Crassostrea ariakensis Fujita, 1913

Lam, Katherine & Morton, Brian, 2004, The Oysters Of Hong Kong (Bivalvia: Ostreidae And Gryphaeidae), Raffles Bulletin of Zoology 52 (1), pp. 11-28 : 18-19

Crassostrea ariakensis Fujita, 1913 , ex Wakiya MS

Common name: Suminoe oyster ( Figs. 4 A-C)

Ostrea rivularis Lischke, 1869: 176 , Pl. 14, Figs. 2 , 3 .

Ostrea ariakensis Fujita, 1913: 519 , ex Wakiya MS; Wakiya, 1929: 363-364, Pl. 9, Figs. 1 , 2 .

Crassostrea ariakensis - Torigoe, 1981: 377, Pl. 15; Lam, 2003: 109-110, Pls. 9, 10.

Material examined. – Type not examined, its location being unknown (K. Torigoe, pers. comm.).

Description. – Shell outline is orbicular to spatulate; outline tongue-shaped. External surface is whitish with broken rays of purplish brown. The scales of the outer shell layer radiating from the dorsal side are brown to light brown. This species is sympatric with Crassostrea hongkongensis and is, thus, bored and heavily encrusted by other marine invertebrates in a similar way. Left valve deeply cupped. Right valve usually either flat or slightly convex dorsally and slightly concave near the ventral margin. Internally whitish with hollow chambers of irregular size and position. Hinge line short; the ligament occupying its full length. Attachment area of the left valve is variably small, usually not more than half the shell height. Adductor muscle scar is D-shaped with the outline on the dorsal side being concave and with light to dark purplish growth lines which vary in colour among individuals. Colour may be different on right and left valves in the same individual.

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 oyster farms in Deep Bay and is thus cultured with Crassostrea hongkongensis but in lower numbers. The species is reported to occur naturally in Japan and along the southern coast of China to Vietnam. It is also cultivated in Oregon and Washington, the United States of America (Robinson & Langdon, 1992).

Remarks. – Crassostrea ariakensis has been called C. rivularis , which has been determined to be invalid ( Torigoe, 1981). This is because the holotype of Ostrea rivularis Gould, 1861 , from the East China Sea was in fact a species of Lopha and is different from O. rivularis as described by Lischke, 1869 (Habe & Kosuge, 1967). It is difficult to separate C. ariakensis and C. hongkongensis using external shell characters. In general, however, the former is more rounded and scales on both valves are generally more erect. The definitive morphological distinction between the two is the shape of the adductor muscle. In C. ariakensis , the adductor muscle scar is crescent-shaped, whereas in C. hongkongensis the outline of the muscle on the dorsal side is straight to slightly convex.

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.

Fig. 2. Neighbour-joining tree based on Kimura 2-parameter distances with 1000 bootstraps using COI mtDNA partial sequences (~618 nt) of Crassostrea. The bootstrap percentages supporting each node of the neighbour-joining and most parsimonious trees are indicated above and below the branches, respectively.Only values>70 are shown.The scale represents percentage substitutions.Ostrea chilensis is employed as outgroups. YSO1 to 6, HHW1 to 6 and TT1 to 3 are samples collected from Yung Shue O, Hoi Ha Wan and Tai Tam, Hong Kong, respectively.

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.

Fig. 1. A map of Hong Kong showing the locations where oysters were collected.