Aenigmula sinensis Tang, Han & Kong sp. nov.

Figs 4, 5, 6

Type material.

Holotype (Fig. 4 A). 1 specimen; Qikou Village, Jimo District, Qingdao, Shandong, China; 36°36'12"N, 120°50'20"E; coll. Jiaze Tang; 27 March 2024; accession no. LSGB -mg 266200-0901. ParIatypes (Fig. 4 B, C). 2 specimens; same collection data as holotype; LSGB -mg 266200-0902, LSGB -mg 266200-0903.

Measurements.

(Table 3)

Diagnosis.

Shell elongate-conic, pale yellow and translucent; whorls convex, smooth except for growth lines, devoid of other sculpture, imperforate, protoconch dome-shaped. Aperture teardrop-shaped with one straight side, lacking external varix. Radula with main cusp of central tooth markedly prominent, nearly level with the single pair of basal denticles; lateral teeth curved, triangular in profile. Operculum paucispiral, with an eccentric nucleus.

Description.

Shell (Fig. 4 a – d). Small (<3.5 mm), elongate-conic, thin, pale yellow, and translucent; spire comprising 5–6 whorls, convex, well-inflated; suture distinct, simple. Protoconch dome-shaped. Teleoconch smooth, sculptured only with fine growth lines, lacking other sculpture. Aperture D-shaped; outer lip thin, slightly expanded; inner lip narrow, without external varix. Imperforate.

Radula (Fig. 5 A – E). Central tooth 3 + 1 + 3 / 1 1, trapezoidal, bearing a single slender conical main cusp at the apex and three sharp denticles on each side (distinctly shorter than the main cusp and decreasing in length laterally), with one pair of well-developed, elongated conical basal denticles (Fig. 5 D). Lateral teeth 5–6 + 1 + 5, distinctly asymmetrical; main cusp offset inward; inner denticles comb-like and subequal in size; outer denticles arranged more sparsely and gradually diminishing in size outward (Fig. 5 C). Marginal teeth elongated and curved, comprising inner and outer marginal teeth: inner marginal teeth with relatively broad base, bearing 20–22 sharp cusps externally and only 3–5 small denticles internally (Fig. 5 B); outer marginal teeth with 10–15 cusps confined to the inner margin (Fig. 5 E).

Operculum (Fig. 5 F). Operculum D-shaped; paucispiral, with an eccentric nucleus situated near the anterior umbilical margin; the surface exhibits only faint spiral growth lines near the nucleus.

Head-foot (Fig. 6). Cephalic region pale yellow, darker on the head; periocular areas with pale yellow chromatophores. Posterior margin of foot simple, lacking caudal cirri.

Distribution and habitat.

Aenigmula sinensis inhabits a rocky habitat in the mid-intertidal zone of muddy flats, where the rocks are partially buried in mud, creating narrow gaps between the rock base and the mud surface; individuals are distributed on the underside of rocks, predominantly within concavities and rough crevices.

Etymology.

The specific name sinensis refers to the country where the new species was discovered. This represents the first record of the genus Aenigmula in China.

Remarks.

This species is small-sized and was collected from beneath rocks in muddy intertidal zones, where it occurs in dense aggregations. This marks the first record of the genus in China, and it has not been observed in other regions of the country to date. Compared to the type locality of A. criscionei (Rapid Creek, Australia), the two species are geographically distant and exhibit markedly different habitat and climatic characteristics: A. sinensis sp. nov. was discovered beneath rocks in muddy intertidal zones under a temperate monsoon climate zone, while A. criscionei was collected from a mangrove forest in a tropical savanna climate zone.

In morphology, adults of this species can be readily distinguished from A. criscionei (detailed differences are shown in Table 4). Although its subadults closely resemble A. criscionei in shell shape, this species can be identified by its pale-yellow shell, more prominent whorls, exceptionally thin aperture, and a straight inner lip without any curvature. Molecular data further support this morphological differentiation and confirm their distinct species status.