World Aquaculture March 2019

48 MARCH 2019 • WORLD AQUACULTURE • WWW.WA S.ORG probes and maintained near saturation. The photoperiod was natural daylight but luminous intensity was reduced partially by covering the tanks. The diet in the larval phase was composed of a mixture of microalgae produced in 80-L plastic sleeves. In this phase, several microalgae species were supplied to cover the nutritional requirements of the sea urchin larvae: Isochrysis aff. galbana, Nannochloropsis occulata and Tetraselmis sp. imp3 and diatoms Chaetoceros calcitrans, Phaeodactylum tricornutum , Skeletonoma costatum . Algae was provided 2-3 times throughout the day to provide an algae concentration between 75,000 and 300,000 cells/ mL for each tank. Water renewal of rearing tanks was low (5 percent/hr) because of cultivation constraints. The small size and morphology of urchin larvae required the use of a very small mesh for larval retention, with periods of no water exchange when adding the microalgae, resulting in less waste. Microalgae consumption was 5-20 L/ day during the larval period and, with flow-through operation, the culturist may need a larger supply of microalgae. Tanks were cleaned by siphoning and mortality evaluated. Metamorphosis and Early Growth Larval stages (Fig. 6) presented major difficulties. On the cultivation of the sea urchin, which was still in a preliminary phase and with only incipient knowledge of requirements, survival was the biggest challenge to the development of aquaculture of this species. At EPPO, samples from tanks were collected daily for counting and estimating survival. A 70 percent survival rate at 7 DAH decreased to about 30 percent at 14 DAH. After this period, the estimation of survival by counting was no longer practical because of the onset of settlement and consequent passage to a benthic life stage. During metamorphosis, there is high mortality associated with the failure of settling and transition to the benthic stage. This is one of the aspects that lacked good research work to improve production performance. Relating to sea urchin larval morphology, two measurements were chosen as standard throughout the first phase: total length and the diameter measured by the maximum amplitude of the arms (Castilla-Gavilán et al . 2018). Larval growth was not continuous over time (Fig. 7), presenting growth that reflected morphological alterations of the body, including the emergence of new arms (front side) and enlargement of the upper part (pre-adult). Growing Benthic Juveniles Between 14 and 17 days old, depending on water temperature, larvae started settlement. At 1 month old, all sea urchins reached the juvenile and benthic stage (Fig. 8). At that time, it was essential FIGURE 5. Embryonic development of purple sea urchin at 22 C. The fertilized egg diameter varies between 70-75.5 µm. FIGURE 6. Larval development of purple sea urchin. (A and B) The 4-arm stage occurs from 0-12 days after hatch (DAH), (C) the 6-arm stage begins 12 DAH, and (D) the 8-arm phase begins 16 DAH.