Unionicola (Dimockatax) haungthayawensis, Chapurina & Kondakov & Vikhrev & Bolotov & Konopleva & Win & Lunn, 2022

Chapurina, Yulia E., Kondakov, Alexander V., Vikhrev, Nyein Chan Ilya V., Bolotov, Ivan N., Konopleva, Ekaterina S., Win, Than & Lunn, Zau, 2022, A new species Unionicola (Dimockatax stat. rev.) haungthayawensis sp. nov. (Trombidiformes: Unionicolidae) from the freshwater mussel Lamellidens generosus (Gould, 1847) in Myanmar, Ecologica Montenegrina 56, pp. 28-39 : 32-38

Unionicola (Dimockatax) haungthayawensis sp. nov.

Figs. 2 , 4 , 5

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Holotype. MYANMAR: Haungthayaw River , upstream of Kawkareik town, 16.4714°N, 98.2183°E, Salween River basin, from the gills of Lamellidens generosus [host voucher RMBH biv363], 09.ii.2018, 1♂ [slide RMBH Hyd 363], Vikhrev, Bolotov, and Nyein Chan leg.

Paratypes. MYANMAR: the type locality, the same host, date, and collectors, 4♀ [slide RMBH Hyd 363] and two sequenced specimens [sample ID RMBH Hyd_363_1; COI sequence accession number MT724616 ; sample ID RMBH Hyd_363_2; COI sequence accession number MT724617 ], Vikhrev, Bolotov, and Nyein Chan leg. ; Winyaw River , 15.6685°N, 97.9496°E, Ataran River basin, from the gills of Lamellidens generosus [host voucher RMBH Biv621], 20.xi.2018, 4♀ [slide RMBH Hyd 621] and two sequenced specimens [sample ID RMBH Hyd_621_19_1; COI sequence accession number MT724660 ; sample ID RMBH Hyd_621_19_3; COI sequence accession number MT724661 ], Vikhrev, Bolotov, and Nyein Chan leg.

Reference DNA sequence data. Three COI sequences and two 28S sequences were obtained from whole mites. These specimens were completely used for DNA extraction but the mites of the same sample were studied by us morphologically, and are included here in the type series (see sequence accession numbers and locality data in the list of paratypes above).

Type host. Lamellidens generosus (Gould, 1847) ( Bivalvia: Unionida : Unionidae : Parreysiinae ) ( Fig. 3 ).

Etymology. The name of the new species refers to the Haungthayaw River, its type locality.

Differential diagnosis. Features of the subgenus Dimockatax stat. rev. Dorsum apparently lacking a dorsal plate. Posterior coxal plates with distinct margins; female genital plates with an elongate, bird-bill-like spinous process; all tarsal claws of walking legs bifid with the dorsal clawlet shorter than the ventral clawlet.

Diagnosis. Coxal plates III and IV with incomplete suture ( Fig. 2 AB; Fig. 5 ); female genital field with 31-50 acetabula on each plate, female genital plates with one obvious seta and one or two fine setae on every side at the base of spinous flap and two setae on top of the flap on every side ( Fig. 2 E , Fig. 4 G ); male genital plates bearing 22-23 acetabula per side, with one fine short seta on a plate ( Fig. 4 F ); pedipalps stocky, well-sclerotized with one dorsoventral seta on P-4 segment and 2-5 setae on P-2 segment (the same individual may have a different number of setae on P-2 ( Fig. 4 AB; KL). Pedipalps with indistinct structure of the clawlet both for males and females ( Fig. 2 D, G ): at its top, the claw seems to consist of several parallel processes with indistinct contours, one of which is longer than the others. Male and female walking legs similar to other members of the subgenus ( Fig. 4 C, E, H, J ); obvious, distal, dorsal, expanded, dropshaped setae above tarsal claws of walking legs ( Fig. 2 CF, Fig. 4 DI); IV-L-6 straight; male’s I- L-6 is slightly chaetose (3 setae)

Description. MALE MORPHOLOGY. I-L: I-L-1 with 1 claw-like lateral seta and 3 claw-like dorsal setae. I-L-2 with 1 claw-like dorsal seta, 2 lateral peg-like setae, 2 short setae: laterally and distally. I-L-3 with 3 ventral setae, two of them are slender, 3 lateral setae. I-L-4 with a ventral row of awl-shaped curved setae, 2 lateral peg-like setae. I-L-5 with 3 ventral claw-like setae. I-L-6 with 3 hair-like setae. On its tip the tarsus bifurcates, forming 2 claws with teardrop-shaped setae above each claw, and each with a short dorsal and a major ventral clawlet.

