Comparative study on apparent digestibility coefficients of crude protein and amino acids from ten novel protein sources in Chinese mitten crab (Eriocheir sinensis)

The global shortage and rising cost of fish meal have driven growing interest in novel protein sources for aquafeed. Advanced processing technologies, including enzymatic hydrolysis, fermentation, extrusion, and hydrolysis, have enabled the production of various deeply processed protein ingredients; however, their digestive utilization in aquatic animals remains insufficiently characterized. As a commercially important crustacean, Chinese mitten crab (Eriocheir sinensis) possesses a digestive physiology that differs fundamentally from that of fish, and the direct application of fish-derived digestibility data to crab feed formulation can lead to substantial errors. However, systematic digestibility data for novel protein sources in this species remain scarce, and whether crude protein apparent digestibility coefficients (ADCs) can reliably predict amino acid availability in highly processed ingredients has not been empirically examined. This study aimed to evaluate the ADCs of ten novel protein ingredients in Chinese mitten crab and to investigate the discrepancy between crude protein and amino acid digestibility across different processing technologies. Ten commercial protein ingredients were tested against fish meal (FM) as the control, including enzymatically hydrolyzed chicken meal (EHCM), feather meal (EHFM), full-fat soybean (EHFS), and soy protein (EHSP); fermented cottonseed meal (FCM) and soybean meal (FSM); extruded soybean meal (ESM) and feather meal (EFM); hydrolyzed feather meal (HFM) and fish meal (HF). Male crabs initial body weight (125.0 ± 6.1) g were assigned to 12 groups (3 replicates, 6 crabs each). Using yttrium oxide (Y₂O₃, 0.1%) as an inert marker, ADCs of dry matter, crude protein, crude lipid, and 17 amino acids were determined via the 70% basal diet + 30% test ingredient substitution method, with fecal collection conducted over 80 days. Ingredient type and processing technology significantly affected all nutrient ADCs (P < 0.05). Crude protein ADCs ranged from 62.95% (ESM) to 92.80% (EHCM). EHCM and EFM (90.77%) were comparable to FM (88.46%) (P > 0.05), while ESM, HFM (65.28%), EHFS (65.11%), and HF (69.22%) were significantly lower (P < 0.05). Notably, amino acid ADCs did not consistently align with crude protein ADCs. Despite its lowest crude protein ADC, ESM maintained ADCs above 90% for methionine (94.35%), histidine (95.08%), lysine (96.78%), and alanine (93.69%). Principal component analysis of essential amino acid digestibility explained 86.4% of total variance, and PC1 scores were significantly correlated with the digestibility discrepancy index (DDI = crude protein ADC − mean essential amino acid ADC) (r = 0.796, P = 0.003). EHCM and FCM exhibited the largest positive DDI values (+30.4% and +27.0%), indicating that non-protein nitrogen substantially inflated crude protein ADC relative to actual amino acid utilization. Conversely, ESM showed the largest negative DDI (−21.8%), demonstrating that crude protein ADC underestimated its amino acid utilization potential. In conclusion, EHCM and EFM are high-quality alternative protein sources for Chinese mitten crab with protein digestibility comparable to fish meal. This study reveals that crude protein ADC is an unreliable predictor of amino acid availability in deeply processed ingredients. It is recommended that amino acid-based digestibility data be incorporated into feed formulation systems to correct evaluation biases introduced by processing technologies.