Didimacar bajelaopsis Oliver, Al-Kandari & Salvi sp. nov.

Striarca tenebrica — Glayzer et al. 1984: 324.

Striarca sculptilis — Jones 1986: 131, pl. 29.

Didimacar tenebrica — Oliver in Dance 1995: 212, fig. 933.

Didimacar tenebrica — Al-Kandari et al. 2020: 12, pl. 1 G.

Didimacar tenebrica — Oliver et al. 2023: 488, pl. 1, fig. 7.

Didimacar tenebrica — AlShuaibi et al. 2025: 146.

Material examined.

Including those sequenced (Table 1).

Holotype Kuwait • 1 sh.; Kuwait Bay, Al-Judailiat, 29.3796°N, 47.7531°E, Intertidal; 27 Apr. 2025; M. Al-Kandari leg.; NMW.Z.2021.009.044. [M 4-27].

Paratypes Kuwait • 1 sh.; Auha Island, 29.3791°N, 48.4388°E; 1 Feb. 2025; M. Al-Kandari leg.; NMW.Z.2021.009.045. [M 1-37]. Kuwait • 2 sh.; Kuwait Bay, Al-Judailiat, 29.3796°N, 47.7531°E, Intertidal; 27 Apr. 2025; M. Al-Kandari leg.; NMW.Z.2021.009.046 [M 4-25] / 047 [M 4-30]. Kuwait • 2 sh.; Kuwait Bay, Sharq, Ras Ajuza, 29.3916°N, 47.9951°E, Intertidal; 29 Jun. 2025; M. Al-Kandari leg.; NMW.Z.2021.009.048 [M 13-63] / 049 [M 13-65]. Kuwait • 4 sh.; Al-Salmiyah, Belajat Beach, 29.3376°N, 48.0965°E; 1 Jun. 2025; M. Al-Kandari leg.; NMW.Z.2021.009.050 / 051 [M 33-152] / 052 [M 33-155] / 053 [M 33-157]. Kuwait • 20 + spec. in 70 % ethanol, 4 sh. dry (figured), 2 sh. on SEM stub (figured); Shuwaikh, 29.357°N, 47.947°E; Feb. 2026; PG Oliver leg.; NMW.Z.2021.009.054. Kuwait • 2 sh.; Kuwait Bay, West Doha, 29.3706°N, 47.7699°E; 27 April 2025; M. Al-Kandari leg.; NMW.Z.2021.009.056, [M 5-36] NMW.Z.2021.009.057 [M 5-37].

Other material examined.

Kuwait • 8 sh.; Kuwait Bay, Al-Judailiat, Intertidal; 27 Apr. 2025; M. Al-Kandari leg.; KISR. Kuwait • 10 sh.; Dasman, Kuwait Towers Beach, Intertidal; 28 May 2025; M. Al-Kandari leg.; KISR. Kuwait • 6 sh.; Sharq, Ras Ajuza Beach, Intertidal; 29 May 2025; M. Al-Kandari leg.; KISR. Kuwait • 8 sh.; Al-Messilah, Intertidal; 23 Jan. 2023; M. Al-Kandari leg.; KISR Kuwait • 6 sh.; Al-Salmiyah, Belajat Beach, Intertidal; 01 Jun. 2025; M. Al-Kandari leg.; KISR. Kuwait • 10 spec. in 70 % ethanol; Al-Shumaimah, 29.657°N, 48.131°E, under stones, mid-tide level; Feb. 2023; PG Oliver leg.; NMW.Z.2021.009.055.

Comparative material examined.

Arca tenebrica Reeve, 1844, Philippines • 3 sh.; syntypes; Samar; NHMUK 1968246.

All labelled Didimacar tenebrica: Pakistan • 24 sh. + 2 v.; Karachi; NHMUK 1953.1. 23.345–364. India • 18 sh. + 3 v.; Bombay (Mumbai); NHMUK 1994082. Japan • 5 sh. + 2 v.; Fuhusaka, Kyushu; NHMUK 1994081. India • 1 sh.; Tamil Nadu, Krusadai; NHMUK. India • 1 sh., Mandwa, Mumbai; NHMUK 1953.1. 23.438. Iran • 3 sh.; Bushire; Biggs leg.; NHMUK 225. Australia • 2 sh.; Queensland, Port Curtis; NHMUK 81.12.10.71 - 72. Philippines • 1 sh.; Macandrew leg.; NHMUK 2026. Malaysia • 1 sh.; Derhala Island, Sandakan, Sabah, Borneo; Mary Saul leg.; NMW.Z.1975.045.0009. India • 1 sh.; Tamil Nadu, Tuticorin (Thoothukudi); NMW.1955.158.01896. Japan • 2 sh.; Ehime Prefecture, Iyo; NMW.1955.158.10373. Australia • 1 sh. + 1 v.; Boyne River, Port Curtis, N. Trevor leg.; NMW.Z.1984.090.0003 / 4. China • 4 sh., + 1 v, Cape D’Aguilar, Hong Kong Island, low water mark; 1986; PG Oliver leg.; NMW.Z.1896.080.0004 / 0007 / 0010. China • 3 spec; Mirs Bay, Hong Kong, dredged from 20 m; 1986; PG Oliver leg.; NMW. Z. 1989.099. China • 4 sh. + 1 v.; Shek Ngau Chau, Mirs Bay, 10-16 m; 1986; PG Oliver leg.; NMW.Z.1986.080.0005 / 6 / 8 // 9.