IV-L: IV-L-1 with 3 blunt dorsal setae and 1 distal peg-like seta. IV-L-2 with 2 dorso-distal, 1 lateral and 1 lateral setae. IV-L-3 with 2 dorso-distal claw-like setae, 2 distal and 7 lateral blunt setae. IV-L-4 with 1 dorsal and 3 lateral short setae, 2 slender distal setae, 9 venter and 2 distal awl-shaped setae. IV-L-5 with 2 dorsal slender setae and 12 ventral awl-shaped setae. IV-L-6 with 2 dorsal slender setae and 7 ventral awl-shaped, pointed setae. On its tip the tarsus bifurcates, forming 2 claws with teardrop-shaped, minute setae above each claw, and each with a short dorsal and a long ventral clawlet. Length ratio IV-L-6 to IV-L-5 is 0.78.

Genital field consists of one pair of plates each with 22-23 acetabula of various size, without flaps.

FEMALE MORPHOLOGY. I-L: I-L-1 with 3 dorsal setae, two of them are blunt, and 2 lateral setae (one peg-like and one thin). I-L-2 with 2 dorsal, 1 dorso-distal and 1 lateral setae. I-L-3 with 2 heavy and 1 blunt jagged lateral setae, 1 ventro-distal and 1 lateral thin setae and 2 ventral slender setae. I-L-4 with 5 awl-shaped curved setae, 5 lateral setae, 2 of them are blunt and jagged, 1 is thin and 1 is claw-like and 1 distal thin seta. I-L-5 with 4 ventral claw-like setae, 1 dorsal and 2 lateral thin setae and 1 distal peg-shaped seta. I- L-6 with 2 dorsal and 1 latero-distal thin setae. On its tip the tarsus bifurcates, forming 2 claws with teardrop-shaped setae above each claw, and each with a short dorsal and a major ventral clawlet.

IV-L: IV-L-1with 3 dorsal peg-like setae and 1 latero-distal awl-shaped pointed seta. IV-L-2 with 2 lateral blunt setae, 1 dorso-distal peg-like seta and 1 ventral claw-like seta. IV-L-3 with 1 dorsal thin seta, 1 dorso-distal and 1 latero-distal short blunt setae, 4 blunt and 4 awl-shaped, pointed various in size lateral setae and 1 distal blunt seta. IV-L-4 with 2 ventral, 6 lateral and 4 distal awl-shaped, pointed setae, 3 lateral, 1 dorso-distal and 2 latero-distal short blunt setae. IV-L-5 with 10 lateral, 4 ventral and 1 ventro-distal awl-shaped, pointed setae, and 1 thin dorso-distal seta. IV-L-6 with 2 thin dorsal setae, 7 latero-ventral awl-shaped, pointed setae and 1 lateral small peg-like seta. The structure of the claws is the same as in male’s IV leg. Length ratio IV-L-6 to IV-L-5 is 0.56.

The genital field consists of one pair of plates with many acetabula. Female genital field with inner, elongate, spinous flaps typical for the subgenus with 1 slender, 2 thin setae on the each flap and 1 or 2 thin setae on the top of the beak-shaped outgrowth of the flap.

MALE MEASUREMENTS (N = 1, the holotype). Length of the idiosoma 1127; length of the posterior coxal group 275; width of the posterior coxal group 233; dorsal lengths of the P-1–5 segments: 23,

111, 51, 84, 66; dorsal lengths of the I-L-1–6 segments: 75, 100, 125, 163, 165, 125; dorsal lengths of the II-L-1–6 segments: 77, 120, 168, 215, 229, 156; dorsal lengths of the III-L-1–6 segments: 84, 112, 154, 201, 246, 171; dorsal lengths of the IV-L-1–6 segments: 155, 125, 216, 304, 395, 258; length of the genital field 221, width of the genital field 257.

FEMALE MEASUREMENTS (N = 8). Length of the idiosoma 1952 (1495–2380); length of the posterior coxal group 340 (268–402); width of the posterior coxal group 286 (224–326). Dorsal lengths of the P-1–5 segments: 31 (20–41), 123 (110–134), 57 (28–78), 99 (78–107), 69 (58–83);

dorsal lengths of the I-L-1–6 segments: 87 (64–112); 129 (109–143); 162 (139–185); 220 (183–251); 223 (178–252); 142 (128–158);

dorsal lengths of the II-L-1–6 segments: 90 (75–101), 150 (131–171), 224 (156–289), 313 (261– 361), 318 (265–357), 182 (153–206);

dorsal lengths of the III-L-1–6 segments: 98 (78–111), 149 (117–184), 214 (177–237), 303 (240– 345), 348 (296–378), 208 (185–240);

dorsal lengths of the IV-L-1–6 segments: 198 (167–233), 173 (142–188), 341 (289–392), 509 (416– 584), 618 (497–701), 344 (300–392).

