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A comprehensive morphological description of Argyrosomus regius (Asso, 1801) gills was conducted through the gross and scanning electron microscopic examinations in addition to the morphometric analysis of the number, length of the gill rakers and the space between them. The medial surface of the 1st gill arch was differed than that of the other three gill arches. Four gill arche's lateral surfaces contained pear-shaped, circular, and oval-shaped spine groups with varying sizes. The medial surface of the 1st gill arch only had different sizes of pear-shaped and oval groups of spines, while the other gill arches had the same-shaped groups of spines beside the presence of small circular groups. The lateral rakers of the 1st gill arch were long, and only one border carried pointed spines, while its medial rakers were triangular. The rakers of the 2nd, 3rd, and 4th gill arches appeared flask like medially and laterally. There were few folds and tubercles appearing on the primary gill filament surfaces; some folds appeared like the helix on the surface of their parts near the gill arch convex border. This is the first anatomical study on Argyrosomus regius gills and we aimed to reveal the unique structural specifications for gill rakers that related to its feeding behavior and we will contribute to a better understanding of its ecological niche and feeding strategies in its natural environment. Highlights • The lateral surface of the four gill arches of Argyrosomus regius had spines in the form of pear-shaped and circular groups that differed in size beside the presence of oval-shaped groups. • The medial surface of Argyrosomus regius 1st gill arch only had different sizes of pear-shaped groups and oval groups. While the medial surface of the 2nd, 3rd, and 4th gill arches had the same-shaped groups of spines beside the presence of small circular groups. • The Argyrosomus regius 1st gill arch had long lateral rakers and triangular medial rakers, while the medial and lateral rakers of the other three gill arches were flask like • The study revealed that Argyrosomus regius gill rakers poss unique structural specifications related to its feeding behavior.

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.

Experiments were conducted to investigate the health of fish Channa punctatus inhabiting heavy metal-loaded waste water. Heavy metals in the order of Fe > Mn > Zn > Co > Ni > Cu = Cr were present in the waste water. Gills had high metal load followed by liver and then kidney. Albumin, albumin to globulin (A:G) ratio, triglyceride, high density lipoprotein (HDL) and very low density lipoprotein (VLDL) were found to be lower but phospholipid, low density lipoprotein (LDL), total protein, lipid and cholesterol were higher as compared to the reference. Oxidative stress markers such as superoxide dismutase (SOD), catalase (CAT), glutathione S transferase (GST) and lipid peroxidation (LPO) were significantly higher in all tissues, whereas reduced glutathione (GSH) levels were comparatively low. Damage to DNA was observed with significantly higher mean tail length of comets in the exposed fish gill cells (30.9 µm) followed by liver (24.3 µm) and kidney (20.6 µm) as compared to reference fish (5.2, 4.8 and 5.9 µm respectively). Histopathology in gill, liver and kidney also showed marked damage. Integrated biochemical, oxidative stress, genotoxicity and histopathological findings are valuable biomarkers for native fish adaptive patterns, and monitoring of water quality/pollution of freshwater ecosystems.

Rainbow trout ( Oncorhynchus mykiss ) are frequently used as experimental animals in ecotoxicological studies, in which they are experimentally exposed to defined concentrations of test substances, such as heavy metals, pesticides, or pharmaceuticals. Following exposure to a broad variety of aquatic pollutants, early morphologically detectable toxic effects often manifest in alterations of the gills. Suitable methods for an accurate and unbiased quantitative characterization of the type and the extent of morphological gill alterations are therefore essential prerequisites for recognition, objective evaluation and comparison of the severity of gill lesions. The aim of the present guidelines is to provide practicable, standardized and detailed protocols for the application of unbiased quantitative stereological analyses of relevant morphological parameters of the gills of rainbow trout. These gill parameters inter alia include the total volume of the primary and secondary gill lamellae, the surface area of the secondary gill lamellae epithelium ( i . e ., the respiratory surface) and the thickness of the diffusion barrier. The featured protocols are adapted to fish of frequently used body size classes (300–2000 g). They include well-established, conventional sampling methods, probes and test systems for unbiased quantitative stereological analyses of light- and electron microscopic 2-D gill sections, as well as the application of modern 3-D light sheet fluorescence microscopy (LSFM) of optically cleared gill samples as an innovative, fast and efficient quantitative morphological analysis approach. The methods shown here provide a basis for standardized and representative state-of-the-art quantitative morphological analyses of trout gills, ensuring the unbiasedness and reproducibility, as well as the intra- and inter-study comparability of analyses results. Their broad implementation will therefore significantly contribute to the reliable identification of no observed effect concentration (NOEC) limits in ecotoxicological studies and, moreover, to limit the number of experimental animals by reduction of unnecessary repetition of experiments.

