Liver development in Atlantic cod (Gadus morhua L.) larvae: Histomorphological analysis of biliary ABC transporters and hepatic vacuolization

Atlantic cod (Gadus morhua) larvae begin exogenous feeding while organ systems are still immature. Although the liver is morphologically differentiated at hatching, its functional development remains poorly understood. This study characterized histomorphological liver development in cod larvae using (1) monoclonal antibodies C219 and C494 to assess biliary secretory capacity across the hepatocyte canalicular membrane via ATP-binding cassette transporters, and (2) hepatic vacuolization as an indicator of energy storage maturation. Larvae were fed four formulated diets in a 2 × 2 factorial design from 17-61 days post-hatching (dph) to test effects of phospholipid level (~ 7 vs. 6 % dry matter) and bile salt supplementation (0 vs. 0.04 % dry matter). Only C219 labeled bile canaliculi, evident before first-feeding (2 dph) and peaking at 30 dph (9.2 mm standard length), suggesting enhanced capacity for phosphatidylcholine (Abcb4) and/or bile salt (Abcb11) efflux prior to metamorphic climax. Hepatic vacuolization remained low (2.3 %) until late metamorphosis, then increased 14-fold (35.9 %) in a sigmoid pattern with larval size. The steepest rise occurred at 17.7 mm standard length, marking the onset of hepatic lipid storage as larvae transition to juveniles. Dietary treatments did not significantly affect C219 labeling, vacuolization or histological biomarkers of nutritional status. These results indicate that cod liver functionality develops progressively during the larval stage and is largely unaffected by dietary phospholipid level and bile salt supplementation under the tested conditions.