Biology and functions of fish thrombocytes: A review

This comprehensive review examines the role of fish thrombocytes, cells considered functionally analogous to platelets in terms of coagulation, but which differ in their origin and morphology. Despite the evolutionary distance between teleosts and mammals, genomic studies reveal conserved patterns in blood coagulation, although there are exceptions such as the absence of factors belonging to the contact system. Beyond coagulation, fish thrombocytes have important immunological functions. These cells express both proinflammatory genes and genes involved in antigen presentation, suggesting a role in both innate and adaptive immune responses. Moreover, having demonstrated their phagocytic abilities, crucial in the fight against pathogenic microorganisms, underscores their multifaceted involvement in immunity. Finally, the need for further research on the functions of these cells is highlighted, in order to better understand their involvement in maintaining the health of aquaculture fish. The use of standardized and automated methods for the analysis of these activities is advocated, emphaiszing their potential to facilitate the early detection of stress or infection, thus minimizing the economic losses that these adverse situations can generate in the field of aquaculture.

Identification of maturation-inducing steroid in sturgeons via comprehensive analyses of steroids produced during oocyte maturation

Although 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) and 17α, 20β, 21-trihydroxy-4-pregnen-3-one (20β-S) have been identified as maturation-inducing steroids (MIS) in several teleosts, to date, no MISs have been identified in sturgeons. As it remains possible that an unidentified steroid is an MIS in sturgeons, this study aimed to identify a sturgeon MIS via comprehensive analyses and maturation-inducing (MI) assay of C21 steroids. In vivo and in vitro comprehensive analyses of C21 steroids revealed that serum DHP concentrations were rapidly elevated in the oocyte maturation phase and the DHP production level was notably high among C21 steroids. MI assay indicated that the MI activity of DHP, 17α-hydroxyprogesterone (17OHP), a precursor of DHP, 17α, 20α-dihydroxy-4-pregnen-3-one (αDHP), and 20β-S was high among C21 steroids, but the MI activity of these steroids were similar. In the C21 steroids produced in ovarian follicles during oocyte maturation, 17OHP, αDHP, and unidentified compounds had a low production level, and 20β-S was suggested to be metabolized from DHP after oocyte maturation. Against this background, this study concluded that DHP is a steroid that possesses strong MI activity and is highly produced during oocyte maturation. Although this study could not identify an MIS in sturgeons by fractionation of plasma and subsequent bio assay, it was suggested that DHP is a major MIS in sturgeons.

Immune aging in annual killifish

Turquoise killifish (Nothobranchius furzeri) evolved a naturally short lifespan of about six months and exhibit aging hallmarks that affect multiple organs. These hallmarks include protein aggregation, telomere shortening, cellular senescence, and systemic inflammation. Turquoise killifish possess the full spectrum of vertebrate-specific innate and adaptive immune system. However, during their recent evolutionary history, they lost subsets of mucosal-specific antibody isoforms that are present in other teleosts. As they age, the immune system of turquoise killifish undergoes dramatic cellular and systemic changes. These changes involve increased inflammation, reduced antibody diversity, an increased prevalence of pathogenic microbes in the intestine, and extensive DNA damage in immune progenitor cell clusters. Collectively, the wide array of age-related changes occurring in turquoise killifish suggest that, despite an evolutionary separation spanning hundreds of millions of years, teleosts and mammals share common features of immune system aging. Hence, the spontaneous aging observed in the killifish immune system offers an excellent opportunity for discovering fundamental and conserved aspects associated with immune system aging across vertebrates. Additionally, the species' naturally short lifespan of only a few months, along with its experimental accessibility, offers a robust platform for testing interventions to improve age-related dysfunctions in the whole organism and potentially inform the development of immune-based therapies for human aging-related diseases.

In situ observation of a macrourid fish at 7259 m in the Japan Trench: swimbladder buoyancy at extreme depth

A macrourid, Coryphaenoides yaquinae sp. inc., was observed to be attracted to bait and exhibiting normal foraging behaviour during a period of 80 min within view of a baited video camera on the sea floor at 7259 m - the deepest ever observation of a fish species with a swim bladder. The buoyancy provided by an oxygen-filled swim bladder at 74.4 MPa pressure was estimated to be 0.164 N, at a theoretical energy cost of 20 kJ, 200 times less than the cost of equivalent lipid buoyancy. During normal metabolism, 192 days would be required to fill the swimbladder. At these depths, oxygen is very incompressible, so changes in volume during ascent or descent are small. However, swimbladder function is crucially dependent on a very low rate of diffusion of oxygen across the swimbladder wall. The oxygen in the swimbladder could theoretically sustain aerobic metabolism for over 1 year but is unlikely to be used as a reserve.

A review of soluble factors and receptors involved in fish skin immunity: The tip of the iceberg

The immune system of fish possesses soluble factors, receptors, pathways and cells very similar to those of the other vertebrates' immune system. Throughout evolutionary history, the exocrine secretions of organisms have accumulated a large reservoir of soluble factors that serve to protect organisms from microbial pathogens that could disrupt mucosal barrier homeostasis. In parallel, a diverse set of recognition molecules have been discovered that alert the organism to the presence of pathogens. The known functions of both the soluble factors and receptors mentioned above encompass critical aspects of host defense, such as pathogen binding and neutralization, opsonization, or modulation of inflammation if present. The molecules and receptors cooperate and are able to initiate the most appropriate immune response in an attempt to eliminate pathogens before host infection can begin. Furthermore, these recognition molecules, working in coordination with soluble defence factors, collaboratively erect a robust and perfectly coordinated defence system with complementary specificity, activity and tissue distribution. This intricate network constitutes an immensely effective defence mechanism for fish. In this context, the present review focuses on some of the main soluble factors and recognition molecules studied in the last decade in the skin mucosa of teleost fish. However, knowledge of these molecules is still very limited in all teleosts. Therefore, further studies are suggested throughout the review that would help to better understand the functions in which the proteins studied are involved.

Interleukin-22 induces immune-related gene expression in the gills of Japanese medaka Oryzias latipes

The cytokine interleukin (IL)-22 has been identified in several fish species; however, its functional significance in the gills of these fish species remains unclear. In this study, we analyzed the expression of proinflammatory cytokines, antimicrobial peptides, and IL-22 binding protein in the gills of wild-type and IL-22-knockout (IL-22 KO) medaka under dextran sulfate sodium-induced inflammation. We also produced medaka recombinant IL-22 (rIL-22) and analyzed the expression of immune-related genes in rIL-22-stimulated primary cell cultures from gills. The il1b, il6, tnfa, and hamp genes were significantly upregulated in wild-type gills upon dextran sulfate sodium stimulation compared with the naïve state but not in IL-22 KO gills. il22bp transcripts were barely detectable in the IL-22 KO medaka gills. However, the expression of il1b, il6, hamp, and il22bp was upregulated in rIL-22-stimulated gill cell culture. These results suggest IL-22 could be involved in immune responses through inflammatory cytokine and antimicrobial peptide production in fish gills.

