Semisulcospira nakanoi Sawada, 2023

Sawada, Naoto & Fuke, Yusuke, 2023, Diversification in ancient Lake Biwa: integrative taxonomy reveals overlooked species diversity of the Japanese freshwater snail genus Semisulcospira (Mollusca: Semisulcospiridae), Contributions to Zoology 92 (1), pp. 1-37 : 28-30

Semisulcospira nakanoi Sawada sp. nov.

[New Japanese name: Chikubu-kawanina] ( table 2 , supplementary table S2 ; figs 10 , 12M, N ) urn:lsid:zoobank.org:act:9CB53BD3-F776-4ACB-9C9D-DE0755AADA16

Melania niponica – Annandale, 1916: 45, pl. 3, fig. 3B (part).

Melania biwae – Annandale, 1916: 45, pl. 3, fig. 4 . Semisulcospira niponica – Kuroda, 1929: 186, 189, pl. 5, fig. 31 (part); Hirase & Taki, 1951: pl. 82, fig. 15; Kuroda, 1962: 86 (part; TAU); Kuroda & Habe, 1965: 57 (part); Habe & Kosuge, 1967: 28, pl. 11, fig. 21 (part); Watanabe, 1984: fig. 3b, f ; Watanabe, 1989: 11–13, 47, figs 10 , 27, 28 (part). Semisulcospira niponica var. biwae – Fukuoka, 1933: 114, 117, fig. 6 .

Semisulcospira (Biwamelania) niponica – Nishino, 1991: 9–10, unnumbered figures (part); Kihira et al., 2009: 24–25 (part).

Semisulcospira nipponica biwae – Higo & Goto, 1993: 97; Goto & Poppe, 1996: 204.

Semisulcospira (Biwamelania) niponica (biwae type) – Watanabe & Nishino, 1995: 17–18, fig. 5b (part).

Biwamelania niponica – Nishino & Tanida, 2018: 56, 251 (part).

Material examined: Holotype: KUZ Z4122, adult female with embryonic shells treated with sodium hypochlorite, collected from depth of 3.0 m of Lake Biwa at Chikubu-shima, Nagahama City, Shiga Prefecture, Japan, on 9 September 2020 by first author . Paratypes: KUZ Z4123–Z4126, 3 adult females, 1 adult male, collected with holotype .

Additional materials: KUZ Z4127, 11 females, KUZ Z4128, 2 males, collected with type specimens; KUZ Z4129, 16 females, KUZ Z4130, 5 males, collected from Onoe Port on 25 July 2021 .

Etymology: The specific name is dedicated to Dr Takafumi Nakano, who greatly supported the first author’s study of Japanese Semisulcospira .

Diagnosis: Viviparous semisulcospirid. Adult shell medium sized (SH 30.6 ± 2.7, 23.1 ± 3.0 mm; BWL 18.9 ± 1.3, 14.8 ± 1.7 mm), nearly triangular (SA 21.5 ± 2.6, 21.4 ± 3.5 degrees); color in black background; outer lip of aperture moderately swell (ASL 0.20 ± 0.05, 0.24 ± 0.06 mm); 2.5 ± 0.5, 2.3 ± 0.6 BCN; longitudinal ribs relatively few, prominently nodulated on penultimate whorl (RN 11.3 ± 1.3, 9.4 ± 1.2); 3.7 ± 0.6, 3.4 ± 0.5 SCN; 1.6 ± 0.1, 1.6 ± 0.1 ASR; 2.9 ± 0.2, 3.2 ± 0.6 WER. Embryonic shell small, with distinct nodes on surface; color in black background or beige background with 3 brown or black bands.

Description of holotype: Adult shell ( fig. 10A–C ): AH 11.5 mm, AL 12.5 mm, ASR 1.48, AW 8.5 mm, BCN 2, BWL 19.7 mm, FWL 4.4 mm; NL 0.18 mm; PWL 7.3 mm, RN 9, SA 23.1 degrees, SCN 4, SH 34.1 mm, SW 15.7 mm, TWL 5.9 mm, WER 2.61; WN 4.75; shell nearly thick triangular; suture hardly undulating; whorl sides slightly convex; outer lip of aperture moderately swell; longitudinal ribs slightly curved, oblique, prominently nodulated in upper to body whorl; apex of shell eroded; shell colored black background without color band.

Operculum ( fig. 10D ): 8.9 mm in long diameter; nearly egg-shaped subcircular, paucispiral, comprising around 3 whorls. Nucleus subcentral.

Embryonic shells ( fig. 10E–G ): EN 62, RNE 12, SHE 2.3 mm, SWE 2.2 mm, WNE 2.5; shell globose; suture prominently undulating; remarkable nodes on central part of whorls and weak keels on lower part of whorls; shell colored beige background with 3 brown or black bands on upper and/or lower part of each whorl and/or on basal part of shell.

Radula: Taenioglossa consisting of rachidian in single, lateral teeth, interior and exterior marginal teeth in double row. Rachidian roughly triangular with large central denticle and 3 minor pointed triangular cusps on each side. Lateral teeth with large central denticle, 3–4 inner and outer pointed cusps. Central denticle tip of rachidian and lateral teeth mostly flat; central denticle of rachidian approximately regular triangular, about 2.5 times longer than other triangular cusps; central denticle of lateral teeth irregular triangular, about 1.5 times longer than other triangular cusps. Interior and exterior marginal teeth spoon-shaped with 5–6 rounded denticles.

