Planostrea pestigris (Hanley, 1846)

Lam, Katherine & Morton, Brian, 2004, The Oysters Of Hong Kong (Bivalvia: Ostreidae And Gryphaeidae), Raffles Bulletin of Zoology 52 (1), pp. 11-28 : 21-22

Planostrea pestigris (Hanley, 1846)

Common name: Flat oyster ( Figs. 4 J-L)

Ostrea pestigris Hanley, 1846: 106-107 ; Morris, 1985:129, Pl. 4A- D; Scott, 1994: 69, Pl. 5B.

Ostrea paulucciae Crosse, 1869: 188 .

Ostrea palmipes Sowerby, 1871 , sp. 56, Figs. a-c.

Planostrea pestigris Harry, 1985: 143 , Fig. 22.

Material examined. – Holotype – dry shell (height, 75 mm)( U. S. National Museum , USNM 666809 ), ‘‘ Isle of Luzon; on rocks’’, coll. H. Cuming, no date.

Other – Syntype of Ostrea palmipes – One dry shell ( NHM 1907.10 .28.77 & 1907.12.30.10-11), no other data .

Description. – Shell of moderate size, up to 80mm high. Outline subquadrate, attenuated dorsally. Shell laterally compressed, generally in one plane. The smaller right valve fits closely within the margin of the left. Right valve smooth, with the outer shell layer continuous and having few growth increments. Little shell erosion and few encrustations. Cream to yellowish, usually with dark lavender irregular rays. Left valve usually either light or white, with 6-10 low, widely-spaced ribs which may have obsolete hyote spines at some growth increments. Attachment area is small at the left umbo. Most are attached to shells of other gastropods and bivalves. Some are unattached. Chomata are small, uniform, closelyspaced and in straight lines along the anterior and posterior margins near the hinge. Hinge line straight and short. Ligament area slightly extended in both valves. Marginal commissural shelf on the left valve is wide and flat with a well-defined inner edge. The interior of the shell is white with a pearly nacre. Chalky deposits are prominent on the commissural shelf of the left valve only. The adductor muscle scar is white, large and elongate. It is positioned approximately at the middle of the shell, slightly towards the posterior.

Distribution. – Intertidal zone to 10m on rocks and corals. Indo-West Pacific, the Philippines, Taiwan, Thailand, North Borneo, Mauritius, South and East China Seas, Yellow Sea, Shandong Province, China to Honshu, Japan, Townsville, Queensland, Australia. Hong Kong; 8-10 m in Tolo Channel and southern waters of Hong Kong.

Remarks. – Harry (1985) proposed a new genus, Planostrea , exclusively for Ostrea pestigris . This genus is described as of moderate size (to 75 mm), very compressed in one plane, being non-lamellose and with a continuous outer shell layer. Well-developed chomata line up along the dorsal margins near the hinge, with a wide and flat marginal commissural shelf and a lavender outer surface with radial stripes. This species can be distinguished easily from other oysters because of the consistent shape of a smooth, flattened, circular disc. Unlike other oysters, it is usually free from encrustations and the influence of the substratum to which it is attached. This species has been synonymised with Ostrea paulucciae Cross, 1869 and O. palmipes Sowerby, 1871 in terms of similar shell characters (Tchang & Lou, 1956).

Figures

Fig. 4. Crassostrea, Saccostrea and Planostrea from Hong Kong showing external and internal views of the right valves and the internal views of the left valves. A-C. C. ariakensis (NHM 20030470). D-F. S. cucullata (NHM 20030496). G-I. S. mordax (NHM 20030492). J- L. P. pestigris (SBMNH 345716). Scale bar = 20 mm.