Transcriptomics-based analysis of sex-differentiated mechanisms of hepatotoxicity in zebrafish after long-term exposure to bisphenol AF

Bisphenol AF (BPAF) is an emerging endocrine-disrupting chemical (EDC) prevalent in the environment as one of the main substitutes for bisphenol A. Sex-specific effects of EDCs have been commonly reported and closely linked to sexually dimorphic patterns of hormone metabolism and related gene expression during different exposure windows, but our understanding of these mechanisms is still limited. Here, following 28-day exposure of adult zebrafish to an environmentally relevant concentration of BPAF at 10 μg/L, the global transcriptional networks applying RNA sequencing (RNA-seq) and Ingenuity Pathway Analysis (IPA) were respectively investigated in the male and female fish liver, connecting the sex-dependent toxicity of the long-term exposure of BPAF to molecular responses. As a result, more differentially expressed genes (DEGs) were detected in males (811) than in females (195), and spermatogenesis was the most enriched Gene Ontology (GO) functional classification in males, while circadian regulation of gene expression was the most enriched GO term in females. The expression levels of selected DEGs were routinely verified using qRT-PCR, which showed consistent alterations with the transcriptional changes in RNA-seq data. The causal network analysis by IPA suggested that the adverse outcomes of BPAF in males including liver damage, apoptosis, inflammation of organ, and liver carcinoma, associated with the regulation of several key DEGs detected in RNA-seq, could be linked to the activation of upstream regulatory molecules ifnα, yap1, and ptger2; while, the inhibition of upstream regulators hif1α, ifng, and igf1, leading to the down-regulated expression of several key DEGs, might be involved in BPAF's effects in females. Furthermore, BPAF exposure altered hepatic histological structure and inhibited antioxidant capability in both male and female livers. Overall, this study revealed different regulation networks involved in the sex-dependent effects of BPAF on the fish liver, and these detected DEGs upon BPAF exposure might be used as potential biomarkers for further assessing sex-specific hepatotoxicity following environmental EDC exposure.

Hepcidin and piscidin modulation and antibacterial response in gilthead seabream (Sparus aurata) infected with Vibrio harveyi

Vibriosis is an infectious disease that generates large economic losses in Mediterranean aquaculture. Vibrio harveyi is one of the marine bacteria causing this disease, it is widespread in the Mediterranean Sea and causes ulcers on the skin of the fish it infects. In addition, the skin is a route of entry and colonization of this pathogen. In this study, one group of fish was injected intraperitoneally with phosphate buffered saline (control group) and another with V. harveyi (infected group). At 4 h after injection, samples of skin mucus, blood, skin, head kidney, liver, and spleen were collected to study the immune response generated. Liver histology showed notable alterations in hepatocyte morphology, such as increased vacuolization. Bactericidal activity was measured in skin mucus and serum against V. harveyi and V. anguillarum, different changes in this activity were recorded depending on the bacteria target and sample (skin mucus or serum) used. Gene expression of genes encoding hepcidins and piscidins (antimicrobial peptides) was performed in the mentioned organs. The results indicated a different expression according to the type of AMP and the tissue studied. Hepcidin appeared involved in all tissues studied while piscidins were in the spleen. In this study we have integrated hepcidin-piscidin modulation with the effects of infection on skin mucosa, serum and hepatocyte morphology. Knowing the changes produced in all these parameters improves the understanding of the infection in the first hours in sea bream and could have applications in the diagnosis or treatment of vibriosis in fish farms.

cAMP signaling in ovarian physiology in teleosts: A review

Ovarian function in teleosts, like in other vertebrates, is regulated by two distinct gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin effects are mediated by membrane-bound G protein-coupled receptors localized on the surface of follicle cells. Gonadotropin receptor activation results in increased intracellular cAMP, the most important second cellular signaling molecule. FSH stimulation induces the production of 17β-estradiol in the cells of growing follicles to promote vitellogenesis in oocytes. In contrast, in response to LH, fully grown post-vitellogenic follicles gain the ability to synthesize maturation-inducing steroids, which induce meiotic resumption and ovulation. All these events were induced downstream of cAMP. In this review, we summarize studies addressing the role of the cAMP pathway in gonadotropin-induced processes in teleost ovarian follicles. Furthermore, we discuss future problems concerning cAMP signaling in relation to teleost ovarian function and the differences and similarities in the gonadotropin-induced cAMP signaling pathways between mammals and teleosts.

Evolutional perspective and functional characteristics of interleukin-17 in teleosts

Interleukin (IL)-17 is a proinflammatory cytokine and plays essential roles in adaptive and innate immune responses against bacterial and fungal infections. Especially in mammalian mucosal tissues, it is well known that innate immune responses via IL-17A and IL-17F, such as the production of antimicrobial peptides, are very important for microbiota control. In contrast, interesting insights into the functions of IL-17 have recently been reported in several teleost species, although little research has been conducted on teleost IL-17. In the present review, we focused on current insights on teleost IL-17 and speculated on the different or consensus parts of teleost IL-17 signaling compared to that of mammals. This review focuses on the role of teleost IL-17 in intestinal immunity. We expect that this review will encourage a further understanding of the roles and importance of IL-17 signaling in teleosts.

Platyfish bypass the constraint of the caudal fin ventral identity in teleosts

BACKGROUND

The caudal fin of teleosts is characterized by dorsoventral symmetry. Despite this external morphology, the principal rays of this appendage connect to bones below the notochord, indicating the ventral (hypochordal) identity of this organ.

RESULTS

Here, we report that this typical architecture of the caudal fin is not fully conserved in the platyfish (Xiphophorus maculatus) and the guppy (Poecilia reticulata), representatives of the Poeciliidae family. We show that in these species, 3-4 principal rays connect to bones above the notochord, suggesting an epichordal contribution. Consistently, as examined in platyfish, dorsal identity genes zic1/4 were highly expressed in these rays, providing molecular evidence of their epichordal origin. Developmental analysis revealed that the earliest rays above the notochord emerge at the 10-ray stage of fin morphogenesis. In contrast to zebrafish and medaka, platyfish and guppies display a mirrored shape of dorsal and ventral processes of the caudal endoskeleton. Our study suggests that an ancestral bauplan expanded in poeciliids by advancing its symmetrical pattern.

CONCLUSION

The platyfish evolved a fin architecture with the epichordal origin of its upper principal rays and a high level of symmetry in the caudal endoskeleton. This innovative architecture highlights the adaptation of the teleost skeleton.

Chitosan-nanoparticles effects on mucosal immunity: A systematic review

Nanoparticles-based treatments is of utmost importance for aquaculture. In this scenario, chitosan-based nanoparticles have been proposed due to the properties of chitosan, which include mucoadhesiveness. Nevertheless, pivotal parameters of chitosan, such as degree of acetylation and molecular weight, are commonly underestimated in the available literature despite the influence they seem to have on the properties of chitosan-based nanoparticles. In this systematic review, the immunomodulator capacity of chitosan nanoparticles used as mucosal vaccines on teleost fish has been evaluated paying special attention to the chitosan properties. Four databases were used for literature search, yielding 486 documents, from which 14 meet the inclusion criteria. Only 21% of the available studies reported properly chitosan properties, which should be improved in future works to generate reproducible data as well as valuable information. To the best of our knowledge, this work objectively compares for the first time, by quantifying the mg of chitosan/g of fish applied in each study, the chitosan nanoparticle preparation and doses applied to fish, as well as the effects of the treatments applied on fish immune status.

Two duplicated piscidin genes from gilthead seabream (Sparus aurata) with different roles in vitro and in vivo

From the discovery of pleurocidin in skin mucus of winter flounder, many new related sequences have been found, forming a fish-exclusive family of antimicrobial peptides (AMP) called piscidin. Their mature peptides have a broad-spectrum antimicrobial activity and can be involved in the innate immune response. In the present work, two paralogous tripartite piscidin genes are formally described for the first time in gilthead seabream (Sparus aurata), an important marine farmed fish. Gene synteny and protein phylogeny clearly indicated a massive pisc gene expansion in a cluster of the chromosome 22 as well as a special evolution of piscidin in gilthead seabream compared to the rest of piscidins studied in other fish species. Despite being highly similar genes, they show totally different expression patterns in tissues and head-kidney leucocytes under both naïve and Vibrio/nodavirus-stimulated conditions. Moreover, these paralogous genes coded very different proteins according to their physicochemical properties. In this way, these piscidin genes have distinct roles not only related to their microbicide activity but also to their immune modulation. In addition, the present study improves the knowledge of duplication of AMP genes and adaptative diversification of teleost immune system.

