Stranded heavy fuel oil impairs non-specific immune defense in sea cucumber Apostichopus japonicus (Selenka): evidence from cellular, humoral, and oxidative stress biomarkers

Heavy fuel oil (HFO) spills pose persistent threats to coastal ecosystems; however, the immunotoxic effects of stranded HFO on benthic deposit-feeders remain less understood. This study employed an oiled gravel column (OGC) system simulating the weathering and tidal flushing of stranded HFO 380# on gravel substrates and investigated immunotoxic responses in sea cucumbers (Apostichopus japonicus) over 21 d at environmentally relevant oil loadings (0, 2, 8, and 32 mg HFO/g gravel). Results showed that weathering loss of stranded HFO followed characteristic sigmoidal kinetics (R2 = 0.934, 0.898, and 0.902 for 2, 8, and 32 mg/g groups, respectively), while total petroleum hydrocarbon (TPH) concentrations in OGC effluents followed first-order exponential decay (R2 = 0.974, 0.958, and 0.979 for 2, 8, and 32 mg/g groups, respectively). Stranded HFO exposure induced significant dose− and time−dependent effects in innate immune defense. Cellular immune endpoints, including total coelomocyte count (TCC), phagocytic (PAC) activity, and respiratory burst (RBT) capacity, showed initial compensatory activation at lower oil loadings by Day 14, followed by progressive suppression at higher oil loadings by Day 21. Humoral immune biomarkers, including acid phosphatase (ACP) activity, lysozyme (LZM) content, and total nitric oxide synthase (T-NOS) activity, exhibited analogous biphasic patterns characterized by early induction and subsequent inhibition, particularly at higher oil loadings. Reactive oxygen species (ROS) levels increased significantly across all groups. Pearson correlation and principal component analyses identified strong negative correlations between sustained intracellular ROS levels and multiple immune biomarkers, suggesting that petroleum hydrocarbon-induced oxidative stress may represent a key mechanism contributing to the observed immunosuppression. These findings highlight the potential for stranded HFO to progressively compromise innate immune function in deposit-feeding echinoderms, raising concerns for benthic organism health in oil-impacted intertidal zones and warranting further ecological risk assessment of HFO spill residues on gravel shorelines.