Li, Hai-Tao ¹ ; Smit, Harry ² ; Zhan, Bao-Xiang ³ ; Jin, Dao-Chao ⁴ and Guo, Jian-Jun ⁵

¹Institute of Entomology, Guizhou University, Scientific Observing and Experimental Station of Crop Pests in Guiyang, Ministry of Agricultural and Rural Affairs of the P. R. China, Guiyang 550025, P. R. China.
²Naturalis Biodiversity Center, P.O. Box 9517, 2300 RA Leiden, the Netherlands & Museums Victoria Research Institute, Museums Victoria, Melbourne, Australia.
³Institute of Entomology, Guizhou University, Scientific Observing and Experimental Station of Crop Pests in Guiyang, Ministry of Agricultural and Rural Affairs of the P. R. China, Guiyang 550025, P. R. China.
⁴Institute of Entomology, Guizhou University, Scientific Observing and Experimental Station of Crop Pests in Guiyang, Ministry of Agricultural and Rural Affairs of the P. R. China, Guiyang 550025, P. R. China.
⁵✉ Institute of Entomology, Guizhou University, Scientific Observing and Experimental Station of Crop Pests in Guiyang, Ministry of Agricultural and Rural Affairs of the P. R. China, Guiyang 550025, P. R. China.

2025 - Volume: 65 Issue: 3 pages: 786-814

Original research

Keywords

Abstract

Introduction

The pontarachnid mites are the only family of the true water mites occurring in the marine habitats (Chatterjee et al. 2019). A few species are not directly marine, but inhabit nearby estuary or lakes (Cook 1986, 1996; Smit 2007; Pešić 2013). The family Pontarachnidae contains only two genera, i.e. Pontarachna Philippi, 1840 and Litarachna Walter, 1925, with 30 and 24 species respectively (Chatterjee et al. 2019; Montes-Ortiz et al. 2021).

Many experts use ''peculiar'' to describe this group because the presence of so-called ''wheel-like acetabula'' (Cook 1996; Smit and Alberti 2010; Chatterjee et al. 2019). Cook (1996) was aware of the significant phylogenetic value of this character. Smit and Alberti (2010) did careful observations with the help of SEM and TEM, and believed that this organ was used for osmoregulation. However, they were uncertain whether the wheel-like acetabula were homologues of the acetabula of freshwater mites. Taxonomists have paid more attention to the description of this character, but some other characters have not been given enough attention, especially the mouth parts. This to some extent limits our understanding of this group.

In China, three species have been reported, i.e. Litarachna hongkongensis Smit, 2002 (Hong Kong), Pontarachna formosae Lohmann, 1909 (Taiwan) and Pontarachna australis Smit, 2003 (Taiwan) (Pešić et al. 2008; Chatterjee et al. 2019; Jin et al. 2024). After recent collections from Hainan Island, we obtained three species, i.e. Litarachna curtipalpis Smit, 2003, Litarachna thetis Pešić & Smit, 2016 and Pontarachna australis Smit, 2003. More detailed characters were observed by camera, optical microscope and SEM, which will be discussed in this paper.

Material and methods

The collection and preservation of water mites and preparation of slides followed Gu et al. (2021). Micrographs of living watermite were taken by a Canon EOS R8 digital camera with a Mitutoyo Plan NIR 10 lens, a Godox MF12 flash was used as the light source. Helicon Focus (vers. 8.1.0) was used for image stacking. All optical microscope images, examinations and measurements (given in μm) were made with a Nikon Ni-E (with Nikon DS-Ri2 camera). The measurements method of idiosoma, genital field, excretory pore, gnathosoma, chelicera, palp and legs followed Li et al. (2024). The treatments of SEM (scanning electron microscope) referred to Li et al. (2022). All images were modified and grouped into plates in Adobe Photoshop CS6.

The terminology and abbreviations used were updated from Jin (1997): a.s.l. = above sea level, A₁ = preantennal glandularia, A₂ = postantennal glandularia, Ae = anterior extension, C₂ and C₄ = coxoglandularia 2 and 4, Cx-I–IV = coxae I–IV, El₁ = the first pair of eye lenses, El₂ = the second pair of eye lenses, D₁ –D₄ = dorsoglandularia 1–4, I-L-1–6, etc. = the first–sixth segment of the first leg, etc., L = length, L₁ –L₄ = lateroglandularia 1–4, Ma = medial apodeme, O₁ = preocularia, O₂ = postocularia, P-1–5 = the first–fifth segments of the palp (from most proximal to most distal), So₁ –So₅ = slit organs 1–5, V₁ –V₄ = ventroglandularia 1–4, W = width, Wa = wheel-like acetabula.

