Effects of environmental salinity on global and endocrine-specific transcriptomic profiles in the caudal neurosecretory system of salmonid fishes

The caudal neurosecretory system (CNSS) is a fish-specific neuroendocrine complex whose function(s) remain uncertain despite over 60 years of research. Osmoregulatory roles for the CNSS have been hypothesized, but molecular regulation of the CNSS following changes in environmental salinity remains poorly characterized. Therefore, we performed transcriptomics on the CNSS of rainbow trout (Oncorhynchus mykiss) to establish: (1) how the CNSS responds following seawater (SW) transfer, and (2) which endocrine systems contribute to osmoregulatory responses in the CNSS. Responses following SW transfer varied at 24 h versus 168 h, with changes primarily affecting membrane transport and transcriptional processes at 24 h and neuronal processes at 168 h. Components of several osmoregulation-associated endocrine systems were affected (e.g., corticosteroid receptors), including some that have not previously been identified in the CNSS (e.g., calcitonin). Additionally, transcript levels of corticotropin-releasing factor (CRF) peptides-which have osmoregulatory functions and were highly abundant in the CNSS-were approximately twofold higher after 24 h in SW. Therefore, we performed additional experiments investigating CRF peptides in a more euryhaline salmonid, Atlantic salmon (Salmo salar). Smolts had up to 12-fold higher levels of CRF peptide transcripts than parr, but abundance declined following SW transfer. Additionally, CRF transcripts were lower 24 h following freshwater transfer of SW-acclimated salmon. These results suggest that CRF peptides acutely aid in coordinating physiological responses following fluctuations in environmental salinity via anticipatory and/or responsive mechanisms. Collectively, our data indicate that CNSS-mediated production of CRF peptides has osmoregulatory functions and provide a resource for investigations of novel CNSS functions.

Endocrine control of gill ionocyte function in euryhaline fishes

The endocrine system is an essential regulator of the osmoregulatory organs that enable euryhaline fishes to maintain hydromineral balance in a broad range of environmental salinities. Because branchial ionocytes are the primary site for the active exchange of Na+, Cl-, and Ca2+ with the external environment, their functional regulation is inextricably linked with adaptive responses to changes in salinity. Here, we review the molecular-level processes that connect osmoregulatory hormones with branchial ion transport. We focus on how factors such as prolactin, growth hormone, cortisol, and insulin-like growth-factors operate through their cognate receptors to direct the expression of specific ion transporters/channels, Na+/K+-ATPases, tight-junction proteins, and aquaporins in ion-absorptive (freshwater-type) and ion-secretory (seawater-type) ionocytes. While these connections have historically been deduced in teleost models, more recently, increased attention has been given to understanding the nature of these connections in basal lineages. We conclude our review by proposing areas for future investigation that aim to fill gaps in the collective understanding of how hormonal signaling underlies ionocyte-based processes.

Cortisol in fish scales remains stable during extended periods of storage

Measurement of cortisol in fish scales is attracting considerable attention as a non-invasive indicator of chronic stress in wild populations. For many fish species of management and conservation interest, extensive scale collections exist that could provide extended records of individual stress responses, by combining cortisol measurements with life history information. However, it is not yet known how well cortisol is preserved in the scale during storage. To investigate the stability of scale cortisol, we accelerated potential degradation by storing scales from an individual farmed Atlantic salmon (Salmo salar) in an oven at 50°C for between 2 and 12 weeks. We found no significant relationship between scale cortisol concentration and either storage time or storage temperature. Cortisol concentrations in scales from the same fish were consistent (18.54-21.82 ng. g-1; coefficient of variation (CV) = 3.6%), indicating that scale cortisol can be reliably quantified, even in scales stored for varying periods of time or under different conditions. We also examined the effects of storage in real time using Atlantic salmon scales that were stored in paper envelopes at room temperature for between 3 and 32 years and found no significant relationship between scale cortisol concentration and storage time. Scale cortisol concentrations ranged from 4.05 to 135.37 ng.g-1 and levels of between-individual variability were high (CV = 61%). Given that scale cortisol does not degrade during long-term storage, historical scale collections and associated data describing fish life histories could potentially be used to develop bioindicators of physiological responses in fish populations. Further research is needed to understand scale cortisol variability and its biological relevance.

Isolation and characterisation of two epithelial-like cell lines from the gills of Chrysophrys auratus (Australasian snapper) and Oncorhynchus tshawytscha (Chinook salmon) and their use in aquatic toxicology

In vitro gill models are becoming increasingly important in aquatic toxicology, yet the fish gill invitrome is underrepresented, encompassing approximately 0.1% of extant species. Here, we describe the establishment and characterisation of two gill-derived, epithelial-like cell lines isolated from fish species of significant importance to New Zealand: Chrysophrys auratus (Australasian snapper) and Oncorhynchus tshawytscha (Chinook salmon). Designated CAgill1PFR (Chrysophrys auratus, gill 1, Plant & Food Research) and OTgill1PFR (Oncorhynchus tshawytscha, gill 1, Plant & Food Research), these cell lines have each been passaged greater than each 70 times over several years and are considered spontaneously immortalised. Both cell lines required serum for growth and exhibited differential responses to basal media formulations. CAgill1PFR was sensitive to low temperatures (4 °C) but replicated at high temperatures (30 °C), whereas OTgill1PFR was sensitive to high temperatures but remained viable at low temperatures, mirroring the natural environment of their host species. Immunostaining revealed expression of epithelial cell markers cytokeratin and E-cadherin, alongside positivity for the mesenchymal cell marker, vimentin. CAgill1PFR was more sensitive to the environmental toxin 3,4 dichloroaniline than OTgill1PFR through measurements of metabolic activity, membrane integrity, and lysosomal function. Furthermore, CAgill1PFR produced less CYP1A activity, indicative of ongoing biotransformation processes, in response to beta-naphthoflavone than OTgill1PFR. These cell lines expand the toolbox of resources and emphasise the need for species-specific aquatic toxicology research.

Genetic Disruption of cyp21a2 Leads to Systemic Glucocorticoid Deficiency and Tissues Hyperplasia in the Teleost Fish Medaka (Oryzias latipes)

cytochrome P-450, 21-hydroxylase (cyp21a2), encodes an enzyme required for cortisol biosynthesis, and its mutations are the major genetic cause of congenital adrenal hyperplasia (CAH) in humans. Here, we have generated a null allele for the medaka cyp21a2 with a nine base-pair insertion which led to a truncated protein. We have observed a delay in hatching and a low survival rate in homozygous mutants. The interrenal gland (adrenal counterpart in teleosts) exhibits hyperplasia and the number of pomca-expressing cells in the pituitary increases in the homozygous mutant. A mass spectrometry-based analysis of whole larvae confirmed a lack of cortisol biosynthesis, while its corresponding precursors were significantly increased, indicating a systemic glucocorticoid deficiency in our mutant model. Furthermore, these phenotypes at the larval stage are rescued by cortisol. In addition, females showed complete sterility with accumulated follicles in the ovary while male homozygous mutants were fully fertile in the adult mutants. These results demonstrate that the mutant medaka recapitulates several aspects of cyp21a2-deficiency observed in humans, making it a valuable model for studying steroidogenesis in CAH.

The Caudal Neurosecretory System: A Still Enigmatic Second Neuroendocrine Complex in Fish

The caudal neurosecretory system (CNSS) is a neuroendocrine complex, whose existence is specific to fishes. In teleosts, it consists of neurosecretory cells (Dahlgren cells) whose fibers are associated with a neurohemal terminal tissue (urophysis). In other actinopterygians as well as in chondrichthyes, the system is devoid of urophysis, so that Dahlgren cells end in a diffuse neurohemal region. Structurally, it has many similarities with the hypothalamic-neurohypophysial system. However, it differs regarding its position at the caudal end of the spinal cord and the nature of the hormones it secretes, the most notable ones being urotensins. The CNSS was first described more than 60 years ago, but its embryological origin is still hypothetical, and its role is poorly understood. Observations and experimental data gave some evidences of a possible involvement in osmoregulation, stress, and reproduction. But one may question the benefit for fish to possess this second neurosecretory system, while the central hypothalamic-pituitary complex already controls such functions. As an introduction of our review, a brief report on the discovery of the CNSS is given. A description of its organization follows, and our review then focuses on the neuroendocrinology of the CNSS with the different factors it produces and secretes. The current knowledge on the ontogenesis and developmental origin of the CNSS is also reported, as well as its evolution. A special focus is finally given on what is known on its potential physiological roles.

