The culture of the tropical sea urchin Salmacis sphaeroides: A new candidate for aquaculture in South China

Sea urchins produced by aquaculture could enhance declining wild populations and provide a sufficient roe product to satisfy increasing market demand. This study reports a method for large-scale rearing of the edible tropical sea urchin Salmacis sphaeroides, from induction of larval metamorphosis and settlement, to juvenile rearing and grow-out. Larvae were reared in 500-L hatchery tanks, on a diet of the microalga Chaetoceros muelleri at concentrations ranging from 2500 to 24,000 cells mL⁻¹, with daily water renewal of one-third to two-thirds of the volume depending on their developmental stage. The larvae breeding protocol proved suitable for this species, and competent larvae occurred at 12 days post-fertilization. To induce metamorphosis, we tested exposure to potassium chloride (KCl) at different concentrations (0, 25, 50, 100, 200, and 400 mmol L⁻¹) and for different durations (5, 10, and 20 min). Exposure to 200 mmol L⁻¹ KCl for 5 min resulted in the highest proportion of metamorphosed individuals (41.67 ± 2.89 %). Using this method, the densities of juveniles attaching to settlement plates (232.50 ± 41.23 ind. per plate; settlement percentage ca. 3.88 ± 0.69 %) were approximately twice as high as those of the controls (105.75 ± 15.52 ind. per plate; settlement percentage ca. 1.76 ± 0.26 %). Next, two batches of juveniles were grown out in sea cages. In the first batch (deployed for ca. 4 months), the absolute growth rate of the test diameter (AGR_D) was 12.79 ± 0.98 mm month⁻¹ from May to September; in the second batch (deployed for 6 months), the AGR_D was 5.68 ± 0.90 mm month⁻¹ from October to May of the following year. Survival of the juvenile S. sphaeroides in the sea cages was high (86.56 %–100 %). Finally, the S. sphaeroides cultivated in the second batch were harvested (at a mean test diameter of 71.24 ± 1.45 mm, and wet weight of 101.24 ± 1.91 g) on May 28 of the second year; all animals had matured and could be successfully utilized as broodstock. The fatty acid composition of the gonads was then compared between cultivated and wild-collected adult S. sphaeroides. The proportion of polyunsaturated fatty acids in the cultivated sea urchins (16.59 ± 0.67 %) exceeded that in wild-collected individuals (13.13 ± 0.42 %). From these experiments we conclude that S. sphaeroides is a sea urchin species with excellent potential for aquaculture; the present methods and data will provide helpful information for their mass production in South China.