Growth Performance and Postprandial Metabolic Routing in Red Abalone (Haliotis rufescens): Comparing Bacillus-Supplemented Plant- and Fishmeal-Based Diets to Fresh Macrocystis pyrifera

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.