Length of the genital field 268 (161–371), width of the genital field 391 (266–442).

Distribution. Haungthayaw River, upstream of Kawkareik town, Salween River basin; Winyaw River, Ataran River basin, Myanmar.

Ecology. The new species is a gill mite. It is known to occur in Lamellidens generosus . The mites were observed in two localities, in one of the sites (date of collection – 20.xi.2018) there were eggs in the gills in one of the individuals of the host mussel. However, we cannot say unequivocally whether they belong to the same species of mites or not. The data on the mite infestation in both localities are presented in Table 2.

Comments. It is the second Unionicola ( Dimockatax stat. rev.) species from SE-Asia. The holotype of Unionicola ( Dimockatax stat. rev.) tumidoides, Vidrine 1986 , the other SE-Asian species, found in Ensidens ingallsianus (Lea, 1852) from Thailand. The new species strongly reminds Unionicola ( Dimockatax stat. rev.) tumidoides , i.e., the male idiosoma size and P-5 length are comparable in these two species, but the leg segments of U. tumidoides are shorter ( Table 3). Females of the new species are larger in all measured parameters ( Table 3). The chaetotaxy of the male I-L-4 – I-L-6 slightly differs between these species. Unionicola tumidoides male has 4 ventral setae on I-L-4, while U. haungthayawensis sp. nov. has 5 ventral setae on I-L-4; the chaetotaxy of segments I-L-5 and I-L-6 is more developed in the male of U. tumidoides than in that of U. haungthayawensis sp. nov.

two collecting localities, Myanmar.

The length ratio IV-L-6 to IV-L-5 for the females of U. haungthayawensis sp. nov. is 0.56, the same value as that for U. tumidoides females. ( Table 4); for U. neocooki , U. bishopi , and U. tumida females the average ratio IV-L-6 to IV-L-5 is 0.62-0.64, and for U. ernstingi females this ratio is 0.72. For males this ratio is more than 0.7 in three species: U. ernstingi , U. tumida and U. tumidoides ; U. bishopi and U. haungthayawensis sp. nov. share similar ratios between 0.65-0.69; and for males U. neocooki this ratio is 0.61.

segments between Unionicola tumidoides and Unionicola haungthayawensis sp. nov.

Discussion

We propose to restore Dimockatax stat. rev. as a valid subgenus. We consider that all species assigned to this subgenus, including the new species described herein, share the morphological features of the subgenus such as bird-bill-like spinous process on female genital plates and others. Our phylogenetic tree ( Fig. 1 ) demonstrates that COI gene sequences of the species U. tumida , U. ernstingi and U. haungthayawensis sp. nov. belong to one clade with high bootstrap support (BS = 91), corresponding to the subgenus Dimockatax . We believe that the species-rich genus Unionicola needs to be divided into subgenera at least for the intrageneric classification purpose and definition. It was found in our previous work ( Chapurina et al., 2021) that the clades of a reconstructed COI phylogeny correspond well to the subgenera established on the basis of morphological data.

Unionicola haungthayawensis sp.nov. is the sixth species in the subgenus Dimockatax stat. rev. and the second species of this subgenus in Asia. It is known that Dimockatax stat. rev. species are cryptic and that their species-level identification based on morphological features alone is complicated ( Edwards et al., 2008; Ernsting et al., 2008; Edwards & Vidrine, 2013). We suppose that the tarsal clawlet of the pedipalp in Unionicola haungthayawensis sp. nov. differs from that of all other species in the subgenus ( Fig. 2 D, G ). The idiosoma size of U. haungthayawensis sp. nov. is larger than that of U. tumidoides ( Table 3), and resembles that of its Nearctic consubgeners. The largest female we have observed on slides has the idiosoma lengh of 2380 µm. The chaetotaxy also differs between two Asian Dimockatax stat. rev. species. We noticed that the length ratio of tarsus to tibia of IV leg for females differs both in U. tumidoides and U. haungthayawensis sp. nov. compared to the rest species ( Table 4). Hence, the females of the new species are comparable in mean size, but differ in the ratio of the lengths of the distal segments of the IV-L to those of the Nearctic species of the subgenus.

