National Committee on Marine Sciences (NCMS)
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- Growth and metabolic responses of the giant clam-zooxanthellae symbiosis in a reef-fertilisation experimentBelda-Baillie, C. A.; Leggat, W.; Yellowlees, D. (Inter-Research Science Center, 1998)To evaluate the impact of elevated nutrients on reef organisms symbiotic with zooxanthellae, giant clams Tridacna maxima were exposed daily to increased ammonia and phosphate (N, P, N+P) in their natural reef environment for 3 to 6 mo. The results strongly corroborate the major responses of the symbiotic association to nutrient enrichment previously observed (with T. gigas) under controlled outdoor conditions. Exposure of the clams to elevated N (10 µM) increased zooxanthellae density, reduced zooxanthellae size, down-regulated N uptake by zooxanthellae freshly isolated from their hosts, and reduced glutamate in the clam haemolymph, with increased pools of some free amino acids (methionine, tyrosine) in the zooxanthellae. These results confirm that the zooxanthellae in giant clams are N limited in situ and have free access to inorganic N from the sea water. There is also corroborating evidence that the zooxanthellae are P limited in situ as well, possibly due to host interference. While the N:P ratios of the animal host reflected ambient N and P concentrations in the sea water, those of the zooxanthellae did not. Regardless of P exposure (2 µM P) of the clams, zooxanthellae N:P ratios were consistently high(>30:1) and phosphate concentrations in the clam haemolymph bathing the zooxanthellae tube system consistently low (<0.1 µM). These field findings, consistent with previous laboratory observations, confirm the limiting roles of both N and P in the giant clam-zooxanthellae symbiosis. That significant changes occurred earlier and at lower nutrient loading compared to some reef organisms investigated within the same experimental framework further demonstrates organism-level responses of a potential bio-indicator of the early onset of eutrophication in reef waters.
- Genetic structure of giant clam (Tridacna derasa) populations from reefs in the Indo-PacificMacaranas, J. M.; Ablan, C. A.; Pante, M. J. R.; Benzie, J. A. H.; Williams, S. T. (Springer, 1992-06)Large genetic differences were observed among the Great Barrier Reef (GBR), Fiji and Philippine populations of Tridacna derasa (Roding) sampled in 1989 and 1990 (Nei's unbiased genetic distance, D,=0.137 to 0.341). This result contrasted strongly with the low genetic distance (D=0.032) reported previously for the giant clam T. maxima over similar geographical scales. No significant genetic differentiation was observed among most populations from the GBR (mean D=0.007), consistent with the high gene exchange expected in this highly connected reef system. However, significant differentiation resulting from differences in the frequencies of less common alleles between the North-Central GBR and South GBR (Swain region) were observed. Historical isolation of blocks of the Central Indo-West Pacific from the GBR and present-day restrictions to gene exchange between the GBR, Fiji and the Philippines as a result of oceanographic current patterns, were thought to be responsible for the high degree of genetic differentiation of T. derasa populations. The relevance of these findings to clam mariculture and reef restocking are briefly discussed.
- Status of giant clam resources of the PhilippinesJuinio, Marie Antoinette R.; Meñez, Lambert Anthony B.; Villanoy, Cesar L.; Gomez, Edgardo D. (Oxford University Press (OUP), 1989-11-01)Field surveys were conducted to determine distribution and abundance of giant dam resources in the Philippines. All seven known species of giant clams were recorded with Tridacna crocea, T. maxima and T. squamosa occurring the most frequently. The larger species T. derasa, T. gigas, Hippopus hippopus and H. porcellanus were relatively rare. The reduction of giant dam stocks in the Philippines is due to the uncontrolled exploitation of this resource which is primarily gathered for shellcraft and as supplementary diet in coastal villages.