The First Report of a Water Mite Unionicola (Trombidiformes: Unionicolidae) Infection in Filopaludina spp. (Gastropoda: Viviparidae) from Thailand with a Description of a New Species of Unionicola (Polyatax) kasetbangkhenensis sp. nov.

Simple Summary Simple Summary: Water mites are a significant population of Arachnida in aquatic ecosystems, with over 7500 species described worldwide. The genus Unionicola, belonging to the family Unionicolidae, is known for its parasitic behavior, often consuming the mucus and tissue of its molluscan hosts. While most species in this genus parasitize mussels, some have been found in gastropods. In our study, we discovered water mites infesting two species of gastropods: Filopaludina sumatrensis polygramma and F. martensi martensi. Through a comparison with known species, we identified this mite as a new species within the genus Unionicola, subgenus Polyatax, and named it “Unionicola (Polyatax) kasetbangkhenensis sp. nov.”. Abstract Two species of gastropods, Filopaludina sumatrensis polygramma and F. martensi martensi, were found infested with water mites of the genus Unionicola in Bangkok and Nonthaburi provinces, Thailand. Morphological studies on these water mites, based on the characteristics of their genital acetabular and female genital fields, identified them as a new member of subgenus Polyatax. Unionicola (Polyatax) kasetbangkhenensis sp. nov. was named after the first location where this parasitic water mite was discovered. This species is distinguished from others by the pattern of their female genital field, with their anterior acetabular plates each bearing two acetabula and a short thick spine on the inner margin. Additionally, it differs from other species by the structure of the spines in its pedipalps and legs, as well as in the shapes of its coxal plates.


Introduction
Water mites (Acariformes: Hydrachnidia) are a significant population of Arachnida in aquatic ecosystems, with over 7500 species described worldwide [1].Most aquatic habitats where water mites live contain freshwater, while most members of the family Pontarachnidae are found in marine environments [2].Most water mites are predators [3], but some groups of water mites have been reported as parasites of other aquatic animals, such as unionicolid mites (Family Unionicolidae), which are associated with freshwater Animals 2024, 14, 1627 2 of 16 sponge, mussels, and snails [4,5].This family is distributed worldwide and consists of five genera with at least 40 described species [6].Significantly, the genus Unionicola is frequently documented as being parasitic, ingesting the mucus and tissue of its molluscan hosts [7,8].In Thailand, six species in four subgenera have been reported: Unionicola (Fulleratax) robacki Vidrine, 1984 [7,[9][10][11].
In our research, we conducted a survey of parasites in freshwater snails from the central and eastern provinces of Thailand.Remarkably, studies on water mites infesting freshwater snails from this area have been limited.Therefore, this research represents a novel report of the unionicolid water mite group found in the viviparid snail genus Filopaludina in Thailand.

Snail Specimen Collection
A total of 39 canals and ponds in eight provinces-Bangkok, Nakhon Pathom, Nonthaburi, Samut Sakhon, Samut Prakan, and Pathum Thani for the central provinces, and Chonburi and Rayong for the eastern provinces-were designated as snail sampling sites.The freshwater snails were randomly sampled by hand using a hand net between February 2020 and March 2024 (3 times/year) and following the counts per unit of time sampling method by Olivier and Schneiderman (1956) [12].The method involved a single collector spending approximately 20 min collecting snail samples at each sampling site.All snails collected at each sampling site were kept in plastic bags with perforation and then transported to the laboratory of the Department of Zoology, Faculty of Science, Kasetsart University, for examination of any water mite infections.Species identification of the collected snails was conducted based on their morphological features, following the identification keys of Brandt (1974) and Upatham et al. (1983) [13,14].

Examination of Water Mite Infection
The snail specimens were anesthetized by immersion in 5% ethanol before being euthanized by gentle pressing between a pair of petri dishes.The pressed snail bodies were then examined under a stereomicroscope to explore the appearance of water mites in their tissue.Water mite specimens were collected using a needle and glass dropper and then washed several times in distilled water before preservation in 96% ethanol.The number of water mites and infected snails was recorded to calculate their prevalence and mean intensity, following the method of Bush et al. 1997 [15].

