Contributions of phototrophic and heterotrophic nutrition to the metabolic and growth requirements of four species of giant clam (Tridacnidae)

We compare the relative contributions of phototrophy (translocation of photosynthates from zooxanthellae) and heterotrophy (filtered particles) towards the carbon requirements for tissue and shell growth, and metabolism in 4 species of giant clam from the Great Barrier Reef. The primary aims were to determine whether the differences in growth rates of various clam species could be due to nutrition, and to quantify the relative roles of phototrophy and heterotrophy in the nutrition of tridacnids. The species examined were distinguishable by both absolute C flux and relative proportions of components of the C budget. For example, Tridacna gigas was photosynthetically the most efficient, gaining twice as much nutrition as T. crocea, and an order of magnitude more than Hippopus hippopus. In the case of the smallest clams tested (0.1 g tissue wt), intake of C via filter feeding was also highest in T. gigas, being 10 times that of the other species. These interspecific differences declined with clam size. Tridacna gigas, T. crocea, and T. squamosa were able to satisfy all their growth and metabolic requirements from the intake of photosynthate and particulate food, in some cases with considerable energy to spare. In contrast, small H. hippopus gained 80 % of total C needs from these sources. We confirm that phototrophy is the most significant source of energy to clams. In all but the smallest H. hippopus, this source provides sufficient C for growth and metabolic requirements. Filter-feeding decreases in importance with increasing size of clam. Ingested C provides 61 to 113% of total needs in 40 to 80 mm T. gigas and 36 to 44 % in H. hippopus, but was less significant to the other species (10 to 20 %). H. hippopus allocated the highest proportion of C expenditure to growth (30 to 90 %), up to half of which went into shell. T. gigas and T. squamosa both put 20 to 40 % of C into growth, compared with only 10 to 20 % in T. crocea. There was no simple nutritional basis to the differences in growth of the 4 species. T. gigas has the greatest excess of energy available for growth, and the highest growth rate in terms of shell length. However, the connection between available energy and growth rate was not consistent across species. Actual growth in units of C was similar in T. gigas and H. hippopus, yet small individuals of the latter species appear limited by availability of C. Despite a relatively high calculated 'scope' for growth, T. crocea exhibited the lowest gr