Melatonin integrates multidimensional regulation of Na+/K+-ATPase in ionocytes and promotes stress and ease response in hypoxia-induced air-breathing fish: lessons from integrative approach

As circadian regulator, melatonin is involved in many physiological processes including ionosmotic regulation in fishes. Na⁺/K⁺-ATPase (NKA), an ubiquitous Na⁺/K⁺ transporter in ionocyte epithelia that drives electrochemical Na⁺ gradients and systemic osmotic integration, is a target of stress in fish. However, it is not certain how melatonin regulates NKA functions in ionocyte epithelia and how it modulates the adaptive response such as stress and ease response in fish particularly in hypoxia condition. We, thus, examined the short-term in vivo action of melatonin on the dynamics of NKA regulation in branchial, renal and intestinal ionocytes of hypoxia-induced air-breathing fish (Anabas testudineus Bloch). Interestingly, we found a rise in plasma melatonin in fish when kept for 30 min of forced submergence in water and that indicates a role for melatonin in hypoxia tolerance. A fall in blood [Na⁺ _, K⁺] occurred in these hypoxic fish which later showed a recovery after melatonin treatment. Similarly, melatonin favored the fall in NKA activity in branchial and renal epithelia of hypoxic fish, though it remarkably stimulated its activities in non-stressed fish. Likewise, melatonin that produced differential pattern of mRNA expression in nkaα1-subunit isoforms (nkaα1a, nkaα1b and nkaα1c) and melatonin receptor isoforms (mtnr1a, mtnr1bb, mtnr1bb _x1x2 ) in the tested ionocyte epithelia, showed reversed expression in hypoxic fish. In addition, the rise in NKAα-protein abundance in branchial and renal epithelia of melatonin-treated hypoxic fish indicated a recovery action of melatonin. A higher NKAα-immunoreactivity was found in the immunohistochemical and immunofluorescent images of branchial ionocytes and renal proximal and distal ionocytes of hypoxic fish treated with melatonin. Furthermore, an activation of PKA and PKG-dependent phosphorylation was found in branchial epithelia of hypoxic fish. The generated integrative parabola model showed that melatonin has a maximum targeted action on NKA function in the renal epithelia, suggesting its lead role in the integration of ionosmotic balance during the recovery or ease response. Over all, the data indicate a multidimensional and preferential action of melatonin on NKA regulation in fish ionocytes that integrate the recovery action against hypoxia, thus pointing to a major role for melatonin in stress and ease response in this fish.

The effect of environmental stressors on growth in fish and its endocrine control

Fish body growth is a trait of major importance for individual survival and reproduction. It has implications in population, ecology, and evolution. Somatic growth is controlled by the GH/IGF endocrine axis and is influenced by nutrition, feeding, and reproductive-regulating hormones as well as abiotic factors such as temperature, oxygen levels, and salinity. Global climate change and anthropogenic pollutants will modify environmental conditions affecting directly or indirectly fish growth performance. In the present review, we offer an overview of somatic growth and its interplay with the feeding regulatory axis and summarize the effects of global warming and the main anthropogenic pollutants on these endocrine axes.

Effects of acute low-salinity stress on osmoregulation, antioxidant capacity, and growth of the black sea bream (Acanthopagrus schlegelii)

The black sea bream (Acanthopagrus schlegelii) is an important marine economic fish found on the southeast coast of China. Because of the frequent climate change, the salinity of the waters inhabited by A. schlegelii often decreases, which interferes with the fish's physiological homeostasis. The isotonic salinity of teleosts are usually lower than that of seawater, so maximum economic benefits cannot be obtained from conventional mariculture. This study was performed to preliminarily clarify the osmotic regulation and antioxidant mechanism of juvenile A. schlegelii and find an appropriate culture salinity value. We selected 5 psu, 10 psu, 15 psu, and 25 psu (control) to conduct physiological experiments for 96 h and growth experiments for 60 days. We found that the juvenile A. schlegelii could adjust their osmotic pressure within 12 h. The growth hormone and cortisol were found to be seawater-acclimating hormones, whereas prolactin was freshwater-acclimating hormone. The activity and mRNA expression of Na⁺/K⁺-ATPase showed a U-shaped trend with the decrease of in salinity at 12-96 h. Serum ion concentration and osmotic pressure remained at a relatively stable level after being actively adjusted from 6 to 12 h. At 96 h, the osmotic pressure of the serum isotonic point of juvenile A. schlegelii was approximately equal to that of water with 14.94 salinity. The number and volume of Cl^--secreting cells in the gills decreased. The glomeruli were more developed and structurally sound, with the renal tubules increasing in diameter and the medial brush border being more developed; this may indicate a decrease in salt secretion and an enhanced reabsorption function in the low salinity groups. The activities of superoxide dismutase and catalase and concentration of malondialdehyde were the lowest in the 15 psu group. In addition, the culture conditions of the 15 psu group improved the feed conversion rate without significant differences in weight gain when compared with the control group. Our results show that 15 psu salinity may be the best parameter for obtaining the maximum economic benefits.

Crosstalk between Growth and Osmoregulation of GHRH-SST-GH-IGF Axis in Triploid Rainbow Trout (Oncorhynchus mykiss)

Smolting is an important development stage of salmonid, and an energy trade-off occurs between osmotic regulation and growth during smolting in rainbow trout (Oncorhynchus mykiss). Growth hormone releasing hormone, somatostatin, growth hormone and insulin-like growth factor (GHRH-SST-GH-IGF) axis exhibit pleiotropic effects in regulating growth and osmotic adaptation. Due to salmonid specific genome duplication, increased paralogs are identified in the ghrh-sst-gh-igf axis, however, their physiology in modulating osmoregulation has yet to be investigated. In this study, seven sst genes (sst1a, sst1b, sst2, sst3a, sst3b, sst5, sst6) were identified in trout. We further investigated the ghrh-sst-gh-igf axis of diploid and triploid trout in response to seawater challenge. Kidney sst (sst1b, sst2, sst5) and sstr (sstr1b1, sstr5a, sstr5b) expressions were changed (more than 2-fold increase (except for sstr5a with 1.99-fold increase) or less than 0.5-fold decrease) due to osmoregulation, suggesting a pleiotropic physiology of SSTs in modulating growth and smoltification. Triploid trout showed significantly down-regulated brain sstr1b1 and igfbp2a1 (p < 0.05), while diploid trout showed up-regulated brain igfbp1a1 (~2.61-fold, p = 0.057) and igfbp2a subtypes (~1.38-fold, p < 0.05), suggesting triploid trout exhibited a better acclimation to the seawater environment. The triploid trout showed up-regulated kidney igfbp5a subtypes (~6.62 and 7.25-fold, p = 0.099 and 0.078) and significantly down-regulated igfbp5b2 (~0.37-fold, p < 0.05), showing a conserved physiology of teleost IGFBP5a in regulating osmoregulation. The IGFBP6 subtypes are involved in energy and nutritional regulation. Distinctive igfbp6 subtypes patterns (p < 0.05) potentially indicated trout triggered energy redistribution in brain and kidney during osmoregulatory regulation. In conclusion, we showed that the GHRH-SST-GH-IGF axis exhibited pleiotropic effects in regulating growth and osmoregulatory regulation during trout smolting, which might provide new insights into seawater aquaculture of salmonid species.

Inducible Nitric Oxide Synthase/Nitric Oxide System as a Biomarker for Stress and Ease Response in Fish: Implication on Na+ Homeostasis During Hypoxia

The cellular and organismal response to stressor-driven stimuli evokes stress response in vertebrates including fishes. Fishes have evolved varied patterns of stress response, including ionosmotic stress response, due to their sensitivity to both intrinsic and extrinsic stimuli. Fishes that experience hypoxia, a detrimental stressor that imposes systemic and cellular stress response, can evoke disturbed ion homeostasis. In addition, like other vertebrates, fishes have also developed mechanisms to recover from the impact of stress by way of shifting stress response into ease response that could reduce the magnitude of stress response with the aid of certain neuroendocrine signals. Nitric oxide (NO) has been identified as a potent molecule that attenuates the impact of ionosmotic stress response in fish, particularly during hypoxia stress. Limited information is, however, available on this important aspect of ion transport physiology that contributes to the mechanistic understanding of survival during environmental challenges. The present review, thus, discusses the role of NO in Na⁺ homeostasis in fish particularly in stressed conditions. Isoforms of nitric oxide synthase (NOS) are essential for the synthesis and availability of NO at the cellular level. The NOS/NO system, thus, appears as a unique molecular drive that performs both regulatory and integrative mechanisms of control within and across varied fish ionocytes. The activation of the inducible NOS (iNOS)/NO system during hypoxia stress and its action on the dynamics of Na⁺/K⁺-ATPase, an active Na⁺ transporter in fish ionocytes, reveal that the iNOS/NO system controls cellular and systemic Na⁺ transport in stressed fish. In addition, the higher sensitivity of iNOS to varied physical stressors in fishes and the ability of NO to lower the magnitude of ionosmotic stress in hypoxemic fish clearly put forth NO as an ease-promoting signal molecule in fishes. This further points to the signature role of the iNOS/NO system as a biomarker for stress and ease response in the cycle of adaptive response in fish.

Expression Analysis in Atlantic Salmon Liver Reveals miRNAs Associated with Smoltification and Seawater Adaptation

Simple Summary

Smoltification is a developmental process that preadapts Atlantic salmon for a life in seawater. Suboptimal smoltification and poor timing of transfer to seawater is associated with increased mortality. MicroRNAs (miRNAs) are small non-coding genes. They regulate gene expression post-transcriptionally as part of the miRNA induce silencing complex (miRISC) where they guide miRISC to particular mRNAs (target genes). The aim of this study was to identify Atlantic salmon miRNAs expressed in liver that are associated with smoltification and adaptation to seawater as well as to predict their target genes. In total, 62 guide miRNAs were identified, and by their expression patterns they were clustered into three groups. Target gene predictions followed by gene enrichment analysis of the predicted targets indicated that the guide miRNAs were involved in post-transcriptional regulation of important smoltification associated biological processes. Some of these were energy metabolism, protein metabolism and transport, circadian rhythm, stress and immune response. Together, the results indicate that certain miRNAs are involved in the regulation of many of the important changes occurring in the liver during this developmental transition.

