Changes in gene expression in gills of the euryhaline killifish Fundulus heteroclitus after abrupt salinity transfer

Effects of short-term salt exposure on gill damage, serum components and gene expression patterns in juvenile Largemouth bass (Micropterus salmoides)

The Largemouth bass (Micropterus salmoides; LMB) is a freshwater fish that plays a significant role in aquaculture, and its cultural base is expanding into inland saline water areas. To study the effect of short-term salt exposure on LMB, fish with an average body weight of 11.69 (±1.82) g were cultured for 14 days at three different salt concentrations (0 ‰, 6 ‰, and 12 ‰). After 14 days, the second gill arch was collected for tissue sectioning and transcriptome sequencing, while serum samples were collected to analyze serum components. The results showed that the mortality rate in the 0 ‰ and 6 ‰ groups was 0 %, whereas the mortality rate in the 12 ‰ group was 62 %. In the gill tissue sections, no apparent damage was observed in the 0 ‰ and 6 ‰ groups. However, in the 12 ‰ group, the secondary lamellae became shorter, thicker, and exhibited a disordered arrangement. The serum component test results showed that osmolality and K+ significantly increased in the 12 ‰ group, while Na+, K+, and Cl- concentrations showed slight increases, but the differences were not significant. Comparative transcriptome analysis revealed that, along the salinity gradient, gene expression exhibited five profiles. Genes related to ion transport and immunity were highly expressed in the 6 ‰ and 12 ‰ groups, while genes associated with biosynthesis and ATP production showed decreased expression levels as salinity increased. Notably, seven solute carrier genes, two Na+/K+-ATPase genes, and two insulin-like growth factor genes were significantly highly expressed in the 12 ‰ salinity group, playing important roles in the transmembrane transport of ions. Based on the results, the LMB can acclimatize to a salt concentration of at least 6 ‰. However, exposure to 12 ‰ salinity can lead to a series of adverse effects, including organ damage, reduced energy metabolism efficiency, and disruption of ion homeostasis.

Regulation of Branchial Anoctamin 1 Expression in Freshwater- and Seawater-Acclimated Japanese Medaka, Oryzias latipes

In euryhaline teleosts, the cystic fibrosis transmembrane conductance regulator (CFTR) in seawater (SW)-type chloride cells facilitates apical Cl- secretion for SW adaptation, while alternative Cl- excretion pathways remain understudied. This study investigates the role of the calcium-activated chloride channel, Anoctamin 1 (ANO1), in the gills of the euryhaline Japanese medaka (Oryzias latipes) under hyperosmolality and cortisol (CORT) influence. Acclimation to artificial SW, NaCl, mannitol, or glucose significantly upregulated ANO1 and CFTR mRNA expression in gills, unlike urea treatment. In situ hybridization revealed ANO1 mRNA in chloride cells co-expressing CFTR and Na+, K+-ATPase under hyperosmotic conditions. ANO1 inhibition elevated plasma Cl- concentration, indicating impaired Cl- excretion. CORT or dexamethasone administration in freshwater (FW) fish significantly increased branchial ANO1 and CFTR mRNA expression, an effect attenuated by the glucocorticoid receptor (GR) antagonist RU486. Hyperosmotic treatment of isolated gill tissues rapidly induced ANO1 mRNA expression independent of CFTR mRNA changes, and this induction was unaffected by RU486. These findings highlight the dual regulation of ANO1 expression via hyperosmolality-induced cellular response and the CORT-GR system. Thus, branchial ANO1 may likely complement CFTR in Cl⁻ excretion, playing a key role in the hyperosmotic adaptation of euryhaline teleosts.

Gene expression signatures between Limia perugiae (Poeciliidae) populations from freshwater and hypersaline habitats, with comparisons to other teleosts

Salinity gradients act as strong environmental barriers that limit the distribution of aquatic organisms. Changes in gene expression associated with transitions between freshwater and saltwater environments can provide insights into organismal responses to variation in salinity. We used RNA-sequencing (RNA-seq) to investigate genome-wide variation in gene expression between a hypersaline population and a freshwater population of the livebearing fish species Limia perugiae (Poeciliidae). Our analyses of gill gene expression revealed potential molecular mechanisms underlying salinity tolerance in this species, including the enrichment of genes involved in ion transport, maintenance of chemical homeostasis, and cell signaling in the hypersaline population. We also found differences in gene expression patterns associated with cell-cycle and protein-folding processes between the hypersaline and freshwater L. perugiae. Bidirectional freshwater-saltwater transitions have occurred repeatedly during the diversification of fishes, allowing for broad-scale examination of repeatable patterns in evolution. Therefore, we compared transcriptomic variation in L. perugiae with other teleosts that have made freshwater-saltwater transitions to test for convergence in gene expression. Among the four distantly related population pairs from high- and low-salinity environments that we included in our analysis, we found only ten shared differentially expressed genes, indicating little evidence for convergence. However, we found that differentially expressed genes shared among three or more lineages were functionally enriched for ion transport and immune functioning. Overall, our results-in conjunction with other recent studies-suggest that different genes are involved in salinity transitions across disparate lineages of teleost fishes.