Massive gene expansion of hepcidin, a host defense peptide, in gilthead seabream (Sparus aurata)

Host defense peptides (HDP) are among the most ancient immune molecules in animals and clearly reflect an ancestral evolutionary history involving pathogen-host interactions. Hepcidins are a very widespread family of HDPs among vertebrates and are especially diverse in teleosts. We have investigated the identification of new hepcidins in gilthead seabream (Sparus aurata), a fish farmed in the Mediterranean. Targeted gene predictions supported with expressed sequence tags (ESTs) derived from Hidden Markov Models were used to find the hamp genes in the seabream genome. The results revealed a massively clustered hamp duplication on chromosome 17. In fact, the seabream genome contains the largest number of hepcidin copies described in any vertebrate. The evolutionary history of hepcidins in seabream, and vertebrates generally, clearly indicates high adaptation in teleosts and novel subgroups within hepcidin type II. Furthermore, basal hepcidin gene expression analysis indicates specific-tissue expression profiles, while the presence and distribution of transcription factor binding sites (TFBS) in hamp promoters as well as their transcription profile upon bacterial challenge indicates different immune roles depending on the type of hepcidin and tissue. This massive duplication of HDP genes in a bony fish could point to a far more specific and adaptive innate immune system than assumed in the classic concept of immunity in mammals. Hence, a new world of knowledge regarding hepcidins in fish and vertebrates is being initiated.

Gene duplication of C-type natriuretic peptide-4 (CNP4) in teleost lineage elicits subfunctionalization of ancestral CNP

The diversified natriuretic peptide (NP) family, consisting of four CNPs (CNP1-4), ANP, BNP, and VNP, has been identified in the eel. Here, we successfully cloned additional cnp genes from the brain of eel (a basal teleost) and zebrafish (a later branching teleost). The genes were identified as paralogues of cnp4 generated by the third round of whole genome duplication (3R) in the teleost lineage, thereby being named eel cnp4b and zebrafish cnp4-like, respectively. To examine the histological patterns of their expressions, we employed a newly developed in situ hybridization (ISH) chain reaction using short hairpin DNAs, in addition to conventional ISH. Eel cnp4b was expressed in the medulla oblongata, while mRNAs of eel cnp4a (former cnp4) were localized in the preoptic area. In the zebrafish brain, cnp4-like mRNA was undetectable, while the known cnp4 was expressed in both the preoptic area and medulla oblongata. Together with the different mRNA distribution of cnp4a and cnp4b in eel peripheral tissues determined by RT-PCR and ISH, it is suggested that subfunctionalization by duplicated cnp4s in ancestral teleosts has been retained only in basal teleosts. Intriguingly, cnp4b-expressing neurons in the glossopharyngeal-vagal motor complex of the medulla oblongata were co-localized with choline acetyltransferase, suggesting an involvement of Cnp4b in swallowing and respiration functions that are modulated by the vagus. Since teleost Cnp4 is an ortholog of mammalian CNP, the identified localization of teleost Cnp4 will contribute to future studies aimed at deciphering the physiological functions of CNP.

Duration of Induced Metallothionein is a Critical Factor in the Cadmium-Resistance Among Goldfish (Carassius auratus) and Common Carp (Cyprinus carpio)

According to the Environmental Protection Agency in Taiwan, the common carp (Cyprinus carpio) is one species of fish for acute toxic test. It has been found to be extremely sensitive to the toxicity of Cd²⁺; Furthermore, the goldfish (Carassius auratus) has a higher resistance than common carp upon Cd²⁺ exposure, but both fish are the same family. The aim of the study was to compare the physiological and histo-pathological responses between goldfish and common carp under exposure to sublethal concentrations of Cd²⁺ in order to understand the reasons behind the Cd²⁺-resistance. Results showed that metallothionein (MT) protein levels in visceral tissues were exceptionally increased and elevated at an earlier time in goldfish than in common carp. Meanwhile, the amount of Cd²⁺ accumulation in goldfish was higher than common carp after Cd²⁺ exposure. The histo-pathological results revealed that the density of gill mucus cells and the thickness of gill epithelium in common carp were raised earlier than in goldfish, but the histo-pathological findings resemble each other. According to the data, we suggested the efficient response of MT proteins may contribute to goldfish with a higher Cd²⁺ tolerance.

Molecular evolution of hatching enzymes and their paralogous genes in vertebrates

BACKGROUND

Hatching is identified as one of the most important events in the reproduction of oviparous vertebrates. The genes for hatching enzymes, which are vital in the hatching process, are conserved among vertebrates. However, especially in teleost, it is difficult to trace their molecular evolution in detail due to the presence of other C6astacins, which are the subfamily to which the genes for hatching enzymes belong and are highly diverged. In particular, the hatching enzyme genes are diversified with frequent genome translocations due to retrocopy.

RESULTS

In this study, we took advantage of the rapid expansion of whole-genome data in recent years to examine the molecular evolutionary process of these genes in vertebrates. The phylogenetic analysis and the genomic synteny analysis revealed C6astacin genes other than the hatching enzyme genes, which was previously considered to be retained only in teleosts, was also retained in the genomes of basal ray-finned fishes, coelacanths, and cartilaginous fishes. These results suggest that the common ancestor of these genes can be traced back to at least the common ancestor of the Gnathostomata. Moreover, we also found that many of the C6astacin genes underwent multiple gene duplications during vertebrate evolution, and the results of gene expression analysis in frogs implied that genes derived from hatching enzyme genes underwent neo-functionalization.

CONCLUSIONS

In this study, we describe in detail the molecular evolution of the C6astacin gene in vertebrates, which has not been summarized previously. The results revealed the presence of the previously unknown C6astacin gene in the basal-lineage of jawed vertebrates and large-scale gene duplication of hatching enzyme genes in amphibians. The comprehensive investigation reported in this study will be an important basis for studying the molecular evolution of the vertebrate C6astacin genes, hatching enzyme, and its paralogous genes and for identifying these genes without the need for gene expression and functional analysis.

Species variations in XRCC1 recruitment strategies for FHA domain-containing proteins

DNA repair scaffolds XRCC1 and XRCC4 utilize a phosphopeptide FHA domain binding motif (FBM) of the form Y-x-x-pS-pT-D-E that supports recruitment of three identified FHA domain-containing DNA repair proteins: polynucleotide kinase/phosphatase (PNKP), aprataxin (APTX), and a third protein, APLF, that functions as a scaffold in support of non-homologous end joining (NHEJ). Mammalian dimeric XRCC4 is able to interact with two of these proteins at any given time, while monomeric XRCC1 binds only one. However, sequence analysis indicates that amphibian and teleost XRCC1 generally contain two FHA binding motifs. X1-FBM1, is similar to the single mammalian XRCC1 FBM and probably functions similarly. X1-FBM2, is more similar to mammalian XRCC4 FBM; it is located closer to the XRCC1 BRCT1 domain and probably is less discriminating among its three likely binding partners. Availability of an additional PNKP or APTX recruitment motif may alleviate the bottleneck that results from using a single FBM motif for recruitment of multiple repair factors. Alternatively, recruitment of APLF by X1-FBM2 may function to rescue a misdirected or unsuccessful SSB repair response by redirecting the damaged DNA to the NHEJ pathway, - a need that results from the ambiguity of the PARP1 signal regarding the nature of the damage. Evaluation of XRCC4 FBMs in acanthomorphs, which account for a majority of the reported teleost sequences, reveals the presence of an additional XRCC4-like paralog, distinct from other previously described members of the XRCC4 superfamily. The FBM is typically absent in acanthomorph XRCC4, but present in the XRCC4-like paralog. Modeling suggests that XRCC4 and its paralog may form homodimers or XRCC4-XRCC4-like heterodimers.

Endohelminth Fauna of Teleost Fishes from Coasts of Şile Region of the Black Sea

A total of 508 fish specimens belonging to 19 species collected in the coast of Şile region of the Black sea were examined to detect the presence of endohelminths. Of this, 357 (70.27%) were found to be infected with parasites. 15 distinct helminths species were recovered including four species of nematodes, seven digenean species, one species of cestodes and three species of acanthocephalans. It was also determined that the species of nematode Hysterothylacium aduncum was the most common parasite and the most diverse endohelminth fauna was found in Gobius niger and Solea vulgaris, with five species. Furthermore, it should be noted that Capillaria gracilis is reported for the first time from the Turkish coasts. The infection rates, hosts, and parasites are listed in this paper.