Abstract

Optimal smoltification is crucial for normal development, growth, and health of farmed Atlantic salmon in seawater. Here, we characterize miRNA expression in liver to reveal whether miRNAs regulate gene expression during this developmental transition. Expression changes of miRNAs and mRNAs was studied by small-RNA sequencing and microarray analysis, respectively. This revealed 62 differentially expressed guide miRNAs (gDE-miRNAs) that could be divided into three groups with characteristic dynamic expression patterns. Three of miRNA families are known as highly expressed in liver. A rare arm shift was observed during smoltification in the Atlantic salmon-specific novel-ssa-miR-16. The gDE-miRNAs were predicted to target 2804 of the genes revealing expression changes in the microarray analysis. Enrichment analysis revealed that targets were significantly enriched in smoltification-associated biological process groups. These included lipid and cholesterol synthesis, carbohydrate metabolism, protein metabolism and protein transport, immune system genes, circadian rhythm and stress response. The results indicate that gDE-miRNAs may regulate many of the changes associated with this developmental transition in liver. The results pave the way for validation of the predicted target genes and further study of gDE-miRNA and their targets by functional assays.

The Origination of Growth Hormone/Insulin-Like Growth Factor System: A Story From Ancient Basal Chordate Amphioxus

The growth hormone/insulin-like growth factor (GH/IGF) system, also called the pituitary-liver axis, has a somatotrophic role in the body. Although the GH/IGF system has always been regarded as a vertebrate-specific endocrine system, its actual origin remained unknown for a long time. The basal chordate, amphioxus, occupies an evolutionary position between vertebrates and invertebrates. Impressively, most of the members of the GH/IGF system are present in the amphioxus. The GH-like molecule in the amphioxus is mainly expressed in Hatschek's pit. It functions similarly to vertebrate GH and has a GH receptor-like binding partner. The amphioxus IGF-like peptide shows mitogenic activity and an expression pattern resembling that of vertebrate IGF-I. The receptor of IGF-like peptide and IGF binding protein (IGFBP) have also been demonstrated to exist in the amphioxus. These results reveal the origin of the gene families in the GH/IGF system, providing strong evidence that this system emerged in the amphioxus.

Genome-Wide Analysis of Alternative Splicing (AS) Mechanism Provides Insights into Salinity Adaptation in the Livers of Three Euryhaline Teleosts, including Scophthalmus maximus, Cynoglossus semilaevis and Oncorhynchus mykiss

Simple Summary

Alternative splicing (AS) is a key post-transcriptional regulatory mechanism that acts an important regulator in response to environmental stimuli in organisms. In the present study, 18 RNA-Seq datasets were utilized to investigate the potential roles of AS in response to different salinity environments in the livers of three euryhaline teleosts, including turbot (Scophthalmus maximus), tongue sole (Cynoglossus semilaevis) and steelhead trout (Oncorhynchus mykiss). The results indicated that different salinity environments changed the splicing patterns of numerous RNA splicing regulators, which might affect the splicing decisions of many downstream target genes in response to salinity changes. This study provides preliminary evidence for the important roles of AS events in salinity adaptation in teleosts.

Abstract

Salinity is an important environmental factor that directly affects the survival of aquatic organisms, including fish. However, the underlying molecular mechanism of salinity adaptation at post-transcriptional regulation levels is still poorly understood in fish. In the present study, 18 RNA-Seq datasets were utilized to investigate the potential roles of alternative splicing (AS) in response to different salinity environments in the livers of three euryhaline teleosts, including turbot (Scophthalmus maximus), tongue sole (Cynoglossus semilaevis) and steelhead trout (Oncorhynchus mykiss). A total of 10,826, 10,741 and 10,112 AS events were identified in the livers of the three species. The characteristics of these AS events were systematically investigated. Furthermore, a total of 940, 590 and 553 differentially alternative splicing (DAS) events were determined and characterized in the livers of turbot, tongue sole and steelhead trout, respectively, between low- and high-salinity environments. Functional enrichment analysis indicated that these DAS genes in the livers of three species were commonly enriched in some GO terms and KEGG pathways associated with RNA processing. The most common DAS genes work as RNA-binding proteins and play crucial roles in the regulation of RNA splicing. The study provides new insights into uncovering the molecular mechanisms of salinity adaptation in teleosts.

Neuroendocrine Regulation of Plasma Cortisol Levels During Smoltification and Seawater Acclimation of Atlantic Salmon

Diadromous fishes undergo dramatic changes in osmoregulatory capacity in preparation for migration between freshwater and seawater. One of the primary hormones involved in coordinating these changes is the glucocorticoid hormone, cortisol. In Atlantic salmon (Salmo salar), cortisol levels increase during the spring smoltification period prior to seawater migration; however, the neuroendocrine factors responsible for regulating the hypothalamic-pituitary-interrenal (HPI) axis and plasma cortisol levels during smoltification remain unclear. Therefore, we evaluated seasonal changes in circulating levels of cortisol and its primary secretagogue-adrenocorticotropic hormone (ACTH)-as well as transcript abundance of the major regulators of HPI axis activity in the preoptic area, hypothalamus, and pituitary between migratory smolts and pre-migratory parr. Smolts exhibited higher plasma cortisol levels compared to parr across all timepoints but circulating ACTH levels were only elevated in May. Transcript abundance of preoptic area corticotropin-releasing factor b1 and arginine vasotocin were ~2-fold higher in smolts compared to parr in February through May. Smolts also had ~7-fold greater hypothalamic transcript abundance of urotensin 1 (uts-1a) compared to parr in May through July. When transferred to seawater during peak smolting in May smolts rapidly upregulated hypothalamic uts-1a transcript levels within 24 h, while parr only transiently upregulated uts-1a 96 h post-transfer. In situ hybridization revealed that uts-1a is highly abundant in the lateral tuberal nucleus (NLT) of the hypothalamus, consistent with a role in regulating the HPI axis. Overall, our results highlight the complex, multifactorial regulation of cortisol and provide novel insight into the neuroendocrine mechanisms controlling osmoregulation in teleosts.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Osmoregulatory Plasticity of Juvenile Greater Amberjack (Seriola dumerili) to Environmental Salinity

Osmotic costs in teleosts are highly variable, reaching up to 50% of energy expenditure in some. In several species, environmental salinities close to the isosmotic point (~15 psu) minimize energy demand for osmoregulation while enhancing growth. The present study aimed to characterize the physiological status related to osmoregulation in early juveniles of the greater amberjack, Seriola dumerili, acclimated to three salinities (15, 22, and 36 psu). Our results indicate that plasma metabolic substrates were enhanced at the lower salinities, whereas hepatic carbohydrate and energetic lipid substrates decreased. Moreover, osmoregulatory parameters, such as osmolality, muscle water content, gill and intestine Na+-K+-ATPase activities, suggested a great osmoregulatory capacity in this species. Remarkably, electrophysiological parameters, such as short-circuit current (Isc) and transepithelial electric resistance (TER), were enhanced significantly at the posterior intestine. Concomitantly, Isc and TER anterior-to-posterior intestine differences were intensified with increasing environmental salinity. Furthermore, the expression of several adeno-hypophyseal genes was assessed. Expression of prl showed an inverse linear relationship with increasing environmental salinity, while gh mRNA enhanced significantly in the 22 psu-acclimated groups. Overall, these results could explain the better growth observed in S. dumerili juveniles kept at salinities close to isosmotic rather than in seawater.

Transport and Recovery of Turbot (Scophthalmus maximus) Sedated with MS-222 and Eugenol: Effects on Intermediary Metabolism and Osmoregulation

Simple Summary

The waterless transportation of live fish is considered to be a green and economic solution to achieve a high survival rate. However, the effects of cold domestication and the environment after leaving the water are prone to cause stress in fish. In this experiment, the turbots were anesthetized with 40 mg/L MS-222 and 20 mg/L eugenol solution. Then, turbots were cooled to dormancy, followed by waterless transport for 18 h. The results of blood physiological indices indicated that the MS-222 and eugenol could reduce the stress on turbot during cooling and simulated waterless transport.

Abstract

This study focused on the anesthetic waterless keep-alive transport technique for turbot. MS-222 and eugenol were used to anesthetize turbot and then the waterless keep-alive transport was conducted. The blood physiological changes and flesh quality changes of turbot were evaluated after cooling and during the simulated waterless transport. The results show that the temperature lowered from 13 to 2 °C, resulting in a decrease in moisture, fat and protein contents of all samples. Compared to the control turbot, the turbots treated with MS-222 and eugenol presented higher pH and glycogen content. During the simulated waterless transport, the pH, ATP and glycogen contents in MS-222- and eugenol-treated turbots decreased and the IMP and lactate levels increased. For the blood biochemical indices, blood glucose, cortisol and urea nitrogen increased with the increase in transport time in MS-222- and eugenol-treated turbots. At sampling time, the changes in blood physiological indices were significantly higher in the control samples than those in the MS-222- and eugenol-treated samples. The results indicate that the turbot samples treated with MS-222 or eugenol could reduce stress during cooling and simulated waterless transport.