Salinity and prolactin regulate anoctamin 1 in the model teleost, Fundulus heteroclitus

To maintain internal ion balance in marine environments, teleost fishes leverage seawater (SW)-type ionocytes to actively secrete Na+ and Cl- into the environment. It is well established that SW-type ionocytes use apically expressed cystic fibrosis transmembrane conductance regulator 1 (Cftr1) as a conduit for Cl- to exit the gill. Here, we investigated whether the Ca2+-activated Cl- channel, anoctamin 1 (Ano1), provides an additional path for Cl--secretion in euryhaline mummichogs (Fundulus heteroclitus). Two ano1 gene isoforms, denoted ano1.1a and -1.1b, exhibited higher expression in the gill and opercular epithelium of mummichogs long-term acclimated to SW versus fresh water (FW). Branchial ano1.1b and cftr1 expression was increased in mummichogs sampled 24 h after transfer from FW to SW; ano1.1a and -1.1b were upregulated in the gill and opercular epithelium following transfer from SW to hypersaline SW. Alternatively, the expression of ano1.1a, -1.1b, and cftr1 in the gill and opercular epithelium was markedly decreased after transfer from SW to FW. Given its role in attenuating ion secretion, we probed whether prolactin downregulates ano1 isoforms. In addition to attenuating cftr1 expression, a prolactin injection reduced branchial ano1.1a and -1.1b levels. Given how Ano1 mediates Cl- secretion by mammalian epithelial cells, the salinity- and prolactin-sensitive nature of ano1 expression reported here indicates that Ano1 may constitute a novel Cl--secretion pathway in ionocytes. This study encourages a wider evaluation of this putative Cl--secretion pathway and its regulation by hormones in teleost fishes.NEW & NOTEWORTHY In this study, we provide evidence in a teleost fish that the Ca2+-activated Cl- channel, anoctamin 1 may provide an additional path for Cl- secretion by seawater-type ionocytes. Not only is this the first report of a Cftr-independent Cl--secreting pathway conferring survival in seawater but also the first description of its regulation by the pituitary hormone prolactin.

Hypersalinity tolerance of mummichogs (Fundulus heteroclitus): A branchial transcriptomic analysis

Along the east coast of North America, mummichogs (Fundulus heteroclitus) are subjected to a broad range of salinities in their nearshore habitats. However, there is a paucity of information regarding the molecular and cellular processes that mummichogs (and other highly osmotolerant fishes) engage to survive environmental salinities greater than seawater (SW). To reveal branchial processes underlying their extraordinarily broad salinity tolerance, we performed an RNA-Seq analysis to identify differentially expressed genes (DEGs) in mummichogs residing in 3, 35, and 105 ppt conditions. We identified a series of DEGs previously associated with both freshwater (FW)- and SW-type ionocytes; however, the heightened expression of anoctamin 1a, a Ca2+-activated Cl- channel, in 35 and 105 ppt indicates that an undescribed Cl--secretion pathway may operate within the SW-type ionocytes of mummichogs. Concerning FW-adaptive branchial processes, we identified claudin 5a as a gene whose product may limit the diffusive loss of ions between cellular tight junctions. Further, in response to hypersaline conditions, we identified DEGs linked with myo-inositol synthesis and kinase signaling. This study provides new molecular targets for future physiological investigations that promise to reveal the mechanistic bases for how mummichogs and other euryhaline species tolerate hypersaline conditions.

Subcellular localization of Na+/K+-ATPase isoforms resolved by in situ hybridization chain reaction in the gill of chum salmon at freshwater and seawater

The Na+/K+-ATPase (NKA) α1-isoforms were examined by in situ hybridization chain reaction (ISHCR) using short hairpin DNAs, and we showed triple staining of NKA α1a, α1b, and α1c transcripts in the gill of chum salmon acclimated to freshwater (FW) and seawater (SW). The NKA α1-isoforms have closely resembled nucleotide sequences, which could not be differentiated by conventional in situ hybridization. The ISHCR uses a split probe strategy to allow specific hybridization using regular oligo DNA, resulting in high specificity at low cost. The results showed that NKA α1c was expressed ubiquitously in gill tissue and no salinity effects were observed. FW lamellar ionocytes (type-I ionocytes) expressed cytoplasmic NKA α1a and nuclear NKA α1b transcripts. However, both transcripts of NKA α1a and α1b were present in the cytoplasm of immature type-I ionocytes. The developing type-I ionocytes increased the cytoplasmic volume and migrated to the distal region of the lamellae. SW filament ionocytes (type-II ionocytes) expressed cytoplasmic NKA α1b transcripts as the major isoform. Results from morphometric analysis and nonmetric multidimensional scaling indicated that a large portion of FW ionocytes was NKA α1b-rich, suggesting that isoform identity alone cannot mark the ionocyte types. Both immature or residual type-II ionocytes and type-I ionocytes were found on the FW and SW gills, suggesting that the chum salmon retains the potential to switch the ionocyte population to fit the ion-transporting demands, which contributes to their salinity tolerance and osmoregulatory plasticity.

