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For large-scale artificial breeding of sea cucumber (Holothuria leucospilota), it is necessary to understand how environmental parameters affect larval rearing. In this study, the effects of salinity changes on survival, growth, and development of the mid- and late-auricularia of H. leucospilota were estimated within a 7-day culture period. Salinity shock is usually accompanied by water replacement during hatching. To replicate the occurrence of salinity changes in actual larval production, 1/2 of the larval rearing water (initial salinity ∼31.6 ppt) was replaced daily by the experimental waters at different salinity levels (18, 22, 26, 30, 34, 38, and 42 ppt). On day 7, larval survival rates varied with salinity, while the highest survival rates (44.37–46.63% for mid-auricularia; 50.71–53.98% for late-auricularia) were observed in normal salinities (30 and 34 ppt) and the lowest survival rates (15.71–23.96% for mid-auricularia; 24.11–29.57% for late-auricularia) were observed in extreme salinities (18, 22, and 42 ppt). For any sampling event, the body length of mid-auricularia in normal salinities (30 and 34 ppt) were continued to increase during the 7-day culture period. The highest length of late-auricularia was reached on day 3 (880.01 ± 84.38 and 889.06 ± 112.11 µm, respectively), then decreased to 745.31 ± 127.64 and 742.94 ± 98.43 µm on day 7 due to metamorphosis. On day 7, the proportion of mid-auricularia larvae without development potential was the lowest (36.67%) at a salinity level of 34 ppt, and the proportions were increased with both decreasing/increasing salinities. However, this proportion was much lower for the late-auricularia at the same salinity. The decrease in salinity could promote metamorphosis of the late-auricularia, and the highest proportion of doliolaria (24.32%) was observed at salinity level of 26 ppt on day 7. In extreme salinities (18, 22, and 42 ppt), the hyaline spheres (indication of larval competence) in late-auricularia were less obvious than those in other salinities. The ability in resisting salinity shock was increased with larval development, where late-auricularia had better tolerance to salinity shock than mid-auricularia, whereas larvae at both stages were more tolerant to hypertonic than hypotonic conditions. Based on our results, we propose to rear the mid- and late-auricularia at a comparatively high level of salinity (e.g., ∼31 ppt) to promote their survival and performance, while to decrease the salinity (e.g., ∼26 ppt) in the last period of late-auricularia to increase their competence to metamorphose into doliolaria.

Because most tropical sea cucumbers have been overexploited around the world, the sea cucumber Holothuria leucospilota has become increasingly commercially important in recent years. Restocking and aquaculture of H. leucospilota using hatchery-produced seeds could both enhance declining wild populations and provide sufficient beche-de-mer product to satisfy increasing market demand. Identifying an appropriate diet is important for successful hatchery culture of the H. leucospilota. In this study, we trialed different ratios of microalgae Chaetoceros muelleri (2.00–2.50×106 cells/mL) and yeast (Saccharomyces cerevisiae, ~2.00×106 cells/mL) in diets for H. leucospilota larvae (6 d after fertilization, referred to as “day 0”) at proportions 4 : 0, 3 : 1, 2 : 2, 1 : 3, and 0 : 4 by volume, in 5 treatments (A, B, C, D, and E, respectively). Larval survival rates in these treatments decreased over time, with the survival highest in treatment B (59.24±2.49%) on day 15 (double that of the lowest rate in treatment E (28.47±4.23%)). For any sampling event, larval body length in treatment A was always the lowest after day 3, and that for treatment B was always the highest, except on day 15. The maximum percentage of doliolaria larvae occurred in treatment B (23.33%) on day 15, followed by treatments C, D, and E (20.00%, 10.00%, and 6.67%, respectively). No doliolaria larvae occurred in treatment A, and pentactula larvae occurred only in treatment B (3.33%). On day 15 in all treatments, late auricularia larvae had hyaline spheres, but these were not prominent in treatment A. Densities of juveniles attaching to settlement plates varied with treatments, and the values were very low for the larvae only fed microalgae (A, 2.39±1.95 ind per plate) or yeast (E, 2.13±1.05 ind per plate)—only ~5% of the maximum number settling in treatment B (45.56±7.24 ind per plate). Increased larval growth, survival and development, and juvenile attachment indicates that diets combining microalgae and yeast are more nutritionally balanced than single diets for hatchery of H. leucospilota. A combined diet of C. muelleri and S. cerevisiae at a 3 : 1 ratio is optimum for the larvae. Based on our results, we propose a larval rearing protocol to facilitate mass production of H. leucospilota.