The material is deposited in the Institute of Entomology, Guizhou University, Guiyang, P. R. China (GUGC).

Systematics

Family Pontarachnidae Koenike, 1910

Genus Litarachna Walter, 1925

Litarachna curtipalpis Smit, 2003 (Figs 1–10, 24A–B)

Material examined

4♂, 4♀, Lingshui Li Autonomous County, Hainan Province, P. R. China (18.41°N, 110.06°E, 2.8 m a.s.l.) (Fig. 1A), leg. Hai-Tao Li & Lu-Ping Zhao, 17-II-2025, Slides No. HN-PO-2025021701–2025021708.

Habitat

Coastal tidal flats, depth of water 0.5m, with a large amount of organic detritus at the bottom. (Fig. 1B–D).

Diagnosis

Coxal field pink; first coxal plates separated medially; suture lines of Cx-II/III, and Cx-III/IV incomplete; a pair of lateral platelets with three pores posterior to the gonopore; a pair of wheel-like acetabula posteriorly to the genital field; P-2 having a large protrusion without seta on the ventral margin; two long fine setae near a stout seta at the middle of the ventral margin of P-4.

Male — Genital field with a sclerotized ring with a long anterior extension, and numerous (approximately 100) perigenital setae surrounding this ring.

Female — Pregenital sclerite arrow-shaped; postgenital sclerite lunate; an extra pair of wheel-like acetabula located posterior to the genital field.

Description

Living specimen: oval body color usually brownish-red (Fig. 2A, E–F), occasionally red (Fig. 2C–D), eye pigment obvious, coxal field pink (Fig. 2B).

Male (n=4) — In dorsal view: A₁ not visible; two pairs of El rounded (Fig. 3D), lying beneath the integument; idiosoma without sclerites (Fig. 3A). In ventral view: Cx-I separated medially; suture lines of Cx-II/III, and Cx-III/IV incomplete; medial apodeme of Cx-IV longer than lateral apodeme, C₄ in the middle between the apodemes; rounded V₃ much larger than other glandularia, without accompanying seta (Fig. 4C); genital field with a sclerotized ring bearing 4–5 pairs of setae and with a long anterior extension (Fig. 4A); numerous perigenital setae surrounded the genital field (Fig. 3E); a pair of lateral platelets with three pores posterior to the gonopore; a pairs of wheel-like acetabula posteriorly of the genital field (Fig. 3B, E).

Gnathosoma not fused with ACG with a short rostrum bearing two pairs of setae, apodeme located inside the idiosoma and appearing trapezoid in ventral view (Fig. 3F–G). Chelicera consisting of two segments (Fig. 3I); cheliceral claw having a narrow groove with fence-like structure, and dense minute seta-like projections in medial view (Fig. 5B); many teeth ventrally in lateral view (Fig. 5C). Palp five-segmented (Fig. 6A, E); P-1 with one dorsal seta; P-2 with 4–5 dorsal setae, and a large protrusion without seta ventrally (Fig. 6B, F); dorsal P-3 with a fine seta terminally; two long fine setae near a stout seta at the middle of the ventral margin of P-4, and a fine anteroventral seta (Fig. 6C, G); stout P-5 bearing six setae in total, with two stout claws, and two seta-like claws (Fig. 6D, H).

All legs simple, without modifications; claws well-developed and trifid, some nicks on the dorsal surface (Figs 5I; 7C, I; 8D). I–II-L with terminal ''swimming setae'' of the fourth and fifth segments (Fig. 7A–B, G–H); III–IV-L-4–5 with one and two terminal swimming setae, respectively (Figs 5G–H, 8).

Female (n=4) — In most aspects similar to male (Figs 9, 10). Sexual dimorphism exhibited in the following characters. Genital field situated between Cx-IV, pregenital sclerite arrow-shaped, postgenital sclerite lunate (Fig. 9E). An extra pair of wheel-like acetabula compared to males, located posterior to the genital field (Fig. 9B).

Measurements

Male (n=4) and Female (n=4) — See Table 1 for details.

Remarks

The diagnostic characters of the specimens from this study match the description of L. curtipalpis (Fig. 24A–B), a species known from Western Australia and Singapore (Smit 2003, 2009). However, this conclusion is only temporary, as the two localities are far apart. In the future, DNA barcoding based research may provide us with new insights.

Litarachna thetis Pešić & Smit, 2016 (Figs 11–19)

Material examined

1♂, 3♀, Lingshui Li Autonomous County, Hainan Province, P. R. China (18.41°N, 110.06°E, 2.8 m a.s.l.) (Fig. 1A), leg. Hai-Tao Li & Lu-Ping Zhao, 17-II-2025, Slides No. HN-PO-2025021709–2025021712.