The role of salinity in recovery of white sturgeon (Acipenser transmontanus) from stimulated angling stress

White sturgeon (Acipenser transmontanus) in the Lower Fraser River are the focus of a catch-and-release angling fishery in British Columbia, Canada. However, the lower region of the catch area includes areas where tidal waters invade, and the consequence of salinity levels on recovery from an angling challenge are not characterized in sturgeon, despite theoretical implications of its import. We acclimated white sturgeon to various salinities (0, 10 and 20‰ (parts per thousand)) to investigate the effects of acclimation on recovery from stimulated angling stress that was induced through manual chasing. This challenge elicited the traditional physiological responses such as ion homeostasis disturbance, increases in secondary stress indicators and metabolic acidosis; however, environmental salinity altered the timing of recovery in some of the parameters measured. In addition, the severity of the intracellular pH disturbance in both heart and red blood cell seemed to be mediated in fresh water, yet the recovery pattern of plasma chloride and bicarbonate ions seemed to be facilitated by higher salinity. In general, responses were similar but not identical, leading us to conclude that the role of salinity on recovery from exercise is complex but not insignificant. Salinity may be important to behaviours exhibited by white sturgeon (such as migrations) in their respective saline environments, but less so around the impact of an angling stressor. Further exploration of this response may provide insight on whether the current tidal boundaries for angling white sturgeon are appropriate.

Retention of ion channel genes expression increases Japanese medaka survival during seawater reacclimation

Euryhaline teleosts exhibit varying acclimability to survive in environments that alternate between being hypotonic and hypertonic. Such ability is conferred by ion channels expressed by ionocytes, the ion-regulating cells in the gills or skin. However, switching between environments is physiologically challenging, because most channels can only perform unidirectional ion transportation. Coordination between acute responses, such as gene expression, and long-term responses, such as cell differentiation, is believed to strongly facilitate adaptability. Moreover, the pre-acclimation to half seawater salinity can improve the survivability of Japanese medaka (Oryzias latipes) during direct transfer to seawater; here, the ionocytes preserve hypertonic acclimability while performing hypotonic functions. Whether acclimability can be similarly induced in a closed species and their corresponding responses in terms of ion channel expression remain unclear. In the present study, Japanese medaka pre-acclimated in brackish water were noted to have higher survival rates while retaining higher expression of the three ion channel genes ATP1a1a.1, ATP1b1b, and SLC12a2a. This retention was maintained up to 2 weeks after the fish were transferred back into freshwater. Notably, this induced acclimability was not found in its close kin, Indian medaka (Oryzias dancena), the natural habitat of which is brackish water. In conclusion, Japanese medaka surpassed Indian medaka in seawater acclimability after experiencing exposure to brackish water, and this ability coincided with seawater-retention gene expression.

Molecular targets of prolactin in mummichogs (Fundulus heteroclitus): Ion transporters/channels, aquaporins, and claudins

Prolactin (Prl) was identified over 60 years ago in mummichogs (Fundulus heteroclitus) as a "freshwater (FW)-adapting hormone", yet the cellular and molecular targets of Prl in this model teleost have remained unknown. Here, we conducted a phylogenetic analysis of two mummichog Prl receptors (Prlrs), designated Prlra and Prlrb, prior to describing the tissue- and salinity-dependent expression of their associated mRNAs. We then administered ovine Prl (oPrl) to mummichogs held in brackish water and characterized the expression of genes associated with FW- and seawater (SW)-type ionocytes. Within FW-type ionocytes, oPrl stimulated the expression of Na⁺/Cl^- cotransporter 2 (ncc2) and aquaporin 3 (aqp3). Alternatively, branchial Na⁺/H⁺ exchanger 2 and -3 (nhe2 and -3) expression did not respond to oPrl. Gene transcripts associated with SW-type ionocytes, including Na⁺/K⁺/2Cl^- cotransporter 1 (nkcc1), cystic fibrosis transmembrane regulator 1 (cftr1), and claudin 10f (cldn10f) were reduced by oPrl. Isolated gill filaments incubated with oPrl in vitro exhibited elevated ncc2 and prlra expression. Given the role of Aqps in supporting gastrointestinal fluid absorption, we assessed whether several intestinal aqp transcripts were responsive to oPrl and found that aqp1a and -8 levels were reduced by oPrl. Our collective data indicate that Prl promotes FW-acclimation in mummichogs by orchestrating the expression of solute transporters/channels, water channels, and tight-junction proteins across multiple osmoregulatory organs.

