Palm oil-based diet impairs low-salinity tolerance in Scylla paramamosain: Metabolomic insights into osmoregulatory dysfunction, mitochondrial β-oxidation impairment and endoplasmic reticulum stress.

The aquaculture industry increasingly adopts palm oil (PO) as a sustainable alternative to fish oil (FO). However, whether this substitution induces adverse physiological responses under low-salinity stress remains poorly understood, especially for estuarine species. This study investigated the effects of long-term (10 weeks) dietary intake of FO and PO on growth, osmoregulation, mitochondrial function, lipid and energy metabolism, endoplasmic reticulum stress (ERS), and metabolomics in Scylla paramamosain under normal salinity (25‰, NS) and low salinity (6‰, LS). The results showed that crabs in the LS-PO group had significantly reduced percent weight gain compared to those in the LS-FO group. Additionally, LS-PO crabs demonstrated elevated hemolymph and posterior gill apoptosis, accompanied by severe gill structural damage and mitochondrial impairment in antennal glands. Osmoregulatory capacity was further compromised, as evidenced by decreased activities of Na⁺/K⁺-ATPase, V-ATPase, and CA compared to NS-FO and LS-FO groups. Mitochondrial dysfunction was indicated by reduced membrane potential, elevated ROS and MPTP, decreased SDH and ATP contents in the gills, and impaired mitochondrial respiratory chain complexes I-IV related genes expression in both posterior gills and hepatopancreas (nduf6, cox1, cyc1). PO further exacerbated the expression of ERS-related proteins (Grp78, P-Perk, Atf4, P-Ire1) in the hepatopancreas under low salinity. Hepatopancreatic lipid dysregulation was aggravated in LS-PO crabs, characterized by accumulated lipid droplets and downregulated expression of cpt1, hsl, acox1, and acox3. Notably, LS-PO crabs accumulated long-chain acylcarnitines and exhibited depletion of TCA intermediates (α-ketoglutarate), indicating severe suppression of mitochondrial β-oxidation. Correlation and interaction network analyses identified phosphorylcholine, glycerophosphocholine, na+/k+-atpase, hspa5, pelle, and relish as potential hub nodes. These findings indicate that, under the tested low-salinity conditions, dietary palm oil was less effective than fish oil in supporting osmoregulatory and metabolic homeostasis in Scylla paramamosain, highlighting the need to consider dietary lipid source when formulating feeds for low-salinity mud crab aquaculture.