Common name: Portuguese oyster (Figs. 3 D-F)

Gryphaea angulata Lamarck, 1819: 198.

Crassostrea angulata Ranson, 1967: 127-199; Lam, 2003: 107-108, Pls. 3-6.

Material examined. – Holotype – right valve only (length 103 mm)(MNHM, no registration number, from Lamarck’s collection), no locality, coll. Sallé, 1819.

Description. – Hong Kong individuals up to ~ 60mm in height. Shell outline variable with usually spatulate to oval individuals. Attachment area of left valve depends on the substratum. Both valves concave with left more deeply cupped and hence a deep umbonal cavity. Both valves have dichotomous radial ribs from the umbo; the tops of the ribs are well-rounded and radial ribs with growth squamae are recognisable on both valves. The older part of the right valve, i.e. the dorsal surface, is usually eroded. Wavy plications along the shell margins reflect the positions and height of the ribs. No chomata. Adductor muscle scar reniform, anterodorsal border concave and close to the postero-ventral shell margin from the centre of the valve. External coloration of both valves is variable from white and light purple with deep purple lines radiating from the umbo. Internally, the shell is white with patches of chalky deposits and hollow chambers. The muscle scar is colourless with occasional purple growth lines.

Distribution. – This species originated in Taiwan and was introduced into Portugual and Spain for cultivation. It is one of the important commercial oyster species in Taiwan and Western Europe. Although this species is cultured in Taiwan extensively, it occurs as wild populations and in low abundance in Hong Kong. Only two bays with constant freshwater inputs, i.e. Hoi Ha Wan and Tai Tam Bay, have been identified as local habitats.

Remarks. – The shell and anatomy of this species cannot be distinguished morphologically from Crasssotrea gigas. The C. angulata specimens collected from Hong Kong have been genetically identified using mitochondial DNA sequences. Crassostrea angulata from Hong Kong (this study) and Portugal [GenBank accession number AF152567] (O’ Foighil et al., 1998) showed an intraspecific K2P sequence divergence value of 0.43%. Hong Kong C. angulata morphologically resembles and occurs in low abundance at the same shore level as the rock oyster, Saccostrea cucullata. This may be why C. angulata has not been recorded from Hong Kong hitherto. The two can be distinguished by examining the eroded dorsal side of the right valve. Alternate white calcareous and brown conchiolin layers are exposed in S. cucullata whereas only white calcareous layers occur in C. angulata.