Physiological and behavioural strategies of aquatic animals living in fluctuating environments

Shallow or near-shore environments, such as ponds, estuaries and intertidal zones, are among the most physiologically challenging of all aquatic settings. Animals inhabiting these environments experience conditions that fluctuate markedly over relatively short temporal and spatial scales. Living in these habitats requires the ability to tolerate the physiological disturbances incurred by these environmental fluctuations. This tolerance is achieved through a suite of physiological and behavioural responses that allow animals to maintain homeostasis, including the ability to dynamically modulate their physiology through reversible phenotypic plasticity. However, maintaining the plasticity to adjust to some stresses in a dynamic environment may trade off with the capacity to deal with other stressors. This paper will explore studies on select fishes and invertebrates exposed to fluctuations in dissolved oxygen, salinity and pH. We assess the physiological mechanisms these species employ to achieve homeostasis, with a focus on the plasticity of their responses, and consider the resulting physiological trade-offs in function. Finally, we discuss additional factors that may influence organismal responses to fluctuating environments, such as the presence of multiple stressors, including parasites. We echo recent calls from experimental biologists to consider physiological responses to life in naturally fluctuating environments, not only because they are interesting in their own right but also because they can reveal mechanisms that may be crucial for living with increasing environmental instability as a consequence of climate change.

Early and abrupt salinity reduction impacts European eel larval culture

Reducing water salinity towards iso-osmotic conditions is a common practice applied in euryhaline fish farming to limit osmoregulation costs and enhance growth. In this respect, the present study investigated the timing of salinity reduction in an abrupt manner during European eel (Anguilla anguilla) larval culture by examining associated impacts on morphological and molecular levels. Larvae from 3 different parental combinations (families) were reared at constant 36 psu for 6 days (control) or subjected to a direct reduction to 18 psu on 1, 2, or 3 days post-hatch. Overall, salinity reduction enhanced growth and survival, resulting from more efficient energy resource utilization. In the control group, expression of growth-related igf2 remained constant, demonstrating a steady growth progression, while igf1 expression increased over time only for the salinity reduced treatments, potentially qualifying as a useful biomarker for growth performance. Even though each parental combination seems to have a different capacity to cope with salinity alterations, as observed by family-driven water-transport-related aquaporin (aqp1, aqp3) gene expression, it could be inferred that the abrupt salinity change is generally not stressful, based on non-upregulated heat shock proteins (hsp70, hsp90). However, the applied salinity reduction (irrespective of timing) induced the development of pericardial edema. As such, we conclude that despite the positive effect of salinity reduction on early growth and survival, the long-term benefit for eel larval culture lies in establishing a protocol for salinity reduction, at a precise developmental time point, without causing pericardial malformations.

The time course of molecular acclimation to seawater in a euryhaline fish

The Arabian pupfish, Aphanius dispar, is a euryhaline fish inhabiting both inland nearly-freshwater desert ponds and highly saline Red Sea coastal lagoons of the Arabian Peninsula. Desert ponds and coastal lagoons, located respectively upstream and at the mouths of dry riverbeds (“wadies”), have been found to potentially become connected during periods of intense rainfall, which could allow the fish to migrate between these different habitats. Flash floods would therefore flush Arabian pupfish out to sea, requiring a rapid acclimation to a greater than 40 ppt change in salinity. To investigate the molecular pathways of salinity acclimation during such events, a Red Sea coastal lagoon and a desert pond population were sampled, with the latter exposed to a rapid increase in water salinity. Changes in branchial gene expression were investigated via genome-wide transcriptome measurements over time from 6 h to 21 days. The two natural populations displayed basal differences in genes related to ion transport, osmoregulation and immune system functions. These mechanisms were also differentially regulated in seawater transferred fish, revealing their crucial role in long-term adaptation. Other processes were only transiently activated shortly after the salinity exposure, including cellular stress response mechanisms, such as molecular chaperone synthesis and apoptosis. Tissue remodelling processes were also identified as transient, but took place later in the timeline, suggesting their importance to long-term acclimation as they likely equip the fish with lasting adaptations to their new environment. The alterations in branchial functional pathways displayed by Arabian pupfish in response to salinity increases are diverse. These reveal a large toolkit of molecular processes important for adaptation to hyperosmolarity that allow for successful colonization to a wide variety of different habitats.

Molecular characterization of transient receptor potential vanilloid 4 (TRPV4) gene transcript variant mRNA of chum salmon Oncorhynchus keta in response to salinity or temperature changes

Transient receptor potential vanilloid 4 (TRPV4) is an osmosensory cation channel that respond to an increase in cell volume and participates in various physiological functions. Among organisms in aquatic environments, euryhaline teleost is are suitable experimental models to study ion channel proteins related to physiological functions involving osmosensing. Among the studies of various regulatory molecules that mediate osmotic regulation in fish, however, information is lacking, particularly on the TRP family. This study investigated the structural characteristics of theTRPV4 gene of chum salmon (Oncorhynchus keta) and their responses to changes in salinity and temperature. Interestingly, TRPV4 generates transcript variants of the intron-retention form through alternative splicing, resulting in a frameshift leading to the generation of transcripts of different structures. In particular, TRPV4 x1 and TRPV x2 mRNAs were predominant in the gill and skin including at the lateral line. The expression levels of chum salmon TRPV4 x1 were significantly increased with increase in salinity and temperature, whereas TRPV4 x2 mainly responded to temperature decrease. Overall, these results demonstrate for the first time the effects of salinity and temperature on the expression of two salmonid TRPV4 transcript variants, suggesting their contribution to the regulation of hydromineral balance.

