Parvamussium strongae, Dijkstra & Maestrati, 2015

Dijkstra, H. H. & Maestrati, P., 2015, Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the southwestern Indian Ocean, African Invertebrates 56 (3), pp. 585-585 : 608-610

Parvamussium strongae sp. n.

Fig. 4G–M

Etymology: After Dr Ellen E. Strong, Research Zoologist, Curator of Mollusca at the Department of Invertebrate Zoology of the National Museum of Natural History, Smithsonian Institution, Washington DC, U.S.A. Participant on the Mainbaza campaign.

Description: Shell up to 27 mm in height, fragile, inequivalve, inequilateral, both valves flattened, left valve slightlY more inflated than right valve, auricles unequal in shape and size, umbonal angle 105°. Colour left valve creamy with white and orange spots, transparent, right valve whitish, opaque. Prodissoconch 190 µm long. Left valve glossy in early growth stage (c. 8 mm), shell disc smooth and transparent throughout to the subventral margin. Submarginal region (c. 3–5 mm) opaque with delicate, closely spaced (6 per mm) commarginal lamellae.Auricles smooth in early growth stage with close-set commarginal lamellae near the periphery. Hinge line slightly raised. Right valve also glossy and transparent with whitish dots in early growth stage (c. 5 mm), with faint commarginal close-set growth lines, transformed into more prominent commarginal lamellae (c. 6 per mm) near the ventral margin.Anterior auricle sligthly curved with very close-set commarginal lirae, more prominent near the hinge. Posterior auricle almost smooth in early growth stage with commarginal lamellae near the margin. Marginal apron broken off. Rudimentary riblets anteriorly and posterior, 8 on each side of left valve, 7 on each side of right valve. Byssal notch small, byssal fasciole narrow.

Dimensions: Holotype: Height 22.1 mm, width 20.0 mm, convexity 3.9 mm.

Type material examined: Holotype, pr ( MNHN IM-2007-39087). Type locality: NW MADAGASCAR: Between Nosy-Bé and Banc du Leven (12°34'S 47°54'E), - 367–369 m, live, campaign Miriky, stn DW3212, 30.vi.2009 . Paratype (lv): Northwest region : between Nosy-Bé and Banc du Leven (12°55'S 48°11'E), - 260–319 m, live, campaign Miriky, stn DW3228, 02.vii.2009 ( MNHN 39052 , MarBol) .

Other material examined:S MADAGASCAR:Manantenina (24°24'S 47°33'E), - 424–438 m, dead, expedition Atimo Vatae, stn DW3528, RV Nosy Bé 11, 01.v.2010 (MNHN, 2 lv); Sainte Luce (24°43'S 47°32'E), - 296–307 m, dead, expedition Atimo Vatae, stn DW3534, RV Nosy Bé 11, 02.v.2010 (MNHN, 2 lv, 3 rv). Distribution and habitat: So far Madagascar Channel, northwestern and southern Madagascar. Living bathyally on soft substrata at - 267–367 m (minimum depth range). Remarks:A closely smooth bathyal species is Parvamussium torresi (E.A. Smith, 1885) , recorded from the southwestern Pacific. This species could be distinguished from P. strongae by the following characters, i.e. in having a smaller size ( P. torresi up to c. 10 mm in height, P. strongae c. 27 mm), in coloration ( P. torresi is uniform whitish, P. strongae creamy with whitish and brownish maculations), and in development of internal ribs ( P. torresi 10 well developed and prominent, P. strongae 14–16 rudimentary).

Two congeneric species with also similar anteriorly and posteriorly rudimentary internal riblets as in the present species are Parvamussium vidalense ( Barnard, 1964) , recorded from South Africa, and Parvamussium vesiculatum Dijkstra, 1995 , known from the southwestern Pacific. These species could be distinguished from P. strongae in having a significant smaller size (up to c. 8 mm in height), in having a well-developed radial and commarginal sculpture on the left valve, and in having fewer rudimentary internal riblets (generally 3–4 anteriorly and posteriorly).

Figures

Fig. 4. Parvamussium spp.: (A–C) P. scitulum (E.A. Smith, 1885): campaign Mainbaza, stn CP3130, (A) external view of the left valve, H = 6.2 mm, scale bar = 2 mm; (B) external view of the right valve, H = 5.7 mm, scale bar = 2 mm; (C) details of the left valve, H = 6.2 mm, scale bar = 0.1 mm (MNHN). (D–F) P.siebenrocki (Sturany,1901):campaign Mainbaza, stn CC3160, (D) external view of the left valve, H = 6.9 mm, scale bar = 2 mm; (E) external view of the right valve, H = 5.9 mm, scale bar = 2 mm; (F) details of the left valve, H = 6.9 mm, scale bar = 0.1 mm (MNHN). (G–M) P. strongae sp. n., holotype: campaign Miriky, stn DW3212, (G) external view of the left valve, H = 21.9 mm, scale bar = 1 cm; (H) internal view of the left valve, H = 21.9 mm, scale bar = 1 cm; (I) external view of the right valve, H = 17.4 mm, scale bar = 1 cm; (J) internal view of the right valve, H = 17.4 mm, scale bar = 1 cm; (K) prodissoconch of the left valve, H = 21.9 mm, scale bar = 0.2 mm; (L) details of the left valve, H = 21.9 mm, scale bar = 0.1 mm; (M) details of the right valve, H = 17.4 mm, scale bar = 0.1 mm (MNHN 39087).