Magallana belcheri (GB Sowerby II, 1871)

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 : 172

Magallana belcheri (GB Sowerby II, 1871)

( Figs. 1, 2A, B, 3A–I, 6A–C, 7, 8)

Ostrea belcheri GB Sowerby II, 1871 (in 1870–1871): unnumbered caption page to pl., pl. 7, fig. 11 (type locality: “Eastern Seas?”).

Ostrea siamensis Mörch, 1853: 62 (type locality: “Singapuhra”) — nomen nudum.

Published Singapore records.

Ostrea siamensis – Mörch, 1853: 62; Lynge, 1909: 162; Huber, 2010: e-table (after Mörch, 1853) — a nomen nudum ( Ranson, 1967: 172; Huber, 2010: e-table); see Remarks.

Crassostrea belcheri – Ranson, 1967: 171, 172; Bussarawit et al., 2010: 15 (after Ranson, 1967).

Crassostrea belcherii [sic] – Morton, 1977: 503 (Lim Chu Kang [“mangrove close to the village of Lim Chu Kang, on the NW shore of Singapore ”]).

Crassostrea gigas – Tan KS & Chou, 2000: 128, 2 unnumbered figs.; Tan SK & Woo, 2010: 8 (after Tan & Chou, 2000) — not Ostrea gigas Thunberg, 1793 = Magallana gigas ( Thunberg, 1793) .

Material examined. Singapore: 1 ex. (dry) ( ZRC.MOL.8583), Seletar, on mangrove root, coll. Lim CF, 1964 ; 2 ex. (dry) ( ZRC.MOL.9711), Lim Chu Kang mangrove, coll. Lim CF, 13 January 1975 ; 6 ex. (dry) ( ZRC.MOL.8612), Johor Strait , kelong no. E10, near Pulau Tekong, coll. Yang SL, 1986 ; 4 ex. (dry) ( ZRC.MOL.8621), Pulau Ketam, coll. Tan SK, 27 February 2000 ; 2 ex. (dry & wet) ( ZRC.MOL.27722), Pulau Sarimbun, coll. Tan HH et al., 20 January 2004 ; 3 ex. (dry) ( ZRC.MOL.8581), Pulau Ubin , intertidal, on rocks, coll. Lim CF, 5 October 2007 ; 1 ex. (dry) ( ZRC.MOL.8638), Seletar “Wet Gap”, coll. Low MEY, 2008 ; 1 ex. (dry) ( ZRC. MOL.8634), Pulau Semakau , Phase II lagoon, coll. Tan SK et al., 2–3 March 2011 ; 2 ex. (dry) ( ZRC.MOL.28668), Pulau Ketam, mangroves, coll. Lai JCY, 23 November 2011 ; 2 ex. (dry & wet) ( ZRC.MOL.27608), Pulau Sekudu, coll. Tan SK, 25 December 2011 ; 4 ex. (dry & wet) ( ZRC.MOL.27609), National Service Resort & Country Club, coll. Tan SK et al., 6 July 2016 ; 2 ex. (dry & wet) ( ZRC.MOL.12727), Seletar Dam , mangrove, coll. Tan SK et al., 10 May 2017 ; 3 ex. (dry) ( ZRC.MOL.27065), Tuas West , Merawang Beacon, on muddy rocks facing rocky reef flat, coll. Tan SK et al., 29 May 2017 ; 1 ex. (dry) ( ZRC.MOL.27723), National Service Resort & Country Club, coll. Tan HH et al., 14 July 2022 ; 1 ex. (dry & wet) ( ZRC.MOL.27632 [DNA #SG 47]) , 1 ex. (dry & wet) ( ZRC.MOL.27633 [DNA #SG 48]) , 1 ex. (dry & wet) ( ZRC.MOL.27634), East Coast Park , Fort Road canal, N1°17.659′ E103°53.440′, concrete sloping wall, coll. Tan KS et al., 22 February 2023 ; 1 ex. (dry & wet) ( ZRC.MOL.27635 [DNA #SG 50]) , 1 ex. (dry & wet) ( ZRC.MOL.27669 [DNA #SG 49]) East Coast Park , N1°18.583′ E103°55.038′, Siglap canal under culvert (East Coast Road), coll. Tan KS et al., 22 February 2023 ; 1 ex. (dry & wet) ( ZRC.MOL.27636 [DNA #SG 64]) , 1 ex. (dry & wet) ( ZRC.MOL.27672 [DNA #SG 63]), Changi Creek mouth, N1°23.351′ E103°59.498′, mangroves, coll. Tan KS et al., 23 February 2023 ; 1 ex. (dry & wet) ( ZRC.MOL.27637 [DNA #SG 70]) , 2 ex. (dry & wet) ( ZRC.MOL.27638), Sungei Simpang Kiri , N1°27.380′ E103°50.448′, on sloping concrete canal wall, coll. Tan KS et al., 24 February 2023 .

