Heterotrophic Nutrition of Giant Clams from the Jordanian Coast of the Gulf of Aqaba, Red Sea

This study was carried out to investigate the Heterotrophic nutrition of the giant clams (Tridacnidae) in the Jordanian coast of the Gulf of Aqaba, Red Sea. In the present experimental work, a Tridacna maximum was used as a target species, because of its abundance in natural stocks as compared to the other species. Different species of the microalgae Isochrysis galbana, Nannochloropsis oculata, Tetraselmis suecica, and natural phytoplankton were used in the feeding of three size groups (3-5, 5-7, 7-9 cm's) of Juvenile Tridacna maxima. Within the different microalgae species Isochrysis galbana was found to be a good food for all growth stages of giant clams. The results showed a significant difference in the grazing rate of different microalgae species in both short and long-term experiments. The grazing rates of 1.64, 1.22, 0.96 and 0.72 µg l-1 d-1 chlorophyll a (chll a) for Isochrysis galbana, Tetraselmis suecica, Nannochloropsis and natural phytoplankton, respectively. Significant differences in the grazing rate of the different size classes of the juveniles were also observed. However, the grazing rate of the different shell-length classes was in the following order 3-5 > 5-7 > and 7-9 cm. Sampling time does not seem to have any significant effect on the grazing rate of T. maxima on the selected microalgae species. There were no significant differences in the grazing rate of T. maxima veliger larvae under two feeding conditions. The grazing rate was 0.13 µg l-1 d-1 chll a for feeding conditions of 600 cell. ml-1 and 0.07 µg l-1 d-1 chll a for feeding conditions of 300 cell.ml-1. The correlation between the grazing rate and veliger number was highly positive.