Gill remodeling increases the respiratory surface area of grass carp (Ctenopharyngodon idella) under hypoxic stress

Hypoxia stress alters gene expression in the gills and spleen of greater amberjack (Serioladumerili)

Greater amberjack (Seriola dumerili) is a fish species that has significant economic and cultural value. It has a large size and grows rapidly. However, the intolerance to hypoxia poses a major obstacle to the growth of its aquaculture industry. This study focuses on the gills and spleen, two organs closely associated with the response to acute hypoxic stress. By simulating the acute hypoxic environment and using Illumina RNA-Seq technology, we explored the gills and spleen transcriptome changes in the acute hypoxia intolerant and tolerant groups of greater amberjack. It was discovered that gill tissues in the tolerant group may maintain a stable intracellular energy supply by promoting glycolysis and β-oxidation compared to the intolerant group. Additionally, it promotes angiogenesis, enhances the ability to absorb dissolved oxygen, and accelerates oxygen transport to the mitochondria, adapting to the hypoxic environment. Anti-apoptotic genes were up-regulated in gill tissues in the tolerant group compared to the intolerant group, thereby minimizing the damage of acute hypoxia. On the other hand, the spleen inhibited the TCA and energy-consuming lipid synthesis pathways to supply energy under acute hypoxic stress. Pro-angiogenic genes were down-regulated in the spleen of individuals in the tolerant group compared to the intolerant group, which may be related to organ function. The suppressed reactive oxygen species (ROS) production and the impaired immune response function of the spleen were also found. The study explored the acute hypoxic stress response in greater amberjack and the molecular mechanisms underlying its tolerance to acute hypoxia.

Gill morphology adapted to oxygen-limited caves in Astyanax mexicanus

Sensing and acquiring dissolved oxygen is crucial for nearly all aquatic life. This may become even more vital as dissolved oxygen concentrations continue to decline in many aquatic environments. While certain phenotypes that enable fish to live in low oxygen have been characterized, adaptations that arise following sudden, drastic reductions in dissolved oxygen are relatively unknown. Here, we assessed the blind Mexican cavefish, Astyanax mexicanus, for alterations to gill morphology that may be adaptive for life in hypoxic caves. The Astyanax system provides the unique opportunity to compare gill morphology between stereotypical "surface" adapted morphotypes and obligate cave-dwelling conspecifics. While the surface environment is well-oxygenated, cavefish must cope with significantly reduced oxygen. We began by quantifying traditional morphological gill traits including filament number and length as well as lamellar density and height in surface fish and two distinct cave populations, Pachón and Tinaja. This enabled us to estimate total lamellar height, a proxy for gill surface area. We then used immunohistochemical staining to label 5-HT-positive neuroepithelial cells (NECs), which serve as key oxygen sensors in fish. We discovered an increase in gill surface area for both cavefish populations compared to surface, which may enable a higher capacity of oxygen acquisition. Additionally, we found more NECs in Pachón cavefish compared to both surface fish and Tinaja cavefish, suggesting certain selective pressures may be cave-specific. Collectively, this work provides evidence that cavefish have adapted to low oxygen conditions via alterations to gill morphology and oxygen sensing, and informs evolutionary mechanisms of rapid adaptation to dramatic, chronic hypoxia.

Structure and gene expression changes of the gill and liver in juvenile black porgy (Acanthopagrus schlegelii) under different salinities

Black porgy (Acanthopagrus schlegelii) is an important marine aquaculture species in China. It is an ideal object for the cultivation of low-salinity aquaculture strains in marine fish and the study of salinity tolerance mechanisms in fish because of its strong low-salinity tolerance ability. Gill is the main osmoregulatory organ in fish, and the liver plays an important role in the adaptation of the organism to stressful environments. In order to understand the coping mechanisms of the gills and livers of black porgy in different salinity environments, this study explored these organs after 30 days of culture in hypoosmotic (0.5 ppt), isosmotic (12 ppt), and normal seawater (28 ppt) at histologic, physiologic, and transcriptomic levels. The findings indicated that gill exhibited a higher number of differentially expressed genes than the liver, emphasizing the gill's heightened sensitivity to salinity changes. Protein interaction networks and enrichment analyses highlighted energy metabolism as a key regulatory focus at both 0.5 ppt and 12 ppt salinity in gills. Additionally, gills showed enrichment in ions, substance transport, and other metabolic pathways, suggesting a more direct regulatory response to salinity stress. The liver's regulatory patterns at different salinities exhibited significant distinctions, with pathways and genes related to metabolism, immunity, and antioxidants predominantly activated at 0.5 ppt, and molecular processes linked to cell proliferation taking precedence at 12 ppt salinity. Furthermore, the study revealed a reduction in the volume of the interlamellar cell mass (ILCM) of the gills, enhancing the contact area of the gill lamellae with water. At 0.5 ppt salinity, hepatic antioxidant enzyme activity increased, accompanied by oxidative stress damage. Conversely, at 12 ppt salinity, gill NKA activity significantly decreased without notable changes in liver structure. These results underscore the profound impact of salinity on gill structure and function, highlighting the crucial role of the liver in adapting to salinity environments.

