NUTRITION OF THE GIANT CLAM TRIDACNA-GIGAS (L) .1. CONTRIBUTION OF FILTER FEEDING AND PHOTOSYNTHATES TO RESPIRATION AND GROWTH

The total carbon requirements (growth + respiration) of the host tissues of the giant clam Tridacna gigas from Davies Reef on the Great Barrier Reef were measured, and compared with rates with which nutrients were acquired from the two potential sources, translocated photosynthates (estimated from oxygen flux-CZAR method) and filter feeding. Results showed that the giant clam is an efficient utilizer of particulate organic matter available in reef waters (Davies Reef mean: 97-mu-g C.l-1), retaining on average 75% of particles between 2 and 50-mu-m, and absorbing from them 54% of C. Clearance rates (CR, 1.h-1) of clams were size dependent as defined by the function: CR = 1.85 W0.58 (r2 = 0.85, n = 56). There are major differences between typical non-symbiotic bivalves and Tridacna gigas regarding the relative allocations of energy to respiration and growth. The proportion of carbon deposited in tissues relative to that respired is high in giant clams relative to completely heterotrophic bivalves. We conclude that autotrophy is the major source of carbon to this clam, potentially capable of satisfying all respiratory requirements of the host. However, the potential importance of heterotrophy to total energy needs of the host is also significant and changes with the size of clam. The spectacular rates of growth in this clam are such that filter feeding is able to provide 65% of the total carbon needed both for respiration and growth in small clams (100 mg dry tissue wt), whereas large clams (10 g) acquire only 34% of their carbon from this source.