Megastygarctides hekatae sp. nov.

Figs 3, 4, 5, 6, Table 3

Diagnosis.

Megastygarctides with three body plates and three accessory plates encircling the body. Cephalic plate is divided into an anterior part and a posterior part. Ventral cephalic plate is present. Primary clavae club-shaped. Secondary clavae dome-shaped. The genital ducts of the two oval seminal receptacles are extended out of the body as two short hollow spines with inferior hooks.

Type locality.

Station 850, 17°11.31'N, 21°57.20'W, 102.4 m b.s.l., Senghor Seamount, summit central, September 2009, biogenic coarse sand, MUC sample. Paratype was collected from the same site.

Type material.

Holotype: • 1 ♀ (M 79-3-850-2_4v74, SMF 1484) on slide. Paratype: • 1 ♀ (M 79-3-850-2_1v74, SMF 1482) prepared for SEM, on stub.

Description.

Holotype. Mature female (193.3 µm long) with cuticular armour divided into cephalic plate, three body plates, three accessory plates and caudal plate (Figs 3, 4 A, B). The cephalic plate is divided into the anterior and posterior part. On the ventral side of anterior cephalic plate, a ventral cephalic plate is present originating anteriorly from the mouth cone, and ends posteriorly almost at the cephalic plate division. All cephalic cirri arise from a cirrophorus and consist of scapus and flagellum. Internal cirrus is 23.3 µm long. External cirrus is 36.3 µm long and arises from a well-developed cirrophorus. Median cirrus is 28.0 µm long. Elongated club-shaped primary clava (17.0 µm long) and cirrus A (28.6 µm long, the other cirrus flagellum is broken) arises from the same lateral head lobe, and a common membrane (cirrophorus) surrounds the base of the primary clava and, with a van der Land’s body inside the base (Fig. 4 A, B). Secondary clava is kidney-shaped with a thick cuticle. Mouth cone is retracted. Pharyngeal bulb measures 13.8 μm × 13.0 μm. The other structures of the bucco-pharyngeal apparatus are not visible. Cuticle of body plate is not punctate. Cirrus E (64.5 µm long, measured for the whole structure with cirrophorus) arises from cirrophorus and divides into the telescopically inserted annulated scapus and the flagellum (Fig. 4 B, C). Sensory organs only on legs IV. Leg IV sensory organ (3.6 µm long) as spherical papilla resting on a short peduncle that contains a van der Land’s body (Fig. 4 C). All claws with dorsal spur. Leg IV claws are the longest (19.5 µm long) and the widest, followed by claws on leg I (16.5 µm long) and leg II (14.5 µm long). Leg III claws are the thinnest and with 14.0 µm length the shortest. Rosette-shaped female gonopore present on the ventral side of the caudal plate (Fig. 4 E). Seminal receptacles as oval pouches connected by seminal ducts with openings posterior to gonopore and in the form of a hollow spine with short inferior hook (Fig. 4 D, E). Single ovary consists of numerous oocytes (Fig. 4 A). Anus closed by two large lateral lobes, and small anterior and posterior lobes. Anus is 13.0 µm posteriorly to gonopore.

Remarks on paratype.

The single paratype SMF 1482 is another adult female that corresponds to the morphology of the holotype. The primary clavae and cirri A cirrophori exhibit better for observation (Fig. 5 A). Gonopore, genital spines and anus are present and well observable (Fig. 5 B, C). Only a single seminal receptacle filled with spermatozoa (Fig. 5 C). The SEM investigation (Fig. 6) helped to observe cephalic structures, such as prominent ventral cephalic plate, claws, and caudal region.

Differential diagnosis.

Megastygarctides hekatae sp. nov. is most similar to M. christinae Hansen & Kristensen, 2006 due to presence of well-developed body plates, ventral cephalic plate and, most importantly, genital spines — unique structures for tardigrades that are only reported for M. christinae so far. However, M. hekatae sp. nov. can be distinguished from M. christinae by the shape of primary clavae that are more elongated in the new species. Hansen and Kristensen (2006) reported male of M. christinae with the same form and length of primary clavae as females. Therefore, we consider this trait to be stable for morphological differentiation of species, although we have so far, no male specimen of the new species. Cirri E (measured for both species from cirrophorus to the tip end of flagellum) of M. hekatae sp. nov. are distinctly longer: 61–64.5 µm compared to 36–43 µm in M. christinae. Megastygarctides hekatae sp. nov. lacks the second accessory body plate between body plate I and body plate II as present in M. christinae. The length of the claws on all limbs of M. hekatae sp. nov. is noticeably greater than in M. christinae: for leg I 14.4–16.5 vs 9–10 µm, for leg II 13.9–14.5 vs 9–11 µm, for leg III 14 vs 10–12 µm, for leg IV 18.7–19.5 vs 12–13 µm. However, genital spines of M. hekatae sp. nov. are shorter than genital spines of M. christinae. Megastygarctides hekatae sp. nov. has more elongated seminal receptacles and anus.

Etymology.

Hekate (Ancient Greek: Ἑκάτη derived from ἑ κάς (hekás, “ far away ”) is a Greek mythological goddess often associated with crossroads, borders, the night, and witchcraft. Her name originates from the numeral hekaton (Ancient Greek: ἑκατόν) meaning “ one hundred ”, referring to the species type locality depth of approximately 100 meters — the deepest record for all the known Megastygarctides spp. so far. We decided to use the species epithet “ hekatae ” instead of the “ hekaton ” because of the tradition of feminine species names in the genus Megastygarctides (e. g., M. christinae, M. gerdae).