Effects of constant and diel cyclic temperatures on the liver and intestinal phospholipid fatty acid composition in rainbow trout Oncorhynchus mykiss during seawater acclimation

Background

Rainbow trout is an economically important fish in aquaculture and is a model species in environmental physiology. Despite earlier research on the seawater adaptability of rainbow trout at different temperature regimes, the influence on the liver and intestine in this species is still unknown. Two trials were conducted to investigate the effects of constant and diel cyclic temperatures on phospholipid fatty acid (PLFA) composition in the liver and intestine of rainbow trout during seawater acclimation.

Results

At the end of growth trial 1, fish at 9 and 12.5 °C showed significantly higher ratios of unsaturated to saturated (U/S) and unsaturation index (UI) than those at 16 °C in liver and intestine phospholipids. After day 1 of seawater acclimation, the U/S, UI, and average chain length (ACL) of liver and intestinal phospholipids in fish at 16 °C significantly increased. Two weeks after seawater acclimation, the liver and intestinal PLFA composition adapted to salinity changes. In trial 2, significantly higher U/S, UI, and ACL were found in intestinal phospholipids at 13 ± 2 °C. On the first day after seawater acclimation, UI and ACL in liver phospholipids significantly increased at 13 °C, while fish at 13 ± 2 °C showed significantly decreased U/S, UI, and ACL in the intestine. At the end of growth trial 2, liver PLFA compositions were stable, whereas intestinal PLFA at 13 and 13 ± 1 °C showed significantly decreased U/S, UI, and ACL. A two-way analysis of variance and principal component analysis revealed significant effects of different constant temperatures, seawater acclimation, and their interaction on the liver and intestinal phospholipids, a significant effect of diel cyclic temperature on intestinal phospholipids, and the effects of seawater acclimation and its interaction with diel cyclic temperature on liver phospholipids.

Conclusion

Temperatures of 9 and 12.5 °C could elevate membrane fluidity and thickness in the liver and intestine of rainbow trout in freshwater, whereas no significant effects were found with diel temperature variations. After seawater acclimation, constant and diel cyclic temperatures significantly influenced the membrane fluidity and thickness of the liver and intestine. Compared with constant temperature, diel temperature variation (13 ± 2 °C) can enhance the adaptability of rainbow trout during seawater acclimation.

Environmental Salinity Modifies Mucus Exudation and Energy Use in European Sea Bass Juveniles

The European sea bass (Dicentrarchus labrax) is a euryhaline marine teleost that can often be found in brackish and freshwater or even in hypersaline environments. Here, we exposed sea bass juveniles to sustained salinity challenges for 15 days, simulating one hypoosmotic (3‰), one isosmotic (12‰) and one hyperosmotic (50‰) environment, in addition to control (35‰). We analyzed parameters of skin mucus exudation and mucus biomarkers, as a minimally invasive tool, and plasma biomarkers. Additionally, Na⁺/K⁺-ATPase activity was measured, as well as the gill mucous cell distribution, type and shape. The volume of exuded mucus increased significantly under all the salinity challenges, increasing by 130% at 50‰ condition. Significantly greater amounts of soluble protein (3.9 ± 0.6 mg at 50‰ vs. 1.1 ± 0.2 mg at 35‰, p < 0.05) and lactate (4.0 ± 1.0 µg at 50‰ vs. 1.2 ± 0.3 µg at 35‰, p < 0.05) were released, with clear energy expenditure. Gill ATPase activity was significantly higher at the extreme salinities, and the gill mucous cell distribution was rearranged, with more acid and neutral mucin mucous cells at 50‰. Skin mucus osmolality suggested an osmoregulatory function as an ion-trap layer in hypoosmotic conditions, retaining osmosis-related ions. Overall, when sea bass cope with different salinities, the hyperosmotic condition (50‰) demanded more energy than the extreme hypoosmotic condition.

Immunologic Profiling of the Atlantic Salmon Gill by Single Nuclei Transcriptomics

Anadromous salmonids begin life adapted to the freshwater environments of their natal streams before a developmental transition, known as smoltification, transforms them into marine-adapted fish. In the wild, smoltification is a photoperiod-regulated process, involving radical remodeling of gill function to cope with the profound osmotic and immunological challenges of seawater (SW) migration. While prior work has highlighted the role of specialized "mitochondrion-rich" cells (MRCs) and accessory cells (ACs) in delivering this phenotype, recent RNA profiling experiments suggest that remodeling is far more extensive than previously appreciated. Here, we use single-nuclei RNAseq to characterize the extent of cytological changes in the gill of Atlantic salmon during smoltification and SW transfer. We identify 20 distinct cell clusters, including known, but also novel gill cell types. These data allow us to isolate cluster-specific, smoltification-associated changes in gene expression and to describe how the cellular make-up of the gill changes through smoltification. As expected, we noted an increase in the proportion of seawater mitochondrion-rich cells, however, we also identify previously unknown reduction of several immune-related cell types. Overall, our results provide fresh detail of the cellular complexity in the gill and suggest that smoltification triggers unexpected immune reprogramming.

Effects of different temperatures on seawater acclimation in rainbow trout Oncorhynchus mykiss: osmoregulation and branchial phospholipid fatty acid composition

This study aimed to investigate the effects of different temperatures on seawater acclimation in rainbow trout (Oncorhynchus mykiss), in terms of growth performance, osmoregulatory capacity, and branchial phospholipid fatty acid (PLFA) composition. The fish (initial weight, 94.73 g) were reared at 9, 12.5, and 16 °C for 28 days, then acclimated to seawater, and subsequently reared for 14 days. Sampling points were on the last day in freshwater, and the 1st, 4th, 7th, and 14th days after the salinity reached 30. The results showed the final weight, percent weight gain, and specific growth rate of rainbow trout at 12.5 °C were significantly higher than those at 9 °C, while the thermal growth coefficient at 16 °C was significantly lower than that in other treatments. The branchial PLFA composition in rainbow trout changed more rapidly at 9 and 12.5 °C than at 16 °C. The branchial PLFA composition was significantly affected by temperature and salinity and their interaction. The polyunsaturated fatty acid content of phospholipids in the gill at 9 and 12.5 °C was significantly higher than those at 16 °C. Low temperature (9 °C) and seawater acclimation significantly increased the degree of unsaturation of membrane, enhancing membrane fluidity, which is related to Na⁺-K⁺ ATPase activity. Responses of plasma ion, Na⁺-K⁺ ATPase activity, and plasma glucose followed a similar pattern at different temperatures. Overall, the study suggests that 12.5 °C is the ideal temperature for seawater acclimation in rainbow trout.

Pipefish Locally Adapted to Low Salinity in the Baltic Sea Retain Phenotypic Plasticity to Cope With Ancestral Salinity Levels

Genetic adaptation and phenotypic plasticity facilitate the migration into new habitats and enable organisms to cope with a rapidly changing environment. In contrast to genetic adaptation that spans multiple generations as an evolutionary process, phenotypic plasticity allows acclimation within the life-time of an organism. Genetic adaptation and phenotypic plasticity are usually studied in isolation, however, only by including their interactive impact, we can understand acclimation and adaptation in nature. We aimed to explore the contribution of adaptation and plasticity in coping with an abiotic (salinity) and a biotic (Vibriobacteria) stressor using six different populations of the broad-nosed pipefishSyngnathus typhlethat originated from either high [14–17 Practical Salinity Unit (PSU)] or low (7–11 PSU) saline environments along the German coastline of the Baltic Sea. We exposed wild caught animals, to either high (15 PSU) or low (7 PSU) salinity, representing native and novel salinity conditions and allowed animals to mate. After male pregnancy, offspring was split and each half was exposed to one of the two salinities and infected withVibrio alginolyticusbacteria that were evolved at either of the two salinities in a fully reciprocal design. We investigated life-history traits of fathers and expression of 47 target genes in mothers and offspring. Pregnant males originating from high salinity exposed to low salinity were highly susceptible to opportunistic fungi infections resulting in decreased offspring size and number. In contrast, no signs of fungal infection were identified in fathers originating from low saline conditions suggesting that genetic adaptation has the potential to overcome the challenges encountered at low salinity. Offspring from parents with low saline origin survived better at low salinity suggesting genetic adaptation to low salinity. In addition, gene expression analyses of juveniles indicated patterns of local adaptation,trans-generational plasticity and developmental plasticity. In conclusion, our study suggests that pipefish are locally adapted to the low salinity in their environment, however, they are retaining phenotypic plasticity, which allows them to also cope with ancestral salinity levels and prevailing pathogens.

