Abstract: To investigate the ion regulation of large yellow croaker (Larimichthys crocea) under hypoxia and acidification stresses, we investigated the effects of hypoxia (dissolved oxygen DO 3.5 mg·L^-1, pH 8.1), acidification (DO 7.0 mg·L^-1, pH 7.35) and combined stresses of hypoxia and acidification (DO 3.5 mg·L^-1, pH 7.35) on gill tissue structure and physiological indices related to ion regulation of juvenile L. croaker. The results showed that, under hypoxia stress, gill Na⁺/K⁺-ATPase activity, serum Na⁺, Ca²⁺ and Cl^- contents of juvenile L. croaker decreased first and then increased. Under acidification stress, gill Ca²⁺-ATPase activity, serum Na⁺ and Ca²⁺ contents of juvenile L. croaker increased first and then decreased. Under the combined stresses of hypoxia and acidification, Na⁺/K⁺-ATPase activity and Na⁺, K⁺ and Ca²⁺ contents increased first and then decreased, while Ca²⁺-ATPase activity and Cl^- content decreased first and then increased. The results of gill histology showed that hypoxia and acidification stresses led to the detachment of gill epithelial cells, and the combined stresses of hypoxia and acidification led to proliferation, hypertrophy and swelling of gill epithelial cells. Comprehensive analysis showed that hypoxia and acidification stress affected the activities of major ion regulatory enzymes in juvenile L. croaker and caused different degrees of damage to gill tissue, resulting in imbalanced ion regulation in juvenile L. croaker.

Key words: hypoxia, acidification, large yellow croaker, Na⁺/K⁺-ATPase, Ca²⁺-ATPase, serum ion