Concavibalcis haterumaensis, Takano & Tsuzuki & Kano, 2022

Takano, Tsuyoshi, Tsuzuki, Shouji & Kano, Yasunori, 2022, Description of a second species of the eulimid genus Concavibalcis (Gastropoda: Vanikoroidea), Zootaxa 5093 (3), pp. 397-400 : 398

Concavibalcis haterumaensis n. sp.

( Fig. 1 )

Type locality. Nishi-hama , Hateruma Island, Taketomi, Okinawa, Japan (24°04'06"N, 123°45'46"E), sandy bottom at a depth of 4 m.

Type material. Holotype NSMT-Mo 79320; 8 paratypes NSMT-Mo 79321; 3 paratypes MPM Coll. No. 21794a–c, all collected from the type locality on May 30, 2019. 2 paratypes ( MPM Coll. No. 21794a, b) coated with platinum for SEM observation.

Other material examined. 12 specimens MPM Coll. No. 21795, collected together with the type specimens .

Etymology. The name refers to the type locality, Hateruma Island.

Distribution. Known only from the type locality.

Diagnosis. Species of Concavibalcis with a small, thick, white translucent shell, lacking chalky appearance. Teleoconch surface decorated with very fine spiral ridges. Aperture ovate, callused; outer lip simple, curved, constricted below suture, with most protruding part at half of aperture height in lateral view.

Description. Shell conical with blunt apex, up to 3.1 mm high, thick, solid, white translucent ( Fig. 1A, B ). Protoconch dome-shaped, smooth, glassy translucent white, with 1.3 whorls; demarcation line with teleoconch faint, straight, opisthocline; exposed part 220 μm wide and 250 μm high ( Fig. 1C ). Up to 6.4 teleconch whorls, concave near shoulder and convex below; suture well defined and encircled by a rounded keel ( Fig. 1B ); incremental scars distinct ( Fig. 1D ), situated at 0.7, 1.6, 2.0, 2.7, 3.4, 4.1, 4.5, 5.0 and 5.5 whorls from aperture in holotype. First two to three teleoconch whorls bear short, oblique, slightly curved ridges on upper two-thirds and continuous spiral ridges on lower one-third of each whorl ( Fig. 1E ). Later whorls bear spirals only, which are stronger and more widely spaced towards suture of next whorl ( Fig. 1D ). Last adult whorl occupies 55% of total shell height. Aperture ovate; outer lip simple, curved, retracted near suture, sigmoid with most protruding part at half of aperture height in lateral view; parietal wall and columellar lip joining in a slight arc; columellar and parietal callus well developed; basal lip rounded.

Dimensions. Holotype: 3.0 mm in SH and 1.2 mm in SD; 11 paratypes: 2.0– 3.1 mm in SH and 0.8–1.2 mm in SD.

Remarks. The assignment of the present new species to Concavibalcis is supported by its shouldered, concave teleoconch whorls, and its blunt apex with a dome-shaped paucispiral protoconch ( Fig. 1 ; Warén 1980b). The former condition is rare among the Eulimidae and found in only a few other genera, including Oceanida de Folin, 1870 and Scalaribalcis Warén, 1980 . However, Oceanida differs in having a taller, pupiform protoconch, and Scalaribalcis in having an ovate outline of the teleoconch with a much broader aperture ( Warén 1983a). The shell apex is blunt in such genera as Fusceulima Laseron, 1955 , Halielloides Bouchet & Warén, 1986 and Teretianax Iredale, 1918 , but their teleoconch features differ greatly from those of the present species and of C. scalaris , the type and only known species of Concavibalcis ( Warén 1983b; Souza & Pimenta 2014; Hori & Matsuda 2017).

Concavibalcis haterumaensis n. sp. is distinguished from C. scalaris in having (1) a smaller, thicker shell and (2) fine spiral ridges on teleoconch whorls. The two species have nearly the same number of teleoconch whorls (up to 6.4 and 6.5, respectively), meaning that C. haterumaensis n. sp. (3.1 mm high) is more tightly coiled with a lower expansion rate of the aperture than C. scalaris (5.1 mm high). The presence or absence of fine spirals is a species-specific character also in other eulimid genera (e.g. Annulobalcis Habe, 1965 , Melanella Bowdich, 1822 and Trochostilifer Warén, 1980 ; Warén 1980b; Gofas et al. 2011; Dgebuadze et al. 2012). Furthermore, (3) the polished translucent shell of the new species differs from the chalky shell of the type species, although the latter chalky appearance may be attributable to corrosion in the specimens examined (see Warén 1980b). Their habitats also differ from each other: (4) C. haterumaensis n. sp. was collected from a shallow subtidal site, whereas C. scalaris lives in deeper, bathyal waters (365–400 m; Warén 1980b). Several eulimid genera exhibit a similar broad bathymetric range (e.g. Melanella , Pelseneeria Koehler & Vaney, 1908 , Stilapex Iredale, 1925 and Thyca H. Adams & A. Adams, 1854 ; Warén 1980 a, 1981, 1983a; Hori & Matsuda 2017; T. Takano personal observation).

The genus Concavibalcis remains to be the least studied among the Eulimidae from phylogenetic and ecological perspectives. Eulimid species in closely related genera exploit hosts of the same echinoderm class ( Warén 1983a; Takano & Goto 2021). Future phylogenetic reconstruction for Concavibalcis based on live-taken specimens may lead to a better understanding of its parasitic ecology and vice versa.

Figures

FIGURE 1. Concavibalcis haterumaensis n. sp. A. Holotype NSMT-Mo 79320, 3.0 mm in shell height. B–E. Paratype MPM Coll. No. 21794a, 2.0 mm, SEM. B. Lateral view showing incremental scars in penultimate and last whorls. C. Close-up view of blunt apex; arrowheads denote demarcation line between protoconch and teleoconch. D. Penultimate whorl with an incremental line and fine spiral ridges. E. Surface of first teleoconch whorl showing short oblique ridges and continuous spiral ridges. Scale bars = 1 mm (A), 500 µm (B) and 100 µm (C–E).