Morphological Study
Both fresh and preserved water mite specimens were examined and photographed for morphological characteristics using an Olympus BX51 compound light microscope with a DP70 camera (Olympus Corporation, Tokyo, Japan).The photographs of the water mites were then utilized to measure their morphological characteristics via ImageJ software version 1.54 [16].The measured features and their methodology of measurement, modified from Vidrine (1996) [17], were as follows: TL-total body length, including the capitulum; TW-total body width, using the widest part; CL-chelicera length; LDS-length of the whole dorsal shield (both the anterior and posterior shields); LADS-length of the anterior dorsal shield; LPDS-length of the posterior dorsal shield; AC-length of the anterior coxal group (coxal plates I-II); PC-length of the posterior coxal group (coxal plates III-IV); GFL-genital field length; AGFW-width of both anterior genital field plates; AGFWSwidth of a single anterior genital field plate; PGFW-width of both posterior genital field plates (present only in females); PGFWS-width of a single posterior genital field plate; GS-genital spine; P-pedipalp; DLP-dorsal length of the pedipalp segments; LIfirst walking leg (leg I); LII-second walking leg (leg II); LIII-third walking leg (leg III); LIV-fourth walking leg (leg IV); TCt-trochanter (segment of a leg proximal to the coxa); BFe-basifemur (segment of a leg immediately distal to the coxa); Fe-femur (segment of the pedipalps immediately distal to the coxa); TFe-telofemur; Ge-genu (segment of the pedipalps and walking legs immediately proximal to the tibia); Ti-tibia (penultimate segment of the pedipalps and legs); and Ta-tarsus (ultimate or distal-most segment of the pedipalps and legs).The measurements were expressed in microns (µm) and as means (range: min-max).All of the type specimens were deposited in the Zoological Museum at Kasetsart University, Thailand (ZMKU).

Scanning Electron Microscope (SEM) Study
The protocol for preparing well-preserved specimens of water mites for scanning electron microscopy was adapted from Chiangkul et al. (2021) [18].Initially, all specimens were fixed in absolute ethanol for 24 h before being critically point-dried using a critical point dryer (CPD) (Quorum Technology, Lewes, UK: Polaron-model CPD7501).Subsequently, the dried specimens were mounted onto the stubs using carbon tape and coated with gold particles using a sputter coater (Quorum Technology-model SC7620).Finally, all coated specimens were examined and micrographed using a Quanta 450 scanning electron microscope at the Scientific Equipment Center, Faculty of Science, Kasetsart University.

Host and Infection Rate
All of the freshwater snails collected from the 39 canals and ponds in Central and Eastern Thailand between February 2020 and March 2024 were classified into 16 genera in 10 families (as seen in Table 1).Among the nineteen species examined from Bangkok and Nonthaburi provinces, two species were found to be infected with water mites: Filopaludina martensi martensi (Frauenfeld, 1864) and F. sumatrensis polygramma (Martens, 1860) (Figure 1).The first location where water mites were found was the water pond in Warunawan Urban Forest Park, Kasetsart University, Bangkok province (13°50′50.9598″100°33′45.8382″),followed by Klong Phra Pimol Racha, Nonthaburi province (13°55′2.8842″100°25′11.9274″).These two host species had different prevalences, with F. sumatrensis polygramma having a higher prevalence than F. martensi martensi (17.10% and 3.83%, respectively).However, the mean intensities of the two host species were 2.49 and 2.18 parasites/infected host (Table 2).

General Description
All of the collected water mites were classified as a single species in the family Unionicolidae, subfamily Unionicolinae, which is characterized by their anterior legs often having strong, movable setae; all their legs bearing simple claws; their genital fields having numerous pairs of acetabula; their females having two pairs of genital plates and heavy setae; and their palps having five segments, with P4 having prominent, pointed ventral and lateral extensions.This novel water mite species was classified as a member of the genus Unionicola due to its Cx III+IV being large and mostly rectangular in shape, its coxal surface having a generally reticular structure, its LI being generally stout with simple setae, and its genital field having many pairs of acetabula on its two pairs of genital plates [19].This complex genus contains more than 45 subgenera; however, only subgenus Polyatax has been reported to parasitize freshwater mussels and viviparid snails in Asia and North America [20].The water mites in the subgenus Polyatax typically have dark bodies, usually around 2.0 mm in length; pedipalps that are distinctively subcylindrical; and bifid tarsal claws midway on the dorsal side of their walking legs [17].
With characteristics of this genus exhibited in the collected water mites, particularly the shapes of their coxal plates, the characteristics of their female genital plates (two pairs of genital plates and heavy setae), the shape of their palps, and the characteristics of the setae in their leg, these specimens could be inferred to belong to the genus Unionicola, subgenus Polyatax.Additional distinctive morphological characteristics of this novel species are described as follows.

Habitat
Type locality: Unionicola (Polyatax) kasetbangkhenensis sp.nov.parasitize viviparid snails (F.sumatrensis polygramma and F. martensi martensi) found in the benthic zone of ponds at Kasetsart University, main campus (Bangkhen), Bangkok, Thailand.The ponds have shallow water with a depth of approximately less than one meter, including numerous aquatic plants (duckweed, water hyacinths, creeping burhead, etc.) growing under water.

Table 1 .
List of freshwater snails found in central and eastern Thailand in this study.

Table 2 .
Prevalence and mean intensity of water mite infection in freshwater snail Filopaludina spp.No.ExaminedNo.Infected Prevalence (%)