Saccostrea mordax (Gould, 1850)

Lam, Katherine & Morton, Brian, 2004, The Oysters Of Hong Kong (Bivalvia: Ostreidae And Gryphaeidae), Raffles Bulletin of Zoology 52 (1), pp. 11-28 : 21

Saccostrea mordax (Gould, 1850)

Common name: Crenulate rock oyster ( Figs. 4 G-I)

Ostrea cucullata Lamarck,1819: 200 .

Ostraea mordax Gould, 1850: 346 ; Sowerby, 1871, Pl. 15, Figs. 31a, b.

Ostrea forskali var. mordax Lamy, 1929: 159 .

Ostrea amasa Iredale, 1939: 399 , Pl. 17, Fig. 8.

Crassostrea amasa Thomson, 1954: 154 , Pl. 7, Figs. 1 , 2 .

Crassostrea tuberculata Thomson, 1954: 157 , Pl. 8, Fig. 3 .

Saccostrea mordax Torigoe, 1981: 328 , Pl. 17; Qi & Choe, 2000: 138-139, Figs. 3 C-E; Lam, 2003: 112-113, Pl. 13.

Saccostrea cucullata - Morris, 1985: 125-128, Pl. 3, Figs. A, B, C, D.

Material examined. – Syntypes – three dry shells ( NMNH 5958 ), ‘‘ Feejee Islands’ ’( Fiji), coll. United States Exploring Expedition, no date .

Description. – Shell medium, up to 60 mm in length, 40 mm in width. Outline triangular or an elongate D-shape depending upon substratum and space. The left valve is flat and completely attached. It is very thin and easily broken with a hard hit when a specimen is collected. The margin of the left valve is built up steeply along the anterior and posterior sides. The margins are thick, with packed layers of growth squamae running along the margin and perpendicular to these are closely and evenly spaced ribs ending as marginal crenulations. In most cases, where the oyster is growing on unlimited flat rock surfaces, the anterior margin is reduced so that the right valve is in touch with the substratum. When viewed along the mouth-anus axis, the shape of the oyster is almost a right-angled triangle with its height given by the height of the posterior margin and its base by the left valve. The hinge line is straight and short. The ligament area is elongated to form an obvious left beak. The left valve of juveniles is a thin white calcite pad attached completely to the substratum.

The right valve is convex and white with purple patches on the less eroded ventral area. It is usually eroded with exposed conchiolin scales near the dorsal end. Parallel grooves extend from half way along the dorso-ventral axis to the ventral shell margin. These grooves reveal uneroded growth lines on the right valve. They are also present on juveniles which have a fan-like, triangular shell. Evenly-spaced crenulations around the shell margin are more obvious anteriorly and posteriorly.

The interior of the shell is white with a shiny, pearly appearance. A band of dark green conchiolin usually occurs near the ventral margin of the right valve. This band may be absent in some specimens. It appears that the conchiolin and prismatic calcite layers are deposited alternatively at the right, inner, ventral shell margin. The adductor muscle scar is relatively large and positioned in the posterior ventral half of the pallial area. It is circular to elongate-oval according to the growth form of the shell and mostly white, slightly stained with growth bands or purple. The chomata are white, short and rod-shaped and arranged in a single line around the inner margin. These are more prominent dorsally and fade out as they extend towards the ventral shell margin.

Distribution. – Saccostrea mordax occurs only on oceanic, exposed rocky shores such as Cape d’Aguilar and Big Wave Bay. At Big Wave Bay, it occurs as individuals among the Septifer and Tetraclita zone on rocky slopes. At Cape d’Aguilar, it occurs in rock pools frequently refreshed by strong waves. This species is widely distributed in the Indo-West Pacific, e.g. Japan, Korea, Taiwan, Hong Kong, South China Sea, Peninsula Malaysia, Singapore, Indonesia, New Hebrides and Australia.

Remarks. – Saccostrea mordax has been identified previously as S. amasa and S. tuberculata (Thompson, 1954) . This species is often confused with S. cucullata because of similar shell characters. Like S. cucullata , S. mordax is probably a superspecies. Two genetic lineages of this species from the Indo-West Pacific have been differentiated by mitochondrial DNA sequences (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.

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

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.