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

Reductionist approaches to the study of ionoregulation in fishes

The mechanisms underlying ionoregulation in fishes have been studied for nearly a century, and reductionist methods have been applied at all levels of biological organization in this field of research. The complex nature of ionoregulatory systems in fishes makes them ideally suited to reductionist methods and our collective understanding has been dramatically shaped by their use. This review provides an overview of the broad suite of techniques used to elucidate ionoregulatory mechanisms in fishes, from the whole-animal level down to the gene, discussing some of the advantages and disadvantages of these methods. We provide a roadmap for understanding and appreciating the work that has formed the current models of organismal, endocrine, cellular, molecular, and genetic regulation of ion balance in fishes and highlight the contribution that reductionist techniques have made to some of the fundamental leaps forward in the field throughout its history.

Differential expression of two ATPases revealed by lipid raft isolation from gills of euryhaline teleosts with different salinity preferences

In euryhaline teleosts, Na⁺, K⁺-ATPase (NKA) and V-type H + -ATPase A (VHA A) are important ion-transporters located in cell membrane. Lipid rafts (LR) are plasma membrane microdomains enriched in cholesterol, sphingolipids, and proteins (e.g., flotillin). Flotillin is a LR-associated protein, commonly used as the LR marker. Previous mammalian studies showed that LR may play a crucial role in ion exchanges. Meanwhile, studies on mammals and rainbow trout showed that NKA were found to be present mainly in LR. However, little is known about LR in fish. Therefore, the present study aimed to investigate the involvement of branchial LR in osmoregulation of tilapia and milkfish, two euryhaline teleosts with different salinity preferences, by (i) extracting LR from the gills of euryhaline teleosts; (ii) detecting the abundance of LR marker protein (flotillin-2) and ion-transporters (NKA and VHA A) in branchial LR and non-LR of fresh water- and seawater-acclimated milkfish and tilapia. The results indicated that the protein abundance of LR marker, flotillin-2, changed with environmental salinities in branchial LR of tilapia. In addition, flotillin-2 and NKA were only found in LR in both tilapia and milkfish gills, while VHA A were mainly present in non-LR. Relative protein abundance of NKA was found to be significantly higher in gills of freshwater milkfish and seawater tilapia, while VHA A was significantly higher in gills of freshwater tilapia and milkfish. This study illustrated differential distribution and salinity-dependent expression of NKA and VHA A in cell membrane of gill tissues of euryhaline teleosts with different salinity preferences.

Rearing environment affects the genetic architecture and plasticity of DNA methylation in Chinook salmon

Genetic architecture and phenotypic plasticity are important considerations when studying trait variation within and among populations. Since environmental change can induce shifts in the genetic architecture and plasticity of traits, it is important to consider both genetic and environmental sources of phenotypic variation. While there is overwhelming evidence for environmental effects on phenotype, the underlying mechanisms are less clear. Variation in DNA methylation is a potential mechanism mediating environmental effects on phenotype due to its sensitivity to environmental stimuli, transgenerational inheritance, and influences on transcription. To characterize the effect of environment on methylation, we created two 6 × 6 (North Carolina II) Chinook salmon breeding crosses and reared the offspring in two environments: uniform hatchery tanks and seminatural stream channels. We sampled the fish twice during development, at the alevin (larval) and fry (juvenile) stages. We measured DNA methylation at 13 genes using a PCR-based bisulfite sequencing protocol. The genetic architecture of DNA methylation differed between rearing environments, with greater additive and nonadditive genetic variance in hatchery fish and greater maternal effects in seminatural channel fish, though gene-specific variation was evident. We observed plasticity in methylation across all assayed genes, as well as gene-specific effects at two genes in alevin and six genes in fry, indicating developmental stage-specific effects of rearing environment on methylation. Characterizing genetic and environmental influences on methylation is critical for future studies on DNA methylation as a potential mechanism for acclimation and adaptation.

Immunohistochemical characterization and change in location of branchial ionocytes after transfer from freshwater to seawater in the euryhaline obscure puffer, Takifugu obscurus

The obscure puffer Takifugu obscurus is a euryhaline fish species suitable for studying the molecular mechanism of osmoregulation. The distributional changes of branchial ionocytes were detected following the transfer from freshwater (FW) to seawater (SW) based on two main ion transporters, Na⁺/K⁺-ATPase (NKA) and Na⁺/K⁺/ 2Cl^- cotransporter 1 (NKCC1). The mRNA and protein expression levels of NKA and NKCC1 in the gills all increased rapidly in the first four days after transfer to SW. Double immunofluorescence staining showed that NKCC1 and NKA were colocalized in the branchial ionocytes and the immunoreaction of NKCC1 was stronger after transfer. Moreover, following transfer to SW, the number of lamellar ionocytes in the gills is reduced and the number of filament ionocytes is increased significantly. Taken together, these findings indicated that SW transfer of obscure puffer promotes the changes of distribution, function and size of branchial ionocytes.