Diet, Immunity, and Microbiota Interactions: An Integrative Analysis of the Intestine Transcriptional Response and Microbiota Modulation in Gilthead Seabream (Sparus aurata) Fed an Essential Oils-Based Functional Diet

Essential oils (EOs) are promising alternatives to chemotherapeutics in animal production due to their immunostimulant, antimicrobial, and antioxidant properties, without associated environmental or hazardous side effects. In the present study, the modulation of the transcriptional immune response (microarray analysis) and microbiota [16S Ribosomal RNA (rRNA) sequencing] in the intestine of the euryhaline fish gilthead seabream (Sparus aurata) fed a dietary supplementation of garlic, carvacrol, and thymol EOs was evaluated. The transcriptomic functional analysis showed the regulation of genes related to processes of proteolysis and inflammatory modulation, immunity, transport and secretion, response to cyclic compounds, symbiosis, and RNA metabolism in fish fed the EOs-supplemented diet. Particularly, the activation of leukocytes, such as acidophilic granulocytes, was suggested to be the primary actors of the innate immune response promoted by the tested functional feed additive in the gut. Fish growth performance and gut microbiota alpha diversity indices were not affected, while dietary EOs promoted alterations in bacterial abundances in terms of phylum, class, and genus. Subtle, but significant alterations in microbiota composition, such as the decrease in Bacteroidia and Clostridia classes, were suggested to participate in the modulation of the intestine transcriptional immune profile observed in fish fed the EOs diet. Moreover, regarding microbiota functionality, increased bacterial sequences associated with glutathione and lipid metabolisms, among others, detected in fish fed the EOs supported the metabolic alterations suggested to potentially affect the observed immune-related transcriptional response. The overall results indicated that the tested dietary EOs may promote intestinal local immunity through the impact of the EOs on the host-microbial co-metabolism and consequent regulation of significant biological processes, evidencing the crosstalk between gut and microbiota in the inflammatory regulation upon administration of immunostimulant feed additives.

Effects of a temperature rise on melatonin and thyroid hormones during smoltification of Atlantic salmon, Salmo salar

Smoltification prepares juvenile Atlantic salmon (Salmo salar) for downstream migration. Dramatic changes characterize this crucial event in the salmon's life cycle, including increased gill Na⁺/K⁺-ATPase activity (NKA) and plasma hormone levels. The triggering of smoltification relies on photoperiod and is modulated by temperature. Both provide reliable information, to which fish have adapted for thousands of years, that allows deciphering daily and calendar time. Here we studied the impact of different photoperiod (natural, sustained winter solstice) and temperature (natural, ~ + 4° C) combinations, on gill NKA, plasma free triiodothyronine (T3) and thyroxine (T4), and melatonin (MEL; the time-keeping hormone), throughout smoltification. We also studied the impact of temperature history on pineal gland MEL production in vitro. The spring increase in gill NKA was less pronounced in smolts kept under sustained winter photoperiod and/or elevated temperature. Plasma thyroid hormone levels displayed day-night variations, which were affected by elevated temperature, either independently from photoperiod (decrease in T3 levels) or under natural photoperiod exclusively (increase in T4 nocturnal levels). Nocturnal MEL secretion was potentiated by the elevated temperature, which also altered the MEL profile under sustained winter photoperiod. Temperature also affected pineal MEL production in vitro, a response that depended on previous environmental acclimation of the organ. The results support the view that the salmon pineal is a photoperiod and temperature sensor, highlight the complexity of the interaction of these environmental factors on the endocrine system of S. salar, and indicate that climate change might compromise salmon's time "deciphering" during smoltification, downstream migration and seawater residence.

Transcriptomic Analysis of Gill and Kidney from Asian Seabass (Lates calcarifer) Acclimated to Different Salinities Reveals Pathways Involved with Euryhalinity

Asian seabass (or commonly known as barramundi), Lates calcarifer, is a bony euryhaline teleost from the Family Latidae, inhabiting nearshore, estuarine, and marine connected freshwaters throughout the tropical Indo-West Pacific region. The species is catadromous, whereby adults spawn in salinities between 28 and 34 ppt at the mouth of estuaries, with resultant juveniles usually moving into brackish and freshwater systems to mature, before returning to the sea to spawn again as adults. The species lives in both marine and freshwater habitats and can move quickly between the two; thus, the species' ability to tolerate changes in salinity makes it a good candidate for studying the salinity acclimation response in teleosts. In this study, the transcriptome of two major osmoregulatory organs (gills and kidneys) of young juvenile Asian seabass reared in freshwater and seawater were compared. The euryhaline nature of Asian seabass was found to be highly pliable and the moldability of the trait was further confirmed by histological analyses of gills and kidneys. Differences in major expression pathways were observed, with differentially expressed genes including those related to osmoregulation, tissue/organ morphogenesis, and cell volume regulation as central to the osmo-adaptive response. Additionally, genes coding for mucins were upregulated specifically under saline conditions, whereas several genes important for growth and development, as well as circadian entrainment were specifically enriched in fish reared in freshwater. Routing of the circadian rhythm mediated by salinity changes could be the initial step in salinity acclimation and possibly migration in euryhaline fish species such as the Asian seabass.

The Role of the Thyroid Axis in Fish

In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.

Changes in PRL Gene Expression During Upstream Movement of the Japanese Eel, Anguilla japonica

Changes in mRNA levels of prolactin (PRL) during upstream movement were examined in juvenile Japanese eels, Anguilla japonica. Glass eels and elvers were collected from 2007 to 2009 near the entrance of Hamana Lake, and in a small inflowing stream, the Egawa River. Quantification of mRNA was performed by real-time PCR and expressed as whole-body content. PRL mRNA levels of glass eels caught in the coastal zone and tidal area were low. Eels that moved downward in the tidal zone and migrated upstream to enter into freshwater showed increased levels of PRL mRNA. These changes suggest the importance of up-regulation of PRL gene expression in juvenile eels during their upstream movement from seawater to fresh water, particularly in relation to hyperosmoregulation.

Salmonid gene expression biomarkers indicative of physiological responses to changes in salinity and temperature, but not dissolved oxygen

An organism's ability to respond effectively to environmental change is critical to its survival. Yet, life stage and overall condition can dictate tolerance thresholds to heightened environmental stressors, such that stress may not be equally felt across individuals and at all times. Also, the transcriptional responses induced by environmental changes can reflect both generalized responses as well as others that are highly specific to the type of change being experienced. Thus, if transcriptional biomarkers specific to a stressor, even under multi-stressor conditions, can be identified, the biomarkers could then be applied in natural environments to determine when and where an individual experiences such a stressor. Here, we experimentally challenged juvenile Chinook salmon (Oncorhynchus tshawytscha) to validate candidate gill gene expression biomarkers. A sophisticated experimental design manipulated salinity (freshwater, brackish water and seawater), temperature (10, 14 and 18°C) and dissolved oxygen (normoxia and hypoxia) in all 18 possible combinations for 6 days using separate trials for three smolt statuses (pre-smolt, smolt and de-smolt). In addition, changes in juvenile behaviour, plasma variables, gill Na+/K+-ATPase activity, body size, body morphology and skin pigmentation supplemented the gene expression responses. We identified biomarkers specific to salinity and temperature that transcended the multiple stressors, smolt status and mortality (live, dead and moribund). Similar biomarkers for dissolved oxygen were not identified. This work demonstrates the unique power of gene expression biomarkers to identify a specific stressor even under multi-stressor conditions, and we discuss our next steps for hypoxia biomarkers using an RNA-seq study.

Differential Expression and Localization of Branchial AQP1 and AQP3 in Japanese Medaka (Oryzias latipes)

Aquaporins (AQPs) facilitate transmembrane water and solute transport, and in addition to contributing to transepithelial water transport, they safeguard cell volume homeostasis. This study examined the expression and localization of AQP1 and AQP3 in the gills of Japanese medaka (Oryzias latipes) in response to osmotic challenges and osmoregulatory hormones, cortisol, and prolactin (PRL). AQP3 mRNA was inversely regulated in response to salinity with high levels in ion-poor water (IPW), intermediate levels in freshwater (FW), and low levels in seawater (SW). AQP3 protein levels decreased upon SW acclimation. By comparison, AQP1 expression was unaffected by salinity. In ex vivo gill incubation experiments, AQP3 mRNA was stimulated by PRL in a time- and dose-dependent manner but was unaffected by cortisol. In contrast, AQP1 was unaffected by both PRL and cortisol. Confocal microscopy revealed that AQP3 was abundant in the periphery of gill filament epithelial cells and co-localized at low intensity with Na⁺,K⁺-ATPase in ionocytes. AQP1 was present at a very low intensity in most filament epithelial cells and red blood cells. No epithelial cells in the gill lamellae showed immunoreactivity to AQP3 or AQP1. We suggest that both AQPs contribute to cellular volume regulation in the gill epithelium and that AQP3 is particularly important under hypo-osmotic conditions, while expression of AQP1 is constitutive.

