Dipodarctus susannae

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 : 413-416

Description of Dipodarctus susannae nov. sp.

Diagnosis. A small Dipodarctus -species. Primary clavae are ca. half of the total body length in the females. The kidney-shaped secondary clavae located rostrally between the internal and external cirri. The first three leg pairs with a long, dorsal digit IV with two spiralled folds. The fourth leg pair with two very long median digits with two spiraled folds. The sense organ of the fourth leg pair with a swollen base and a club-shaped scapus with a tiny spine (flagellum).

Type material. The holotype (Fig. 4) is an adult female [ ZMUC TAR 1288] with duct openings for seminal receptacles and fully developed female gonopore; collected on 11 January 1999 from carbonate sand inside Fish Rock Cave. A total of 15 Dipodarctus specimens were found in the samples of which 10 clearly belong to D. susannae nov. sp. ( Table 1 ). Two specimens were mounted on SEM stubs (Fig. 5). The type material is deposited in The Natural History Museum of Denmark, University of Copenhagen, Denmark.

Etymology. The name honors Susanna de Zio Grimaldi for her continuous and outstanding contribution to the knowledge of marine tardigrades.

Description of the holotype. The holotypic female is 78 µm long and has a maximum width of 32 µm between second and third leg pairs (Fig. 4). Small head, giving the animal a box-like body form. Anteriorly the head is bents slightly ventrally with a complete set of Halechiniscus -like cephalic appendages (Figs 5A and 5B; SEM of paratype). The median cirrus of the holotype is 13 µm long, simple in form, without a cirrophorus or scapus. The 10 µm internal cirrus comprises a scapus and a thin composite flagellum divided into proximal and distal sections. The 8 µm external cirrus is comprises a scapus and a simple flagellum. The lateral cirrus and primary clava have a common cirrophorus. The 12 µm lateral cirrus consists of a scapus and flagellum. The 37 µm long primary clavae extends from a cirrophorus in which a “van der Land’s organ” is visible (Fig. 4). The rostral secondary clavae are situated between the internal and the external cirrus above the mouth cone.

The buccal tube is 15 µm long and ends in a minor bulbous tip with the mouth opening in the center of the cone. The needle-shaped stylets are 13 µm long. The straight stylet supports are 3 µm long and attached to the buccal tube and stylet furcae. Three slightly hooked-shaped placoids are visible in the three-lobed pharyngeal bulb. The pharyngeal bulb is connected to a large intestine through a short esophagus. The intestine ends at a posterior anus. When closed, the anus is covered with two overlapping folds giving it a typical ovoid appearance with a wave-like midline.

The reproductive system comprises a single ovary containing one large ovum as well as a few (dorsal) immature oocytes. The duct openings of the seminal receptacles are visible laterally between leg pair III and IV. Clearly visible are only the outer cuticular characters of the receptacles, therefore the ampullar part could not be described, and no spermatozoa are visible. The gonopore is located on a large ovoid papilla surrounded by a rosette of six cells typical of arthrotardigrades. The distance between gonopore and anus is 6 µm.

Each leg consists of a coxa, femur, tibia and tarsus. Coxa and femur are not clearly telescopic. Coxa of the first three pair of legs bearing simple leg sense organs consisting of spines; measuring 5 µm (leg I) 8 µm (leg II) 12 µm (leg III). The 13 µm long somatic sense organ on leg IV consists of a papilla-like base (cirrophorus with a “van der Land’s organ”), a club-shaped scapus and a tiny flagellum.

Each tarsus has four digits. The digits on the first three pair of legs (I–III) are of different length, three digits (ventral) of equal length (4 µm) with a calcar in the claws and one 7 µm long digit (dorsal) that bears a simple claw with a calcar but without accessory spine (Figs 5A and 5C). Horizontal peduncles are present at the base of the digits in the first three pair of legs. Peduncles are absent in the fourth leg pair (IV) on which the two external digits are shorter than their two internal counterparts. The long digit IV on leg pairs I–III have two joint-like folds (digits with parallel folds or spiraled digits). On leg pair IV the two internal digits have the same two joint-like folds. The two internal (median) digits are 8 µm long and have crescent-shaped claws with accessory spines. The external digits on leg pair IV are 4 µm long.

Claw glands are visible in the coxal part of each leg. Claws may be covered by a membranous sheath. A 25 µm long cirrus E is attached on the trunk just anteriorly to the insertion of the coxa of leg IV; it consists of a 3 µm long cirrophore and a 22 µm long flagellum (Fig. 5D). The scapus is indistinct and only visible by SEM (Fig. 5D). Morphometric data of the holotypic female of D. susannae nov. sp. is presented in Table 2.

FIGURE 4. Drawing of the holotypic female of Dipodarctus susannae nov. sp. viewed from the ventral surface. Abbreviations: an—anus; bt—buccal tube; cE—cirrus E; ce—lateral cephalic projection; cg—claw gland; ec—external cirrus; go—gonopore; ic—internal cirrus; lc—lateral cirrus; mc—median cirrus; mg—midgut; nu—nucleus in oocyte; oo—oocyte; ov—ovary; pb—pharyngeal bulb; pc—primary clava; pl—placoid; sc—secondary clava; se1–4—leg sense organs 1–4; sr—seminal receptacle; ss—stylet support; st—stylet; vo1—“van der Land’s organ” of primary clava; vb4—“Van der Land’s organ” of se4.

FIGURE 5. SEM of Dipodarctus susannae nov. sp. A. Frontal/ventral view. B. Close-up of head. C. Close-up of tarsus of first leg. D. Close-up of cirrus E. Abbreviations: ba—bacteria; ci—cirrophorus; co—coccolithophore plate; dd—dorsal digit; ec—external cirrus; fl—flagellum; ic—internal cirrus; lc—lateral cirrus; mc—median cirrus; mo—mouth cone; pc—primary clava; sa—scapus; se1—leg sense organ 1.

Remarks. Gallo D’Addabbo et al. (2001) have reported that Dipodarctus subterraneus is the most common species in two well-investigated caves of San Domino Island (Mediterranean Sea). They found more than 500 specimens of this species together with four specimens of a new species of Dipodarctus closely related to D. anaholiensis . They illustrate the adult female of D. subterraneus from the Italian caves with a long dorsal toe on legs I–III. However, in the original description by Renaud-Mornat (1959) all toes are of similar length. It is clear that our new species is related to D. subterraneus , but differs in the finer details; as regards all of the cephalic appendages, leg spines and cirri E. The sensory structure of the fourth leg pair is notably very different from all other Dipodarctus -species. In Dipodarctus susannae nov. sp. this organ has a swollen basis (cirrophorus) with the “van der Land’s organ”, a long club-shaped scapus and a very short spine (flagellum). Furthermore the parallel folds of the digits in D. australiensis nov. sp. and D. susannae nov. sp. have not been reported from any described Dipodarctus species. The presence of rod-like peduncles in the fourth pair of legs in D. australiensis is very interesting as these structures are missing in all other Dipodarctus species.