commercialis, but almost non-existent in S. manilai. Mean umbo depths were 1.49 cm for S. cucullata, 0.7 cm for S. commercialis and 0.42 cm for S. manilai. Colour of the adductor scar was
also of taxonomic value, with 75 of S. cucullata having black scars, while scars were white or brown in the other two species. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Saccostrea; Shell morphology; Taxonomy; Thailand
1. Introduction
Rock oysters of the genus Saccostrea are widely distributed around the coast of Thailand occurring both offshore and in near-shore brackish environments on substrates
that vary from rocks to the stilt roots of mangroves. In spite of the diverse habitats that these oysters occupy and their highly variable shell morphology, most populations have
been thought to be a single species. However, authors have disagreed whether to call
Ž . Ž
this species Saccostrea cucullata Born Amornjaruchit, 1988; Yusuk, 1988; Natee-
. Ž
. wathana, 1995; Sanpanich, 1998
or S. commercialis Iredale and Roughley
Ž .
Ž Brohmanonda et al., 1988; Tookwina, 1991 . Two other species, S. echinata Quoy and
. Ž
. Gaimard and S. mordax Gould , have also been included in species lists for offshore
Ž .
Ž islands in the Andaman Sea Nielsen, 1976 and the Gulf of Thailand Bussarawit,
. 1995 , respectively. The identification of these species is also problematic as a number
of authors consider S. mordax and S. echinata to be synonyms for S. cucullata Ž
. Thomson, 1954; Morton and Morton, 1983 .
Saccostrea species are grown commercially so that incorrect identification of species has economic implications. Culture of rock oysters in Thailand has relied almost entirely
upon natural spatfall. However, oyster harvests have declined in recent years, and this Ž
decline has been attributed to a shortage of spat Jarayabhand and Thavornyutikarn, .
1995 . One of the responses to this perceived shortfall was the introduction in 1985 of a programme of selective breeding on rock oysters that were presumed to be S. cucullata
Ž .
Jarayabhand and Thavornyutikarn, 1995 . Such programmes could well fail if brood- stock are not conspecific. Transplantation of rock oysters has also been used to re-stock
Ž .
culture areas Musig, 1987 . If environmental conditions are not appropriate for the introduced oysters, heavy mortalities and low yields may result. For example, S.
commercialis thrives in estuarine environments whereas most other Saccostrea species including S. cucullata require fully marine conditions. Correct selection of oysters for
conditions prevailing at culture sites necessarily depends on their correct identification.
Allozyme electrophoresis is often effective at identifying shellfish. For example, use of allozymes allowed recognition of a previously unknown sibling species of S.
Ž .
malabonensis from the Philippines Buroker et al., 1979b . However, it is relatively time-consuming and costly and may be impractical for hatcheries, let alone local
fishermen. Our aim in this study was to resolve the number of species present in Thailand, and to then test methods of identification that may be suitable for both
laboratory and commercial use. We take a dual approach by using allozymes to resolve individuals according to species, and then test the effectiveness of different morphologi-
cal factors for discriminating between those same species.
2. Materials and methods