Acute inflammatory response in the skin of gilthead seabream (Sparus aurata) caused by carrageenin

Although inflammation is a well-characterized process in mammals, few studies have dealt with the mechanisms involved in this process in fish. The present study evaluated the expression of inflammation-related genes in the skin of fish injected with carrageenin, which has previously been used in inflammatory models in mammals. In our case, fish were injected subcutaneously with PBS (as control) or carrageenin (1%), and skin samples from the injection site were collected 1.5, 3 and 6 h post-injection. The gene expression of inflammatory markers (csfr1, mhc-ii and phox40), several pro-inflammatory cytokines (il1b, tnfa, il6, il8 and il18) and other molecules related (such as myd88 and c-rel) were up-regulated at 1.5 and 3 h in fish injected with carrageenin compared with control levels. By contrast, the gene expression of anti-inflammatory molecules (nlrx1, nlrc5 isoform 1, ctsd and ctss) was down-regulated in fish injected with carrageenin and sampled 3 h post injection, again compared to the gene expression in control fish. According to our results, carrageenin can be considered not only a good stimulator to study skin inflammation in gilthead seabream but also this method might be use to study the modulation of fish inflammatory process caused by internal or external factors.

Fish Sedation and Anesthesia

Veterinarians often need to sedate or anesthetize fish to perform physical examinations or other diagnostic procedures. Sedation may also be required to transport fish. Painful procedures require complete anesthesia with appropriate antinociceptive agents. Regulations and withdrawal times apply to food animal species in many countries. Specific protocols are therefore warranted in commercial fish versus ornamentals. Tonic immobility of elasmobranchs and electric anesthesia should never be used to perform painful procedures. Anesthetic monitoring in fish remains challenging. This review summarizes ornamental fish anesthesia and discusses techniques used in the commercial fish industry and in field conditions.

Three new species of Creptotrema (Trematoda, Allocreadiidae) with an amended diagnosis of the genus and reassignment of Auriculostoma (Allocreadiidae), based on morphological and molecular evidence

Diversity of Creptotrema was investigated using morphological and molecular tools, including data for Creptotrema creptotrema (type-species). Three new species, parasites of Brazilian fishes, are described: Creptotrema conconae n. sp. (type-host, Imparfinis mirini Haseman), Creptotrema schubarti n. sp. (type-host, Characidium schubarti Travassos) and Creptotrema megacetabularis n. sp. (type-host, Auchenipterus osteomystax (Miranda Ribeiro)). The diagnosis of the genus was amended to include new features. The new species differ from each other mainly in terms of body shape, relative sucker size, and testes position. DNA sequences were obtained from Creptotrema spp. from Brazil, including 28S, ITS and COI. Genetic divergences among the new species and C. creptotrema varied from 2.1 to 5.2% (21–49 bp) for 28S, and 6.6 to 16.4% (21–45 bp) for COI. Phylogenetic analysis (28S) placed the newly generated DNA sequences of Creptotrema in a clade (Creptotrema clade sensu stricto) composed of C. creptotrema, the new species described herein, and all species previously described as Auriculostoma, revealing that Auriculostoma is best interpreted as a synonym of Creptotrema based on the principle of priority of zoological nomenclature. Creptotrema funduli, the single sequence of the genus previously available, was not grouped within the Creptotrema clade sensu stricto, suggesting the need for reevaluation of the taxonomic status of this species. Our results showed that Creptotrema represents a monophyletic genus of trematodes widely distributed across the Americas, which currently comprises 19 valid species of parasites of teleosts and anurans.

In silico and gene expression analysis of the acute inflammatory response of gilthead seabream (Sparus aurata) after subcutaneous administration of carrageenin

Inflammation is one of the main causes of loss of homeostasis at both the systemic and molecular levels. The aim of this study was to investigate in silico the conservation of inflammation-related proteins in the gilthead seabream (Sparus aurata L.). Open reading frames of the selected genes were used as input in the STRING database for protein-protein interaction network analysis, comparing them with other teleost protein sequences. Proteins of the large yellow croaker (Larimichthys crocea L.) presented the highest percentages of identity with the gilthead seabream protein sequence. The gene expression profile of these proteins was then studied in gilthead seabream specimens subcutaneously injected with carrageenin (1%) or phosphate-buffered saline (control) by analyzing skin samples from the injected zone 12 and 24 h after injection. Gene expression analysis indicated that the mechanisms necessary to terminate the inflammatory response to carrageenin and recover skin homeostasis were activated between 12 and 24 h after injection (at the tested dose). The gene analysis performed in this study could contribute to the identification of the main mechanisms of acute inflammatory response and validate the use of carrageenin as an inflammation model to elucidate these mechanisms in fish.

Seasonality, sex-specificity and transcriptional regulation of hepatic leptin system in spotted snakehead Channa punctata

The present study deals with sex-specific reproductive phase-dependent variation and sex steroids-induced transcriptional regulation of hepatic lep and lepr in nutritionally valuable spotted snakehead, Channa punctata. The data on seasonality reveals sex-specific variation in pattern of lep transcription where a high level was recorded during resting and postspawning quiescent phases in female while during resting and spawning phases in male. Unlike lep, lepr exhibited similar expression pattern along the reproductive phases in both the sexes. As compared to female, a three-fold higher expression of lep was detected in male during reproductively active phase only. However, no sexual dimorphism was evidenced in lepr either during active or quiescent phase. To explore the implication of sex steroids in regulation of leptin system, we correlated levels of plasma testosterone (T) and 17β-estradiol (E₂) with leptin system in males as well as females. Further, criss-cross in vivo and in vitro experiments with dihydrotestosterone (DHT) and E₂ were conducted in male and female spotted snakehead. The leptin system was downregulated after DHT administration in both the sexes. However, with E₂, a marked decrease was evidenced in male only. The sex-wise variable response of leptin system to sex steroids was validated by in vitro experiments wherein liver fragments from male and female fish were incubated individually with both the sex steroids. In conclusion, sex steroids modulate hepatic leptin system differentially depending on sex and reproductive state of spotted snakehead.

Protein kinase A activity and NO are involved in the regulation of crucian carp (Carassius carassius) red blood cell osmotic fragility

Activation of the cAMP pathway by β-adrenergic stimulation and cGMP pathway by activation of guanylate cyclase substantially affects red blood cell (RBC) membrane properties in mammals. However, whether similar mechanisms are involved in RBC regulation of lower vertebrates, especially teleosts, is not elucidated yet. In this study, we evaluated the effects of adenylate cyclase activation by epinephrine and forskolin, guanylate cyclase activation by sodium nitroprusside, and the role of Na⁺/H⁺-exchanger in the changes of osmotic fragility and regulatory volume decrease (RVD) response in crucian carp RBCs. Western blot analysis of protein kinase A and protein kinase G substrate phosphorylation revealed that changes in osmotic fragility were regulated via the protein kinase A, but not protein kinase G signaling pathway. At the same time, the RVD response in crucian carp RBCs was not affected either by activation of adenylate or guanylate cyclase. Adenylate cyclase/protein kinase A activation significantly decreased RBC osmotic fragility, i.e., increased cell rigidity. Inhibition of Na⁺/H⁺-exchanger by amiloride had no effect on the epinephrine-mediated decrease of RBC osmotic fragility. NO donor SNP did not activate guanylate cyclase, however affected RBCs osmotic fragility by protein kinase G-independent mechanisms. Taken together, our data demonstrated that the cAMP/PKA signaling pathway and NO are involved in the regulation of crucian carp RBC osmotic fragility, but not in RVD response. The authors confirm that the study has no clinical trial.