Effects of structural remodelling on gill physiology

The complex relationships between the structure and function of fish gills have been of interest to comparative physiologists for many years. Morphological plasticity of the gill provides a dynamic mechanism to reversibly alter its structure in response to changes in the conditions experienced by the fish. The best known example of gill remodelling is the growth or retraction of cell masses between the lamellae, a rapid process that alters the lamellar surface area that is exposed to the water (i.e. the functional lamellar surface area). Decreases in environmental O2 availability and/or increases in metabolic O2 demand stimulate uncovering of the lamellae, presumably to increase the capacity for O2 uptake. This review addresses four questions about gill remodelling: (1) what types of reversible morphological changes occur; (2) how do these changes affect physiological function from the gill to the whole animal; (3) what factors regulate reversible gill plasticity; and (4) is remodelling phylogenetically widespread among fishes? We address these questions by surveying the current state of knowledge of gill remodelling in fishes, with a focus on identifying gaps in our understanding that future research should consider.

Effects of vitamin C combined with sodium alginate on serum biochemistry, oxidative stress, gill tissue morphology, and muscle quality of pearl gentian grouper during waterless transport

This research examined the effects of sodium alginate (SA) and vitamin C (Vc) soaking of pearl gentian grouper before waterless transportation from the perspectives of serum parameters, oxidative stress, muscle quality, and gill tissue morphology. After the fish reached semi-dormancy with a cooling rate of 3 °C/h, fish (420 ± 25 g) were distributed to 4 treatments as follows: S1 group (50 mg/L Vc and 0.1% SA were added), S2 group (50 mg/L Vc and 0.3% SA were added), S3 group (50 mg/L Vc and 0.5% SA were added), and control group (without soaking in protective fluid). After oxygenated packaging, samples were taken at 0, 8, and 16 h of waterless transportation and 12 h after rehydration, respectively. It was found that after 16 h of waterless transport, compared with the control group, cortisol, glucose, blood urea nitrogen (BUN), uric acid (UA), creatinine (CREA), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and malondialdehyde (MDA) levels were significantly decreased (p < 0.05), while albumin, lysozyme (LZM), muscle pH, and total free amino acid (TFAA) contents were significantly elevated (p < 0.05) in the S3 group. Moreover, by gill tissue microscopy, it was found that the protective solution of group S3 did not cause serious deleterious morphological changes to the gill epithelium. The results showed that the grouper was soaked by protective fluid before waterless could maintain surface moisture, reduce gill and kidney function and oxidative stress damage, and maintain the stability of muscle quality. This study provides a novel transportation method for waterless preservation, which helps to reduce transportation costs and improve transportation efficiency.

Cell proliferation and regeneration in the gill : By

Seminal studies from the early 20th century defined the structural changes associated with development and regeneration of the gills in goldfish at the gross morphological and cellular levels using standard techniques of light and electron microscopy. More recently, investigations using cell lineage tracing, molecular biology, immunohistochemistry and single-cell RNA-sequencing have pushed the field forward and have begun to reveal the cellular and molecular processes that orchestrate cell proliferation and regeneration in the gills. The gill is a multifunctional organ that mediates an array of important physiological functions, including respiration, ion regulation and excretion of waste products. It is comprised of unique cell types, such as pavement cells, ionocytes, chemoreceptors and undifferentiated stem or progenitor cells that regulate growth and replenish cell populations. The gills develop from the embryonic endoderm and are rich in cell types derived from the neural crest. The gills have the capacity to remodel themselves in response to environmental change, such as in the case of ionocytes, chemoreceptors and the interlamellar cell mass, and can completely regenerate gill filaments and lamellae. Both processes of remodeling and regeneration invariably involve cell proliferation. Although gill regeneration has been reported in only a limited number of fish species, the process appears to have many similarities to regeneration of other organs in fish and amphibians. The present article reviews the studies that have described gill development and growth, and that demonstrate a suite of genes, transcription factors and other proteins involved in cell proliferation and regeneration in the gills.