Habitat

Same as Litarachna curtipalpis.

Diagnosis

Coxal field amber-colored; Cx-I separated medially; suture lines of Cx-II/III incomplete, suture lines of Cx-III/IV complete; V₃ almost at the same level of the two pairs of wheel-like acetabula; P-2 with a shallow anteroventral extension; the two long ventral setae of P-4 not lying at the same level; claws of legs well-developed, usually five-branched, but sometimes 4- or 6-branched.

Male — Genital field with a sclerotized ring bearing four pairs of setae with an anterior extension, and numerous perigenital setae (approximately 130).

Female — Genital field situated between Cx-IV.

Description

Living specimen: rounded body color red, eye pigment obvious, coxal field amber-colored (Fig. 11).

Male (n=1) — In dorsal view: A₁ not visible; El₁ larger than El₂ (Fig. 12D), lying beneath the integument; idiosoma without sclerites (Fig. 12A). In ventral view: Cx-I separated medially; suture lines of Cx-II/III incomplete, suture lines of Cx-III/IV complete; medial apodeme of Cx-IV longer than lateral apodeme, C₄ close to the tip of the lateral apodeme; V₃ much larger than other glandularia, almost at the same level of the two pairs of wheel-like acetabula; genital field with a sclerotized ring bearing four pairs of setae with an anterior extension, and numerous perigenital setae (approximately 130) (Figs 4D, 12E); two pairs of wheel-like acetabula posteriorly of the genital field (Fig. 12B).

Gnathosoma not fused with ACG, with a short rostrum and two pairs of setae, apodeme located inside the idiosoma and appearing as a rectangle in the ventral view (see Fig. 18H–I of the female). Chelicera consisting of two segments (Fig. 12G); cheliceral claw having a deep groove, and a few teeth in medial view (see Fig. 4F of the female), and many teeth ventrally (see Fig. 4G of the female). Palp five-segmented (Fig. 12F); P-1 with one dorsal seta (Fig. 13A, E); P-2 with four dorsal setae, and a shallow anteroventral extension (Fig. 13B, F); P-3 dorsally with a fine terminal seta; two long ventral setae of P-4 not lying at the same level, and three dorsal setae located terminally (Fig. 13C, G); P-5 with six setae in total (Fig. 13D, H).

All legs simple, without modifications; claws well-developed, usually five-branched, but sometimes 4- or 6-branched, some nicks on the dorsal surface (Figs 14–16C, F; 17J, K). Only one ''swimming setae'' (but it's not sure if it's strictly speaking a swimming seta, as it's noticeably thinner than the swimming setae of III–IV-L) on I-L-5 and II-L-5 (Figs 14–15A, D); III–IV-L-4–5 with one and two terminal swimming setae, respectively (Figs 16A, D; 17A–C).

Female (n=3) — In most aspects similar to male (Figs 18–19). Genital field situated between Cx-IV and medial apodemes of Cx-IV longer than lateral apodemes, pregenital sclerite arrow-shaped, postgenital sclerite lunate (Fig. 18E).

Measurements

Male (n=1) and Female (n=3) — See Table 2 for details.

Remarks

In most aspects, e.g. (1) Cx-I separated medially, suture lines of Cx-II/III incomplete and suture lines of Cx-III/IV complete, (2) P-2 with shallow anteroventral extension and (3) two long ventral setae of P-4 not lying at the same level, the specimens from this study match the description of L. thetis, a species known from South Korea and Japan (Pešić and Smit 2016). However, it is worth mentioning that the shallow anteroventral extension of P-2 of our specimens is more obvious than in the original descriptions. Different geographical populations may be a reasonable explanation for this phenomenon. For the time being we assign our specimens to this species, but more research is needed, especially with molecular techniques.

Genus Pontarachna Philippi 1840

Pontarachna australis Smit, 2003 (Figs 20–23)

Material examined

1♂, 1♀, Lingshui Li Autonomous County, Hainan Province, P. R. China (18.41°N, 110.06°E, 2.8 m a.s.l.) (Fig. 1A), leg. Hai-Tao Li & Lu-Ping Zhao, 17-II-2025, Slides No. HN-PO-2025021713–2025021714.

Habitat

Same as L. curtipalpis.

Diagnosis

L₄ close to D₃ ; Cx-I separated medially; two pairs of small wheel-like acetabula located posteriorly to the genital field; cheliceral claw stylet-like; P-2 without sclerotized ventral extension; P-4 ventrally straight, without setal tubercles.

Male — Genital field consisting of a sclerotized ring, without an anterior extension, with 12–16 setae.