Mucosal immunoglobulins of teleost fish: A decade of advances

Immunoglobulins (Igs) are complex glycoproteins that play critical functions in innate and adaptive immunity of all jawed vertebrates. Given the unique characteristics of mucosal barriers, secretory Igs (sIgs) have specialized to maintain homeostasis and keep pathogens at bay at mucosal tissues from fish to mammals. In teleost fish, the three main IgH isotypes, IgM, IgD and IgT/Z can be found in different proportions at the mucosal secretions of the skin, gills, gut, nasal, buccal, and pharyngeal mucosae. Similar to the role of mammalian IgA, IgT plays a predominant role in fish mucosal immunity. Recent studies in IgT have illuminated the primordial role of sIgs in both microbiota homeostasis and pathogen control at mucosal sites. Ten years ago, IgT was discovered to be an immunoglobulin class specialized in mucosal immunity. Aiming at this 10-year anniversary, the goal of this review is to summarize the current status of the field of fish Igs since that discovery, while identifying knowledge gaps and future avenues that will move the field forward in both basic and applied science areas.

Epigenetic mapping of the somatotropic axis in Nile tilapia reveals differential DNA hydroxymethylation marks associated with growth

In vertebrates, the somatotropic axis comprising the pituitary gland, liver and muscle plays a major role in myogenesis. Its output in terms of muscle growth is highly affected by nutritional and environmental cues, and thus likely epigenetically regulated. Hydroxymethylation is emerging as a DNA modification that modulates gene expression but a holistic characterization of the hydroxymethylome of the somatotropic axis has not been investigated to date. Using reduced representation 5-hydroxymethylcytosine profiling we demonstrate tissue-specific localization of 5-hydroxymethylcytosines at single nucleotide resolution. Their abundance within gene bodies and promoters of several growth-related genes supports their pertinent role in gene regulation. We propose that cytosine hydroxymethylation may contribute to the phenotypic plasticity of growth through epigenetic regulation of the somatotropic axis.

Social network dynamics predict hormone levels and behavior in a highly social cichlid fish

Group living confers many benefits while simultaneously exposing group members to intense competition. An individual's rise to prominence within a group may conflict with the overall functioning of the group. There is therefore a complex and dynamic relationship between the behavioral displays that directly benefit an individual, the consequences of these actions for the community, and how they feed back on individual-level fitness. We used a network analysis approach to study the link between behavior, social stability, and steroid hormone levels in replicate communities of the cichlid fish, Astatotilapia burtoni, which live in social groups with a dominance hierarchy. We demonstrate that individual behavior can have direct and indirect effects on the behavior of others while also affecting group characteristics. Our results show that A. burtoni males form stable social networks, where dominant individuals act as hubs for social interactions. However, there was variation in the temporal stability in these networks, and this variation in stability impacted hormone levels. Dominant males had higher testosterone levels, however, the differences in testosterone levels between dominant and subordinate males were greatest in stable communities. In sum, our analyses provide novel insights into the processes by which individual and community properties interact.

Four polyopisthocotyleans (Platyhelminthes: Monogenea) from carangid fishes in the Mediterranean, off the Algerian coasts

Four polyopisthocotyleans were collected from the gill filaments of carangids from off the Algerian coast, southern Mediterranean. Specimens of Gastrocotyle trachuri van Beneden & Hesse, 1863 (Gastrocotylidae) and Cemocotyle cf. trachuri Dillon & Hargis, 1965 (Heteraxinidae) from the Mediterranean horse mackerel Trachurus mediterraneus (Steindachner), Zeuxapta seriolae (Meserve, 1938) (Heteraxinidae) from the greater amberjack Seriola dumerili (Risso) and Pyragraphorus hollisae Euzet & Ktari, 1970 (Pyragraphoridae) from the pompano Trachinotus ovatus (Linnaeus) are redescribed based on newly collected specimens. Their taxonomically important morphological features (male copulatory organ and clamp sclerites) are described and illustrated, and the morphometric variation between Mediterranean and oceanic specimens is highlighted. Careful examination of the specimens of Cemocotyle Sproston, 1946 from the Mediterranean revealed that they exhibited unusual features compared with Cemocotyle trachuri Dillon & Hargis, 1965 from the Pacific, mainly the absence of the terminal lappet, thus questioning previous records of this species in the Mediterranean. New geographical locality records are provided for Z. seriolae and P. hollisae. The presence of C. cf. trachuri and Z. seriolae in the Mediterranean is noteworthy as these monogeneans were initially described in the Pacific Ocean. This study extends the geographical range of Z. seriolae to the southern Mediterranean.

Influence of temperature on larval ontogenesis of Geophagus brasiliensis (Quoy & Gaimard, 1824) (Pisces: Cichlidae)

Studies on the larval development of fish are essential for conservation and improvements in cultivation techniques. Geophagus brasiliensis popularly known as Cará has potential as a fish of interest in ornamental aquaculture. Wild adults of G. brasiliensis were kept in an aquarium for spontaneous reproduction. Newly hatched larvae were transferred to 5-litre aquaria at 22, 26 and 30°C until total yolk sac resorption. Histological slides were used for biometric analysis and monitoring of larval ontogenesis at different temperatures. Histologically, from the first to the fourth days it was possible to identify myomeres, optic vesicle, yolk syncytial layer, brain, heart and differentiation of the eye layers. From the fourth to the seventh days, it was possible to identify mandibular and gill cartilages, swim bladder, liver, prismatic epithelium with striated border in intestine and renal epithelium. All biometric measurements increased over the days, except height and length of the yolk sac, which gradually decreased until the complete resorption of the yolk sac that occurred on the fifth day at a temperature of 30°C, the sixth day at 26°C and the seventh day at 22°C. Morphological events at 30°C such as the reabsorption of the yolk sac, the appearance of cartilage in the branchial arches and differentiation of the layers of the eyes occurred faster compared with the other temperatures tested. Opening of the mouth and digestive tract occurred at a similar time on the fourth day in all temperatures.

Evolution of Tandemly Arranged Repetitive DNAs in Three Species of Cyprinoidei with Different Ploidy Levels

Polyploid species represent a challenge for both cytogenetic and genomic studies due to their high chromosome numbers and the morphological similarity between their paralogous chromosomes. This paper describes the use of low-coverage high-throughput sequencing to identify the 14 most abundant tandemly arranged repetitive elements in the paleotetraploid genome of the crucian carp (Carassius carassius, 2n = 100). These repetitive elements were then used for molecular cytogenetic studies of a closely related functionally triploid form of the Prussian carp (Carassius gibelio, 3n = 150 + Bs) and a relatively distant diploid species, the tench (Tinca tinca, 2n = 48). According to their distribution on the chromosomes of the 3 aforementioned species, the repetitive elements here identified can be divided into 5 groups: (1) those specific to a single genomic locus in both Carassius species, despite the recent carp-specific genome duplication; (2) those located in a single genomic locus of T. tinca, but amplified in one or both Carassius species; (3) those massively amplified in the B chromosomes of C. gibelio; (4) those located in a single locus in C. gibelio, but amplified in many blocks in C. carassius; and (5) those located in multiple pericentromeric loci in both Carassius species. Our data indicate that some of the repetitive elements are highly conserved in cyprinoid species and may serve as good cytogenetic and genomic markers for discriminating paralogous chromosomes, while others are evolutionarily recent, and their amplification may be related to the last whole-genome duplication event.

Understanding fish cognition: a review and appraisal of current practices

With over 30,000 recognized species, fishes exhibit an extraordinary variety of morphological, behavioural, and life-history traits. The field of fish cognition has grown markedly with numerous studies on fish spatial navigation, numeracy, learning, decision-making, and even theory of mind. However, most cognitive research on fishes takes place in a highly controlled laboratory environment and it can therefore be difficult to determine whether findings generalize to the ecology of wild fishes. Here, we summarize four prominent research areas in fish cognition, highlighting some of the recent advances and key findings. Next, we survey the literature, targeting these four areas, and quantify the nearly ubiquitous use of captive-bred individuals and a heavy reliance on lab-based research. We then discuss common practices that occur prior to experimentation and within experiments that could hinder our ability to make more general conclusions about fish cognition, and suggest possible solutions. By complementing ecologically relevant laboratory-based studies with in situ cognitive tests, we will gain further inroads toward unraveling how fishes learn and make decisions about food, mates, and territories.

