Aequipecten commutatus peripheralis, Dijkstra & Kilburn, 2001

Dijkstra, H. H. & Kilburn, R. N., 2001, The family Pectinidae in South Africa and Mozambique (Mollusca: Bivalvia: Pectinoidea), African Invertebrates 42, pp. 263-321 : 307-309

Aequipecten commutatus peripheralis subsp. n.

( Figs 46–48 )

Description of holotype (LV): Shell rather compressed, up to ca. 20 mm high, suborbicular, higher than wide, inequilateral (posteriorly obliquely ovate), auricles slightly unequal in size, umbonal angle 90°. LV sculptured with 19 radial costae, solid and nearly smooth, anteriorly and posteriorly secondary 3–4 radial riblets, preradial stage of granular microsculpture, radial stage with very weak interstitial microscopic antimarginal striae near ventral margin, interstitial commarginal lamellae, commencing at early radial stage and extending to central part of disc. Auricles with 7 scaly radial

48

riblets, anterior auricle marginally repaired. Hinge line straight. Interior marginal carinate. Prominent auricular crura. Resilifer triangular, slightly oblique. Colour creamy with white and orange spots and streaks (V-shaped markings). Dimensions of holotype: height 19.4 mm, length 18.4 mm, depth (one valve) 2.7 mm ).

Additional description of paratypes (E8964): RV more convex than holotype, up to ca. 30 mm high, usually smaller, suborbicular, inequilateral. Auricles slightly unequal (anterior larger than posterior). Interstitial commarginal lamellae usually extending to ventral margin. Macrosculpture more prominent than in holotype, radial ribs near ventral margin sometimes tripartite. Anterior auricle of RV with more prominent radial riblets than on posterior. Byssal fasciole small, byssal notch obsolete. Active ctenolium with 3–5 teeth. Coloration of RV weaker.

Type locality: SOUTH AFRICA: Zululand : off Matigulu River mouth, 29°21.9'S, 31°56.2'E, 200–220 m, bottom of mud, coarse sand and Dendrophyllia , dead, NMDP Stn ZR 9, 16.vi.1989 .

Type material: Holotype NMSA E8964 /T1731, data as above, 1 LV.

Paratypes (44 LV, 64 RV) : SOUTH AFRICA: Zululand : off Matigulu River mouth, 300 m, soft mud, dead ( NMSA E8875 /T1733: 2 LV); same loc., 200–220 m, mud, coarse sand, Dendrophyllia , dead ( MNHN: 1 LV, 1 RV; NMSA V7830 /T1732: 15 LV, 28 RV; ZMA Moll. 3.99.004: 1 LV, 1 RV) . Transkei : off Whale Rock , 430–450 m, fine muddy sand, dead ( NMSA C8887 /T1734: 2 LV); same loc., 500 m, fine sandy mud, dead ( NMSA C9242 /T1735: 1 LV); same loc., 350 m, fine muddy sand, dead ( NMSA C9408 /T1736: 1 RV); same loc., 250–280 m, sand and shell rubble, dead ( NMSA C8620 / T1737: 1 LV); off Mendu Point, 300 m, coarse sand, dead ( NMSA C6548 /T1738: 1 RV); same loc., 250 m, coarse sand, rubble, few sponges, dead ( NMSA C6393 /T1739: 3 RV); off Qolora River , 290–300 m, fine muddy sand, broken shell, dead ( NMSA C6983 /T1730: 3 LV, 1 RV); off Qora River, 400 m, sand, dead ( NMSA C4898 /T1741: 15 LV, 20 RV); off Kei River, 390 m, coarse sand, dead ( NMSA C3854 /T1742: 2 LV, 8 RV) .

Distribution: Continental slope of KwaZulu-Natal and Transkei, South Africa.

Habitat: Bathyal (200–500 m) on soft sediments (mud, muddy sand or sand). No live specimens have been collected.

Discussion: A. commutatus peripheralis differs only in small degree from A. commutatus commutatus ( Monterosato, 1875) of the Mediterranean and W. Africa, but the two populations are widely allopatric. In the south-east African population the shell is less solid, more obliquely elongated, the RV more depressed, and with a finer sculpture of commarginal lamellae; also interstitial secondary radial riblets (1–2 per interval) are sometimes present near the ventral margin of the LV. Worn juveniles can be easily confused with Cryptopecten bullatus ( Dautzenberg & Bavay, 1912) , but the latter has more numerous radial costae (22–24) and is ornamented with vesicular radial costae.

Etymology: peripheralis = at the periphery - alluding to its origin as a population of A. commutatus now isolated on the extreme periphery of its previous range.

Figures

Figs 46–48. Aequipecten commutatus peripheralis subsp. n. 46–47. Holotype, NMSA E8964, off Matigulu R., Zululand, 200–220 m, 19.4 x 18.4 mm. 46. LV exterior. 47. LV interior. 48. Paratype, NMSA E8964, same loc., 19.4 x 18.3 mm, RV interior.