Slow or nonexistent natural recovery of the Caribbean long-spined sea urchin (Diadema antillarum) following a mass mortality event in [1983][1984] has prompted interest in hatchery-origin production and restocking to aid coral-reef restoration. A critical rst step is the ability to propagate D. antillarum from gametes, at scale. However, a unique larval biology and di cult and lengthy culture period of ~ 40 days has resulted in inconsistent success over the past 20-plus years. The purpose of this study was to develop protocols for rearing D. antillarum within a novel 1800-L recirculating aquaculture system capable of scaled production. Five separate experiments investigated larval development in response to diet quantity, diet composition, and initial stocking density within 40-L replicate culture tanks. The initial experiment was used to develop a microalgae reference diet consisting of Tisochrysis lutea and Chaetoceros sp. and revealed similar growth and survival between high quantity (40.0×10 3 cells ml -1 ) and low quantity (10.0×10 3 cells ml -1 ) treatments at 21 DPF. Experiments 2-4 examined diet quality by comparing carbon-equivalent microalgae compositions. Mixed diets containing Rhodomonas lens outperformed the reference diet in multiple experiments and a tripartite diet containing all three species resulted in signi cantly higher survival at 42 DPF. The highest growth overall occurred from a monoalgal R. lens diet, which indicated that this species is critically important. Further observations of density-dependent growth dynamics revealed that initial stocking densities > 1 larvae ml -1 signi cantly reduced growth over 28 DPF. Data generated were used to establish fundamental larviculture protocols that have since led to the production of over 1,000 juveniles. Adult D. antillarum were collected in March 2018 from reefs at < 8m depth off Marathon, Florida by the Florida Fish and Wildlife Research Institute under Florida Keys National Marine Sanctuary permit # FKNMS-2018-023. These broodstock were transported to a land-based restoration aquaculture facility operated by The Florida Aquarium in Apollo Beach, FL. Urchins were quarantined in a RAS within a greenhouse for 45 days. Following quarantine, 14 broodstock were transferred to 450-L berglass tanks within a 2,400-L seawater RAS. Broodstock husbandry adhered to methods previously described in Pilnick et al. (2021).Spawning was induced via thermal shock following methods described in Leber et al. (2009) andMoe (2014). Broodstock were transferred to a polyethylene tank lled with 150-L of 1-µm ltered 35 ppt arti cial seawater (ASW) (Tropic Marin, Wartenburg, Germany). Water was heated to 5°C above holding temperature and aeration was provided. Eggs were collected in a 60-mL syringe following release from females and gently rinsed in 2-L egg collectors with 35-µm mesh. Typically, males released