Evolution of sex hormone binding globulins reveals early gene duplication at the root of vertebrates

Sex hormone-binding globulin (Shbg) is an important vertebrate blood carrier protein synthetized in the liver and involved in the transport and local regulation of sex steroids in target tissues. A novel shbg gene (shbgb) with a predominant ovarian expression was recently characterized. Being initially found only in salmonids, this shbgb was originally thought to result from the Salmonid-specific whole genome duplication. Using updated transcriptomic and genomic resources we identified Shbgb orthologs in non-salmonid teleosts (European eel, arowana), holosteans (spotted gar, bowfin), polypteriformes (reedfish), agnatha (sea lamprey) and in amphibians, and found that the classical Shbg gene (Shbga) displays a predominant hepatic expression whereas Shbgb has a predominant gonadal expression. Together, these results indicate that these two Shgb genes most likely originate from a whole genome duplication event at the root of vertebrate evolution, followed by numerous and independent losses and by tissue expression specialization of Shbga and Shbgb paralogs.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Red blood cell carbonic anhydrase mediates oxygen delivery via the Root effect in red drum

Oxygen (O₂) and carbon dioxide (CO₂) transport are tightly coupled in many fishes as a result of the presence of Root effect hemoglobins (Hb), whereby reduced pH reduces O₂ binding even at high O₂ tensions. Red blood cell carbonic anhydrase (RBC CA) activity limits the rate of intracellular acidification, yet its role in O₂ delivery has been downplayed. We developed an in vitro assay to manipulate RBC CA activity while measuring Hb-O₂ offloading following a physiologically relevant CO₂-induced acidification. RBC CA activity in red drum (Sciaenops ocellatus) was inhibited with ethoxzolamide by 53.7±0.5%, which prompted a significant reduction in O₂ offloading rate by 54.3±5.4% (P=0.0206, two-tailed paired t-test; n=7). Conversely, a 2.03-fold increase in RBC CA activity prompted a 2.14-fold increase in O₂ offloading rate (P<0.001, two-tailed paired t-test; n=8). This approximately 1:1 relationship between RBC CA activity and Hb-O₂ offloading rate coincided with a similar allometric scaling exponent for RBC CA activity and maximum metabolic rate. Together, our data suggest that RBC CA is rate limiting for O₂ delivery in red drum.

Kisspeptin and GnRH interactions in the reproductive brain of teleosts

It is well known that gonadotropin-releasing hormone (Gnrh) has a key role in reproduction by regulating the synthesis and release of gonadotropins from the anterior pituitary gland of all vertebrates. About 25 years ago, another neuropeptide, kisspeptin (Kiss1) was discovered as a metastasis suppressor of melanoma cell lines and then found to be essential for mammalian reproduction as a stimulator of hypothalamic Gnrh and regulator of puberty onset. Soon after, a kisspeptin receptor (kissr) was found in the teleost brain. Nowadays, it is known that in most teleosts the kisspeptin system is composed of two ligands, kiss1 and kiss2, and two receptors, kiss2r and kiss3r. Even though both kisspeptin peptides, Kiss1 and Kiss2, have been demonstrated to stimulate gonadotropin synthesis and secretion in different fish species, their actions appear not to be mediated by Gnrh neurons as in mammalian models. In zebrafish and medaka, at least, hypophysiotropic Gnrh neurons do not express Kiss receptors. Furthermore, kisspeptinergic nerve terminals reach luteinizing hormone cells in some fish species, suggesting a direct pituitary action. Recent studies in zebrafish and medaka with targeted mutations of kiss and/or kissr genes reproduce relatively normally. In zebrafish, single gnrh mutants and additionally those having the triple gnrh3 plus 2 kiss mutations can reproduce reasonably well. In these fish, other neuropeptides known to affect gonadotropin secretion were up regulated, suggesting that they may be involved in compensatory responses to maintain reproductive processes. In this context, the present review explores and presents different possibilities of interactions between Kiss, Gnrh and other neuropeptides known to affect reproduction in teleost fish. Our intention is to stimulate a broad discussion on the relative roles of kisspeptin and Gnrh in the control of teleost reproduction.

Fish nasal immunity: From mucosal vaccines to neuroimmunology

Like terrestrial vertebrates, bony fishes have a nasopharynx-associated lymphoid tissue (NALT) that protects the host against invading pathogens. Despite nasal immunity being a relatively new field in fish immunology, the investigation of nasal immune systems has already illuminated fundamental aspects of teleost mucosal immune systems as well as neuroimmunology. In this review, we highlight the importance of nasal infections in bony fish and the progress that has been made towards understanding how fish respond locally and systemically to nasal infection or vaccination. We also want to highlight the complex interactions between neurons and immune cells that occur in the olfactory organ during the course of an immune response. We predict that similar neuroimmune interactions govern immune responses at all mucosal tissues in bony fish. Understanding the principles of mucosal immune responses in teleost NALT has therefore revealed important aspects of fish mucosal immunity that are critical for mucosal vaccination in aquaculture.

Controlled release of sex steroids through osmotic pump alters brain GnRH1 and catecholaminergic system dimorphically in the catfish, Clarias gariepinus

The present study aimed to evaluate osmotic pump-mediated controlled release of estrogen in males and androgen in females to analyze the impact on gonadotropin-releasing hormone (GnRH1), catecholamines (CAs) and other associated genes in the catfish, Clarias gariepinus. During pre-spawning phase, catfish were separately implanted osmotic pumps loaded with 17β-estradiol (E₂) in males and 17α-methyltestosterone (MT) in females at a dose of 10 μg/100 μl or saline (100 μl) controls into both sexes to release for 21 days and all fishes were maintained as per the duration. Further, GnRH1 expression levels were analysed in the discrete regions of brain after E₂ and MT treatments in male and female catfish, respectively using qPCR which revealed that GnRH1 expression was significantly higher in E₂ treated male as compared to the control. On the other hand, GnRH1 expression was lower in MT treated female when compared to the control in the discrete regions of brain. In addition, certain brain and monoaminergic system related genes showed a differential response. Catfish GnRH1 could be localized in preoptic area-hypothalamus (POA-HYP) that correlated with the expression profile in the discrete regions of catfish brain. Serum levels of sex steroids in the treated male fish indicated that the treatment of E₂ could maintain and impart feminization effect even in the presence of endogenous androgen during gonadal recrudescence while such an effect was not seen in females with androgen treatment. Measurement of CAs, L-3,4-dihydroxyphenylalanine, dopamine and norepinephrine levels in the male and female brain after the controlled release of E₂ and MT, respectively confirmed the modulation of neurotransmitters in the E₂treated male than MT treated female fish. These results collectively suggest the severity of estrogenic over androgenic compounds to alter reproductive status even at a minimal dose by targeting CAs and GnRH1 at the level of brain of catfish. This study provides insights into the reproductive toxicity of sex steroid analogues at the level of brain GnRH1 and CA-ergic system in addition to serum T, 11-KT and E₂ levels during gonadal recrudescence, which is a crucial period of gametogenesis preceding spawning.

Effects of inflammation and/or infection on the neuroendocrine control of fish intestinal motility: A review

Food is the largest expense in fish farms. On the other hand, the fish health and wellbeing are determining factors in aquaculture production where nutrition is a vital process for growing animals. In fact, it is important to remember that digestion and nutrition are crucial for animals' physiology. However, digestion is a very complex process in which food is processed to obtain necessary nutrients and central mechanisms of this process require both endocrine and neuronal regulation. In this context, intestinal motility is essential for the absorption of the nutrients (digestive process determining nutrition). An imbalance in the intestinal motility due to an inadequate diet or an infectious process could result in a lower use of the food and inefficiency in obtaining nutrients from food. Very frequently, farmed fish are infected with different pathogenic microorganism and this situation could alter gastrointestinal physiology and, indirectly reduce fish growth. For these reasons, the present review focuses on analysing how different inflammatory molecules or infections can alter conventional modulators of fish intestinal motility.

