Effects of Bacillus spp. and fermented carbon on performance of planktonic larvae of Holothuria leucospilota

The seedling rearing techniques for sea cucumber (Holothuria leucospilota) are still underdeveloped, particularly with unsatisfactory results during the planktonic larval stage. By feeding with Chaetoceros muelleri, Chlorella powder and Saccharomyces cerevisiae as basic diets, we investigated the effects of supplementing microbial amendments (Specifically fermented carbon, Bacillus subtilis, or B. pumilus) on the larval survival, growth, and development of H. leucospilota, as well as their impact on water quality in the rearing system. Results show that the larvae in the fermented carbon group reached the doliolaria stage at the same time as the control group, and showed no significant difference in the survival rate at the end of experiment (p>0.05). However, the average body length of the doliolaria larvae was (666.25±5.45) μm, significantly higher than that of the control group (641.79±6.63) μm(p<0.05). Conversely, the larvae supplemented with B. subtilis or B. pumilus stopped development at the auricularia stage. Compared with the larvae in the control group and fermented carbon groups, those in B. subtilis group showed a statistically significant decrease in the terminal body length, and all B. pumilus group larvae died by 12^th day (p<0.05). The results show that addition of fermented carbon could reduce the ammonia and nitrite levels in the culture system, but its effect was short-lived. The addition of B. subtilis had a positive effect on controlling the ammonia and nitrite levels, while the addition of B. pumilus had a relatively poor effect on controlling water quality. Based on the cultivation outcomes and water quality parameters, it is suggested to implement continuous supplementation of fermented carbon during the rearing of planktonic larvae of H. leucospilota.