Common name: Black-scar oyster (Figs. 3 A-C)

Ostrea lugubris Sowerby, 1871: Fig. 63.

Ostrea iredalei Faustino, 1932: 546-547, 550, Pl. 1; Talavera & Faustino, 1933: 49-50, Pls. 1-2.

Crassostrea iredalei - Carreon, 1969: 109-110, Pl. IV; Yoosukh & Duangdee, 1999: 367, Fig. 5.

Material examined. – Holotype – dry shells (length by height = 77.57 by 53.23 mm)(NHM 1900.2.13.30), ‘‘ North America’ ’, coll. Hanley (1819-1899), no date.

Description. – Shell outline variable but typically elongated and slipper-shaped when grown singly either on or close to the bottom. Margin non or only weakly plicate. Right valve is flat and covered with dense layers of flat, brown scales with finely pointed margins. Left valve is purple or yellowish white, more deeply cupped (especially those grown in clusters) and with a more produced ligamental area than the right one. The hinge line is straight and short. Ligament area is short. Shell background is internally shiny white with colour patches varying from yellow, orange to purple. Patches of chalky white calcium carbonate may be present on the internal surfaces of the valves, near the dorsal shell margin. No chomata. Muscle scar is deep purple on both valves, reniform and displaced postero-dorsally.

Distribution. – In Hong Kong, this species is restricted to shores along Tolo Channel, e.g. the sandy shore of Starfish Bay (= Hoi Sing Wan), mangroves at Ting Kok and the sandy shore at Yung Shue O. This species has not been documented from Hong Kong because of its limited distribution and difficult-to-access habitats. Individuals usually grow either singly or in clusters on sandy or muddy bottoms at lower intertidal levels.

Remarks. – The holotype of Ostrea lugubris shows very similar shell characters to the type specimen of C. iredalei (figured as drawings in Faustino, 1932: 550, Pl. 1, and photographs in Talavera & Fautino, 1933: 49-50, Pls. 1-2). Ostrea lugubris is considered to be synonymous with C. iredalei (Yoosukh & Duangdee, 1999). Ostrea lugubris differs from other Crassostrea species in having patches of colour, which may be either brownish-yellow, purple or light orange, on the valve interiors. It is also characterised by the purple left valve being sculptured with weak, dense, discontinuous radial ribs which is unusual in the genus.

The result of the phylogentic analysis confirmed that the collected samples from Yung She O, Hong Kong, are Crassostrea lugubris (= C. iredalei). Hong Kong samples and C. iredalei from Thailand [GenBank accession number AY038078] show a divergent K2P sequence divergence, in terms of partial COI gene, i.e. 3.1%. This value is greater than that between C. gigas and C. angulata, i.e. 2.55%. Two populations of New Zealand Ostrea chilensis have also shown pronounced sequence divergence for a 609 nucleotide fragment of COI gene, i.e. 3.1%. The mitochondrial dichotomy between Hong Kong C. lugubris and Thai C. iredalei may be due to the geographical isolation of the former population. Crassostrea iredalei has been reported to occur in the Philippines and Thailand (Yoosukh & Duangdee, 1999) but there is no other identified C. lugubris population in Hong Kong and adjacent waters. The holotype of C. lugubris was collected by Hanley from the China coast but no specific location was given.