Description. Shell: Valves very large (SL commonly around or larger than 15 cm, largest examined SL 20.2 cm [LV]) and heavy, generally somewhat ovate or pear-shaped, often broadest at ventral region. External surface of RV dark yellow to greyish brown, sometimes with darker irregular greyish patches or streaks (often more prominent in smaller specimens), with overlapping thin, foliose flaky commarginal edges, and usually distinctly finely lamellose at the dorsoanterior and posterior edges. Interior surface of both valves slightly nacreous white, often with varying degrees of cloudy grey and yellow blotches, and prominent, somewhat matt, calcareous white patches. Interior margin of RV usually with a well-defined yellowish-brown border. Umbonal cavity of LV shallow. Adductor muscle scar kidney-shaped, similarly white as the rest of the interior surface.

Body. Living animals with mantle edges coloured dark brown to black, visceral mass surface reddish-brown to yellowbrown ( Fig. 6A–C). Mantle edge outermost layer smooth, tentacles absent; outside wall with line of black pigment, inside wall with blotches of black surface pigment. Mantle edge middle layer with tentacles along entire edge; tentacles are dimorphic in size, arranged linearly such that there is one larger tentacle for every 0–5 smaller tentacles. Large tentacles are slightly offset from edge and originate from side of wall facing the innermost layer. Tentacle surfaces with blotches of black surface pigment. Mantle edge innermost layer also with dimorphic tentacles similar in size as those in the middle layer, but the large tentacles are not offset from the smaller ones. Size of tentacles reduced dorsally.

Habitat. Estuarine intertidal hard substrata including sheltered sloping and vertical concrete walls of tidal canals along East Coast Park and Johor Strait. Optimal salinities for settlement and metamorphosis of this species range between 12 and 18 psu ( Tan & Wong, 1996).

Remarks. The species-group name Ostrea siamensis was published by Mörch (1853: 62) without any accompanying figure or diagnosis and is a nomen nudum. Ranson (1967: 172) examined the type of Ostrea siamensis Mörch, 1853 , and determined it is conspecific with Ostrea belcheri GB Sowerby II, 1871 . This record of Ostrea siamensis by Mörch (1853: 62) is therefore the earliest known record of Magallana belcheri (GB Sowerby II, 1871) from Singapore. This species is often known in Thailand as the ‘white scar’ oyster (e.g., Yoosukh & Duangdee, 1999; Bussarawit & Simonsen, 2006), to differentiate it from the co-occurring and externally similar ‘black scar’ oyster M. bilineata (see below), in reference to the colour of their respective adductor muscle scars on the inside surface of the valves. Tan & Chou (2000) recorded the species (as C. gigas ) following the taxonomic descriptions by Harry (1985), which were the most recent available at the time, but with modern understanding of oyster systematics, there is little doubt that the M. gigas in Tan & Chou (2000), and repeated in Tan & Woo (2010), is instead M. belcheri . In Singapore, this species appears to be restricted to more sheltered waters that are often brackish. Smaller specimens are very difficult to distinguish from M. bilineata in the field, with which it occurred together in many of the sampled locations. Specimens larger than an adult’s palm size are almost certainly this species.