Fishes

Benzo(a)pyrene (B[a]P) is a pervasive freshwater pollutant, yet its toxicity to the fish gallbladder remains poorly understood. This study investigated the toxicological impacts of 2.5 and 25 μg/L B[a]P on common carp (Cyprinus carpio) using histological, transcriptomic, and single-cell RNA sequencing (scRNA-seq) analyses. Results showed that the gallbladder is a primary site for B[a]P accumulation. High B[a]P concentrations caused vacuolar degeneration of mucosal epithelial cells and nuclear deformities. Transcriptomic analysis revealed that B[a]P stress triggered autoimmune homeostasis imbalance and overinhibited apoptosis. scRNA-seq identified cellular heterogeneity changes, specifically T-cell impairment and epithelial cell (EC) proliferation. Mechanistically, T-cell reduction was linked to the T-cell 2 subset, while EC proliferation involved EC 0 and EC 4 subsets, all participating in the apoptosis pathway. These findings demonstrate that the apoptosis pathway is a key target of B[a]P toxicity in the gallbladder. This work provides a cellular-level framework for assessing environmental polycyclic aromatic hydrocarbon (PAH) risks in aquaculture.

This study was conducted to explore the impacts of L-carnitine (CAR) on growth performance, antioxidant capacity, hepatic lipid deposition and fatty acid β-oxidation in farmed fish fed a high-fat diet (HFD). A total of 150 juvenile tiger puffer (15.23 ± 0.01 g) were randomly assigned into six tanks (three tanks each group) and fed with a control diet (HFD, 16% lipid) and a HFD supplemented with 1.5% CAR for eight weeks, respectively. The results indicated that the growth performance and the most proximate compositions were not influenced by dietary CAR. However, compared to the control group, the hepatosomatic index and crude lipid content in the liver were significantly reduced in the CAR group, indicating that CAR performed a lipid-lowering effect in tiger puffer. Serum lipid profiles remained stable, implying that the cholesterol metabolism was not influenced by dietary CAR. Furthermore, dietary CAR activated fatty acid β-oxidation (FAO) in both the mitochondria and peroxisomes and drove lipolysis and fatty acid intracellular transport to supply sufficient substrates for FAO, manifested by the activation of related genes, proteins and enzyme activity. Additionally, CAR lowered hepatic MDA levels, indicating improved antioxidant capacity. In conclusion, CAR shows potential as a lipid-lowering feed additive for marine fish under high-fat dietary conditions.

This study investigated the temporal variation and removal efficiency of solid waste, together with the dynamics of water quality parameters and microbial community structure, in a recirculating aquaculture system (RAS) for mandarin fish (Siniperca chuatsi) fed a formulated diet. The average fish weight was 384.62 ± 12.13 g, and the stocking density was 25 kg/m³. The results showed that: (1) the contents of the stomach and intestine exhibited a bimodal pattern, characterized by an initial increase followed by a decrease, and a subsequent secondary increase and decline. In the culture tanks, the total suspended solid (TSS) concentration at the main discharge outlet of the dual-channel bottom drainage system was highest immediately after feeding (0 h), reached its lowest level at 5 h post-feeding, and displayed a double-peak pattern during the 6–24 h post-feeding period. The temporal variations of in-tank TSS and chemical oxygen demand (COD) generally followed the same trend as the TSS concentration at the main discharge outlet. (2) Total ammonia nitrogen (TAN) reached its maximum at 7 h, whereas nitrite nitrogen (NO₂⁻-N) peaked at 9 h. (3) The solid–liquid separation efficiencies of the dual-drain system, vertical flow clarifier, and Rotating drum microfilter were 30.40–58.33%, 51.30–76.61%, and 37.04–68.26%, respectively, with the highest removal efficiencies observed at 0 h post-feeding. (4) In the nitrifying biofilter, the TAN concentrations ranged from 0.22–0.99 mg/L at the inlet to 0.15–0.36 mg/L at the outlet. In contrast, NO₂⁻-N concentrations exhibited negligible differences between the inlet and outlet at the corresponding sampling times. At the phylum level, Pseudomonadota, Bacteroidota, and the superphylum Patescibacteria dominated the biofilter microbial communities. At the genus level, Sediminibacterium and Limnohabitans were predominant in BF_1, whereas taxa affiliated with norank_f__Hyphomicrobiales_ and unclassified_o__Saccharimonadales dominated BF_2. Overall, the results indicated that increasing water circulation and tank flow rate at 0 h and during the 7–14 h post-feeding period may facilitate the timely removal of solid waste, and that the installation of a foam fractionator could contribute to the removal of dissolved and fine organic matter.