Evidence for a role of arginine vasotocin receptors in the gill during salinity acclimation by a euryhaline teleost fish

The nonapeptide arginine vasotocin (AVT) regulates osmotic balance in teleost fishes, but its mechanisms of action are not fully understood. Recently, it was discovered that nonapeptide receptors in teleost fishes are differentiated into two V1a-type, several V2-type, and two isotocin (IT) receptors, but it remains unclear which receptors mediate AVT's effects on gill osmoregulation. Here, we examined the role of nonapeptide receptors in the gill of the euryhaline Amargosa pupfish (Cyprinodon nevadensis amargosae) during osmotic acclimation. Transcripts for the teleost V1a-type receptor v1a2 were upregulated over fourfold in gill 24 h after transferring pupfish from 7.5 ppt to seawater (35 ppt) or hypersaline (55 ppt) conditions and downregulated after transfer to freshwater (0.3 ppt). Gill transcripts for the nonapeptide degradation enzyme leucyl-cystinyl aminopeptidase (LNPEP) also increased in fish acclimating to 35 ppt. To test whether the effects of AVT on the gill might be mediated by a V1a-type receptor, we administered AVT or a V1-type receptor antagonist (Manning compound) intraperitoneally to pupfish before transfer to 0.4 ppt or 35 ppt. Pupfish transferred to 35 ppt exhibited elevated gill mRNA abundance for cystic fibrosis transmembrane conductance regulator (cftr), but that upregulation diminished under V1-receptor inhibition. AVT inhibited the increase in gill Na⁺/Cl^- cotransporter 2 (ncc2) transcript abundance that occurs following transfer to hypoosmotic environments, whereas V1-type receptor antagonism increased ncc2 mRNAs even without a change in salinity. These findings indicate that AVT acts via a V1-type receptor to regulate gill Cl^- transport by inhibiting Cl^- uptake and facilitating Cl^- secretion during seawater acclimation.

Haematological, blood biochemical and immunological responses to gradual acclimation to low-salinity water in Walton’s mudskipper Periophthalmus waltoni Koumans, 1941 (Perciformes: Gobiidae)

The present study investigates and reports the effects of gradual acclimation to low salinity water on some haematological, biochemical and immunological responses in Walton’s mudskipper, Periophthalmus waltoni. For this purpose, mudskippers caught from Persian Gulf coastal area (Bandar Khamir, Hormozgan Province, Iran) were maintained in laboratory aquaria with half seawater (50% SW, 17 ppt) and fed daily with frozen blood worms (Chironomus spp.) for one month prior to the start of experiments. After acclimation, groups of 18 individuals were either directly transferred to 50% SW (control), or acclimated to low salinity water during two sub-periods. In the first sub-period, fish were exposed to low salinity water namely to a gradual water salinity decrease of 1 ppt per day (during 17 days) until the final salinity of 0.4 ppt was reached. Afterwards, fish continued to maintain in this point of salinity (0.4 ppt), for further 15 days until day 32 (second sub-period). Fish were sampled on day 0, 17 and 32. Statistical analysis showed a significant influence of reduced salinity on erythrocytes, haemoglobin, haematocrit, leukocytes, lymphocytes, neutrophils, monocytes and on all biochemical and immunological parameters tested on day 17. However, these indices returned to the control level on day 32. Based on results, the extremely euryhaline p. waltoni can be acclimated to freshwater medium without showing any health disturbance if a gradual decrease in salinity is carried out for a long period of time.

Molecular mechanisms for intestinal HCO3− secretion and its regulation by guanylin in seawater-acclimated eels

The intestine of marine teleosts secretes HCO3− into the lumen and precipitates Ca2+ and Mg2+ in the imbibed seawater as carbonates to decrease luminal fluid osmolality and facilitate water absorption. However, hormonal regulation of HCO3−secretion is largely unknown. Here, mucosally-added guanylin (GN) increased HCO3− secretion, measured by pH-stat, across isolated seawater-acclimated eel intestine bathed in saline at pH 7.4 (5% CO2). The effect of GN on HCO3− secretion was slower than that on the short-circuit current, and the time-course of the GN effect was similar to that of bumetanide. Mucosal bumetanide and serosal 4,4’-dinitrostilbene-2,2’-disulfonic acid (DNDS) inhibited the GN effect, suggesting an involvement of apical Na+-K+-2Cl− cotransporter (NKCC2) and basolateral Cl−/HCO3− exchanger (AE)/Na+-HCO3− cotransporter (NBC) in the GN effect. As mucosal DNDS failed to inhibit the GN effect, apical DNDS-sensitive AE may not be involved. To identify molecular species of transporters involved in the GN effect, we performed RNA-seq analyses followed by quantitative real-time PCR after transfer of eels to seawater. Among the genes upregulated after seawater transfer, AE genes, draa, b, and pat1a, c, on the apical membrane, and NBC genes, nbce1a, n1, n2a, and a AE gene, sat-1, on the basolateral membrane were candidates involved in HCO3− secretion. Judging from the slow effect of GN, we suggest that GN inhibits NKCC2b on the apical membrane and decreases cytosolic Cl− and Na+, which then activates apical DNDS-insensitive DRAs and basolateral DNDS-sensitive NBCs to enhance transcellular HCO3− flux across the intestinal epithelia of seawater-acclimated eels.

Narrowing the Range of Environmental Salinities Where Juvenile Meagre (Argyrosomus regius) Can Be Cultured Based on an Osmoregulatory Pilot Study

Aquaculture in Europe aims to diversify and optimize fish farming. The meagre (Argyrosomus regius) arose as a promising species due to its fast growth rates and flesh quality. Thus, it is currently being produced in several Mediterranean countries, mainly in sea-cages and salt-marshes. However, although meagre naturally spend the first years of life in brackish waters, to date it is cultured in seawater. Here, we show that juveniles may not successfully face either freshwater or hyper-osmotic environments as high as 55 ppt salinity. We found that 55 ppt induced catabolism and mobilization of energy metabolites stored in the liver, probably to maintain its osmotic balance. Furthermore, we found that osmoregulatory tissues such as gills managed to maintain plasma osmolality levels without differences in meagre acclimated at 5, 12 and 39 ppt salinity. Our results demonstrate the euryhaline capacity of this species, highlighting that juveniles may be cultured in a wider range of salinities rather than just at seawater. Future studies should focus on optimal environmental salinities for the growth of A. regius juveniles, including long-term experiments limited to the range of 5 ppt to full-strength seawater. Minimizing fish energy consumption in osmoregulation could be economically beneficial for the aquaculture industry in Europe.

The Amphibious Mudskipper: A Unique Model Bridging the Gap of Central Actions of Osmoregulatory Hormones Between Terrestrial and Aquatic Vertebrates

Body fluid regulation, or osmoregulation, continues to be a major topic in comparative physiology, and teleost fishes have been the subject of intensive research. Great progress has been made in understanding the osmoregulatory mechanisms including drinking behavior in teleosts and mammals. Mudskipper gobies can bridge the gap from aquatic to terrestrial habitats by their amphibious behavior, but the studies are yet emerging. In this review, we introduce this unique teleost as a model to study osmoregulatory behaviors, particularly amphibious behaviors regulated by the central action of hormones. Regarding drinking behavior of mammals, a thirst sensation is aroused by angiotensin II (Ang II) through direct actions on the forebrain circumventricular structures, which predominantly motivates them to search for water and take it into the mouth for drinking. By contrast, aquatic teleosts can drink water that is constantly present in their mouth only by reflex swallowing, and Ang II induces swallowing by acting on the hindbrain circumventricular organ without inducing thirst. In mudskippers, however, through the loss of buccal water by swallowing, which appears to induce buccal drying on land, Ang II motivates these fishes to move to water for drinking. Thus, mudskippers revealed a unique thirst regulation by sensory detection in the buccal cavity. In addition, the neurohypophysial hormones, isotocin (IT) and vasotocin (VT), promote migration to water via IT receptors in mudskippers. VT is also dipsogenic and the neurons in the forebrain may mediate their thirst. VT regulates social behaviors as well as osmoregulation. The VT-induced migration appears to be a submissive response of subordinate mudskippers to escape from competitive and dehydrating land. Together with implications of VT in aggression, mudskippers may bridge the multiple functions of neurohypophysial hormones. Interestingly, cortisol, an important hormone for seawater adaptation and stress response in teleosts, also stimulates the migration toward water, mediated possibly via the mineralocorticoid receptor. The corticosteroid system that is responsive to external stressors can accelerate emergence of migration to alternative habitats. In this review, we suggest this unique teleost as an important model to deepen insights into the behavioral roles of these hormones in relation to osmoregulation.

Serum cortisol level and survival rate of juvenile Epinephelus fuscoguttatus following exposure to different salinities

Background and Aim:

Brown-marbled grouper Epinephelus fuscoguttatus is a premium marine food fish with high demand in Asia. In fish, stress due to environmental changes such as fluctuations in the salinity can result in increased cortisol level. Stress in fish increases susceptibility to diseases ultimately resulting in death. Therefore, the aim of this study was to investigate the salinity tolerance of E. fuscoguttatus and their survival in lower salinities.

Materials and Methods:

In this study, grouper juveniles (92.43±standard error of the mean 0.51 mm) maintained in 31 ppt seawater were transferred into five tanks with seawater diluted to 25, 20, 15, 10, and 5 ppt. The salinity of the control group was not changed and was maintained at 31 ppt. Serum cortisol was measured using ELISA at 0, 30, 60, and 120 min after the fish were transferred to the different concentrations of salinity.

