Paraclepsis dongnaiensis Bolotov, Eliseeva & Kondakov, 2025

Paraclepsis dongnaiensis Bolotov, Eliseeva & Kondakov sp. nov.

= Paraclepsis vulnifera Moore (1935): 68 [nec Harding, 1924; identification error; locality: Bangkok, Thailand; host: Somanniathelphusa sinensis (H. Milne Edwards, 1853) View in CoL ( Decapoda View in CoL : Gecarcinucidae View in CoL )].

Figures 8-12, Bolotov et al. 2025c: Dataset S4; high-resolution images of the holotype and selected paratypes are deposited at figshare (https://doi.org/10.6084/m9.figshare.28922393; Bolotov et al. 2025b).

https://zoobank.org/ urn:lsid:zoobank.org:act:E9738BC4-FA5F-413B-A3CF-EDC60324AD9D

Holotype. RMBH Hir_1190-H; VIETNAM: Cát Tiên National Park, 11.4369°N, 107.4230°E, altitude 130 m, Đồng Nai River basin, Đồng Nai Province, temporary forest pools, on the carapace of freshwater crab Somanniathelphusa triangularis Đăng & Ðỗ, 2005 ( Decapoda : Gecarcinucidae ), October 2024, Ivan N. Marin leg. ( Figure 8A).

Paratypes (n = 16). VIETNAM: the type locality, the same habitat, host, collecting date and collector – 14 specimens (lot RMBH Hir_1190) ( Figure 8 B-D) . VIETNAM: underside large stones in spring at the entrance of a mountain massif, 3.5 km SW of Tri Tôn city, 10.3959°N, 104.9828°E, altitude 38 m, Tri Tôn Rural District, An Giang Province, Mekong Delta Region , 11 November 2023, Dmitry M. Palatov leg. – 2 specimens (lot RMBH Hir_1137) ( Figure 8 E-F) GoogleMaps .

Etymology. This species is named after the Đồng Nai River, in the drainage of which its type locality is situated.

Differential diagnosis. Small leech with leaf-shaped body; three pairs of eyespots in the genuscharacteristic arrangement; four rows of ovate shallow dorsal papillae of moderate size (outer paramedian and outer paramarginal series); two annuli between the male (XI a3 / XII a1) and female (XII a2/a3) genital pores; esophagus with a broad, bag-like pharingeal bulb; two pairs of compact salivary glands; 7 pairs of crop caeca. The new species differs from Paraclepsis ceylanica and Pa. cancricola by having simple crop caeca (vs bilobate).

Molecular diagnosis. The mean uncorrected COI p-distance between the new species and Pa. cancricola is 11.2 ± 0.1 %. The GenBank acc. numbers of reference DNA sequences of the new species (COI and 18S rRNA) are given in Table S1.

Description. Small leech (body length up to 11.6 mm). The measurements of the holotype and paratypes are presented in Dataset S4 on figshare ( Bolotov et al. 2025c). Body is broad, leaf-shaped, elongate-ovate ( Figure 8 A-F). Dorsum with four rows of ovate shallow papillae of moderate size on annulus a2 (outer paramedian and outer paramarginal series). Posterior sucker is small, circular (maximum diameter of 1.5 mm), and ventrally directed. Proboscis pore at the rim of anterior sucker. Coloration of living animals: dorsum is translucent, light-brown, sometimes with sparse green chromatophores; a median series of 7-8 yellow spots of irregular shape, which are larger at the posterior part of the body; four additional series of small yellow round spots at the outer paramarginal and marginal positions; some specimens have dorsum with multiple brown longitudinal lines and a marginal series of light round spots; posterior sucker is brown with light radial stripes ( Figures 9 A-C, 10A-B). It largely agrees with that of the carapace of crab hosts and, at first glance, could be considered a camouflage (cryptic) coloration ( Figures 9 A-C). Coloration of ethanol-preserved animals: dorsum is dark brown, sometimes with numerous darker longitudinal stripes; three longitudinal rows of yellow spots at marginal (large ovate spots at a3), outer paramarginal (smaller ovate spots at a2) and outer paramedian (minute dots at a2) position; the anterior eyespots-bearing part with light margin and large light spot around the third pair of eyespots. Some specimens have yellow dorsum with multiple longitudinal brown lines; median line is broad and broken, consisting of several parts, separated by ground yellow color; other lines are narrow, continuous ( Figure 8 A-F). Three pairs of circular or cup-shaped eyespots are situated on III, IV a1 and V a2 (second and third pairs of eyespots are separated by two annuli) and arranged to a triangle with narrow base: eyespots of the first pair are small, joined to a single spot and usually connected with the second pair; eyespots of the second pair are circular, situated close to each other or connected; eyespots of the third pair are circular, separate ( Figure 10 C-F). Venter is smooth, yellowish to whitish, sometimes with sparse brown to dark green chromatophores and a white patch around the gonopores. Total number of annuli: 70. Somites I-III uniannulate, IV biannulate, V– XXIV triannulate, XXV-XXVI biannulate, XXVII uniannulate. The male and female genital pores are separated by two annuli and are located in furrows XI a3 / XII a1 and XII a2/a3, respectively ( Figure 10G). Reproductive system: six pairs of large, ovate testisacs intersegmentally from XIV/XV to XIX/XX; atrium spherical, the atrial cornua massive, twisted anteriorly; paired ejaculatory ducts long, extending to XVII; paired ovisacs moderately long, arranged as loops ( Figure 11 A-D). Digestive system: proboscis sheath short, of moderate size; esophagus with a broad, bag-like pharingeal bulb; salivary glands compact, two pairs; crop with seven pairs of crop caeca: 1st–6th simple, elongate finger-like, 7th pair (posterior caeca) with four blind processes; intestine with four simple processes.

