Dipodarctus australiensis

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 : 410-412

Description of Dipodarctus australiensis nov. sp.

Diagnosis. A small Dipodarctus -species. The primary clavae are much shorter than half the length of the body. The sausage-shaped secondary clavae are located ventrally behind the internal and external cirri. The head has two triangular projections (cephalic projections) between the common cirrophorus for lateral cirri and primary clavae. Between the two internal cirri a cuticular sheath (frontal cuticular sheath) is present. Two medium-size triangular projections, each with two small posterior indentations, are present between the leg pairs III and IV. The first three leg pairs with a long, dorsal digit IV with three spiraled folds. The leg pair IV with two very long internal digits possessing three spiraled folds, and the two shorter external digits with two spiraled folds. All the four pairs of feet possess two parallel peduncles in the tarsus. The sense organ of leg pair IV has a typical cirrophorus, ovoid scapus and spiny flagellum.

Type material. The holotype (Fig. 2) is an adult male [ ZMUC TAR 1285] with testis, seminal vesicles and fully developed male gonopore; it was collected on 11 January 1999 from carbonate sand inside Fish Rock Cave. Only three specimens of Dipodarctus australiensis nov. sp. were retrieved from the samples ( Table 1 ). One paratype could be seen in frontal view (Fig. 3). The type material is deposited at The Natural History Museum of Denmark, University of Copenhagen, Denmark.

Etymology. The name derives from the continent of Australia.

Description of holotype: The holotypic male (Fig. 2) is 73 µm long, with a maximum width of 33 µm between second and third leg pair. The head is relatively large compared to the slender body. The facial plate may be similar to the frontal ala (“ala anterior”, van der Land (1968)) in Wingstrandarctus . In a frontal view (Fig. 3) the head is somewhat triangular because of the two lateral cephalic projections and the facial plate and it appears distinct from the body. A complete set of cephalic appendages is present and several amoebocytes are embedded in the brain. The amoebocytes are of two sizes: three to four large (1.5–2.0 µm) and 12 to 14 small (0.1–0.3 µm). The 15 µm long median cirrus has a large scapus and a slender composite flagellum divided into proximal and distal sections. The 15 µm internal cirrus consists of a scapus and a thin composite flagellum divided into proximal and distal sections. The 15 µm external cirrus has a long scapus, with a composite flagellum with short proximal and distal sections. The median, internal and external cirri are without distinctive cirrophori. The 13 µm lateral cirrus consists of a flagellum projected from the cirrophorus shared with the 37 µm primary clava. In the common cirrophorus a “van der Land’s organ” is visible (Fig. 2). Large sausage-shaped secondary clavae (13.5 µm long) are lying ventrally on both sides of a small slender mouth cone.

The buccal tube (15 µm long) ends in a small bulbous tip with the mouth opening in the center of the cone. The needle-shaped stylets are 13 µm long. The thin stylet supports are 3 µm long and attached to the buccal tube and the stylet furcae. Three slightly horn-shaped or arc-shaped placoids are visible in the three-lobed pharyngeal bulb. Through a short esophagus the pharyngeal bulb is connected to the large intestine, which ends at a posterior anus. The crescent-shaped male gonopore is placed on a small ovoid papilla 1.5 µm anterior of the anus. The anus is covered with two overlapping folds with a wave-like mid-ventral suture. Crescent-shaped to roundish spermatids and rod-shaped spermatozoa are visible in the seminal vesicles on each side of the gonopore. The large testis is filled with spermatocytes anteriorly and round-headed spermatids posteriorly.

Each leg consists of a coxa, femur, tibia and tarsus. Coxa on leg pair II and III possess simple leg sense organs consisting of spines of equal length; 10 µm (II) 10 µm (III). The leg sense organ on leg pair I (8 µm long) consists of a scapus and a flagellum. The sense organ on leg IV consists of a cirrophore-like base, an ovoid scapus and a short flagellum with a distal spine.

FIGURE 2. Drawing of the holotypic male of Dipodarctus australiensis nov. sp. (ventral view). Abbreviations: am—amoebocyte; an—anus; bt—buccal tube; cE—cirrus E; cg—claw gland; ec—external cirrus; ed1—2—external digit 1—2; fc—facial plate; go—gonopore; ic—internal cirrus; id—internal digit; lc—lateral cirrus; mc—median cirrus; mg—midgut; mo—mouth cone; pc—primary clava; pd—peduncle; pl—placoid; pr4—lateral process 4; sc—secondary clava; se1—4—leg sense organs 1–4; ss—stylet support; st—stylet; te—testis; vo1—“van der Land’s organ” of primary clava.

FIGURE 3. Drawing of the head of Dipodarctus australiensis nov. sp. (frontal view). Abbreviations: am—amoebocyte; ce—lateral cephalic projection; cg—claw gland; ec—external cirrus; fc—facial plate; ic—internal cirrus; lc—lateral cirrus; mc—median cirrus; mo—mouth cone; pc—primary clava; sc—secondary clava; se1—leg sense organ 1; vo1—“van der Land’s organ” of primary clava; vr—spiraled folds.

Each tarsus has four digits. On the first three pairs of legs (I–III), three digits (ventral) are of equal length and have claws with a calcar in addition one long digit (dorsal) bears a simple claw. Claw glands are visible in the coxal part of each leg. Claws may be covered by a membranous sheath. At the base of the digits in all four pairs of legs, two horizontal peduncles are present. In the fourth leg pair the two external digits are 6 µm long and shorter than the two internal which are 9 µm long. The dorsal digit on leg pairs I–III have three spiraled-like folds (digits with parallel folds). On leg pair IV all four digits have the spiraled-like folds. The external digits have two folds, while the internal digits have three folds.

A long cirrus E is attached laterally on the trunk, anteriorly to the insertion of the coxa of leg pair IV. Cirrus E consists of a large cirrophore which is 6 µm long and a 29 µm long flagellum. The scapus is indistinct. A pair of triangular lateral processes, measuring 10 µm in length, is present between leg pair III and IV. The posterior edge of the process has two small indentations. The lateral processes consist only of a thin cuticular membrane and may be folded and therefore overlooked.

Morphometric data of the holotypic male of D. australiensis nov. sp. is presented in Table 2.

Remarks. Only three specimens were found in Fish Rock Cave. D. australiensis is related to D. subterraneus as indicated by the many similar sensory structures, however, the presence of the triangular cephalic processes, the facial plate and the large lateral processes between leg III and leg IV are diagnostic of D. australiensis . de Zio Grimaldi et al. (1995 /96) described two small lateral projections in D. subterraneus from the Mediterranean Sea. In the original description by Renaud-Debyser (1959) these structures were not described in the Bahamas-form even though two small lateral indentations are shown in the drawing. The long dorsal toe of legs I–III in D. australiensis is also observed in D. borrori and D. susannae nov. sp., but not in D. anaholensis and in the original description of D. subterraneus .