Abstract. The importance of auto- and heterotrophy for growth, survival, zooxanthellae development and nutrient uptake of juvenile giant clams Tridacna maxima and T. squamosa was investigated. In addition experiments on the photosynthesis, metabolism and calcification were performed with specimens of T. maxima and T. derasa in an intermittent flow-through respiration system. Animals were reared in a recirculation system under different light conditions and different concentrations of nutrients. Both particulate organic (Tetraselmis algae) and dissolved inorganic (ammonia and phosphate) food was given. Results show that light intensity and spectra have a significant effect on survival and length or weight increase. Nutrient concentrations only seem to play a role when clams are kept in optimum light conditions. In this case fertilization with NH4+ and PO43- is more effective than feeding with algae. However, the concentration of zooxanthellae is only moderately increasing with increasing nutrient concentration. Moreover photosynthesis, respiration rates and calcification seem to be linked to light conditions but not to increasing nutrient concentrations. It is concluded that juvenile clams depend more on autotrophy to satisfy nutritional requirements. The uptake of nutrients is limited and needs to be tested with larger clams and a variety of nutrient combinations