A new marine tardigrade, Moebjergarctus clarionclippertonensis sp. nov., is described based on specimens collected from a manganese nodule area in the Clarion-Clipperton Fracture Zone of the abyssal North-eastern Pacific. The new species is a member of the bathyal/abyssal subfamily Euclavarctinae Renaud-Mornant, 1983. Within the Euclavarctinae, the genus Moebjergarctus Bussau, 1992, with only one described species, M. manganis Bussau, 1992, is characterised by simple claws, club-shaped and anteriorly bent primary clavae, well-developed spherical secondary clavae and cephalic cirri separated into three parts: short cirrophore, long and annulated scapus, and a short flagellum. Moebjergarctus clarionclippertonensis sp. nov. can be distinguished from M. manganis by the morphology of cephalic cirri which have scapi annulated only in the proximal part and by the presence of a caudodorsal bulge covered by a crescent-shaped cuticular thickening.

The knowledge of marine tardigrades in the South China Sea (Western Pacific Ocean) is very scarce, with only four species from shallow waters recorded until now. The first deep-sea (1530–1624 m bsl) tardigrade species from this sea, Halechiniscus janus sp. nov. (Arthrotardigrada: Halechiniscidae), is described here. Specimens of the new species have four wrinkled digits without peduncles on each leg, terminated by simple crescent-shaped claws, and primary clavae clearly shorter than cirri A, both inserted on a common cirrophore, as typical for the genus Halechiniscus Richters, 1908. The new species differs from all other Halechiniscus species by its cylindrical body and conical head; by the presence of semispherical secondary clavae, and by sensory organs on legs IV consisting of a short cirrophore followed by a short papilla terminated in a peculiar short bipartite tip. The discovery of this new bathyal species, with its peculiar morphological traits, brings new insights not only to the biogeography and ecology of tardigrades, but also to the understanding of the only partially resolved systematics of the diversified family Halechiniscidae.

Knowledge about marine tardigrades from the South China Sea is very scarce, with only four species from shallow waters recorded to date. The present study investigated the structure and diversity of tardigrade communities from the deep sea (1517-1725 m) at 8 stations in a polymetallic nodule area of the northern South China Sea. A total of 151 arthrotardigrades were collected belonging to 11 genera (Angursa, Batillipes, Coronarctus, Euclavarctus, Exoclavarctus, Halechiniscus, Moebjergarctus, Raiarctus, Rhomboarctus, Tanarctus and Tholoarctus), representing 17 species. Two Angursa species (Angursa sp. 4 and Angursa sp. 3) were the most abundant (25.2% and 14.6%, respectively), followed by Moebjergarctus sp. (13.9%). Specimens were mostly (90.7%) distributed in the upper layer of the sandy-mud sediment (0-1 cm). The SIMPROF test showed that the composition of tardigrade communities at all stations was not significantly different. At different stations, the number of species, Shannon-Wiener diversity index and Pielou’s evenness index ranged from 4 to 10, 1.94 to 2.87, and 0.75 to 1.00, respectively. The average taxonomic distinctness (Δ+) ranged from 72.50 to 90.00, and the variation in taxonomic distinctness (Λ+) ranged from 316.67 to 1181.25. This study provides some basic information about the biodiversity of the marine tardigrade community in the South China Sea.