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.

A rapid intrinsic heart rate resetting response with thermal acclimation in rainbow trout, Oncorhynchus mykiss

We examined cardiac pacemaker rate resetting in rainbow trout following a reciprocal temperature transfer. In the original experiment, performed in winter, 4°C-acclimated fish transferred to 12°C reset intrinsic heart rate after just 1 h (from 56.8±1.2 to 50.8±1.5 beats min-1); 12°C-acclimated fish transferred to 4°C reset intrinsic heart rate after 8 h (from 33.4±0.7 to 37.7±1.2 beats min-1). However, in a replicate experiment, performed in the summer using a different brood year, intrinsic heart rate was not reset, even after 10 weeks at a new temperature. Using this serendipitous opportunity, we compared mRNA expression changes of a suite of proteins in sinoatrial node (SAN), atrial and ventricular tissues after both 1 h and longer than 3 weeks for both experimental acclimation groups to identify those changes only associated with pacemaker rate resetting. Of the changes in mRNA expression occurring after more than 3 weeks of warm acclimation and associated with pacemaker rate resetting, we observed downregulation of NKA α1c in the atrium and ventricle, and upregulation of HCN1 in the ventricle. However, in the SAN there were no mRNA expression changes unique to the fish with pacemaker rate resetting after either 1 h or 3 weeks of warm acclimation. Thus, despite identifying changes in mRNA expression of contractile cardiac tissues, there was an absence of changes in mRNA expression directly involved with the initial, rapid pacemaker rate resetting with warm acclimation. Importantly, pacemaker rate resetting with thermal acclimation does not always occur in rainbow trout.

Gill Transcriptome Sequencing and De Novo Annotation of Acanthogobius ommaturus in Response to Salinity Stress

Acanthogobius ommaturus is a euryhaline fish widely distributed in coastal, bay and estuarine areas, showing a strong tolerance to salinity. In order to understand the mechanism of adaptation to salinity stress, RNA-seq was used to compare the transcriptome responses of Acanthogobius ommaturus to the changes of salinity. Four salinity gradients, 0 psu, 15 psu (control), 30 psu and 45 psu were set to conduct the experiment. In total, 131,225 unigenes were obtained from the gill tissue of A. ommaturus using the Illumina HiSeq 2000 platform (San Diego, USA). Compared with the gene expression profile of the control group, 572 differentially expressed genes (DEGs) were screened, with 150 at 0 psu, 170 at 30 psu, and 252 at 45 psu. Additionally, among these DEGs, Gene Ontology (GO) analysis indicated that binding, metabolic processes and cellular processes were significantly enriched. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways analysis detected 3, 5 and 8 pathways related to signal transduction, metabolism, digestive and endocrine systems at 0 psu, 30 psu and 45 psu, respectively. Based on GO enrichment analysis and manual literature searches, the results of the present study indicated that A. ommaturus mainly responded to energy metabolism, ion transport and signal transduction to resist the damage caused by salinity stress. Eight DEGs were randomly selected for further validation by quantitative real-time PCR (qRT-PCR) and the results were consistent with the RNA-seq data.

Inter-individual variability in freshwater tolerance is related to transcript level differences in gill and posterior kidney of European sea bass

Acclimation to low salinities is a vital physiological challenge for euryhaline fish as the European sea bass Dicentrarchus labrax. This species undertakes seasonal migrations towards lagoons and estuaries where a wide range of salinity variations occur along the year. We have previously reported intraspecific differences in freshwater tolerance, with an average 30% mortality rate. In this study, we bring new evidence of mechanisms underlying freshwater tolerance in sea bass at gill and kidney levels. In fresh water (FW), intraspecific differences in mRNA expression levels of several ion transporters and prolactin receptors were measured. We showed that the branchial Cl^-/HCO₃^- anion transporter (slc26a6c) was over-expressed in freshwater intolerant fish, probably as a compensatory response to low blood chloride levels and potential metabolic alkalosis. Moreover, prolactin receptor a (prlra) and Na⁺/Cl^- cotransporter (ncc1) but not ncc-2a expression seemed to be slightly increased and highly variable between individuals in freshwater intolerant fish. In the posterior kidney, freshwater intolerant fish exhibited differential expression levels of slc26 anion transporters and Na⁺/K⁺/2Cl^- cotransporter 1b (nkcc1b). Lower expression levels of prolactin receptors (prlra, prlrb) were measured in posterior kidney which probably contributes to the failure in ion reuptake at the kidney level. Freshwater intolerance seems to be a consequence of renal failure of ion reabsorption, which is not sufficiently compensated at the branchial level.

