First Report on Bacillus ‐Mediated Gonad Growth and Coloration in Purple Sea Urchin ( Strongylocentrotus purpuratus ) Using a Plant‐Meal Diet

Sea urchin ranching has been proposed as a potential restorative aquaculture activity to manage sea urchins from barrens affecting kelp forests globally. However, the development of sustainable and economically viable formulated feeds for sea urchin ranching is needed. This study evaluates the effect of a plant‐based diet and the incorporation of Bacillus probiotics on gonad growth, survival, and gonad coloration in purple sea urchin ( Strongylocentrotus purpuratus ). Over 8 weeks, four dietary treatments were assessed: plant‐based diet without Bacillus (AFC) and three probiotic‐supplemented diets (AFSp0, AFSp1, and AFSp2) incorporating 2 × 10 9 CFU kg −1 of B. subtilis , B. velezensis , and B. amyloliquefaciens , respectively. Probiotic treatments achieved mean (±SE) gonadosomatic index (GSI) of 15.30 ± 0.76, 16.10 ± 0.65, and 14.30 ± 0.60 for AFSp0, AFSp1, and AFSp2, respectively. In contrast, S. purpuratus in AFC obtained 8.23% ± 0.58% ( p < 0.05). Gonad weight increase (GWI) reached 634.10% ± 32.10% in AFSp2 versus 150.00% ± 15.90% in AFC ( p < 0.001). Survival rates were higher in probiotic treatments (AFSp0: 81.81% ± 7.87%; AFSp1: 84.84% ± 1.51%; and AFSp2: 92.4% ± 1.51%) compared to AFC (45% ± 2.62%). Low survival in AFC may be related to nondigestible antinutritional factors (ANFs) present in plant ingredients. Finally, spectrophotometric and high‐performance liquid chromatography (HPLC) analyses suggest the presence of putative 9‐cis‐echinenone (9CEC) at high concentrations in probiotic treatments. 9CEC concentration was 13.65 μg/g, 20.40 μg/g, and 17.10 μg/g for AFSp0, AFSp1, and AFSp2, respectively, representing increases of 31.88%, 97.10%, and 65.22% relative to the AFC control treatment (10.35 μg/g). These findings suggest that Bacillus strains improve gonad growth, survival, and gonad coloration in S. purpuratus fed a plant‐meal diet, potentially through ANFs degradation, nutrients absorption, and carotenoid metabolism enhancement. This research aims to provide practical insights for improving sea urchin ranching while assessing the broader potential of probiotic‐enhanced plant‐based nutrition in sustainable aquaculture systems.