Acucapito hainanensis sp. nov., the first record of the family Acucapitidae Wiles, 1996 (Acari, Hydrachnidia) from China

We describe a new species Acucapito hainanensis Li & Guo sp. nov. from Hainan Province, P. R. China. In addition to being the first record of the family Acucapitidae Wiles, 1996 for Chinese water mite fauna, it is only the third species of this rare family in the world. Subtle structures are illustrated in detail with line drawings, light microscope images and scanning electron micrographs.


Introduction
The family Acucapitidae Wiles, 1996 belongs to the superfamily Lebertioidea Thor, 1900 based on the sperchontid-type genital field of the nymph, and also bear similarities to the Oxidae K. Viets, 1926 andthe Lebertiidae Thor, 1900. However, because of great differences (e.g. distinctive median suture of ventral shield, completely different gnathosoma and palp) between acucapitids and above three families, Wiles (1996) established Acucapitidae as a monotypic family for Acucapito naso Wiles, 1996 from Brunei. Another species, Acucapito vietnamensis Tuzovskij, 2009, was described from Vietnam, and until now these two species represented the only members of the Acucapitidae (Wiles 1996, Tuzovskij 2009, Smit 2020. In a field survey of water mites from Hainan Province we found a new species of this rare family that we describe here as Acucapito hainanensis Li & Guo sp. nov. This adds another family of water mites to the Chinese fauna. Furthermore, some structures of acucapitids are distinctive, especially palp structures that are difficult to resolve using light microscopy because of its minute fifth segment. Consequently, we take SEM (scanning electron microscope) micrographs to learn more about these structures.

Material and methods
For details of collection and preservation of water mites and preparation of slides see Gu et al. (2021). A 30 cm in diameter net with fine mesh size (250 μm) is positioned in streams, then turning over the cobblestones at the bottom of the river, so that flowing water will carry mosses, aquatic plants and dislodged organisms into it. Next, two stacked sieves (mesh size 4 mm above, 250 µm below) are used to remove leaves and other large impurities. Finally, a 2-mL dropper and a white tray are used to capture water mites. Specimens are preserved in Koenike's fluid and mounted in gelatin mounting fluid.
Slide-mounted specimens were examined and illustrated using a Leica DM3000 microscope. In the illustrations, dashed lines of eye lenses and the fourth leg sockets mean these structures under the surface, short curved lines on cuticle indicate muscle attachment scars. All photographs and illustrations were edited using Adobe Photoshop CS6. Measurements were taken by Nikon Ni-E (with Nikon DS-Ri2 camera). All measurements are given in μm, the measurements outside parentheses belong to holotype and allotype, the ranges in parentheses are the measurements for all collected specimens.
For SEM, specimens were kept for 12 hours in 2% glutaraldehyde at 4°C. Afterwards, the specimens were dehydrated through a graded ethanol series of 30%, 50%, 75%, 85%, and 95% (for 30 min each), then fully dehydrated in absolute ethanol (seven times for 24 hours each). After drying (40°C) in an electric blast drying oven (WGLL-125BE) for 12 hours, the specimens were attached to a holder using electrical adhesive tape, sputter-coated with gold, examined and photographed with a JCM6000 Desktop SEM (at 15.0 kV).
The type specimens are deposited in the Institute of Entomology, Guizhou University, Guiyang, P. R. China (GUGC).

Systematics
Family Acucapitidae Wiles, 1996 Genus Acucapito Wiles, 1996 Acucapito hainanensis Li & Guo sp. nov. Etymology "hainan-" is derived from the name of the Hainan Province, P. R. China, where the specimen was collected

Diagnosis
Both sexes -In dorsal view: O 2 closer to D 1 than to L 2 ; D 1 and D 2 arranged a rectangle; P-3 very long and slender, dorsum with two fine setae, the sunken area of ventral side of P-3 more than half the total length and with two plumose setae at the terminal; P-4 with a minute stubby seta at the middle of dorsal side, a very fine seta at the distal end and a ventrolateral seta on the  inner side. The numbers of swimming setae on legs: II-Leg-4, 2; II-Leg-5, 4-6; III-Leg-4, 5-6; III-Leg-5, 9-10; IV-Leg-4, 4-5; IV-Leg-5, 7-8; IV-Leg thicker than other legs.

