Helicostoa liuae Zhang, Shi & Chen, 2024

Helicostoa liuae Zhang, Shi & Chen sp. nov.

LSID urn:lsid:zoobank.org:act:07CE74EC-AEC9-4A54- 8883-DCED59B513A6

Holotype: adult female ( IZCAS-FG-609823 ) collected on 8 May 2022 by Xu-Cheng Wei from limestone in Long River, under Long River Bridge , Yizhou District, Hechi City, Guangxi Zhuang Autonomous Region, China .

Paratypes: 1 adult male designated as allotype ( IZCAS-FG-609824 ) GoogleMaps , 16 adult females (5 IZCAS, 5 PKU, 3 SZA, 3 CZY) GoogleMaps , 17 adult males (7 IZCAS, 8 PKU, 1 SZA, 1 CZY) GoogleMaps , 1 limestone with 9 adult females and 7 adult males ( IZCAS) GoogleMaps , 1 sub-adult male ( CZY) GoogleMaps , 8 sub-adult females (3 IZCAS, 5 CZY), all collected on July 18, 2022 by Zi-Ang Shi from limestone in Long River , Yizhou District, Hechi City, Guangxi Zhuang Autonomous Region, China (24.5020206°N, 108.6306600°E) GoogleMaps ; 3 adult females ( ZMB) , 4 adult females ( CZY), all collected on May 8, 2022 by Xu-Cheng Wei from the limestone in Long River , Yizhou District, Hechi City, Guangxi Zhuang Autonomous Region, China; 6 adult females (4 IZCAS, 2 PKU) , 3 adult male (1 IZCAS, 2 CZY) , 1 adult female ( CZY), all collected by local people in the junction of Qian, Yu and Xun River in Guiping City, Guangxi Zhuang Autonomous Region, China ; 1 adult female ( MNHN-IM-2000-38621 , ‘T type’) , 15 adult females ( MNHN-IM-2000-38622 , ‘T type’) .

Etymology: the species is named in honour of Yue-Ying Liu, the late pioneer researcher of Chinese freshwater molluscs.

Description: shell of adult female ( figure 4 a–f View Figure 4 ; electronic supplementary material, figure S 2 a–f View Figure 2 ; table 1 View Table 1 ) 4.17 to 6.75 mm in width, globose, thin but solid, orange yellow to red in colour; low spiral, most with around 3.5 to 4 inflated whorls, protoconch broad and smooth, teleoconch whorls with dense fine ribs; on the middle of the last half whorl upper end and lower end of outer lip extending and merging together, enclosing a wide bow-shaped opening, convex edge of the bow-shaped opening close to the original aperture, seam between both ends of outer lip clearly visible; original aperture large, ovate, but sealed by shell and attached rock, foreign objects sometimes embedded into sealed aperture, inner lip extending and covering umbilicus.

Shell of adult male ( figure 4 g–l View Figure 4 ; electronic supplementary material, figure S 2 g–n View Figure 2 ; table 1 View Table 1 ) 2.61 to 3.51 mm in width, oblate, thin but solid, pale yellow to orange yellow in colour; low spiral, most with 2.25 to 3 inflated whorls, protoconch broad and smooth, teleoconch whorls with dense fine ribs; aperture large, ovate, labral margin obviously extended, umbilicus mostly covered by inner lip, sometimes open.

Protoconch ( figure 5 h View Figure 5 ) broad, low dome-shaped, nucleus of protoconch relatively smooth, the rest of protoconch with fine granule and several spiral striations; no other obvious sexual dimorphism except for the size.

Operculum ( figure 5 a–b View Figure 5 ) calcareous, ovate, thin and fragile, transparent white in colour; on exterior surface a paucispiral nucleus visible, with one anti-clockwise whorl, located on the lower central part, concentric growth lines surrounding the nucleus; on interior surface a large inner opercular region visible, with many grains and veins; no other obvious sexual dimorphism except for the size.

Head, foot and mantle of living animals ( figure 5 c View Figure 5 ) transparent white or grey, with fine dense yellow pigment spots on the upper side of snout, tentacles, region around eyes and mantle edge, and sparse yellow pigment spots and dense fine black spots on the mantle; ethanol-preserved specimens ( figure 5 d–e View Figure 5 ) with pale yellow body colour and irregular black spots on the surface of mantle.

Male genital ( figure 5 g View Figure 5 ) with very long flagellum, about five times as the length of penis; the thickness of penis uniform, tip pointed, length of penial appendix same as that of distal part (from the base of penial appendix to the tip of penis) of penis, length of distal part of penis same as that of proximal part (from the base of penial appendix to the base of penis) of the penis; penis situated in the neck behind the right tentacle of male.

Egg capsules kept in ethanol ( figure 5 f View Figure 5 ) white, with a group size of 6 individuals, clustering together tightly into a round flat shape.

Radula (n = 5, 3 females and 2 males; electronic supplementary material, figure S 3 View Figure 3 ) oblong. Central teeth with 5 to 6 cusps in females or 4 cusps in males on either side of median cusp, median cusp with wider base longer than adjacent cusps; 3 to 4 cusps on either lateral edge of central teeth, inner pair much larger than others. Lateral teeth with 5 to 7 cusps in females or 5 to 6 cusps in males on either side of median cusp, median cusp with wider base longer than adjacent cusps; inner lateral edge of lateral teeth with a concave on lower part, only half of outer lateral edge in length. Marginal teeth slender, slightly oblong; inner marginal teeth with 25 to 41 cusps in females or 29 to 43 cusps in males, the fourth or fifth cusp in females or sixth to seventh cusp in males counting from outer side larger than the other cusps; outer marginal teeth with 23 to 24 cusps in females or 23 to 27 cusps in males.

Morphological comparisons: the shell of Helicostoa liuae sp. nov. displays a striking sexual dimorphism, which is related to different life histories of males and females. The shell of the sessile adult female can be distinguished from that of the vagile adult male based on its much larger size, more whorls, higher spiral, closed primary aperture, and with a bow-shaped opening. The sessile adult female can be distinguished from Helicostoa sinensis based on its much smaller size, conical shell, a wide bow-shaped opening and ovate operculum. The shell of vagile adult male is most similar to that of another bithyniid species Sierraia expansilabrum Brown, 1988 from Sierra Leone, which has a small globose shell and extended labral margin. However, it can be easily distinguished from species of Sierraia based on its much smaller oblate orange shell, operculum characters and many more cusps on the marginal teeth of radula. The shell of juvenile female of Helicostoa liuae sp. nov. can be distinguished from that of the juvenile male based on its higher spire.

Ecology: this species is mainly found on the surface of limestone rocks in big rivers, but it sometime also can be found attached on bricks and snail or mussel shells in big rivers; adult females are sessile and adult males are vagile ( figure 1 f View Figure 1 ); sessile adult females produce egg capsules in the cavity of shell, and capsules cluster together, tightly attaching to the sealed aperture. When kept in aquarium, the sessile females could actively do suspension feeding; the red form of living planktonic algae Chlamydomonas reinhardtii was accepted as food by the snails, and the snails excrete red faeces ( figure 1 g View Figure 1 ); the sessile females could stretch out from the secondary aperture ( figure 1 e View Figure 1 ), but normally only stay close to the secondary aperture in the shell ( figure 1 g View Figure 1 ).

Distribution: the species currently is only found in Long River (龙江), Qian River (黔江) and Xun River (浔江) in Guangxi Zhuang Autonomous Region, China. However, since this species was found syntopic with Helicostoa sinensis , it apparently shares or used to share at least parts of its distribution range with Helicostoa sinensis .