Crassostrea hongkongensis Lam & Morton, 2003

Lam, Katherine & Morton, Brian, 2004, The Oysters Of Hong Kong (Bivalvia: Ostreidae And Gryphaeidae), Raffles Bulletin of Zoology 52 (1), pp. 11-28 : 18

Crassostrea hongkongensis Lam & Morton, 2003

Common name: Hong Kong cultured oyster ( Figs. 3 G-I)

Crasssotrea gigas – Morton & Wong, 1975: 139-149, Pls. 14-16; Morris, 1985: 123-125, Pl. 2, Figs. A-F.

Crassostrea hongkongensis Lam & Morton, 2003a: 1-13 , Fig. 3 .

Material examined. – Holotype – One dry shell and preserved tissues (135 by 82.5 mm)( NHM 20020501 ), ‘‘Lau Fau Shan, Deep Bay , Hong Kong, 837 500N 816 950E’’, coll. K. Lam, 9 Feb.2001.

Paratypes – Three dry shells and preserved tissues (largest 160 by 93 mm)( NHM 20020502 , UMMZ 3000076 , NSMT-Mo 73486), same data as holotype .

Others – Ten dry shells and preserved tissues (largest 129 by 76 mm)( NHM 20020503 ), same data as holotype ; 10 dry shells and preserved tissues (largest 161 by 68 mm)( SBMNH 348407-348416 ), same data as holotype .

Description. – Shell elongate dorsoventrally with a spatulate form, being broader towards the ventral margin and tapering dorsally. Right valve is slightly convex. Left valve is cupped. No ribs on the surface of either the right or left valves. The surface is encrusted with smaller oysters and barnacles and the shell is bored by polychaetes, sponges and the bivalve Aspidopholas obtecta ( Wong, 1975) . Concentric growth increment lines spreading from the umbone of the right valve give rise to brownish-yellow and brittle conchiolin lamellae. Unbroken lamellae have radiating rays which end in the slightly undulating ventral shell margin. The soft flaky lamellae of the outer prismatic layer at the ventral margin forms a flexible edge. At the older parts of the shell, i.e. near the umbones, the lamellae are eroded with the white chalky layer of the shell often exposed. The left valve is covered with white growth squamae. During life, the left dorsal shell margin grows towards the right, producing commarginal and slightly erect layers of old ventral margins during each season of faster growth. Attachment area is small and appears as a patch at the umbone with the rest of the left valve surface unattached.

The hinge line is short and slightly curved. The ligament is large, occupying the full length of the hinge line. The resilifer is typically greatly elongated in both valves with a deep channel formed by the growth increments of the inner fibrous layer of the ligament. The interior of the shell is transclucent white with patches of opaque white chalky deposits. The adductor muscle scar is large, D-shaped and white with light purple growth lines. The adductor muscle scar is located towards the posterior side between the middle and ventral third of the pallial area. Chomata on the inner shell margin and commissural shelf are not developed.

Distribution. – This species is cultivated in the Pearl River Delta, Guangdong Province, China, i.e. under estuarine conditions. In Hong Kong, it occurs only in Deep Bay (= Hau Hoi Wan). It is cultured in oyster farms but also occurs as wild individuals on intertidal and subtidal rocks along the shoreline of Deep Bay. This species may also be cultured in oyster farms along the shores of the Pearl River Delta, and elsewhere in China. Deep Bay, Hong Kong is, however, the only known locality for this species.

Remarks. – In the Pearl River Delta, the cultivation of species of Crassostrea is reputed to have a ~700 year history, the industry in Hong Kong currently being centred around the village of Lau Fau Shan in Deep Bay. There are two methods of oyster cultivation employed in Deep Bay. One involves the collection of spat on concrete tiles (25 13 1.5 cm) or posts deployed on the lower shore and sublittoral mud ( Morton & Wong, 1975) whereas the other is the more recently developed raft culture. Shucked oysters cultivated in Deep Bay are usually either sold fresh to Hong Kong markets or sun-dried for export. Part of the harvest is sold to make oyster sauce, a popular Chinese seasoning.

Figures

Fig. 3. Crassostrea from Hong Kong showing external and internal views of the right valves and the internal views of the left valves. A- C. C. lugubris (NHM 20030490). D-F. C. angulata (NHM 20030491). G-I. C. hongkongensis (NHM 20020501). Scale bars = 20 mm.