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.

Plant-based formulations supplemented with probiotics are a promising strategy to improve nutrient bioavailability in aquaculture, yet mechanistic understanding of microbial modulation of host metabolism remains limited. We evaluated whether supplementation with Bacillus velezensis (Sp1) and B. amyloliquefaciens (Sp3) influences metabolic efficiency and energy allocation in sub-adult red abalone (Haliotis rufescens). Over 180 days, animals were fed a plant-based or a fishmeal-based diet—both probiotic-supplemented and matched in protein and energy—and compared with fresh kelp (Macrocystis pyrifera) as a control. Metabolic performance was assessed from oxygen consumption, relative specific dynamic action (SDA) index, ammonia excretion and Oxygen-to-nitrogen ratio (O:N) ratio. The Bacillus-supplemented plant-based diet achieved metabolic efficiency comparable to M. pyrifera, with minimal SDA (4.7% vs. 4.3%) and commercially viable growth (2.07 mm/month). The O:N ratio indicated diet-specific, time-dependent shifts in substrate use. The fishmeal-based diet—despite identical macronutrient and probiotic composition—produced a markedly higher postprandial metabolic cost (SDA = 21.0%), consistent with reduced compatibility between high animal protein and herbivorous digestive physiology. Overall, probiotic supplementation may help convert plant-based ingredients into metabolically efficient functional feeds, offering a more sustainable approach that reduces dependence on marine-derived ingredients.

The economic output of sea urchin aquaculture depends almost entirely on a single organ. Gonads, marketed as uni, determine value through their size, color, texture and flavor, and feed formulations have been optimized empirically for decades with limited reference to the molecular machinery governing gametogenesis, sex differentiation and gonadal maturation in echinoids. This review integrates four lines of mechanistic evidence around the adult gonad, namely developmental gene regulatory networks, sex-biased gonadal transcriptomics, germ-cell and nutritive-phagocyte biology, and endocrine, neuroendocrine and retinoid signaling, and reads them in terms of the control points connecting molecular state to production outcome. Rather than advancing a unified mechanistic model, which the current evidence does not support, each component is classified by the strength of the evidence attached to it: demonstrated in an echinoid, inferred from another taxon, plausible but untested, or hypothetical. The resulting picture is one in which descriptive resources have outpaced functional tests. Candidate sex-determination transcription factors have been identified but none have yet been directly perturbed in an echinoid, the retinoid pathway is architecturally present yet biochemically incomplete, and responses to exogenous steroids remain heterogeneous across species and protocols. Priority experimental directions are ranked by commercial return and by experimental falsifiability.