Our phylogenetic tree ( Fig. 1 ) indicates that the COI sequences of Unionicola haungthayawensis sp. nov. clustered as a sister group to Unionicola tumida and Unionicola ernstingi and close to Unionicola (Wolcottatax) arcuata (Wolcott, 1898) . These results support an earlier molecular-phylogenetic study revealing that these subgenera are close to each other ( Edwards et al., 2010). They were originally assigned to the same subgenus Parasitatax Viets 1949 ( Edwards & Vidrine, 2013).

Unionicola species are known to have a high degree of differentiation while comparing of COI sequences (uncorrected p-distance>10%) ( Lewisch et al., 2021; Ernsting et al., 2008). U. tumida and U. ernstingi revealed a 12.7% of differentiation ( Ernsting et al., 2008) and they are species of the same subgenus. When comparing the COI gene region of 300 bp between Unionicola haungthayawensis sp. nov. and two Nearctic species, the uncorrected p-distance was 17.3% and 13.6%, respectively. We understand that the compared gene region is quite short, and this could affect the percentages be slightly overestimated. But in general, our data are consistent with those previously identified for these water mites.

U. ( Dimockatax stat. rev.) tumidoides was found in only one freshwater mussel species, Ensidens ingallsianus (Lea, 1852) , and only one male and one female were collected ( Vidrine, 1986). Unionicola haungthayawensis sp. nov. is known to parasitize the gills of Lamellidens generosus ( Fig. 3 ). We do not know whether these mites are narrow host specialists or not, but we have found that L. generosus is an appropriate host for at least 5 species of Unionicola from various subgenera. In addition to individuals of Unionicola haungthayawensis sp. nov., the following mites were found in this mussel species: Unionicola (Prasadatax) diversipes K. Viets, 1926 , one of the Imamuratax species and two species of the Subgenus B, representing a complex of cryptic species, which are strongly resemble Unionicola (Prasadatax) brandti Vidrine, 1985 ( Chapurina et al., 2021).

Unionicola haungthayawensis sp. nov. demonstrates a narrow host specialization. It is noteworthy that Dimockatax stat. rev. species inhabiting such geographically remote regions as North America and SEAsia are nonetheless hardly distinguishable and very similar in morphology despite their considerable genetic distance.

Figures

Figure 1. Maximum likelihood phylogeny of Unionicola based on the dataset COI gene fragment. Numbers near nodes are bootstrap support (BS) values of IQ-TREE. Scale bar indicates the branch lengths. The red color indicates Unionicola (Dimockatax stat. rev.) haungthayawensis sp. nov. sequences from Myanmar.

Figure 2. Unionicola (Dimockatax stat. rev.) haungthayawensis sp. nov. Light microscopy pictures of male and female and details of morphology: (A, B) general view; (C, F) tarsal claw and fragment of IV-L-6; (D, G) tarsus of pedipalp (P-5); (E) spinous flaps of genital plates. Specimens: (A) paratype female RMBH Hyd 363_1; (C, D, E) paratype female RMBH Hyd 621_1; (B, F, G) holotype male RMBH Hyd 363. Scale bars = 200µm (A-B) and 50µm (C-E). (Photos: Yulia E. Chapurina).

Figure 3. The shell of Lamellidens generosus [RMBH biv363_3], the host of Unionicola haungthayawensis sp. nov. Scale bar = 10mm. (Photos: Ekaterina S. Konopleva).

Figure 4. Morphological details of Unionicola (Dimockatax stat. rev.) haungthayawensis sp. nov. The holotype male RMBH Hyd 363: (A, B) pedipalps (P-1–5); (C) first walking leg (I-L-1–6); (D) claw of first walking leg; (E) fourth walking leg (IV-L-1–6); (F) genital field. The paratype female RMBH Hyd 363_1: (G) genital field; (H) first walking leg (I-L1–6); (I) claw of first walking leg; (J) fourth walking leg (IV-L-1–6); (K, L) pedipalp (P-1–5). Scale bars = 100 µm. (Graphics: Yulia E. Chapurina).

Figure 5. Unionicola (Dimockatax stat. rev.) haungthayawensis sp. nov. ventral side of idiosoma with coxal plates and genital fields: (A) holotype male RMBH Hyd 363; (B) paratype female RMBH Hyd 363_1. Scale bar = 200 µm (Graphics: Yulia E. Chapurina).