Photoadaptation in juvenile Tridacna gigas

Photoadaptation was demonstrated in the algal symbiosis of juveniles of the giant clam Tridacna gigas. Juveniles that were grown in unshaded ambient light required higher light intensities (about 200 μE/m²/sec, 95% confidence interval: 194-218 μE/m²/sec), for saturation of oxygen production, while those that were reared in approximately 9OClJo shaded ambient light reached saturation at lower light intensities (ISO μE/m²/sec, 95% confidence interval: 126-175 μE/m²/sec). Shade-reared clams had a higher chlorophyll a concentration per cell, and based on chlorophyll a had lower alpha, or photosynthetic, rates at subsaturating irradiance, and lower Pm, or maximum photosynthetic rates, at saturating irradiance. P:R, specifically the ratio of net zooxallthellae photosynthesis to total respiration over 24 hours, of unshaded clams was significantly higher (p < 0.05) than values for shaded clams. All P:R ratios were higher than 1.0 indicating net primary production. CZAR values, the contribution of photosynthetically fixed carbon to host respiration, were higher in unshaded clams. Greater P:R and CZAR may account for the higher condition index. (wet flesh weight/shell length) in unshaded clams. Kept in darkness, juvenile clams lost condition within a few days and showed heavy mortality after 6 days. This may support the P:R results in showing the dependence of juvenile clams on their algal symbionts for a major part of their nutrition