Heavy metal contamination in a highly consumed Brazilian fish: immunohistochemical and histopathological assessments

Due to industrial, rural, and domestic waste disposal, heavy metals such as cadmium (Cd), chromium (Cr), lead (Pb), zinc (Zn), copper (Cu), and iron (Fe) continually infiltrate aquatic environments. These pollutants do not degrade naturally and, thus, have a high capacity for bioaccumulation in tissues and organs. The present study uses histological and immunohistochemical analyses to evaluate the contamination status of Salminus franciscanus, a large and economically important fish. Levels of Cd, Cr, Pb, Zn, Cu, and Fe were evaluated by atomic absorption spectrometry in the liver and muscle of fish sampled from two tributaries of the upper São Francisco River Basin, Brazil: the Abaeté and Paraopeba Rivers. In addition, histopathological alterations and expressions of three environmental biomarkers were assessed: metallothionein (MT), heat shock protein-70 (HSP70), and cytochrome P450-1A (CYP1A). The results show that fish from the Paraopeba River are unsuitable for human consumption, with several metals being detected above the safe limits established by the World Health Organization. Histopathological alterations in the liver and spleen were also significantly more frequent in fish from the Paraopeba River than in those from the Abaeté River (P < 0.05). Significant differences in the expressions of environmental biomarkers were observed between the rivers. Fish from the Abaeté River presented significantly higher values of the gonadosomatic index (GSI) and lower levels of metal contamination in the liver and muscle.

The gonadotropin-releasing hormones: Lessons from fish

Fish have been of paramount importance to our understanding of vertebrate comparative neuroendocrinology and the mechanisms underlying the physiology and evolution of gonadotropin-releasing hormones (GnRH) and their genes. This review integrates past and recent knowledge on the Gnrh system in the fish model. Multiple Gnrh isoforms (two or three forms) are present in all teleosts, as well as multiple Gnrh receptors (up to five types), which differ in neuroanatomical localization, pattern of projections, ontogeny and functions. The role of the different Gnrh forms in reproduction seems to also differ in teleost models possessing two versus three Gnrh forms, Gnrh3 being the main hypophysiotropic hormone in the former and Gnrh1 in the latter. Functions of the non-hypothalamic Gnrh isoforms are still unclear, although under suboptimal physiological conditions (e.g. fasting), Gnrh2 may increase in the pituitary to ensure the integrity of reproduction under these conditions. Recent developments in transgenesis and mutagenesis in fish models have permitted the generation of fish lines expressing fluorophores in Gnrh neurons and to elucidate the dynamics of the elaborate innervations of the different neuronal populations, thus enabling a more accurate delineation of their reproductive roles and regulations. Moreover, in combination with neuronal electrophysiology, these lines have clarified the Gnrh mode of actions in modulating Lh and Fsh activities. While loss of function and genome editing studies had the premise to elucidate the exact roles of the multiple Gnrhs in reproduction and other processes, they have instead evoked an ongoing debate about these roles and opened new avenues of research that will no doubt lead to new discoveries regarding the not-yet-fully-understood Gnrh system.

Effect of Jasonia glutinosa on immune and oxidative status of gilthead seabream (Sparus aurata L.)

Jasonia glutinosa (rock-tea, RT) has numerous biological activities. In the present work, the beneficial effects of dietary RT on gilthead seabream (Sparus aurata L.) were studied. Fish fed experimental diets containing 0 (control), 10 or 30% of RT for 15 and 30 days. Samples from skin mucus, blood, head kidney, liver and gut were obtained at 15 and 30 days. The antioxidant properties from RT were analysed such as the antioxidant capacity and phenolic content. The heat shock protein 70 level (HSP70) and the total oxidized proteins were evaluated on skin mucus as stress markers. Immune parameters, both humoral (peroxidase activity, immunoglobulin M levels and complement activity) and cellular (phagocytic, respiratory burst and peroxidase activities) were determined in skin mucus, serum or head-kidney leucocytes, respectively. Concomitantly, the expression of different genes related to inflammation and oxidative stress was studied both in liver and gut. Skin mucus peroxidase was significantly increased on fish fed 10% RT for 15 days with respect to the control group. In addition, Serum IgM levels were significantly increased while HSP70 levels and oxidized proteins were significantly decreased on skin mucus from fish fed 30% RT for 30 days, respectively. Besides, cellular immune parameters (phagocytosis, respiratory burst and peroxidase activity) were significantly higher in leucocytes from fish fed the RT diets for 15 days, but not for 30 days. Finally, the gene expression of antioxidant enzymes was up-regulated in liver at 15 and in liver and gut at 30 days. However, the expression of il1b and hsp70 was down-regulated in the liver of fish fed 30% RT for 30 days with respect to the values of control fish. The possible inclusion of RT in fish diets as an additive with antioxidant and/or immunostimulant activities is discussed.

Effects of dietary Origanum vulgare on gilthead seabream (Sparus aurata L.) immune and antioxidant status

Origanum sp. is a very common genus of aromatic plants worldwide distributed around the Mediterranean area and O. vulgare (oregano) is the most important species of this genus throughout the world. Due the known medicinal properties of oregano, the effect of diets enriched with 0% (control), 0.5% and 1% oregano leaves powder was studied on the growth, immune and antioxidant status of gilthead seabream (Sparus aurata L.). Fish fed with oregano 0.5% and 1% enriched diets improved both humoral (IgM and bactericidal activity in skin mucus and protease activity in serum) and cellular (head kidney leucocytes phagocytic ability) immunity at 15 and 30 days. Furthermore, the addition of oregano did not provoke any significant effect neither in the growth promotion nor in the liver antioxidant enzymes activity studied in the serum and skin mucus. The possibility of using O. vulgare as a functional additive to fish diet is discussed.

Phylogeny of teleost connexins reveals highly inconsistent intra- and interspecies use of nomenclature and misassemblies in recent teleost chromosome assemblies

BACKGROUND

Based on an initial collecting of database sequences from the gap junction protein gene family (also called connexin genes) in a few teleosts, the naming of these sequences appeared variable. The reasons could be (i) that the structure in this family is variable across teleosts, or (ii) unfortunate naming. Rather clear rules for the naming of genes in fish and mammals have been outlined by nomenclature committees, including the naming of orthologous and ohnologous genes. We therefore analyzed the connexin gene family in teleosts in more detail. We covered the range of divergence times in teleosts (eel, Atlantic herring, zebrafish, Atlantic cod, three-spined stickleback, Japanese pufferfish and spotted pufferfish; listed from early divergence to late divergence).

RESULTS

The gene family pattern of connexin genes is similar across the analyzed teleosts. However, (i) several nomenclature systems are used, (ii) specific orthologous groups contain genes that are named differently in different species, (iii) several distinct genes have the same name in a species, and (iv) some genes have incorrect names. The latter includes a human connexin pseudogene, claimed as GJA4P, but which in reality is Cx39.2P (a delta subfamily gene often called GJD2like). We point out the ohnologous pairs of genes in teleosts, and we suggest a more consistent nomenclature following the outlined rules from the nomenclature committees. We further show that connexin sequences can indicate some errors in two high-quality chromosome assemblies that became available very recently.

CONCLUSIONS

Minimal consistency exists in the present practice of naming teleost connexin genes. A consistent and unified nomenclature would be an advantage for future automatic annotations and would make various types of subsequent genetic analyses easier. Additionally, roughly 5% of the connexin sequences point out misassemblies in the new high-quality chromosome assemblies from herring and cod.

