Phenotype-Anchored Multi-Omics Reveals Mechanisms Underlying Starvation-Induced Body, Liver, and Gallbladder Discoloration in Rock Carp (Procypris rabaudi).

Visible changes in body and internal organ coloration can serve as practical indicators of nutritional and metabolic stress in aquaculture, but their physiological meaning remains poorly understood. Here, we used starvation as a controlled nutritional stress model to investigate the physiological and molecular changes associated with body color fading and liver and gallbladder discoloration in rock carp (Procypris rabaudi). To assess the effects of food deprivation (0-35 days), fish were examined using growth and biochemical assays, intestinal histology, as well as integrated hepatic transcriptomic and metabolomic profiling. Starvation caused weight loss, reduced condition factor and hepatosomatic index, intestinal atrophy, dynamic changes in digestive enzyme activities, and a transition from early antioxidant activation to late-stage oxidative injury. It also produced a clear sequence of visible color changes, with body pigmentation fading from deep black to reddish-brown, the liver changing from dark red to pink, and the gallbladder changing from dark green or black to pale yellow or yellow-green. Multi-omics analysis associated these phenotypes with three major processes. Body color fading was associated with down-regulation of bcl-2l10, bcl-2l11, and cdk2 and depletion of tyrosine-related metabolites, suggesting possible alterations in melanogenesis-related substrate availability. Liver paling was linked to lipid and glycogen mobilization, as indicated by altered fabp and aldo expression and decreased levels of glycerophosphocholine, phosphatidylethanolamine, and phosphatidylcholine. Gallbladder discoloration was consistent with reduced expression of ho-1 and abcb11, suggesting possible involvement of pathways related to bile pigment formation and bile salt transport. Lysophosphatidic acid and demethylnobiletin were identified as candidate indicators of late-stage oxidative injury. These findings indicate that starvation-induced color changes are informative physiological phenotypes rather than superficial traits, and may support future monitoring of nutritional stress and organ dysfunction in rock carp aquaculture.