Female — Genital field partially located between medial posterior apodemes of Cx-IV, postgenital sclerite with one pair of small wheel-like acetabula.

Description

Female (n=1) — In dorsal view: El₁ slightly larger than El₂ (Fig. 20D), lying beneath the integument; L₄ close to D₃ ; idiosoma without sclerites (Fig. 20A). In ventral view: Cx-I separated medially; suture lines of Cx-II/III incomplete; suture lines of Cx-III/IV posteriorly ending in long apodemes; medial apodeme of Cx-IV longer than lateral apodeme, C₄ located postlaterally the lateral apodeme; V₃ larger than other glandularia, almost at the same level as postgenital sclerite; genital field partially located between medial posterior apodemes of Cx-IV, postgenital sclerite with one pair of small wheel-like acetabula (Figs 20E); two pairs of small wheel-like acetabula (sensu Cook 1996) or specialized glandularia (sensu Tuzovskij 1978) located posteriorly to the genital field (Fig. 20B).

Gnathosoma not fused with ACG, with a short rostrum and bearing two pairs of setae (Fig. 4H). Chelicera consisting of two segments (Fig. 20G); cheliceral claw stylet-like, having a groove medially and some teeth laterally (Fig. 4I). Palp five-segmented (Fig. 20F); P-1 with one dorsal seta; P-2 with four dorsal setae, without sclerotized extension ventrally (Fig. 21B, F); P-3 with a fine dorsal seta; P-4 ventrally straight, without setal tubercles, and with two long subterminal setae (Fig. 21C, G); P-5 tapering (Fig. 21D, H).

All legs simple, without modifications; claws well-developed, four-branched (Fig. 22I). ''Swimming setae'' (but it's not sure if these are strictly speaking swimming setae, as they are noticeably thinner than the swimming setae of III–IV-L) occurring terminally of I–II-L-4–5 (Fig. 22A–D); III–IV-L-4–5 with one and two terminal swimming setae respectively (Fig. 22E–L).

Male (n=1) — In most aspects similar to female (Fig. 23). Genital field consisting of a sclerotized ring, without anterior extension, and with 12 setae on the left side and 16 setae on the right side. (Fig. 23E).

Measurements

Male (n=1) and Female (n=1) — See Table 2 for details.

Remarks

Pontarachna australis was described from a single female from Western Australia (Smit 2003). Pešić et al. (2008) collected this species from Taiwan and added the description of the male. Due to our specimens have following characters: (1) two pairs of small wheel-like acetabula or specialized glandularia located posteriorly of the genital field; (2) the male genital field consisting of a sclerotized ring, without an anterior extension; (3) the female genital field partially located between the medial posterior apodemes; (4) the postgenital sclerite with one pair of small wheel-like acetabula; and (5) a similarly shaped palp (Fig. 24C), we assign our specimens to P. australis.

Discussion

We used camera, optical microscope and SEM to make a comprehensive description of three species from Hainan. Except P. australis these species have been reported from Taiwan by Pešić et al. (2008). The remaining two, i.e. L. curtipalpis Smit, 2003 and L. thetis Pešić & Smit, 2016, are recorded new for Chinese fauna.

Cook (1974) divided Litarachna into two subgenera based on the fusion between Cx-I, but as more species were recorded later on, this system was abandoned (Cook 1986). We found that the characters of the currently known species in this genus can be divided into two groups: (1) incomplete suture lines of Cx-III/IV + gnathosoma as a trapezoid in the ventral view + P-4 with a setal tubercle, (2) complete suture lines of Cx-III/IV + gnathosoma as a rectangle in the ventral view + P-4 without a setal tubercle. Therefore, the taxonomy of this genus needs further research involving molecular techniques.

In addition, we examined the cheliceral claw of three species using SEM. They are different in shape and medial surface, i.e. P. australis is stylet-like and without seta-like projections, L. curtipalpis is ''normal'' and with dense seta-like projections, L. thetis is ''normal'' and without seta-like projections. These very strong differences in the structure of the mouth parts of these three species investigated are strongly indicating completely different dietary habits of these cohabiting species. Unfortunately, respective detailed descriptions of the mouth parts are lacking for other species of the family. Nevertheless, our results show the great potential of these characters for revisional studies. Moreover, further research should include as well molecular studies in order to provide data for phylogenetic studies.

Acknowledgements

We are indebted to Dr. Tom Goldschmidt (Germany) for providing suggestions on text and figures. Moreover, Thanks for Guang-Hui Li, Li-Ping Wang, Ai-Guo Zhao and Gui-Hua Wang (P. R. China), to help in specimen collection. This research was supported by Guizhou Provincial Science and Technology Projects (Qiankehe Pingtai Rencai-GCC [2022]029-1).

References

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