Distribution. Thailand ( Moore 1935) and Southern Vietnam ( Figure 12).

Phoretic/dwelling host. In the type locality, specimens of the new species were collected on the carapace of the freshwater crab Somanniathelphusa triangularis Đăng & Ðỗ, 2005 ( Gecarcinucidae ). In Thailand, it was collected from the gill chamber of another crab species in this genus, S. sinensis (H. Milne Edwards, 1853) ( Moore 1935) .

Primary host. The COI sequences of iDNA that was taken from the gut content of three paratypes reveal that they suck blood of the frog species Hoplobatrachus chinensis (Osbeck, 1765) ( Dicroglossidae ). The accession numbers of the host COI sequences are as follows: PV132093 (paratype RMBH Hir_1137-1); PV132094 (paratype RMBH Hir_1190 DNA-2); and PV132095 (paratype RMBH Hir_1190 DNA-3) ( Bolotov et al. 2025c: Dataset S4).

Habitats. The type series was collected from temporary forest pools (on crabs) and a spring (free-living, beneath stones). The crop of two available specimens (sample RMBH Hir_1137) from a spring is completely filled with frog host blood ( Figure 8 E-F), while that of leeches from crabs (sample RMBH Hir_1190) is less filled with the same content ( Figure 8 A-D). We assume that amphibious crabs may serve as transport hosts and shelter for dispersal through land barriers between separate water bodies, especially during dry season (see Discussion for detail).

Biogeographical notes. Available samples of the new species comprises of two distant genetic lineages with an uncorrected COI p-distance of 3.7 %. The first lineage was discovered in the Cát Tiên National Park (the type locality of the new species) and most likely inhabits the Đồng Nai River basin. The latter basin is situated east of the Mekong Delta (see Figure 12). The second lineage was recorded from a spring in An Giang, belonging to the Mekong River basin and situated west of the Mekong Delta. We were unable to find any diagnostic differences between the two lineages by means of morphological and anatomical analyses, and these samples are here considered conspecific .

The relatively large genetic distance between these populations could be explained by several reasons. First, the deep, turbulent mainstream of the Mekong River itself may serve as an effective barrier to dispersal of the crabs and the leeches using them as transport hosts (e.g., Gascon et al. 2000; Zhang et al. 2010; Wang et al. 2015; Yuan et al. 2016; De Castro Godinho and Silva 2018; Brunke et al. 2019). It is known that the Somanniathelphusa host crabs ( Gecarcinucidae ) are rather amphibious than fully aquatic (Yeo and Nguyen 1999; Shih et al. 2007). They dwell in shallow water bodies such as forest pools, streams, and flooded paddy fields. The leeches we found were collected from crabs that lived in shallow forest pools or small streams, whereas these leeches were never recorded on crabs collected from the beds of the Đồng Nai and Mekong rivers or from paddy fields in the Mekong Delta. These patterns may explain the recent lack of gene flow between populations of leeches.

Second, paleogeographic events such as periodical marine transgressions and regressions might have influence the past connectivity of freshwater leech populations in the southern edge of the the Indochina Peninsula ( Gupta 2009; Tjallingii et al. 2010). There were large fluctuations in the sea level in the Miocene, ranging between -300 and + 180 m ( Miller et al. 2005). Marine regressions connected the Mekong with other basins such as Đồng Nai and Chao Phraya ( Morley 2012; Morley and Morley 2013; Arifin et al. 2022; Giao et al. 2023). In turn, massive marine transgressions led to the flooding of the Mekong Delta with saline waters, crossing the modern borders of Cambodia and reaching the Tonle Sap Lake ( Penny 2006; Miller et al. 2005). In the Pleistocene, the eustatic sea level fluctuations reached smaller magnitudes from -120 to +50 meters ( Haq et al. 1987; Woodruff 2003; Miller et al. 2005). The deglacial sea-level rise and the maximum flooding period (19–8 and 8–6 Kya, respectively) reduced the lower part of the Mekong Delta ( Tjallingii et al. 2010; Thanh et al. 2021) and likely divided the basins of the Mekong and Đồng Nai rivers. The events, mentioned above, might have contributed to the isolation of freshwater animal populations in the region ( De Bruyn et al. 2013; Sholihah et al. 2021; Delrieu-Trottin et al. 2025).