Results:

The survival percentage was recorded for 14 days following the transfer and the results revealed that serum cortisol of fish in a high change in salinity (15, 10, and 5 ppt) was significantly higher than the control group immediately after exposure. At the high salinity change, the cortisol levels gradually decrease at 30 min and 60 min, until no difference in cortisol concentration was observed at 120 min. No mortality was observed in fish exposed to low salinity change (25 and 20 ppt) while in higher salinity change (5 ppt), the survival percentage was 50%.

Conclusion:

The study revealed that the serum cortisol concentration was high initially and continues to decrease to resting cortisol level at 120 min indicating that cortisol hormone is released following acute stress as a primary response in grouper juveniles.

claudin-10 isoform expression and cation selectivity change with salinity in salt-secreting epithelia of Fundulusheteroclitus

To provide insight into claudin (Cldn) tight junction (TJ) protein contributions to branchial salt secretion in marine teleost fishes, this study examined cldn-10 TJ protein isoforms of a euryhaline teleost (mummichog; Fundulus heteroclitus) in association with salinity change and measurements of transepithelial cation selectivity. Mummichogs were transferred from freshwater (FW) to seawater (SW, 35‰) and from SW to hypersaline SW (2SW, 60‰) in a time course with transfer control groups (FW to FW, and SW to SW). FW to SW transfer increased mRNA abundance of cldn-10d and cldn-10e twofold, whilst cldn-10c and cldn-10f transcripts were unchanged. Transfer from SW to 2SW did not alter cldn-10d, and transiently altered cldn-10e abundance, but increased cldn-10c and cldn-10f fourfold. This was coincident with an increased number of single-stranded junctions (observed by transmission electron microscopy). For both salinity transfers, (1) cldn-10e mRNA was acutely responsive (i.e. after 24 h), (2) other responsive cldn-10 isoforms increased later (3-7 days), and (3) cystic fibrosis transmembrane conductance regulator (cftr) mRNA was elevated in accordance with established changes in transcellular Cl^- movement. Changes in mRNA encoding cldn-10c and -10f appeared linked, consistent with the tandem repeat locus in the Fundulus genome, whereas mRNA for tandem cldn-10d and cldn-10e seemed independent of each other. Cation selectivity sequence measured by voltage and conductance responses to artificial SW revealed Eisenman sequence VII: Na⁺>K⁺>Rb⁺∼Cs⁺>Li⁺ Collectively, these data support the idea that Cldn-10 TJ proteins create and maintain cation-selective pore junctions in salt-secreting tissues of teleost fishes.

Reduced thermal tolerance during salinity acclimation in brook trout (Salvelinus fontinalis) can be rescued by prior treatment with cortisol

The aims of this study were to assess whether thermal tolerance of brook trout (Salvelinus fontinalis) is affected during seawater (SW) acclimation and to investigate the role of cortisol in osmoregulation and thermal tolerance during SW acclimation. Freshwater (FW)-acclimated brook trout at 18 °C (Tacc) were exposed to SW for 16 d, whilst maintaining a FW control. Fish were examined for critical thermal maximum (CTmax) 0 (before), 2, 5, and 16 d after SW exposure, and sampled at Tacc and CTmax for analysis of plasma cortisol, glucose, and Cl−, gill Na+/K+-ATPase (NKA) activity and heat shock protein 70 (HSP70) abundance, and white muscle water content. At 2 d in SW, CTmax was significantly reduced (from 31 to 26 °C), then recovered by 16 d. This transient decrease in thermal tolerance coincided with a transient increase in plasma Cl− and decrease in muscle moisture content. Salinity itself had no effect on gill HSP70 abundance compared to the large and immediate effects of high temperature exposure during CTmax testing. To examine the role of cortisol in osmoregulation, brook trout were administered a cortisol implant (5 and 25 µg/g CORT) prior to SW exposure. Both CORT doses significantly increased their capacity to maintain plasma Cl− during SW acclimation. Treatment with the 25 µg/g CORT dose was shown to significantly improve CTmax after 2 d in SW, and CTmax was associated with plasma Cl−. These findings indicate that brook trout are sensitive to temperature during SW acclimation and that thermal tolerance is associated with ion and water balance during SW acclimation.

An Adipokinetic Hormone Acts as a Volume Regulator in the Intertidal Gastropod Mollusk, Aplysia californica

Adipokinetic hormone (AKH) is a multifunctional neuropeptide in the gonadotropin-releasing hormone superfamily. In insects, AKH acts to mobilize energy stores during times of high energetic demand, but has been shown to have other effects. In lophotrochozoans, the presence and function of AKH are less characterized. We have previously identified an AKH in an intertidal gastropod mollusk, the California sea hare (Aplysia californica), and named it ac-AKH. Our previous data showed ac-AKH induced an acute weight loss, suggesting a role in volume regulation. The overarching goals of this study were to test the role of ac-AKH as a volume regulator and examine the mechanism by which ac-AKH induced the acute weight loss. Our results showed that ac-AKH reduced body mass, in part, through the reduction of hemolymph volume without altering hemolymph osmolality or specific osmolytes. The effect of ac-AKH on volume loss was accentuated under a hyposaline condition. We further showed that ac-akh expression was inhibited during a hyposaline challenge, and that the administration of ac-AKH partially reversed the increase in body mass, but not hemolymph osmolality change, caused by the hyposaline challenge. These data collectively show that ac-AKH is a proximate regulator controlling the fluid volume, but not osmolality, in A. californica. Importantly, our results highlight the functional divergence of this structurally conserved neuropeptide in the molluscan lineage.

Stress, Glucocorticoids and Bone: A Review From Mammals and Fish

Glucocorticoids (GCs) are the final effector products of a neuroendocrine HPA/HPI axis governing energy balance and stress response in vertebrates. From a physiological point of view, basal GC levels are essential for intermediary metabolism and participate in the development and homeostasis of a wide range of body tissues, including the skeleton. Numerous mammalian studies have demonstrated that GC hormones exert a positive role during bone modeling and remodeling as they promote osteoblastogenesis to maintain the bone architecture. Although the pharmacological effect of the so-called stress hormones has been widely reported, the role of endogenous GCs on bone mineral metabolism as result of the endocrine stress response has been largely overlooked across vertebrates. In addition, stress responses are variable depending on the stressor (e.g., starvation, predation, and environmental change), life cycle events (e.g., migration and aging), and differ among vertebrate lineages, which react differently according to their biological, social and cognitive complexity (e.g., mineral demands, physical, and psychological stress). This review intends to summarize the endogenous GCs action on bone metabolism of mammals and fish under a variety of challenging circumstances. Particular emphasis will be given to the regulatory loop between GCs and the parathyroid hormone (PTH) family peptides, and other key regulators of mineral homeostasis and bone remodeling in vertebrates.

clc-2c is regulated by salinity, prolactin and extracellular osmolality in tilapia gill

Teleosts inhabiting fresh water (FW) depend upon ion-absorptive ionocytes to counteract diffusive ion losses to the external environment. A Clc Cl^- channel family member, Clc-2c, was identified as a conduit for basolateral Cl^- transport by Na⁺/Cl^- cotransporter 2 (Ncc2)-expressing ionocytes in stenohaline zebrafish (Danio rerio). It is unresolved whether Clc-2c/clc-2c is expressed in euryhaline species and how extrinsic and/or intrinsic factors modulate branchial clc-2c mRNA. Here, we investigated whether environmental salinity, prolactin (Prl) and osmotic conditions modulate clc-2c expression in euryhaline Mozambique tilapia (Oreochromis mossambicus). Branchial clc-2c and ncc2 mRNAs were enhanced in tilapia transferred from seawater (SW) to FW, whereas both mRNAs were attenuated upon transfer from FW to SW. Next, we injected hypophysectomized tilapia with ovine prolactin (oPrl) and observed a marked increase in clc-2c from saline-injected controls. To determine whether Prl regulates clc-2c in a gill-autonomous fashion, we incubated gill filaments in the presence of homologous tilapia Prls (tPrl₁₇₇ and tPrl₁₈₈). By 24 h, tPrl₁₈₈ stimulated clc-2c expression ~5-fold from controls. Finally, filaments incubated in media ranging from 280 to 450 mosmol/kg for 3 and 6 h revealed that extracellular osmolality exerts a local effect on clc-2c expression; clc-2c was diminished by hyperosmotic conditions (450 mosmol/kg) compared with isosmotic controls (330 mosmol/kg). Our collective results suggest that hormonal and osmotic control of branchial clc-2c contributes to the FW adaptability of Mozambique tilapia. Moreover, we identify for the first time a regulatory link between Prl and a Clc Cl^- channel in a vertebrate.