Fast-start escape performance across temperature and salinity gradients in mummichog Fundulus heteroclitus

Fast-start predator-escape performance of mummichogs Fundulus heteroclitus was tested across field-informed variation in temperature (24, 30 and 36°C) and salinity (2, 12 and 32 ppt). Performance was similar across temperatures and salinities when fish were allowed to acclimate to these conditions. However, when mummichogs experienced acute temperature changes, performance exhibited thermal dependence in two contrasting ways. Fast-start turning rates and linear speeds varied directly with the temperature at which the manoeuvre was executed, but these aspects of performance varied inversely with acclimation temperature, with cool-acclimated fish exhibiting faster starts across test temperatures. Temperature effects were consistent across salinities. These results suggest that while mummichogs increase performance with acute temperature increases, long-term rises in sea temperature may cause these fish to become more susceptible to predation during abrupt cooling events, such as when storm events flood shallow water estuaries with cool rainwater.

Salinity-dependent expression of ncc2 in opercular epithelium and gill of mummichog (Fundulus heteroclitus)

Mummichogs (Fundulus heteroclitus) can tolerate abrupt changes in environmental salinity because of their ability to rapidly adjust the activities of ionocytes in branchial and opercular epithelia. In turn, the concerted expression of sub-cellular effectors of ion transport underlies adaptive responses to fluctuating salinities. Exposure to seawater (SW) stimulates the expression of Na⁺/K⁺/2Cl^- cotransporter 1 (nkcc1) and cystic fibrosis transmembrane regulator (cftr) mRNAs in support of ion extrusion by SW-type ionocytes. Given the incomplete understanding of how freshwater (FW)-type ionocytes actually operate in mummichogs, the transcriptional responses essential for ion absorption in FW environments remain unresolved. In a subset of species, a 'fish-specific' Na⁺/Cl^- cotransporter denoted Ncc2 (Slc12a10) is responsible for the uptake of Na⁺ and Cl^- across the apical surface of FW-type ionocytes. In the current study, we identified an ncc2 transcript that is highly expressed in gill filaments and opercular epithelium of FW-acclimated mummichogs. Within 1 day of transfer from SW to FW, ncc2 levels in both tissues increased in parallel with reductions in nkcc1 and cftr. Conversely, mummichogs transferred from FW to SW exhibited marked reductions in ncc2 concurrent with increases in nkcc1 and cftr. Immunohistochemical analyses employing a homologous antibody revealed apical Ncc2-immunoreactivity in Na⁺/K⁺-ATPase-immunoreactive ionocytes of FW-acclimated animals. Our combined observations suggest that Ncc2/ncc2-expressing ionocytes support the capacity of mummichogs to inhabit FW environments.

The acute transcriptome response of the midbrain/diencephalon to injury in the adult mummichog (Fundulus heteroclitus)

Adult fish produce new cells throughout their central nervous system during the course of their lives and maintain a tremendous capacity to repair damaged neural tissue. Much of the focus on understanding brain repair and regeneration in adult fish has been directed at regions of the brainstem and forebrain; however, the mesencephalon (midbrain) and diencephalon have received little attention. We sought to examine differential gene expression in the midbrain/diencephalon in response to injury in the adult fish using RNA-seq. Using the mummichog (Fundulus heteroclitus), we administered a mechanical lesion to the midbrain/diencephalon and examined differentially expressed genes (DEGs) at an acute recovery time of 1 h post-injury. Comparisons of whole transcriptomes derived from isolated RNA of intact and injured midbrain/diencephalic tissue identified 404 DEGs with the vast majority being upregulated. Using qPCR, we validated the upregulation of DEGs pim-2-like, syndecan-4-like, and cd83. Based on genes both familiar and novel regarding the adult brain response to injury, these data provide an extensive molecular profile giving insight into a range of cellular processes involved in the injury response of a brain regenerative-capable vertebrate.

Differential Coping Strategies in Response to Salinity Challenge in Olive Flounder

To examine how different fish coping strategies respond to salinity challenge, olive flounder (Paralichthys olivaceus) with active coping style (AC) and passive coping style (PC) were transferred from seawater (SW) to freshwater (FW) and their behavior and physiology were analyzed. Different behavioral coping strategies, in terms of escape and feeding tendencies, were confirmed in AC and PC fish without FW exposure. Differences in swimming distance between AC and PC flounder were then assessed after 1 and 2 days of FW transfer. Plasma parameters and branchial gene expression were also determined 2, 5, 8, and 14 days after transfer, with comparisons between AC and PC fish and against a SW-acclimated control group. The results showed that: (1) PC flounder exhibited a significant reduction in swimming activity, while AC flounder significantly increased locomotion 2 days after transfer. (2) The plasma osmolality and plasma ionic (Na⁺ and Cl⁻) concentration of FW-acclimated PC flounder declined in a continuous fashion over time but this contrasted against the plasma parameters of AC flounder which fluctuated below the baseline level of a SW-acclimated control group. (3) The expression of NKA-α1 and NHE-3-like mRNA in PC flounder gill increased significantly from 5 days, but the expression of these two genes in AC flounder only increased after 8 days of transfer. (4) There were no remarkable differences observed in Rhcg expressions between AC and PC flounder. This study indicates for the first time that PC flounder adopt a “freeze-passive tolerance” strategy while AC flounder adopt a “flight-active resistance” defense strategy in response to salinity challenge.