Reproductive organ: Female: Long narrow oviduct, entering near seminal receptacle with long protrusions. Ventral edge of spermatophore bursa with curved sperm gutter, extending toward mantle cavity. Brood pouch on dorsal side of spermatophore bursa, inflated dorsally, separated into many cells, including eggs and embryos; eggs and embryos radially developing from base of brood pouch near seminal receptacle and embryos in anterior or dorsal cells more developed.

Variation: Adult shells ( fig. 10M, P ): Measurements and counts shown in table 2 and supplementary table S2 . Shell nearly thick to elongated triangular; surface sculptures almost straight to slightly curved, oblique, rarely ribbed on upper whorls; shell colored black to brackish olive background with or without several color bands on upper and lower part of each whorl and basal part of shell; surface of shells covered with thin algae layer before treatment.

Operculums ( fig. 10I, L, N, Q ): 7.1–8.9 mm in long diameter.

Embryonic shells ( fig. 10J, Q ): Measurements and counts shown in table 2 and supplementary table S2 . Shell with or without keels on lower part of whorls; shell colored blackish brown background or beige background with 3 brown or black bands on upper and/or lower part of each whorl and/or on basal part of shell.

Radulae ( fig. 12M, N ): Rachidian with 2–4 minor pointed triangular cusps on each side; lateral teeth with 2–4 inner and outer pointed cusps; central denticle of rachidian about 1.5 to 4 times longer than other triangular cusps; central denticle of lateral teeth about 1.5 to 4.0 times longer than other triangular cusps; 4–6 rounded denticles in interior and exterior marginal teeth.

Reproductive organs: Male: Gonad consisting of testes, vas deferens, and prostate without penis. Posterior ventral part of inflated prostate with deep groove, forming U-shape in transverse section. Anterior prostate narrowly opening to mantle cavity.

Distribution and ecology: The new species has been found only in Chikubu-shima Island and the coastal Onoe Port in Lake Biwa ( fig. 1 ). Semisulcospira nakanoi sp. nov. inhabits the wharf and rocky bottom of Chikubu-shima Island and concrete blocks around Onoe Port. This species was found with S. morii and S. habei at Chikubu-shima Island.

Remarks: The new species has been identified as S. biwae together with S. salebrosa sp. nov. and has been treated as a junior synonym of S. niponica since Kuroda (1929). Semisulcospira nakanoi sp. nov. resembles

S. salebrosa sp. nov. with well-developed nodes on the shell surface. However, the former species is distinguishable from the latter by the smaller adult and embryonic shell size and less undulating outer lip of the aperture. The new species can be discriminated from other congeners by its large, blackish, triangular adult shell, prominent granulated ribs, the moderately undulating outer lip of the aperture, and globose embryonic shells with several brown bands (supplementary fig. S1 ).

Figures

FIGURE 1 Map of Lake Biwa (A) and Oura Bay (B) showing 15 sampling localities:blue, Semisulcospira niponica; green, S.fuscata; purple, S.watanabei sp.nov.; red, S.nakanoi sp.nov.; orange, S.salebrosa sp.nov; black, putative hybrid between S.fuscata and S.watanabei sp.nov.

FIGURE 3 Results of principal components analysis based on 738 SNPs conducted for five Semisulcospira species. A, Principal component (PC) 1 vs PC2.B, PC2 vs PC3. C, PC3 vs PC4. D, PC4 vs PC5

FIGURE 4 Results of ADMIXTURE analysis based on 738 SNPs conducted for five Semisulcospira species.Bar colours of the species in K = 5 correspond to the colour coding in fig.3

FIGURE 5 Phylogenetic relationships of five Semisulcospira species estimated by the Neighbor-Net reconstructed based on uncorrected p-distances of 738 SNPs

FIGURE 6 Map of Lake Biwa indicating geographical variation in the frequency of sculpture types in the adult females of five Semisulcospira species.Colours of locality names correspond to the colour coding in fig.1

FIGURE 10 Shells of Semisulcospira nakanoi sp. nov. A–G,Holotype, KUZ Z4122.H–J, Paratype, KUZ Z4125. K–L, Paratype, KUZ Z4126.M–Q, Specimens from Onoe Port, KUZ Z4129, Z4130.A–C, H, K, M, P, Adult shell. A–C, H, M, Female.K, P,Male.D, I, L, N, Q, Operculum.E–G, J, O, Embryonic shell.Scale bars: 10 mm (A–D, H–I, K–L, M–N, P–Q), 1 mm (E–G, J, O). All specimens were treated with 3% sodium hypochlorite

FIGURE 12 Radulae of Semisulcospira species.A–G, Semisulcospira niponica: A, Oura Port, KUZ Z4091; B, Nagahama Port, KUZ Z4093; C, Iso, KUZ Z4095; D, Kitakomatsu Port, KUZ Z4097; E, Katata Port, KUZ Z4099; F, Otsu Port, KUZ Z4101; G, Nango, KUZ Z4103.H–J, Semisulcospira fuscata: H, Lake Yogo, KUZ Z4103; I, Oura, KUZ Z4105; J, Nihonmatsu, KUZ Z4107.K–L, Semisulcospira watanabei sp. nov.:K, Kitakomatsu Port, KUZ Z4115;L, Horikiri Port, KUZ Z4120.M–N, Semisulcospira nakanoi sp. nov.: M, Onoe Port, KUZ Z4129;N, Chikubu-shima Island, KUZ Z4127.O–P, Semisulcospira salebrosa sp. nov.:O, Take-shima Island, KUZ Z4138;P, Shiraishi-jima Island, KUZ Z4136.Scale bars:100 Μm