Saccostrea cuccullata ( Born, 1778 )

Lam, Katherine & Morton, Brian, 2009, Oysters (Bivalvia: Ostreidae And Gryphaeidae) Recorded From Malaysia And Singapore, Raffles Bulletin of Zoology 57 (2), pp. 481-494 : 482-483

Saccostrea cuccullata ( Born, 1778)

Common name: Rock oyster (Spiny oyster for the ‘ echinata ’ morphotype) ( Fig. 1 )

Ostrea cuccullata Born, 1778: 100 . Ostrea cucullata – Born, 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. Saccostrea cucullata – Morris, 1985: 125–128, Pl. 3, Fig. E, F, G;

Lam, 2003: 110–112, Pls. 11–12.

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

Specimens in this collection (all preserved in alcohol) - [ NHM] 20090286, “On big mangrove trees, Tanjong Rhu mangrove, Pulau Langkawi, Malaysa” (9) and “On mangrove rocks” (two clusters) coll. B. Morton, 3 & 5 Feb.2004 . [ NHM] 20090287, Pelangi Beach rocks, low intertidal, Pulau Langkawi, Malaysia, coll. B. Morton, 3 & 5 Feb.2004 (33) . [ NHM] 20090288, intertidal, Pelangi Beach groyne, Pulau Langkawi, Malaysia, coll. B. Morton, 3 & 5 Feb.2004 . [ NHM] 20030507, Tanah Merah , Singapore, coll. B. Morton, 9 Jul.2001 (2) . [ NHM] 20030508, Kallang River , Singapore, coll. B. Morton, 9 Jul.2001 (2).

Description. – Shell size small to medium, up to ~ 70 mm in length and ~ 75 mm in height, ~ 5–20 mm in width, with an oval to subtriangular outline, depending upon substratum and available space. Hinge line straight, short, with the ligament occupying its total length. Ligamental area short. External surfaces of both left and right valves white, lilac to purple with dark purple coloration at the shell margin.

Three main growth forms or ecotypes in terms of external shell characters were recorded.

The first ecotype occurs on mangrove trees and attaches to intertidal tree trunks as solitary individuals. Sizes are large, up to ~ 70 mm in length, 75 mm in height and 7 mm in width. Attachment area of left valve almost complete with slightly raised shell margin. Flat left valves usually slightly larger than right valve, shallowly cupped and with slightly raised margins and dense growth squamae. Commissural shelf not obvious. Surface of the right valve in large specimens eroded probably by acidic freshwaters, revealing the alternate white calcareous and brown conchiolin layers of the shell. In smaller specimens, commarginal layers of purple, weak, flaky growth squamae cover the right valves. Ribs not raised but weak crenulations along the ventral shell margin identify the number of ribs present.

The second ecotype occurs on low intertidal beach rocks as solitary individuals. Sizes are medium, up to ~ 45 mm in length, 50 mm in height and 8 mm in width. Flat left valve attached completely to the substratum without raised margin. Commissural shelf not obvious. Flat left valves slightly smaller than right valve, which are slightly convex, covered with commarginal layers of dense, flaky lamellar scales radiating from the ligament. Scales usually eroded along the ligamental side, where commarginal growth squamae are revealed. These end as black or purple conchiolin scales along the ventral shell margin. White or purple radiating stripes occur on the convex right valves of some specimens. Shell margin usually not crenulate or if it

Saccostrea cuccullata Present Oval to subtriangular

Saccostrea mordax Present Triangular or elongate

D-shape

Crasssotrea gigas Absent Spatulate to oval

Planostrea pestigris Present Oval

Dendostrea folium Inconspicuous Orbicular to oval Hyotissa imbricata Present Circular Right valve covered with growth squamae 70 × 75 × 7 with hyote spines

Parallel grooves run from hinge to ventral 47 × 30 × 6 margin

Both valves have radial ribs and right valve 30 × 25 × 15 covered with calcareous growth squamae

Flat and smooth 50 × 40 × 4

Smooth, covered by dichotomous ribs 55 ×30 ×10

Dichotomous ribs, growth squamae and 45 × 50 × 15 ocassion hyote spines on both valves

1 Maximum size is listed here as height (mm) × length (mm) × width (mm)

is, only weakly crenulate reflecting the number and position of the slightly raised radial ribs. Marginal plications weak, usually irregularly-spaced and shallowly-rounded.

The third ecotype occurs in clusters on intertidal rocks on beaches or small hard substrata, such as dead mollusc or barnacle shells. Sizes are small, up to ~ 30 mm in length, 40 mm in height and 12 mm in width. Attachment area of the left valve usually ~ half the shell length. Left valve margin raised and thus deeply cupped. Commissural shelf not obvious. Surface of the left valve with up to about 15 ribs. Commarginal layers of weak, flaky growth squamae occur on the left valve margin that is raised above the substratum. Fine crenulations along the ventral shell margin identify the number of ribs present, forming a flaky shell margin patterned black, purple and white. Flat to slightly convex right valve covered with comarginal layers of dense, flaky lamellar scales radiating from the ligament. These scales are usually eroded along the ligamental side, where alternate white calcareous and brown conchiolin layers are revealed. They end as black conchiolin scales along the ventral shell margin. Shell margin crenulate reflecting the number and position of the radial ribs. Marginal plications small, usually regularly-spaced and sharply-rounded.

Interiors of both valves of all ecotypes usually either iridescent bluish green or opalescent white with patches of bluish green, or olive to yellowish green. Adductor scar large, D-shaped, and may have purple or yellow growth bands paralleling the width of the shell. Scars on the left and right valves of the same specimen of same shape, size, and colour, positioned in the posterior ventral third of the shell interior. 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 and habitat. – Subtropical and tropical Indo- West Pacific, from southern Japan to Australia, as verified by mitochondrial DNA markers (Lam and Morton, 2006). This oyster dominates the eulittoral zone of sheltered rocky shores.

Remarks. – The Saccostrea cuccullata collected from Pulau Langkawi illustrate the effect of substratum on the growth form, resulting in various ecotypes. For example, those growing on mangrove tree stems are laterally compressed, with the left valve possessing a great attachment surface and a raised left shell margin. Individuals occurring on low intertidal beach rocks have a similar growth form as the former except the right valve is convex and the whole of the left valve is attached to the substratum, without a raised margin. This prevents the animal from being dislodged by wave action. S. cuccullata also occurs on intertidal rocky substrata. Individuals from this zone cluster together, compete for attachment area and are of small size (~ 10 mm to 20 mm). The shell margins are plicate and left shell valves have a raised margin and small attachment area.

Figures

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).