Saccostrea cucullata (Born, 1778)

Lam, Katherine & Morton, Brian, 2004, The Oysters Of Hong Kong (Bivalvia: Ostreidae And Gryphaeidae), Raffles Bulletin of Zoology 52 (1), pp. 11-28 : 20

Saccostrea cucullata (Born, 1778)

Common name: Rock oyster (Spiny oyster for the ‘ echinata ’ morpho- or ecotype) ( Figs. 4 D-F)

Ostrea cucullata Born, 1778: 100 ; 1780: 114, Pl. 6, Figs. 11, 12;

Awati & Rai, 1931: 1-107. Ostrea echinata Quoy & Gaimard, 1835: 455 , Pl. 76, Figs. 13, 14. Ostrea commercialis Iredale & Roughley, 1933: 278 . Saccostrea cucullata Stenzel, 1971 : N1134-N1135, Fig. J106;

Morris, 1985: 125-128, Pl. 3, Figs. E, F, G; Lam, 2003: 110-

112, Pls. 11, 12.

Material examined. – Born’s type is deposited in the Naturhistorisches Museum, Vienna ( Morris, 1985): data unknown.

Description. – Shell small, up to ~ 40 mm in length and ~ 30 mm in width, with an oval to subtriangular outline, depending upon substratum and available space. Hinge line is straight, short, with the ligament occupying its total length. Ligamental area short. The external surfaces of both left and right valves are white to lilac with a dark purple coloration at the shell margin. White radiating stripes occur on the right valves of some specimens.

The attachment area of the left valve, i.e., the one attached to the substratum, is usually at least half the shell length, and is often complete. Commissural shelf not obvious. The surface of the left valve has up to about 15 ribs radiating from the umbone. Concentric layers of weak flaky growth squamae with hyote spines at rib-intersecting points occur in specimens in which the left valve is raised above the substratum and sheltered from erosion. Fine crenulations along the ventral shell margin identify the number of ribs present. They form a flaky shell margin patterned black, white and yellow. For individuals with a complete attachment area, the flat left valves are slightly larger than the right, shallowly cupped and have slightly raised margins with dense growth squamae.

The flat to slightly convex right valve is covered with concentric layers of dense, flaky lamellar scales radiating from the ligament. These scales are usually eroded along the ligamental side and where concentric growth squamae are revealed. They end as black or yellow conchiolin scales along the ventral shell margin. The shell margin is crenulate reflecting the number and position of the radial ribs. The marginal plications are small, usually regularly-spaced and sharply-rounded.

The right valve of the ‘ echinata ’ ecotype of Saccostrea cucullata is slightly convex. External surface is eroded but there may be between 10-30 closely spaced radial to subparallel ribs and short, hollow, black hyote spines. Ribs not raised; may retain thick, overlapping, dark greenish-black, conchiolin scales. Where the conchiolin scales are eroded, the exterior surface is either white or purplish black with white streaks radiating from the umbone and at the margins. Shell margin crenulate to irregularly plicate with flaky scales fitting closely within the margin of the left valve. The right valve of juveniles is circular, black and covered densely with short hyote spines.

Interiors of both valves are usually iridescent bluish green or opalescent white with patches of bluish green, or olive to yellowish green. The adductor scar is large, D-shaped, and may have purple or yellow growth bands paralleling the width of the shell. The scars on the left and right valves of the same specimen are of the same shape, size, and colour. The scar is positioned in the posterior ventral third of the pallial area. Chomata and gutters may be present in young individuals. These are well developed from the hinge region to the adductor muscle, but fade beyond.

Distribution. – This oyster dominates the eulittoral zone of sheltered rocky shores such as Hoi Ha Wan, Starfish Bay, Tso Woo Hang, Deep Bay and Tai Tam Bay and mangroves such as Ting Kok ( Morton, 1990; Chiu, 1997, 1998). It forms a clear visible oyster zone restricted by the level of Mean High Water Neap Tides (MHWNT). It is rare on exposed shores such as Cheung Sha and Cape d’Aguilar. It also occurs on submerged, sheltered concrete structures of the pier on East Ping Chau and is scattered individually among the barnacle zone at Big Wave Bay, Hong Kong Island. This species is distributed in both the subtropical and tropical Indo-West Pacific, from southern Japan to Australia (authors’ unpublished data).

Remarks. – Because of shell plasticity, the taxonomy of Saccostrea has been problematic. Some Saccostrea cucullata “forms” have been cross-referred to using different names by different authors, e.g. S. echinata , S. mordax and S. glomerata . Mitochondiral DNA sequence analysis of these oysters has shown that S. cucullata is a superspecies with S. glomerata as a component species and S. echinata as a morpho- or ecotype (authors’ unpublished data).

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.