In an era of striking climate change, increased marine temperature severely affects aquatic animals and cause significant ecological and socioeconomic impacts. At present, there is little information about the effects of heat stress on the physiology of greater amberjack (Seriola dumerili). This study explores the mechanisms whereby juvenile greater amberjacks cope with heat stress. An increase in water temperature (25 to 28°C and 31°C) changed the activity levels of enzymes responsible for antioxidant defense, immune function, and energy metabolism. When water temperatures increased from 25 to 28°C and 31°C, the levels of malondialdehyde (MDA) and glucose (GLU) in the greater amberjacks increased, suggesting that the fish suffered oxidative stress. During the early stage of heat stress, the levels of triglycerides (TG) and glutathione (GSH) and the activities of superoxide dismutase (SOD), catalase (CAT), alkaline phosphatase (ALP), acid phosphatase (ACP), lysozyme (LYZ), alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and succinate dehydrogenase (SDH) increased significantly suggesting that juvenile greater amberjacks adapt to heat stress by enhancing their antioxidant defense, energy metabolism, and immune defense. However, long‐term exposure to heat stress decreased the levels of TG and GSH; decreased the activities of SOD, CAT, ALP, ACP, LYZ, and AST; and increased the levels of MDA and GLU, suggesting that the energy metabolism and immune function of juvenile greater amberjack S. dumerilis are suppressed by oxidative damage. This study provides insights into the physiological adaptation of juvenile greater amberjacks to heat stress.

The greater amberjack (Seriola dumerili) is a gonochoristic fish with no sexual dimorphism in appearance, making sex identification difficult. Piwi-interacting RNAs (piRNAs) function in transposon silencing and gametogenesis and are involved in various physiological processes, including sex development and differentiation. Exosomal piRNAs can be indicators for the determination of sex and physiological status. In this study, four piRNAs were differentially expressed in both serum exosomes and gonads between male and female greater amberjack. Three piRNAs (piR-dre-32793, piR-dre-5797, and piR-dre-73318) were significantly up-regulated and piR-dre-332 was significantly down-regulated in serum exosomes and gonads of male fish, compared to female fish, consistent with the serum exosomal results. According to the relative expression of four marker piRNAs derived from the serum exosomes of greater amberjack, the highest relative expression of piR-dre-32793, piR-dre-5797, and piR-dre-73318 in seven female fish and that of piR-dre-332 in seven male fish can be used as the standard for sex determination. The method of sex identification can ascertain the sex of greater amberjack by blood collection from the living body, without sacrificing fish. The four piRNAs did not show sex-inclined expression in the hypothalamus, pituitary, heart, liver, intestine, and muscle tissue. A piRNA–target interaction network involving 32 piRNA-mRNA pairs was generated. Sex-related target genes were enriched in sex-related pathways, including oocyte meiosis, transforming growth factor-beta signaling pathway, progesterone-mediated oocyte maturation, and gonadotropin releasing hormone signaling pathway. These results provide a basis for sex determination in greater amberjack and improve our understanding of the mechanisms underlying sex development and differentiation in the species.

Background: In recent years, titanium dioxide (TiO₂) nanoparticles (NPs) have been widely used in various industries due to their favorable chemical properties, and their contamination of the environment has attracted much attention, especially to aquatic animals. Methods: Therefore, we assessed the impact of TiO₂ NPs (5 mg/L) on the marine bivalve, pearl oyster (Pinctada fucata martensii), especially gill metabolism. Pearl oysters were exposed to seawater containing 5 mg/L TiO₂ NPs for 14 days, followed by 7 days of recovery in untreated seawater. Gill tissues and hepatopancreatic tissues were sampled on days 0, 14, and 21 of the experiment named C0, E14, and R7, respectively. Results: Metabolomic analysis identified 102 significantly different metabolites (SDMs) on gills tissue in pearl oysters following exposure to TiO₂ NPs (C0 vs. E14). Compared with group C0, group E14 had 76 SDMs (such as acetylcholine, itaconic acid, citric acid, and taurine) with higher concentrations and 26 (including L-arginine and isobutyryl-L-carnitine) with lower concentrations. KEGG (Kyoto Encyclopedia of Genes and Genomes) analysis showed that these SDMs enriched 28 pathways, including glycine, serine, and threonine metabolism, neuroactive ligand–receptor interaction, and taurine and hypotaurine metabolism. In addition, 116 SDMs were identified in E14 and R7 pearl oysters. Compared with group E14, group R7 had 74 metabolites (such as acetylcholine, 6-phosphogluconic acid, isocitric acid, and itaconic acid) with higher concentrations and 42 (including uracil, glycerophosphocholine, N-Acetyl-D-glucosamine) with lower concentrations. The SDMs identified between E14 and R7 enriched 25 pathways, including the pentose phosphate pathway, glutathione metabolism, and citrate cycle (TCA cycle). In addition, analysis of the energy metabolism-associated enzymes revealed that exposure to TiO₂ NPs reduced Ca²⁺/Mg²⁺-ATPase, Na⁺/K⁺-ATPase, and Total-ATPase activities. Conclusions: These findings suggested that TiO₂ NPs may inhibit the energy metabolism function of gill and hepatopancreas of pearl oysters. Meanwhile, TiO₂ NPs may affect the normal functioning of immune and osmoregulatory functions of pearl oysters gill and even may lead to oxidative stress and neurotoxicity. Therefore, this study may provide a reference for analyzing the bioadaptation of marine bivalves to TiO₂ NPs and the potential negative effects of TiO₂ NPs on bivalves.