Photosynthesis and Respiration Intridacna Gigasas a Function of Irradiance and Size

The effects of irradiance level and size on the rate of O2 evolution and consumption was examined in Tridacna gigas using an oxygen electrode. Seven photosynthesis-irradiance (P-I) curves were generated for intact clams ranging from 1 to 23 cm in shell length. Both alpha and Pmax decreased with increasing size of the clam. Oxygen evolution at 1000 µE·m–2·s–1 and consumption in the dark were measured for an additional 9 clams ranging up to 38 cm in shell length. Oxygen evolved per gram clam tissue in the light, dark respiration, and the number of zooxanthellae per gram all decreased with increasing clam size. The average amount of shading experienced by the zooxanthellae in the clam tissues was estimated by comparing the P-I curves of different-sized clams to the P-I curves of zooxanthellae freshly isolated from these clams. Estimates of shading increased from negligible levels in small clams to 80% in the largest clams investigated. Use of these results to chose optimum light intensities for the mariculture of T. gigas is discussed.