Water temperature stands as a crucial environmental element, exerting an impact on the survival and growth of organisms in aquaculture. Heat stress poses a significant threat to the survival and aquaculture of the Hong Kong oyster Magallana hongkongensis (also known as Crassostrea hongkongensis), yet the underlying physiological and molecular mechanisms remain poorly understood. This study investigated the effects of elevated temperatures (35 °C and 37 °C) on survival, DNA damage, antioxidant enzyme activities, and gene expression related to apoptosis, inflammation, and heat shock proteins (HSPs) in M. hongkongensis. The median lethal temperature (LT50) of M. hongkongensis was determined to be 37.09 °C, with significant mortality observed at 35 °C compared with the control (29 °C). Antioxidant enzyme activities (SOD, CAT, and GPx) and T-AOC were up-regulated initially but exhibited divergent patterns under prolonged stress, indicating a temperature-dependent threshold for oxidative defense. Comet assay results also showed that heat stress induced severe DNA damage in hemocytes. Moreover, heat stress significantly up-regulated mRNA expression of apoptosis-related genes (Caspase-2, Caspase-8, Bax, and P53), inflammatory genes (TNF, p38-MAPK, and AP-1), and HSP family members (Hsp70, Hsp90, Hsp27, and Hsp68). The expression peaks of these genes were generally earlier and more pronounced at 37 °C, reflecting intensified cellular damage and protective responses. Collectively, this study demonstrates that M. hongkongensis employs integrated antioxidant, apoptotic, inflammatory, and HSP-mediated mechanisms to counteract heat stress, but temperatures exceeding 35 °C disrupt these defenses, leading to survival impairment. These findings provide critical insights into the heat adaptation strategies of M. hongkongensis and serve as a scientific foundation for developing sustainable aquaculture practices to mitigate summer heat stress.

The Kumamoto oyster (Crassostrea sikamea), an indigenous species in southern China, is ecologically and aquaculturally significant. To explore the associations between oyster-associated bacteria and their environment, we analyzed bacterial communities in oyster gills, seawater, and sediment from Techeng Island (TCCS, TCW, TCS) and Longtousha (LTSCS, LTSW, LTSS) using Illumina MiSeq sequencing of the 16S rRNA gene V3–V4 region. Alpha diversity showed the highest richness and diversity in sediments (Shannon index: 6.40–6.52), followed by seawater (4.15–4.58) and gills (2.64–2.99). Taxonomic analysis revealed 75 phyla, with Pseudomonadota (23.23–49.32%) dominant across all habitats. Habitat-specific patterns were observed: Spirochaetota was enriched in gills (45.31–46.43%), Bacteroidota in seawater (6.70–14.05%), and Thermodesulfobacteriota in sediments (0.31–0.91%). At the genus level, norank_f_Spirochaetaceae, Marinococcus, and Woeseia showed significant differences among groups (p ≤ 0.001). Venn and PCoA analyses indicated closer similarity between gill and seawater communities than between gill and sediments, likely linked to oyster filter-feeding. This study clarifies the association between oyster gill bacteria and their environment, providing a basis for understanding microbial dynamics in oyster aquaculture.

Crassostrea hongkongensis is an economically important mariculture species in southern China, and hybrid breeding has been applied to improve its growth traits. However, the molecular mechanisms underlying growth heterosis remain poorly understood. In this study, inter-population hybrid (HG) and intra-population (IG) groups of C. hongkongensis were compared using integrated transcriptomic and metabolomic analyses to investigate the regulatory basis of growth heterosis. Shell length and shell height were significantly greater in HG than in IG (p < 0.01). Transcriptomic analysis identified 474 false discovery rate (FDR)-supported significant differentially expressed genes (DEGs) (233 upregulated and 241 downregulated), which were treated as the primary statistically robust findings; a broader set of 3161 candidate DEGs, including genes associated with growth regulation and shell biomineralization, was retained for exploratory functional and multi-omics analyses. Metabolomic analysis detected 345 differential metabolites (DMs), which were mainly enriched in energy metabolism, nucleotide metabolism, and arachidonic acid metabolism. Exploratory pathway-level multi-omics integration revealed significant concordance between the transcriptomic and metabolomic profiles (M² = 0.1962, p = 0.001) and highlighted the tricarboxylic acid cycle, oxidative phosphorylation, branched-chain amino acid degradation, nucleotide metabolism, and arachidonic acid metabolism. These findings suggest that growth heterosis in C. hongkongensis is associated with altered energy metabolism, biosynthetic processes, shell formation-related pathways, and potential changes in the growth–defense balance, providing molecular insights for genetic improvement and selective breeding.

Lead (Pb) is a persistent aquatic pollutant that disrupts redox homeostasis in fish. This study investigated hepatic Pb accumulation, reactive oxygen species (ROS) production, antioxidant responses, lipid peroxidation, and transcriptomic alterations in juvenile pufferfish (Takifugu obscurus) exposed to waterborne Pb. Juvenile pufferfish were exposed to 5.98 mg/L waterborne Pb, corresponding to 10% of the 96 h LC₅₀, for 96 h. Liver, blood, and hepatocyte samples were collected at 0, 12, 24, 48, and 96 h, with four biological replicates at each sampling time point. Hepatic Pb accumulation increased over time and reached the highest level at 96 h. ROS levels in blood cells and hepatocytes increased rapidly and peaked at 12 h. Superoxide dismutase (SOD) and catalase (CAT) activities showed early activation followed by late suppression, whereas glutathione peroxidase (GSH-Px) displayed partial adaptive recovery. Malondialdehyde (MDA) content increased progressively and reached approximately 2.8-fold of the control level at 96 h, indicating persistent lipid peroxidation. RNA-seq analysis identified 167, 460, 1398, and 2580 differentially expressed genes at 12, 24, 48, and 96 h, respectively. Enrichment, temporal trend, and weighted gene co-expression analyses indicated that Pb exposure shifted hepatic responses from early redox regulation to later metabolic adaptation, protein processing in the endoplasmic reticulum, proteasome function, and oxidative phosphorylation. qRT-PCR validation of 12 hub genes supported the RNA-seq results. These findings provide integrated biochemical and transcriptomic evidence for oxidative-stress-mediated hepatic toxicity in pufferfish exposed to waterborne Pb.