Effects of microalgae and stocking density on growth and survival rate of giant clam (Tridacna squamosa Lamarck, 1819) larvae

Abstract

Abstract The present study aimed to evaluate the impact of microalgae mixture and stocking density on growth and survival rate of giant clam (Tridacna squamosa) at the veliger stage. In the first experiment, giant clam larvae of the D-stage were reared in the conical fiberglass tanks and fed with combinations of microalgae, including Chaetoceros muelleri and Isochrysis galbana (1:1), and C. muelleri, I. galbana and Nannochloropsis oculata, (1:1:1). One group of giant clam larvae without feeding was used as a control. The experiment was lasted for 7 days, until the Pediveliger stage with six replicate tanks. In the second experiment, the larvae (1 day post hatched, DPH) were reared at densities of 3, 5, 7 and 9 larvae mL-1 (so-called D3, D5, D7 and D9, respectively) for a period of 7 days. Each density was randomly assigned to 6 replicate tanks. The larvae were fed once a day with a mixture of C. muelleri, I. galbana and N. oculata, (5,000 cells/ml each type of algae). At 4DPH, extracted zooxanthelle (Symbiodinium microadriaticum) was seeded into the giant clam larvae at the concentration of 5,000cells/ml once a day. The results showed that larvae fed with C. muelleri, I. galbana and N. oculata (1:1:1) obtained the largest shell length (217.5 ± 3.1 µm), specific growth rate (SGR) 7.18 ± 0.34 (mean±SD) and the survival rate (31.5% ± 1.6%) (P<0,05). In addition, rearing densities significantly affected growth in shell length, SGR and survival rate in giant clam larvae. Of which, the larvae at D3 gained the highest shell length, SGR and survival rate. Meanwhile, the larvae at D9 had the lowest shell length, SGR and survival rate. There was no significant difference in larval survival rate between D3 and D5. Therefore, it is suggested that in the hatchery giant clam larvae reared at 5 larvae mL-1 in order to improve productivity.