Description
Male -(n=5). Idiosoma oval. In dorsal view: integument soft, without sclerotized plates, nine places of muscle attachments clearly visible; lateral eyes in weakly developed capsules; A 1 setae relatively long, thick and plumose; A 2 and the second pair of eye lenses at the same level; O 2 closer to D 1 than to L 2 ; D 1 and D 2 arranged a rectangle ( Figure 1A). In ventral view: a well-developed ventral shield formed by the fused coxae, Cx-I-III narrow, Cx-IV wide and partially enclosing the genital field; under light microscope, Cx-I/Cx-I fused to the MS, the Y-shaped interspace between Cx-I/Cx-I smooth and different from the punctate ornamentation of Cx-I, Cx-I/Cx-II complete and fused to the MS (Figure 2A  Cx-IV ( Figures 1B, 3D); excretory pore slightly distal to V 2 ; one place of muscle attachment well visible between V 4 and the posterior margin of Cx-IV ( Figure 1B). In lateral view: So 1 dorsal to the second pair of eye lenses, So 2 between O 2 and L 1 , So 3 between D 2 and L 3 , So 4 between D 3 and D 4 , So 5 located ventrally; L 2 directly above L 1 , L 2 -L 4 in a straight line ( Figure  1C).
Gnathosoma with exceptionally long, narrow rostrum; a pair of flabellate, sclerotized extensions on the dorsum of basal rostrum (Figures 4A, 5A). Chelicera long and narrow; chela with two rows of teeth ( Figure 5B).
Palp five-segmented ( Figures 4B, 6A, 6C); P-1 short, with a dorsal fine seta; P-2 with two dorsal fine setae; P-3 very long and slender, dorsum with two fine setae, the sunken area of venter more than half the total length and with two plumose setae at the terminal; P-4 with a minute, stubby seta at the middle of dorsum, a fine seta at the terminal and a ventrolateral seta on the inner side ( Figure 6B, D); P-5 greatly reduced, the joint visible on the outer side ( Figure  6B), only the front end visible on the inner side ( Figure 6D).
Legs robust and swimming setae only on the inner side; I-Leg without swimming setae ( Figure 7A

Differential diagnosis
There are six differences between Acucapito hainanensis Li & Guo sp. nov. and the type species A. naso Wiles, 1996: (1)   nov., but a trapezoid in A. naso; (4) in female, V 3 of A. hainanensis Li & Guo sp. nov. is at the margins of Cx-IV, while it is further from the margins of Cx-IV in A. naso; (5) as for palps of both sexes, the sunken area of P-3 at the ventral side is more than half the total length in A. hainanensis Li & Guo sp. nov., but less than half the total length in A. naso; (6) as for legs, the swimming seta counts (both sexes are same) of II-L-4, III-L-4, III-L-5, IV-L-4 and IV-L-5 are 2, 4-6, 9-10, 4-5 and 7-8 in A. hainanensis Li & Guo sp. nov., while in A. naso, II-L-4 and III-L-4 have 3 and 8 which are same in both sexes, III-L-5 of male has 12, IV-L-4 and IV-L-5 of female have 8 and 11 (Wiles 1996).
With regard to differences between Acucapito hainanensis Li & Guo sp. nov. and the remaining described species A. vietnamensis Tuzovskij, 2009, although less information of glandular position could be obtained from original description and illustration due to the bad condition of the specimen, five differences are still found obviously between these two species: (1) in both sexes, seta of A 1 is plumose in A. hainanensis Li & Guo sp. nov., but smooth in A. vietnamensis; (2) in dorsal view of both sexes, So 4 of A. hainanensis Li & Guo sp. nov. is behind D 3 , but So 4 and D 3 are at the same level in A. vietnamensis; (3) in both sexes, a pair of membranous extensions and a pair of plumose setae are existed at the tip of Cx-I in A. hainanensis Li & Guo sp. nov., but only a pair of short plumose setae at the tip of Cx-I in A. vietnamensis; (4) in males, genital flaps have 17-19 median setae and 9-10 lateral ones at each side in A. hainanensis Li & Guo sp. nov., while they have 14-15 median setae and 11-12 lateral ones in A. vietnamensis; (5) as for palp of both sexes, P-3 has a sunken area at the ventral side, P-4 only has one distal seta and P-5 is extended out of P-4 in A. hainanensis Li & Guo sp. nov., while P-3 is flat, P-4 has three distal setae and P-5 is extremely reduced in A. vietnamensis (Tuzovskij 2009).
In addition to the geographical separation between localities of the previously described species (Borneo, Vietnam), the above morphological differences demonstrate that the specimen we collected from Hainan Province, P. R. China represents the third acucapitid mite, Acucapito hainanensis Li & Guo sp. nov. Molecular data (e.g., the barcoding region of COI) would provide valuable additional information about the distinctiveness of these three species and their placement within the Lebertioidea.