Evolution of IFN subgroups in bony fish - 2. analysis of subgroup appearance and expansion in teleost fish with a focus on salmonids

A relatively large repertoire of type I interferon (IFN) genes is apparent in rainbow trout/Atlantic salmon, that includes six different IFN subgroups (IFNa-IFNf) belonging to the three known type I IFN groups (1-3) in bony fish. Whether this is true for other salmonids, and how the various type I subgroups evolved in teleost fish was studied using the extensive genomic resources available for fish. This confirmed that salmonids, at least the Salmoninae, indeed have a complex (in terms of IFN subgroups present) and large (number of genes) IFN repertoire relative to other teleost fish. This is in part a consequence of the salmonid 4 R WGD that duplicated the growth hormone (GH) locus in which type I IFNs are generally located. Divergence of the IFN genes at the two GH loci was apparent but was not seen in common carp, a species that also underwent an independent 4 R WGD. However, expansion of IFN gene number can be found at the CD79b locus of some perciform fish (both freshwater and marine), with expansion of the IFNd gene repertoire. Curiously the primordial gene order of GH-IFNc-IFNb-IFNa-IFNe is largely retained in many teleost lineages and likely reflects the tandem duplications that are taking place to increase IFN gene number. With respect to the evolution of the IFN subgroups, a complex acquisition and/or loss has occurred in different teleost lineages, with complete loss of IFN genes at the GH or CD79b locus in some species, and reduction to a single IFN subgroup in others. It becomes clear that there are many variations to be discovered regarding the mechanisms by which fish elicit protective (antiviral) immune responses.

Hypothalamic- and pituitary-derived growth and reproductive hormones and the control of energy balance in fish

Most endocrine systems in the body are influenced by the hypothalamic-pituitary axis. Within this axis, the hypothalamus delivers precise signals to the pituitary gland, which in turn releases hormones that directly affect target tissues including the liver, thyroid gland, adrenal glands and gonads. This action modulates the release of additional hormones from the sites of action, regulating key physiological processes, including growth, metabolism, stress and reproduction. Pituitary hormones are released by five distinct hormone-producing cell types: somatotropes (which produce growth hormone), thyrotropes (thyrotropin), corticotropes (adrenocorticotropin), lactotropes (prolactin) and gonadotropes (follicle stimulating hormone and luteinizing hormone), each modulated by specific hypothalamic signals. This careful and distinct organization of the hypothalamo-pituitary axis has been classically associated with the existence of many lineal axes (e.g., the hypothalamic-pituitary-gonadal axis) in charge of the control of the different physiological processes. While this traditional concept is valid, it is becoming apparent that hormones produced by the hypothalamo-pituitary axis have diverse effects. For instance, gonadotropin-releasing hormone II has been associated with a suppressive effect on food intake in fish. Likewise, growth hormone has been shown to influence appetite, swimming activity and aggressive behavior in fish. This review will focus on the hypothalamic and pituitary hormones classically involved in regulating growth and reproduction, and will attempt to provide a general overview of the current knowledge on their actions on energy balance and appetite in fish. It will also give a brief perspective of the role of some of these peptides in integrating feeding, metabolism, growth and reproduction.

Digenean parasites of deep-sea teleosts: A progress report

The developments in the study of digeneans of deep-sea fish in the 21st Century are documented and discussed. Most recent work has been on the bathyal fauna (i.e. 1,000m-2,999 m depth), with virtually nothing on the abyssal fauna (i.e. deeper than 3,000 m). The one study on hydrothermal vent digeneans has indicated that these regions probably harbour a distinctive fauna. The demarcation of the deep-sea fauna is blurred at the poles, where the cold-adapted fauna appears similar to the shallower bathyal fauna. The abyssal fauna, however, appears distinct, possibly due to adaptations to variable or ultra-high pressures. The digenean fauna of bathypelagic fishes is depauperate. Recent phylogenetic studies reinforce the view that the typical deep-sea fauna has radiated in the deep-sea. Encroachment into the deep from shallow water is relatively rare. Overall, the digenean fauna in the deep-sea is distinctly less diverse that the equivalent fauna in shallow waters. A major conclusion is that our understanding of the deep-sea digenean fauna is poor, and that much further work over a much wider area is needed.

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Sampling effort in the deep-sea is poor and restricted to a few areas.

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Deep-sea digenean diversity is distinctly lower than in shallow water.

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Much of the deep-sea digenean fauna appears to have radiated in the deep-sea.

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Encroachment into the very deep sea from shallow waters is rare.

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Adaptation to cold temperature appears to be a major factor in deep-sea digeneans.

Investigation on the validity of 3D micro-CT imaging in the fish brain

BACKGROUND

Micro-computed tomography (CT) is a non-invasive technique that is used to obtain three-dimensional (3D) images of tissue structure in small animals. Compared with extensive mammal studies, few 3D imaging studies of fish have been conducted using micro-CT. An optimized method for imaging fish tissue structure is necessary, because they have adapted to diverse environments via functional and structural specialization.

NEW METHOD

Brains of three species with different sizes and habitats were fixed in 4% paraformaldehyde and immersed in non-ionic iodinated contrast agent (Iopamiron). We examined the relationship between Iopamiron concentration and immersion time to determine universally optimal conditions for use in fish.

RESULTS

We reconstructed 3D images of whole fish brains from cross-sections of brains from the Malabar grouper (Epinephelus malabaricus), bastard halibut (Paralichthys olivaceus), and threespot wrasse (Halichoeres trimaculatus). Developmental changes in brain structure were observed in the bastard halibut. Most brain regions of the threespot wrasse were distinguishable, although inner regions of the brain were less visible.

COMPARISON WITH EXISTING METHODS

Histological techniques are typically used to observe fish brain structure, despite its drawbacks in terms of tissue sample preparation (shrinkage and distortion) and image capture (3D image constriction). The technique examined in the present study solves these problems and allows for the simultaneous handling of multiple specimens.

CONCLUSION

Micro-CT imaging is suitable for observing the surfaces and inner structures of fish of various species.

Hemoglobin deoxygenation and methemoglobinemia prevent regulatory volume decrease in crucian carp (Carassius carassius) red blood cells

Fish red blood cells (RBCs) exhibit an oxygen-dependent regulatory volume decrease (RVD) in hypoosmotic environment. In higher vertebrates, membrane-associated hemoglobin is involved in the regulation of osmotic ion movements across the cellular membrane. However, whether the hemoglobin conformational state plays a role in the regulation of osmotic responses in fish red blood cells is still not fully understood. We found that changes in hemoglobin conformation influence the pattern of the regulatory volume decrease in Carassius carassius red blood cells. In oxygenated cells (96.4 ± 3.7% oxygenated hemoglobin), the volume recovery was completed within 125 min. Deoxygenation of hemoglobin (96.5 ± 2.7% of deoxygenated hemoglobin) inhibited the volume decrease in hyposmotically swollen red blood cells. Reoxygenation restored regulatory volume decrease in cells within 5 min. Induced methemoglobinemia (48.4 ± 1.8% of methemoglobin and 41.3 ± 2.3% of deoxygenated hemoglobin) blocked the process of volume recovery and significantly decreased osmotic stability of red blood cells.

Plastics occurrence in the gastrointestinal tract of Zeus faber and Lepidopus caudatus from the Tyrrhenian Sea

The present study investigates the occurrence of plastic pollution in two commercially important marine teleosts (Zeus faber and Lepidopus caudatus) from the northern coasts of Sicily (Tyrrhenian Sea). Plastics occurrence in the gastrointestinal tract was higher in Lepidopus caudatus (78.1%) than Zeus faber (51.4%). Debris characterization, carried out by micro-Raman spectroscopy, allowed identified the main types of found polymers as: polypropylene (PP), polyamide (PA), nylon and, to a lesser extent, polyethylene (PE). Of the two fish species studied, the silver scabbardfish appeared to be the more vulnerable to plastic ingestion. Our study represents a starting point that may pave the way for future investigation of the fate, accumulation and transfer of plastic debris to upper trophic levels, to verify their potential toxicity and to better understand strategies to mitigate this phenomenon.