Didimacar nigra Lamy, 1907. Philippines • 4 sh.; Macandrew leg.; NHMUK 2026. 3 sh. Philippines, NMW.1955.158.10381.

Diagnosis.

A small noetiid, rarely exceeding 15 mm in length, tumid, distinctly umbonate in larger shells. Ligament entirely opisthodetic. Sculpture of dominantly radial riblets increasing in number with size by addition of secondary and tertiary riblets in the interspaces, unlike D. nigra, which is cancellate. Hinge plate narrow, teeth small.

Description.

Figs 3, 4, 5. Morphometrics from Fig. 6, Table 2.

Shell to 15.5 mm in length (holotype 13.8 mm, Fig. 3 A – C); robust; equivalve; inequilateral, beaks in the anterior quarter. Outline oblong, length-to-height ratio 1.25 to 1.63: 1 (x = 1.46). Anterior margin broadly rounded, posterior margin curved, sloping, ventral margin straight. Growth allometric, shells becoming disproportionately tumid with size, length to tumidity ratio varying from 2.06: 1 to 1.39: 1 (x = 1.76); height to tumidity ratio 1.03: 1 to 1.30: 1 (x = 1.20). Umbonate, beaks strongly curving to the anterior. Median area very weakly sulcate. Sculpture of fine radial riblets, about 70 at the margin of a 15 mm shell, initially with c. 20 primary riblets emanating from beak (Fig. 4 B), secondary riblets added between primaries, tertiaries added in resulting interspaces (Fig. 4 A, B). Anterior riblets stronger than median, all with sharp, crested apices; posterior riblets lower, less sharp, evenly beaded. Hinge taxodont, teeth in 2 series, anterior up to 8, posterior up to 30 in number, anterior set and posterior few larger than median teeth, all simple with striated edges (Fig. 8 G). Duplivincular ligament with multiple parallel, transverse, fibrous chevrons entirely behind the beak, heart-shaped elastic ligament below beaks and over anterior set of teeth (Fig. 3 F). Periostracum (Fig. 4) of commarginal flimsy lamellae thickened by erect bristles arising from the interspaces, these forming a dense thatch over the posterior slope. Adductor muscle scars equal in size, both with weak myophoric flanges, pallial line entire, impressed, radial pallial muscle scars prominent, extending towards the umbonal cavity. Prodissoconch domed, 190 µm in diameter, smooth with few commarginal raised lines on PII, PI approximately 70 µm in diameter (Fig. 4 D).

Anatomy (Fig. 5).

Adductor muscles oval, the posterior a little larger. Mantle edges free with folded edges posteriorly, less so anteriorly. Area around the exhalant region thickened, glandular. Single pigment spots (ocelli) on mantle edge above anterior adductor muscle. Gills of both demibranchs, outer demibranch a little smaller than inner demibranch. Labial palps proportionately large with up to 65 sorting ridges and a prominent smooth oral fold. Foot large, heel and toe well developed, deep byssal groove in toe, median byssal notch, byssus of a few fine threads. Small combined byssus retractor / pedal retractor muscles attach dorsally. Small anterior pedal retractor muscles attach dorsally adjacent to the anterior adductor muscle.

Habitat.

Lives attached by a weak byssus under rocks and stones, often in muddy or silty environments. Intertidal, across the middle and lower shores. Widespread.

Etymology.

From “ bajela ”, the Kuwaiti name for a small brown bean and the Latin suffix- opsis denoting resemblance to.

Remarks.

Didimacar bajelaopsis is rather variable in respect to overall shell shape. Much of this variation reflects a size-related increase in tumidity. The onset of this increase is not consistent, appearing in some at 12 mm in length but in others not until 15 mm (Fig. 6 B). A similar relationship is seen when height is plotted against tumidity (Fig. 6 C) but the length to height relationship appears linear (Fig. 6 A). A consequence of this is that making comparisons of shell shape between populations is difficult and requires having good samples covering the whole size range. Tooth number and radial riblet number are also subject to changes with growth, both increasing in number as the shells get bigger.

In the Indo-West Pacific, D. tenebrica is frequently reported and often confused with Striarca symmetrica, but the arrangement of the ligament, opisthodetic in Didimacar and amphidetic in Striarca is diagnostic. Striarca symmetrica is absent from Kuwait (Oliver et al. 2023) although present in the southern Arabian Gulf (Oliver in Dance 1995). A possible explanation for this may be in the adaptive function of the sorting mechanisms in Striarca and Didimacar. The size and number of sorting ridges of the labial palps have been related to survival in turbid conditions (Lim 1966). Didimacar differs anatomically from Striarca in having larger labial palps with three times as many sorting ridges (Oliver 1985, 1990). The large palps may enable Didimacar to flourish in the turbid waters of Kuwait while excluding Striarca.