Nile tilapia ( Oreochromis niloticus ) is valued in aquaculture because of its quick development and ability to thrive in various environments. Myxosporeans are among the fish parasites that affect fish productivity, as they impact fish growth and reproduction, resulting in large fish deaths in farms and hatcheries. This study has been focused on morpho-molecular identification for the myxosporean parasites infecting Nile tilapia from three governorates in Egypt and assessment of gene expression of different cytokines (Interleukin-1βeta ( IL-1β ), major histocompatibility complex class II ( MHC-II ), and clusters of differentiation 4 ( CD-4 ) and 8 ( CD-8 )) in tissues. Additionally, this work aimed to correlate the developed histopathological alterations and inflammatory reactions in gills with immunohistochemical expression of inducible nitric oxide synthase ( iNOS ) and tumor necrosis factor-alpha ( TNF-α ). Finally, the infected fish’s cortisol levels and blood glucose were assessed. Results of BLAST sequence analysis of the 18S rRNA for the collected protozoans confirmed Myxobolus agolus , M . brachysporus , M . tilapiae , and Henneguya species. The molecular characterization of the immunological status of gills revealed marked upregulation of different inflammatory cytokines in the gills of infected fish. There was a significantly increased serum cortisol and glucose level in infected fish compared with control, non-infected ones. Severe histopathological alterations were observed in the infected fish gills, associated with increased expression of iNOS and TNF-α and related to myxosporean infection. The present study provides new insights into oxidative stress biomarkers in Nile tilapia infected with Myxosporeans and elucidates the gill’s immune status changes as a portal of entry for protozoa that contribute to tissue damage.

How new structures emerge during evolution has long fascinated biologists. An example is how the diminutive bones of the mammalian middle ear arose from ancestral fish jawbones 1 . By contrast, the evolutionary origin of the outer ear, another mammalian innovation, remains a mystery, partly because it is supported by non-mineralized elastic cartilage, which is rarely recovered in fossils. Whether the outer ear arose de novo or through the reuse of ancestral developmental programs has remained unknown. Here we show that the outer ear shares gene regulatory programs with the gills of fishes and amphibians for both its initial outgrowth and the later development of the elastic cartilage. Comparative single-nucleus multiomics of the human outer ear and zebrafish gills reveals conserved gene expression and putative enhancers enriched for common transcription factor binding motifs. This is reflected by the transgenic activity of human outer-ear enhancers in gills, and of fish gill enhancers in the outer ear. Furthermore, single-cell multiomics of the cartilaginous book gills of horseshoe crabs reveals a developmental program shared with the distal-less homeobox ( DLX )-mediated gill program of vertebrates, with a book-gill distal-less enhancer driving expression in zebrafish gills. We propose that elements of an invertebrate gill program were reutilized in vertebrates to generate first gills and then the outer ear. A study on the evolutionary origin of the mammalian outer ear finds that it shares genetic programs with the gills of fishes and amphibians, indicating that elements of an ancestral gill have been reused.

Each year from April to May, high mortality rates are reported in red swamp crayfish ( Procambarus clarkii ) cultured in Jiangsu and other regions, in China, and this phenomenon has come to be known as “Black May” disease (BMD). Therefore, in order to investigate the possible causes of this disease, this study gathered BMD-affected P. clarkii samples and performed transcriptome analysis on hepatopancreas, gill, and muscle tissues. A total of 19,995,164, 149,212,804, and 222,053,848 clean reads were respectively obtained from the gills, muscle, and hepatopancreas of BMD-affected P. clarkii , and 114,024 unigenes were identified. The number of differentially expressed genes (DEGs) in gill, muscle, and hepatopancreas was 1703, 964, and 476, respectively. GO and KEGG enrichment analyses of the DEGs were then conducted. Based on KEGG pathway enrichment analysis, the most significantly differentially expressed pathways were mainly those involved with metabolism, human disease, and cellular processes. Further analysis of the significantly DEGs revealed that they were mainly related to the mitochondrial-mediated apoptosis pathway and that the expression of these DEGs was mostly down-regulated. Moreover, the expression of genes related to immune and metabolism-related pathways was also significantly down-regulated, and these significantly-inhibited pathways were the likely causes of P. clarkii death. Therefore, our results provide a basis for the identification of BMD causes.

