Pusillolucina arabica, Taylor, John D. & Glover, Emily A., 2019

Taylor, John D. & Glover, Emily A., 2019, Unloved, paraphyletic or misplaced: new genera and species of small to minute lucinid bivalves and their relationships (Bivalvia, Lucinidae), ZooKeys 899, pp. 109-140 : 109

Pusillolucina arabica sp. nov. Figs 3 , 9 , 10

Type material.

Holotype: NHMUK 20191077 L 2.4 mm. Paratypes: figured L 2.1 mm, L 2.0 mm, L 1.9 mm, unfigured 11 sh, 2 v (NHMUK 20191078).

Type locality.

Arabian Gulf, Tarut Bay, Saudi Arabia, dredged (17.5.1971) K. Smythe collection.

Material examined.

NHMUK consultancy report ECM 5027C/06. Arabian Gulf: station 36 FC, 27°42'38"N, 52°11'16"E 25 m (NHMUK20191080), stn 39 FC, 27°42'05"N, 52°10'50"E 31 m, NHMUK 20191081), stn 41 FC, 27°42'31"N, 52°10'02"E 32 m (NHMUK 20191079), stn 48 FC, 27°45'0.27"N, 52°07'46"E 23 m (NHMUK 20191082).

Etymology.

arabica from Latin arabicus. Used as an adjective.

Diagnosis.

Pusillolucina with posterior lateral teeth divided into four or five cusps and sockets.

Description.

Shell very small, L to 2.4 mm, ovate, umbones prominent, sculpture of closely spaced, narrow, commarginal lamellae, sometimes elevated at posterior and anterior dorsal margins, crossed at anterior and posterior by low radial ribs, juvenile shells with commarginal lamellae only. Colour: white, translucent when wet. Protoconch: P1 84 µm, P1 + P2 = 155 µm, P2 with numerous growth increments. Lunule broadly lanceolate, smooth. Ligament internal, short, set alongside cardinal teeth. Hinge: right valve with single cardinal tooth, anterior lateral tooth located above anterior adductor muscle. Posterior lateral tooth long, divided into four or five cusps, left valve with two cardinal teeth, the anterior larger, anterior lateral tooth small, posterior lateral tooth divided into four or five sockets for cusps of right valve. Anterior adductor muscle scar short, barely detached from pallial line, posterior scar ovoid. Inner shell margin crenulate, more strongly to anterior and posterior.

Remarks.

Pusillolucina arabica differs from the Philippine P. pusilla by the more ovate outline, and the less prominent commarginal and radial sculpture. By comparison, P. africana has finer more closely spaced commarginal sculpture and more cusps (7-8) on the posterior lateral tooth. From South Africa P. denticula has much finer commarginal sculpture and up to 10 smaller cusps on the posterior lateral tooth. From Madagascar P. biritika sp. nov. has only three cusps on the posterior lateral tooth.

Despite its small size, P. arabica has the characteristic, thick inner ctenidial demibranchs with a well-developed bacteriocyte zone packed with symbiotic bacteria ca 3-4 µm ( Fig. 10N, O ) similar to other lucinids.

Figures

Figure 3. Internal drawings of left valves of Rugalucina and Pusillolucina species. Not to scale.

Figure 9. Pusillolucina arabica sp. nov. Tarut Bay, Saudi Arabia, Arabian Gulf. A-C Holotype (NHMUK 20191077) exterior of right valve and interior of left and right valves, L 2.4 mm D, E holotype, interior of left and right valves SEMs F-H paratype (NHMUK 20191078) exterior of right valve and interior of left and right valves, L 2.1 mm I, J paratype F-H (NHMUK 20191078) interior of left and right valves SEM K, L paratype (NHMUK 20191078) exterior of left and right valves, L 2.0 mm M paratype (NHMUK 20191078) dorsal view, L 1.9 mm N protoconch of holotype, scale bar 50 µm O, P paratype I-J detail of multi-cuspate posterior lateral hinge teeth. Scale bar: 200 µm.

Figure 10. Pusillolucina arabica sp. nov. Arabian Gulf NHMUK consultancy samples- see text for details. A Lateral view of right valve stn 41 C (NHMUK 20191079) stained with Rose Bengal, L 1.3 mm B lateral view of left valve stn 41 C (NHMUK 20191079), L 1.2 mm C, D exterior of left (L 1.5 mm) and right (L 1.3 mm) valves stn 39 FC (NHMUK 20191081) E, F Interior of right (L 1.4 mm) and left (L 1.5 mm) valves stn 36 FC (NHMUK 20191080) G dorsal view, stn 39 FC (NHMUK 20191081), L 1.3 mm. H juvenile shell, stn 48 FC (NHMUK 20191082), L 1.1 mm I juvenile shell, stn 36 FC (NHMUK 20191080), L 0.9 mm J-L hinge teeth of left and right valves stn 36 FC (NHMUK 20191080) M detail of posterior lateral tooth of right valve stn 36 FC (NHMUK 20191080) N section through a ctenidial demibranch with thickened bacteriocyte zone, critical point dried preparation O symbiotic bacteria. Scale bars: 300 µm (J-L); 100 µm (M); 50 µm (N); 4 µm (O).