Effects of acute hypoxia and reoxygenation on histological structure, antioxidant response, and apoptosis in razor clam Sinonovacula constricta

Hypoxia is one of the major environmental problems limiting the healthy development of intensive aquaculture. Marine benthic shellfish are encountering heightened problems related to hypoxic stress as a result of ongoing human activities and aquaculture operations. Razor clam Sinonovacula constricta, a commercially valuable shellfish, has not yet been reported in studies on physiological changes caused by hypoxia and reoxygenation. To understand the negative effects of hypoxia and reoxygenation on the clams, we set up two low-oxygen concentration groups (DO 2.0 mg/L and DO 0.5 mg/L) and assessed multiple aspects of oxidative damage to their hepatopancreas and gills. After the hypoxic stress, the two tissues of the razor clam suffered varying degrees of damage, including cell degeneration and disruption of mitochondrial cristae. After reoxygenation, the 2.0 mg/L group recovered substantially, but the clams in the 0.5 mg/L group still unrecovered. The activities of antioxidant enzymes (MDA, T-AOC, SOD, GPX, and CAT) in clams were considerably altered by acute hypoxia and reoxygenation. Briefly, there was a growing and then declining trend in MDA, T-AOC, and SOD activities in the hepatopancreas, whereas GPX and CAT activities showed the converse trend. In the hepatopancreas and gills, the level of anti-apoptotic gene Bcl-2 transcripts gradually decreased with the duration of hypoxia and increased following reoxygenation. However, changes in the transcript level of the pro-apoptotic gene Bax were in contrast to that of Bcl-2. The TUNEL assay revealed that hypoxia caused apoptosis. Furthermore, at DO 0.5 mg/L, the degree of apoptosis was more significant than at DO 2.0 mg/L, and hepatopancreatic apoptosis was more severe than gill apoptosis. Collectively, our findings imply that hypoxia induces oxidative stress, histological damage, and apoptosis in razor clams in a concentration-dependent and tissue-specific manner. These consequences serve as a reminder that prolonged recovery periods may be required for razor clams to fully recover from oxidative damage resulting from hypoxia-reoxygenation episodes.

Different reoxygenation rates induce different metabolic, apoptotic and immune responses in Golden Pompano (Trachinotus blochii) after hypoxic stress

Dissolved oxygen (DO) is essential for teleosts, and fluctuating environmental factors can result in hypoxic stress in the golden pompano (Trachinotus blochii). However, it is unknown whether different recovery speeds of DO concentration after hypoxia induce stress in T. blochii. In this study, T. blochii was subjected to hypoxic conditions (1.9 ± 0.2 mg/L) for 12 h followed by 12 h of reoxygenation at two different speeds (30 mg/L per hour and 1.7 mg/L per hour increasing). The gradual reoxygenation group (GRG), experienced DO recovery (1.9 ± 0.2 to 6.8 ± 0.2 mg/L) within 3 h, and the rapid reoxygenation group (RRG), experienced DO recovery (1.9 ± 0.2 to 6.8 ± 0.2 mg/L) within 10 min. Physiological and biochemical parameters of metabolism (glucose, glycegon, lactic acid (LD), lactate dehydrogenase (LDH), pyruvic acid (PA), phosphofructokinase (PFKA), and hexokinase (HK), triglyceride (TG), lipoprotein lipase (LPL), carnitine palmitoyltransferase 1 (CPT-1)) and transcriptome sequencing (RNA-seq of liver) were monitored to identify the effects of the two reoxygenation speeds. Increased LD content and increased activity of LDH, PA, PFKA, and HK suggested enhanced anaerobic glycolysis under hypoxic stress. LD and LDH levels remained significantly elevated during reoxygenation, indicating that the effects of hypoxia were not immediately alleviated during reoxygenation. The expressions of PGM2, PFKA, GAPDH, and PK were increased in the RRG, which suggests that glycolysis was enhanced. The same pattern was not observed in the GRG. Additionally, In the RRG, reoxygenation may promote glycolysis to guarantee energy supply. However, the GRG may through the lipid metabolism such as steroid biosynthesis at the later stage of reoxygenation. In the aspect of apoptosis, differentially expressed genes (DEGs) in the RRG were enriched in the p53 signaling pathway, which promoted cell apoptosis, while DEGs in the GRG seem to activate cell apoptosis at early stage of reoxygenation but was restrained latterly. DEGs in both the RRG and the GRG were enriched in the NF-kappa B and JAK-STAT signaling pathways, the RRG may induce cell survival by regulating the expression of IL-12B, COX2, and Bcl-XL, while in the GRG it may induce by regulating the expression of IL-8. Moreover, DEGs in the RRG were also enriched in the Toll-like receptor signaling pathway. This research revealed that at different velocity of reoxygenation after hypoxic stress, T. blochii would represent different metabolic, apoptotic and immune strategies, and this conclusion would provide new insight into the response to hypoxia and reoxygenation in teleosts.