species to S. malabonensis that they named S. manilai. For this species, the zone of activity for the Lap locus was slowest of all scored Saccostrea species while the zone of
activity for the Pgi locus was fastest. This was the case for our species ‘B’, suggesting Ž
. that this may be the cryptic species S. manilai Buroker . For S. malabonensis itself,
although mobility’s of common Lap alleles were slower than S. commercialis and S. cucullata, the zone of activity for the Pgi locus was of similar mobility to all species
Ž .
except S. manilai. We therefore identify species ‘B’ as S. manilai Buroker .
4. Discussion
By the use of allozyme variation at four marker loci we were able to assign Saccostrea oysters from Thailand to one of three species that we identified as S.
commercialis, S. cucullata and S. manilai. S. cucullata was restricted to oceanic conditions on offshore islets. The other two species were found in coastal and estuarine
sites throughout the Gulf of Thailand where they were often found in sympatry. The sharing of common alleles between species limited our ability to identify hybrids.
However, in spite of this, we found two hybrids between S. commercialis and S. manilai, one hybrid between S. manilai and S. cucullata, and one hybrid between S.
commercialis and S. cucullata. Fixed genetic markers for these species will be necessary to discover the full extent of hybridisation occurring in nature.
Principal component analysis of shell morphometrics revealed three principal compo- nents that varied between species. These reflected variation in overall size, shape of the
Ž .
left valve degree of cupping , and relative size of the umbo cavity. The best subset of individual measures, for diagnostic purposes, were the depth of the umbo, the thickness
of the right valve and the width of the right valve. The percentage of correctly classified individuals was similar irrespective of whether original measures or principal compo-
nents were used. The distinctive shell morphology of S. cucullata was reflected in a 100 success rate when classifying individuals by morphometrics. The identification
success rate for the other two species averaged 85. Studies on a variety of Saccostrea species have demonstrated that many aspects of shell morphology, most notably external
shell sculpture, and depth, length and thickness of shell, vary with environment Ž
. Thomson, 1954; Stenzel, 1971; Tack et al., 1992 . These are precisely those characters
that are easiest to measure on living shells. Our subset of ‘‘external’’ characters that comprised total depth of the oyster, and length and width of the right valve was still
successful in identifying 90 and 85 of S. cucullata and S. commercialis, respec- tively. However, use of these measures resulted in only 68 successful classifications
for S. manilai. In terms of qualitative characters, the possession of a black adductor muscle scar was almost entirely confined to S. cucullata, although this applied to only
75 of this species. Assessment of chomata spacing is more subjective and varied within all species.
5. Conclusion
