of R. colurus , i.e. very short and with an inverted distal part.

Type material. An adult male (Fig. 11 [ ZMUC TAR 1297]) collected on 11 January 1999 from carbonate sand inside Fish Rock Cave. Two specimens of Raiarctus jesperi nov. sp., a male and a juvenile ( Table 1 ), were obtained from the sample. The type material is deposited at The Natural History Museum of Denmark, University of Copenhagen, Denmark.

Etymology. The name honors Jesper Guldberg Hansen for his contribution to the knowledge of marine tardigrades.

Description of the holotype. The holotypic male is 132 µm long with a maximum width of 52 µm between the second and third leg pair (Fig. 11). The head is broad with a large ventro-lateral appendage (9 µm long), from where the 12 µm long cone-shaped primary clava extends. A “van der Land’s organ” is clearly visible in the connection between the appendage and the primary clava. The animal has a slender body form with a cuticular wing supported by more than 210 minor pillars (5–10 µm long). The wing apparently surrounding the whole body gives the animal an Actinarctus -like body form. Between the fourth pair of legs a caudal body projection with 18 cuticular pillars protrudes (Fig. 11).

FIGURE 11. Drawing of the holotypic male of Raiarctus jesperi nov. sp. (ventral view). Abbreviations: am—amoebocyte; an—anus; bt—buccal tube; cb—caudal body projection; cE—cirrus E; cg—claw gland; ec—external cirrus; es—esophagus; go—gonopore; ic—internal cirrus; lc—lateral cirrus; mc—median cirrus; mg—midgut; mo—mouth cone; pc—primary clava; pl—placoid; se1–4—leg sense organs 1–4; sv—seminal vesicle; sp—spermatid; te—testis; vb1—“van der Land’s organ” of primary clava.

FIGURE 12. LM of Raiarctus jesperi nov. sp. Abbreviations: cE—cirrus E; ec—external cirrus; lc—lateral cirrus; mo—mouth cone; pc—primary clava; se4—leg sense organ 4.

The head has, anteriorly, a complete set of Halechiniscus - type cephalic appendages; however, the secondary clavae are indistinct and could not be observed in the holotype. The 12 µm long median cirrus comprises a prominent scapus and flagellum. The 20 µm internal cirrus has a long scapus with a composite flagellum divided into proximal and distal sections. The 22 µm external cirrus consists of a prominent scapus and a long simple, undifferentiated flagellum. The 25 µm lateral cirrus comprises a long scapus with a long narrow simple, undifferentiated flagellum. The lateral cirrus and primary clava share a distinct common cirrophorus (pedestal). The short primary clava (Fig. 11) has an inverted distal part, which may be everted during preparation (Fig. 12). Four pairs of amoebocytes are embedded in the brain; two minor and a larger pair of amoebocytes are placed anteriorly along the edge of the head and a large pair is placed distally.

The 20 µm long buccal tube extends from the three-lobed pharyngeal bulb with a small cuticular droplet situated midways and ends in a bulbous mouth cone with a central mouth opening surrounded by six triangular mouth papillae. The needle-shaped stylets are 20 µm long with prominent furcae. The curved stylet supports are 7 µm long and attached to the buccal tube and the stylet furcae. Three hooked-shaped placoids are visible in the three-lobed pharyngeal bulb. From the pharyngeal bulb the short oesophagus is connected to the large empty intestine which ends at a posterior anus covered with two overlapping folds giving it a typical ovoid appearance with a wave-like midline. A small third lobe is present posteriorly.

Crescent-shaped to roundish spermatids and rod-shaped spermatozoa can be seen in the seminal vesicles on each side of the crescent-shaped male gonopore, located on a small papilla. The testis extends to the third leg pair and is filled with (anterior) spermatocytes and (posterior) round-headed spermatids. The distance between the male gonopore and the anus is 3 µm.

Each leg, not clearly telescopic, comprises an indistinct separation of coxa and femur, and distinct tibia and tarsus. The leg sense organs (spines) on the coxa of the first three pairs of legs are of equal length ending in a small round cuticular swelling. The spines measure 10 µm in length. The 8 µm long sense organ on leg IV (Fig. 12) consists of a papilla-like cirrophorus, with no visible “van der Land’s organ”, an ovoid-shaped scapus and a tiny flagellum. Lateral to the fourth leg pair, the 22 µm long cirrus E is attached on the trunk, comprising a cone-shaped cirrophorus and a long slender flagellum. Each tarsus has four digits. The digits on the first three pairs of legs (I–III) are of equal shape. The two shorter external digits are 7 µm long, and the two larger internal digits, with a large basal fold, 8 µm long. Both external and internal digits, of the fourth leg pair, measure 8 µm. Each digit has a 3-pointed crescent-shaped claw with an accessory and a secondary hook. Each claw may be covered by a membranous sheath. Prominent Styraconyx -like horizontal peduncles are present at the base of the external digits in all four leg pairs. These peduncles have a characteristic shape as they are very short with two asymmetric wings and a terminal beak. Claw glands are visible in the coxal part of each leg.

Morphometric data of the Australian type of Raiarctus jesperi are presented in Table 2.

Remarks. Raiarctus jesperi nov. sp. is very similar to Raiarctus colurus ; however, several obvious differences are of note. A unique characteristic of R. jesperi nov. sp. is the presence of a small round cuticular swelling on each of the sense organs of the first three pairs of legs. The new species has more than 210 minor pillars in the cuticular wing; other species of Raiarctus have ca. 100–150 pillars. The pillars in the frontal cuticular sheet are reduced in length compared with the other Raiarctus species. The small cuticular droplet on the buccal tube in R. jesperi nov. sp. differs from that described by Renaud-Mornant (1981b) in R. colurus where it is a more ‘arrow-like’. The mouth opening is surrounded by six triangular mouth papillae, which were not recognized in the description of R. colurus . The cephalic appendages also differ from those of R. colurus in that R. jesperi nov. sp. has internal cirri comprising a large scapus and a composite flagellum with a needle-like distal section, with brain-embedded amoebocytes arranged in pairs. Raiarctus variabilis is characterized by a remarkable variability (D’Addabbo Gallo 1986), however, R. jesperi nov. sp. is clearly distinguishable from R. variabilis by the shape of the primary clavae and the presence of the caudal body projection (similar to that seen in R. colurus ).

The Raiarctus jesperi nov. sp. holotype is male, confirming that both sexes occur within the genus Raiarctus . This is the first time a male has been described, however the occurence of males have been mentioned in R. variabilis (D’Addabbo Gallo et al. 1986) .

The reduced pillars in the frontal cuticular sheet of R. jesperi nov. sp. makes it superficially similar to the genus Rhomboarctus (e.g. Hansen et al. 2003; Renaud-Mornant 1984), however, differences in claw morphology and dorsal cuticle (smaller pillars) are evident.