Saccostrea mordax ( Gould, 1850 )

Lam, Katherine & Morton, Brian, 2009, Oysters (Bivalvia: Ostreidae And Gryphaeidae) Recorded From Malaysia And Singapore, Raffles Bulletin of Zoology 57 (2), pp. 481-494 : 483-485

Saccostrea mordax ( Gould, 1850)

Common name: Crenulate rock oyster ( Fig. 2 )

Ostrea cucullata Lamarck,1819: 200 [non Born, 1778]

Ostraea mordax Gould, 1850: 346 ; Sowerby, 1871, Pl. 15, Fig. 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.

Saccostrea mordax – Qi & Choe, 2000: 138–139, Fig. 3C–E ; Lam, 2003: 112–113, Pl. 13.

Saccostrea cucullata – Morris, 1985: 125–128, Pl. 3, Fig. A, B, C, D.

Material examined. – Syntypes: [ USNM] 5958, ‘‘ Feejee Islands’ ’( Fiji), coll. United States Exploring Expedition, no date, 3 dry shells .

Voucher: [ NHM] 20090289, Pelangi Beach rocks, low intertidal, Pulau Langkawi, Malaysia, coll. B. Morton, 4 Feb.2004, two individuals preserved in alcohol .

Description. – Shell medium, up to 47 mm in height, 30 mm in length and 6 mm in width. Outline either triangular or an elongate D-shaped depending upon the substratum. Left valve flat and attached completely.The dorsal margin very thin and easily broken when collected. Margin of left valve built up steeply along the anterior and posterior sides. Margins thick, with packed layers of growth squamae running along the margin. Closely and evenly spaced ribs perpendicular to these end as marginal crenulations. In most cases, where the oyster grows 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 height given by the height of the posterior margin and base given by the left valve. Hinge line straight and short. Ligament area elongated to form an obvious left beak.

Right valve convex, thicker than the left valve and white with purple patches on the less eroded ventral area. Shell 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. Evenly-spaced crenulations around the shell margin are more obvious anteriorly and posteriorly.

Interior of the shell white with a shiny, pearly appearance. A band of dark green conchiolin usually occur 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 alternately at the right, inner, ventral shell surface. Adductor muscle scar relatively large and positioned in the posterior ventral half of the pallial area, circular to elongate-oval according to the growth form of the shell and mostly white, slightly stained with growth bands or purple. Chomata white, short, rod-shaped and arranged in a single line around the inner margin. They are more prominent dorsally and fade as they extend towards the ventral shell margin.

Distribution and habitat. – Saccostrea mordax occurs only on oceanic, exposed rocky shores as individuals among the Septifer and Tetraclita zone on slopes or in pools frequently refreshed by strong waves (Lam & Morton, 2004). This species is widely distributed in the Indo-West Pacific, for example, Japan, Korea, Taiwan, Hong Kong, South China Sea, peninsular Malaysia, Singapore, Indonesia, New Hebrides [now Vanuatu] and Australia (Lam & Morton, 2006).

Remarks. – Saccostrea mordax has been identified previously as S. amasa and S. tuberculata (Thompson, 1954) . This species is often confused with S. cuccullata because of similar shell characters. Lam & Morton (2006) suggest that S. mordax occurs only on exposed, wholly marine rocky shores whereas the other Saccostrea lineages occupy a wide range of habitats from brackish mangroves to somewhat less exposed marine shores. Un-eroded Saccostrea mordax individuals are distinct from other species of Saccostrea in terms of the regularly-spaced grooves radiating from the umbone to the ventral margin of the right valve, its triangular shell shape and finely, regularly, m-shaped plicated valve margin.

Figures

Fig. 2. Saccostrea mordax (Gould, 1850) from Pulau Langkawi, Malaysia (NHM20090289). A, external view of the right shell valve, B, internal view of the right shell valve and C, internal view of the left shell valve.

Fig. 1. Saccostrea cuccullata (Born, 1778) from Pulau Langkawi, Malaysia, showing external and internal views of the right and left shell valves. A–D, ecotype occurring on mangrove trees (NHM20090286), E–H, ecotype occurring on low intertidal beach rocks (NHM20090287), I–L, ecotype occurring on intertidal rocks (NHM20090288).

Fig. 3. Crassostrea gigas (Thünberg, 1793) from Johor Strait, Malaysia (NHM20090290). A, external view of the right shell valve, B, external view of the left shell valve. C, internal view of the right shell valve D, internal view of the left shell valve.