Nitric oxide inhibition of NaCl secretion in the opercular epithelium of seawater-acclimated killifish, Fundulus heteroclitus

Nitric oxide (NO) modulates epithelial ion transport pathways in mammals, but this remains largely unexamined in fish. We explored the involvement of NO in controlling NaCl secretion by the opercular epithelium of seawater killifish using an Ussing chamber approach. Pharmacological agents were used to explore the mechanism(s) triggering NO action. A modified Biotin-switch technique was used to investigate S-nitrosation of proteins. Stimulation of endogenous NO production via the nitric oxide synthase (NOS) substrate l-arginine (2.0 mmol l^-1), and addition of exogenous NO via the NO donor SNAP (10^-6 to 10^-4 mol l^-1), decreased the epithelial short-circuit current (I_sc). Inhibition of endogenous NO production by the NOS inhibitor l-NAME (10^-4 mol l^-1) increased I_sc and revealed a tonic control of ion transport by NO in unstimulated opercular epithelia. The NO scavenger PTIO (10^-5 mol l^-1) supressed the NO-mediated decrease in I_sc, and confirmed that the effect observed was elicited by release of NO. The effect of SNAP on I_sc was abolished by inhibitors of the soluble guanylyl cyclase (sGC), ODQ (10^-6 mol l^-1) and Methylene Blue (10^-4 mol l^-1), revealing NO signalling via the sGC/cGMP pathway. Incubation of opercular epithelium and gill tissues with SNAP (10^-4 mol l^-1) led to S-nitrosation of proteins, including Na⁺/K⁺-ATPase. Blocking of NOS with l-NAME (10^-6 mol l^-1) or scavenging of NO with PTIO during hypotonic shock suggested an involvement of NO in the hypotonic-mediated decrease in I_sc Yohimbine (10^-4 mol l^-1), an inhibitor of α₂-adrenoceptors, did not block NO effects, suggesting that NO is not involved in the α-adrenergic control of NaCl secretion.

Hormonal regulation of aquaporin 3: opposing actions of prolactin and cortisol in tilapia gill

Aquaporins (Aqps) are expressed within key osmoregulatory tissues where they mediate the movement of water and selected solutes across cell membranes. We leveraged the functional plasticity of Mozambique tilapia (Oreochromis mossambicus) gill epithelium to examine how Aqp3, an aquaglyceroporin, is regulated in response to osmoregulatory demands. Particular attention was paid to the actions of critical osmoregulatory hormones, namely, prolactin (Prl), growth hormone and cortisol. Branchial aqp3 mRNA levels were modulated following changes in environmental salinity, with enhanced aqp3 mRNA expression upon transfer from seawater to freshwater (FW). Accordingly, extensive Aqp3 immunoreactivity was localized to cell membranes of branchial epithelium in FW-acclimated animals. Upon transferring hypophysectomized tilapia to FW, we identified that a pituitary factor(s) is required for Aqp3 expression in FW. Replacement with ovine Prl (oPrl) was sufficient to stimulate Aqp3 expression in hypophysectomized animals held in FW, an effect blocked by coinjection with cortisol. Both oPrl and native tilapia Prls (tPrl177 and tPrl188) stimulated aqp3 in incubated gill filaments in a concentration-related manner. Consistent with in vivo responses, coincubation with cortisol blocked oPrl-stimulated aqp3 expression in vitro Our data indicate that Prl and cortisol act directly upon branchial epithelium to regulate Aqp3 in tilapia. Thus, within the context of the diverse actions of Prl on hydromineral balance in vertebrates, we define a new role for Prl as a regulator of Aqp expression.

Effect of the liquorice root derivatives on salt and water balance in a teleost fish, rainbow trout (Oncorhynchus mykiss)

The effect of liquorice root derivatives (LRDs) glycyrrhizic acid (GL) and glycyrrhetinic acid (18βGA) on salt and water balance and end points of gill ion transport in a freshwater teleost, (rainbow trout) was examined after feeding fish diets containing GL or 18βGA (0, 5, 50 or 500 µg/g diet) for a two week period. Serum cortisol levels and gill 11β-hydroxysteroid dehydrogenase type 2 mRNA abundance decreased in fish fed GL but increased (at select doses) in fish fed 18βGA. At higher doses of GL, gill Na(+)-K(+)-ATPase and H(+)-ATPase activity increased, while cystic fibrosis transmembrane conductance regulator type II mRNA abundance significantly decreased at the lowest dose of GL. End points of gill transcellular ion transport were not significantly altered in fish fed 18βGA, except for a reduction in Na(+)-K(+)-ATPase activity at a 50 µg/g dose. In contrast, high doses of GL and 18βGA increased gill transcript abundance of the tight junction protein claudin-31 (cldn-31). Other end points of gill paracellular transport differed in fishes fed LRDs. Tricellulin mRNA abundance was increased by high dose GL and decreased by high dose 18βGA, and cldn-23a and cldn-27b mRNA abundance significantly decreased in response to GL irrespective of dose. Despite the above observations, systemic end points of salt and water balance (i.e. serum [Na(+)] and [Cl(-)] as well as muscle moisture) were unaffected by LRDs. Therefore data suggest that LRDs can alter end points of ion transport in fishes but that overall salt and water balance need not be perturbed.

Role of glucocorticoid in developmental programming: evidence from zebrafish

The vertebrate corticosteroid stress response is highly conserved and a key function is to restore homeostasis by mobilizing and reallocating energy stores. This process is primarily initiated by activation of the hypothalamus-pituitary-adrenal axis, leading to the release of corticosteroids into the circulation. In teleosts, cortisol is the primary corticosteroid that is released into the circulation in response to stress. This steroid activates corticosteroid receptors that are ligand-bound transcription factors, modulating downstream gene expression in target tissues. Recent research in zebrafish (Danio rerio) has identified novel roles for cortisol in early developmental processes, including organogenesis and mesoderm formation. As cortisol biosynthesis commences only around the time of hatch in teleosts, the early developmental events are orchestrated by cortisol that is maternally deposited prior to fertilization. This review will highlight the molecular events leading to the development of the corticosteroid stress axis, and the possible role of cortisol in the developmental programming of stress axis function. Use of zebrafish as a model may lead to significant insights into the conserved role of glucocorticoids during early development with potential implications in biomedical research, including fetal stress syndromes in humans.

Effect of nutritional status and sampling intensity on recovery after dorsal aorta cannulation in free-swimming Atlantic salmon (Salmo salar L.)

Recovery from implantation of a cannula in the dorsal aorta (DA) of Atlantic salmon (Salmo salar) was studied in relation to nutritional status and sampling intensity. The incentive for the study was the inconsistency between published reports and our own experience of recovery and longevity of fish exposed to this protocol. In two studies using starved fish, blood (0.3 ml) was sampled 0, 1 and 24 h after DA-cannulation, and thereafter at 48 and 72 h and thereafter once weekly for four weeks. In a third study using fed fish, four consecutive samples (0, 3, 6 and 12 h after a meal) were obtained twice a week over a four-week period. All fish displayed a sharp increase in pCO(2) and haematocrit (Hct) during surgery, followed by a marked raise in cortisol, glucose, sodium and potassium (1 h). pCO(2), pH and Hct approached baseline levels as early as the 1 h post-surgery sample, while this was not the case for cortisol and electrolytes before the 24 h post-surgery sample. Glucose did not display any significant changes post surgery. From then on, all variables displayed minor but non-significant (P > 0.05) changes indicating a steady state close to baseline values for unstressed fish. This pattern was independent of sampling procedure, i.e. repeated single or multiple samples and thus volume of blood removed. Nutritional status (fed vs. starved) did not affect post-surgical recovery pattern. Only K(+) and Hct displayed consistent and significant post-prandial patterns. We found marked differences between baseline level of cannulated fish and uncannulated control fish, in pH, K(+) and Hct indicating that cannulation may be the preferred method to obtain representative resting values in fish.

Tissue-specific thyroid hormone regulation of gene transcripts encoding iodothyronine deiodinases and thyroid hormone receptors in striped parrotfish (Scarus iseri)

In fish as in other vertebrates, the diverse functions of thyroid hormones are mediated at the peripheral tissue level through iodothyronine deiodinase (dio) enzymes and thyroid hormone receptor (tr) proteins. In this study, we examined thyroid hormone regulation of mRNAs encoding the three deiodinases dio1, dio2 and dio3 - as well as three thyroid hormone receptors trαA, trαB and trβ - in initial phase striped parrotfish (Scarus iseri). Parrotfish were treated with dissolved phase T(3) (20 nM) or methimazole (3 mM) for 3 days. Treatment with exogenous T(3) elevated circulating T(3), while the methimazole treatment depressed plasma T(4). Experimentally-induced hyperthyroidism increased the relative abundance of transcripts encoding trαA and trβ in the liver and brain, but did not affect trαB mRNA levels in either tissue. In both sexes, methimazole-treated fish exhibited elevated dio2 transcripts in the liver and brain, suggesting enhanced outer-ring deiodination activity in these tissues. Accordingly, systemic hyperthyroidism elevated relative dio3 transcript levels in these same tissues. In the gonad, however, patterns of transcript regulation were distinctly different with elevated T(3) increasing mRNAs encoding dio2 in testicular and ovarian tissues and dio3, trαA and trαB in the testes only. Thyroid hormone status did not affect dio1 transcript abundance in the liver, brain or gonads. Taken as a whole, these results demonstrate that thyroidal status influences relative transcript abundance for dio2 and dio3 in the liver, provide new evidence for similar patterns of dio2 and dio3 mRNA regulation in the brain, and make evident that fish exhibit tr subtype-specific transcript abundance changes to altered thyroid status.