Lindane is a broad-spectrum insecticide widely used on fruits, vegetables, crops, livestock and on animal premises to control the insects and pests. The extensive use of pesticides and their residues in the soil and water typically join the food chain and thus accumulate in the body tissues of human and animals causing severe health effects. The study was designed to determine the toxicity effects of sub-lethal concentrations of lindane on hemato-biochemical profile and histo-pathological changes in Rohu ( Labeo rohita ). A significant increase in the absolute (p<0.05) and relative (p<0.05) weights was observed along with severe histo-pathological alterations in liver, kidneys, gills, heart and brain at 30μg/L and 45μg/L concentration of lindane. A significant (p<0.05) decrease in RBCs count, PCV and Hb concentration while a significant (p<0.05) increased leukocytes were observed by 30μg/L and 45μg/L concentrations of lindane at 45 and 60 days of the experiment. Serum total protein and albumin were significantly (p<0.05) decreased while hepatic and renal enzymes were significantly (p<0.05) increased due to 30μg/L and 45μg/L concentrations of lindane at days-45 and 60 of experiment compared to control group. The observations of thin blood smear indicated significantly increased number of erythrocytes having nuclear abnormalities in the fish exposed at 30μg/L and 45μg/L concentrations of lindane. ROS and TBARS were found to be significantly increased while CAT, SOD, POD and GSH were significantly decreased with an increase in the concentration and exposure time of lindane. The results showed that lindane causes oxidative stress and severe hematological, serum biochemical and histo-pathological alterations in the fish even at sub-lethal concentrations.

Cr(VI) is a heavy metal contaminant, can diffuse to ecosystems and harm aquatic animals. Gills, as a vital organ in direct contact with the aquatic environment, have become a key target tissue for assessing the toxicological effects of heavy metal pollution of water bodies due to their sensitivity to heavy metal exposure. However, 3the effects of Cr(VI) on the gill tissues in fish have been less studied. In this study, we revealed the multiple effects of chromium toxicity by assessing the oxidative damage, transcriptomic and metabolomic changes of Cr(VI) on gill tissues of . A total of 270 fishes were stratified into three experimental groups: control group, low-concentration exposure group (0.2 mg/L), and high-concentration exposure group (1 mg/L). In this study, we revealed the multiple effects of chromium toxicity by assessing the oxidative damage, transcriptomic and metabolomic changes of Cr(VI) on gill tissues of . Cr(VI) stress can lead to gill damage with significant reduction in gill filament thickness, significant thinning of gill lamellae, and congestion of epithelial blood vessels. Cr(VI) stress significant increases in H O and MDA levels and significant decreases in antioxidant enzyme activity levels (SOD, GSH-Px, and T-AOC) and energy metabolism-related ATPase activity levels (Na K -ATPase, Ca -ATPase, and Mg -ATPase). Cr(VI) stress induced disturbances in gill arachidonic acid metabolism leading to the release of pro-inflammatory metabolites (e.g., thromboxane A2 and prostaglandin J2) accompanied by the accumulation of oxidised glutathione. However, the synthesis of metabolites with anti-inflammatory/antioxidant functions (e.g. GABA, quinidine and l-artitic acid) was reduced. Transcriptomics and metabolomic coanalyses revealed that Cr(VI) induced inactivation to deregulate , which disrupted arachidonic acid metabolic pathways, leading to oxidative stress, apoptosis, and release of inflammatory factors. Disorders of arachidonic acid metabolism led to the release of proinflammatory metabolites (such as thromboxane A2 and prostaglandin J2), and decreased levels of reduced glutathione. The effects of Cr(VI) exposure on gill gene expression and metabolism were analysed using RT-PCR, transcriptomic, and metabolomic approaches. In summary, we better understand the toxic effects of Cr(VI) on gill tissues of aquatic animals. Targeted activation of and supplementation with anti-inflammatory metabolites such as GABA, quinidine and l- artitic acid may be potential intervention strategies to reverse Cr(VI) toxicity.

Antimicrobial peptides (AMPs) are found widespread in nature and possess antimicrobial and immunomodulatory activities. Due to their multifunctional properties, these peptides are a focus of growing body of interest and have been characterized in several fish species. Due to their similarities in amino-acid composition and amphipathic design, it has been suggested that neuropeptides may be directly involved in the innate immune response against pathogen intruders. In this review, we report the molecular characterization of the fish-specific AMP piscidin1, the production of an antibody raised against this peptide and the immunohistochemical identification of this peptide and enkephalins in the neuroepithelial cells (NECs) in the gill of several teleost fish species living in different habitats. In spite of the abundant literature on Piscidin1, the biological role of this peptide in fish visceral organs remains poorly explored, as well as the role of the neuropeptides in neuroimmune interaction in fish. The NECs, by their role as sensors of hypoxia changes in the external environments, in combination with their endocrine nature and secretion of immunomodulatory substances would influence various types of immune cells that contain piscidin, such as mast cells and eosinophils, both showing interaction with the nervous system. The discovery of piscidins in the gill and skin, their diversity and their role in the regulation of immune response will lead to better selection of these immunomodulatory molecules as drug targets to retain antimicrobial barrier function and for aquaculture therapy in the future.