Magallana gigas ( Thunberg, 1793 )

Tan, Koh Siang, , Siong Kiat Tan, , Sherralee S. Lukehurst, Wells, & Fred E., 2024, Assessing the threat of the oyster genus Magallana (Bivalvia: Ostreidae) in Singapore to the Australian marine environment, Raffles Bulletin of Zoology 72, pp. 162-183 : 176

Magallana gigas ( Thunberg, 1793)

( Figs. 1, 5A–I, 6G–I, 8)

Ostrea gigas Thunberg, 1793: 140–142 , pl. 6 (type locality: Japan).

Published Singapore records:

Crassostrea gigas – Tan KS & Chou, 2000: 128 (in error for M. belcheri ; see above); Ruesink et al., 2005: 657; Lam & Morton, 2009: 486 , 487, 492, fig. 3 (NHMUK20090290, Johor Strait); Tan SK & Woo, 2010: 8 (after Lam & Morton, 2009) — see Remarks.

Material examined. Singapore: 9 ex. (dry) ( ZRC.MOL.8622), purchased from local supermarket by Tan SK, originating from New Zealand; coll. Tan SK, 2000s; 2 ex. (dry & wet) ( ZRC.MOL.27610), purchased from Japanese restaurant by Tan HH, purportedly from Japan, coll. Tan HH, 3 March 2023 . Australia: 2 ex. (dry & wet) ( ZRC.MOL.28996), 1 ex. ( ZRC.MOL.28997), South Australia, Coffin Bay, purchased from fish shop in Perth , Wells FE, September 2023 .

Description. Shell: Valves large (SL commonly reaching 7 cm, largest examined 14.6 cm), often elongately ovate in shape. External surface of RV light yellowish brown, with varying amounts of dark purplish brown markings, usually as radiating streaks and/or dark concentric bands, with overlapping thin, commarginal edges. Interior surface of RV white with a silky lustre, usually with varying degrees of cloudy grey blotches and white calcareous patches. Umbonal cavity of LV usually deep, but this may be due to the specimens being aquacultured. The adductor muscle scar is variable in colour, from white, with or without varying degrees of purplish brown markings, to entirely dark purplish grey.

Body. Ethanol-preserved animals with mantle edges coloured brown to black; visceral mass white. Mantle border with outer narrow light brown region and an inner black region that has a series of elongate triangles resembling a backgammon board. Mantle edge outermost layer narrow, entire, almost uniformly pigmented black, and tentacles absent. Mantle edge middle layer with tentacles along entire edge; tentacles light brown, largely of two sizes, with larger tentacles slightly offset from edge and originating from side of wall facing the innermost layer, while the smaller tentacles originate from the mantle edge. There are 0–5 small tentacles between two large tentacles. Outside wall of middle layer not pigmented, while the inside wall is heavily pigmented black. Mantle edge innermost layer with black dimorphic tentacles especially along the ventral margin that become smaller, spaced farther apart and less black dorsally. Mantle surface on visceral mass greyish-white.

Habitat. Magallana gigas is an estuarine species that is commonly found in dense populations in the rocky intertidal. The species can settle on small hard objects on muddy bottoms. It often settles on other oysters, and can form oyster reefs. However, they are also extensively cultured in temperate latitudes ( Fig. 8), including southern Japan, northern China, Europe, South Africa, Australia, New Zealand, northern USA (both Pacific and Atlantic coasts), Chile, Argentina, and southern Brazil. More recently, they are also grown out in the tropics (see Chávez-Villalba et al., 2021) from spat imported from oyster hatcheries located in the temperate zone.

Remarks. This species is often sold in local restaurants and supermarkets of Singapore. Apart from direct importation, some of these were, or still are, supplied by one or more floating farms off northeastern Singapore. Grow-out of triploid M. gigas has been undertaken in Johor Strait using imported spat from Australia and later Chile ( Lam & Morton, 2009), raising the issue of whether feral populations have developed as the grow-out period may be several months or more. Bayen et al. (2007) conducted a study on the effect and response of this species to marine contaminants, but their specimens were grown from juvenile oysters imported from Australia. The record in Lam & Morton (2009) was similarly noted to be aquacultured with spat from Australia. Ruesink et al. (2005) considered this species to be introduced and unlikely to be established in Singapore. There is hitherto no evidence of feral populations of M. gigas in Singapore. All specimens examined in this study were from the seafood trade.