Differential regulation of select osmoregulatory genes and Na+/K+-ATPase paralogs may contribute to population differences in salinity tolerance in a semi-anadromous fish

The Sacramento splittail (Pogonichthys macrolepidotus) is a species of special concern that is native to the San Francisco Estuary, USA. Two genetically distinct populations exist and differ in maximal salinity tolerances. We examined the expression of 12 genes representative of osmoregulatory functions in the gill over a 14  day time course at two different salinities [11 or 14 PSU (Practical Salinity Units)] and revealed that each population showed distinct patterns of gene expression consistent with population differences in response to osmotic regimes. The relatively more salinity-tolerant San Pablo population significantly upregulated nine out of the 12 transcripts investigated on day 1 of 11 PSU salinity exposure in comparison to the day zero freshwater control. Three transcripts (nka1a, nka1b, and mmp13) were differentially expressed between the populations at 7 and 14 days of salinity exposure, suggesting a reduced ability of the relatively salinity-intolerant Central Valley population to recover. Additionally, a phylogenetic analysis of several Sacramento splittail Na⁺/K⁺-ATPase α1 sequences resulted in grouping by proposed paralog rather than species, suggesting that different paralogs of this gene may exist. These findings, together with prior research conducted on the Sacramento splittail, suggest that the San Pablo population may be able to preferentially regulate select osmoregulatory genes, including different Na⁺/K⁺-ATPase α1 paralogs, to better cope with salinity challenges.

Salinity affects growth but not thermal preference of adult mummichogs Fundulus heteroclitus

The effects of an ecologically relevant range of salinities (2, 12, 22, 32) on thermal preferences and growth of adult mummichogs Fundulus heteroclitus were determined for fish from a southern Chesapeake Bay population. Salinity did not affect the mean temperature selected by F. heteroclitus in a thermal gradient, which was identified as 26.6°C based on observations of 240 individuals. Salinity and temperature had significant and interacting effects on growth rates of F. heteroclitus measured over 12 weeks. Growth rates were highest overall and remained high over a broader range of temperatures at moderate salinities (12 and 22), while high growth rates were shifted toward lower temperatures for fish grown at a salinity of 2 and higher temperatures at a salinity of 32. Significant reductions in growth relative to the optimal conditions (28.6°C, salinity of 22) were observed at the coolest (19.6°C) and warmest (33.6°C) temperature tested at all salinities, as well as temperatures ≥ 26.6°C at a salinity of 2, ≥ 28.6°C at a salinity of 12 and ≤ 26.6°C at a salinity of 32. Growth rates provide a long-term, organismal measure of performance and results of this study indicate that performance may be reduced under conditions that the highly euryhaline F. heteroclitus can otherwise easily tolerate. The combination of reduced salinity and increased temperature that is predicted for temperate estuaries as a result of climate change may have negative effects on growth of this ecologically important species.

Cortisol regulation of Na+, K+-ATPase β1 subunit transcription via the pre-receptor 11β-hydroxysteroid dehydrogenase 1-like (11β-Hsd1L) in gills of hypothermal freshwater milkfish, Chanos chanos

Hypothermal stress changes the balance of osmoregulation by affecting Na⁺, K⁺-ATPase (Na-K-ATPase) activity or inducing modulation to epithelium permeability in fish. Meanwhile, cellular concentrations of cortisol can be modulated by the pre-receptor enzymes 11β-hydroxysteroid dehydrogenase 1 and 2 (11β-Hsd1 and 2). In fish, increasing levels of exogenous cortisol stimulate Na⁺ uptake via specific interaction with cortisol. This study investigated cortisol effects on expression of Na-K-ATPase subunit proteins and activity in gills of milkfish under hypothermal stress and revealed that the plasma cortisol contents as well as gill 11β-hsd1l and na-k-atpase β1 mRNA abundance were decreased in fresh water (FW) milkfish. Meanwhile, in the seawater (SW) milkfish, the plasma cortisol contents and gill 11β-hsd1l and na-k-atpase β1 mRNA abundance was increased under hypothermal stress. On the other hand, the abundance of 11β-hsd2 mRNA increased in both FW and SW. In addition, 11β-hsd1l expression increased in FW milkfish but decreased in SW milkfish after cortisol injection. Accordingly, the results that gill Na-K-ATPase activity of FW milkfish was affected by environmental temperatures as well as cortisol-dependent Na-K-ATPase β1-subunit levels might be due to increased expression of 11β-hsd1l that elevated intracellular cortisol contents. In hypothermal SW milkfish, decreasing abundance of Na-K-ATPase β1 protein due to reduced expression of 11β-hsd1l was found after cortisol injection. Thus, under hypothermal stress, 11β-HSD1L in FW milkfish gills was used to modulate cortisol and the following effects on increasing the transcription of Na-K-ATPase β1 protein.

Influence of salinity and pH on bioconcentration of ionizable pharmaceuticals by the gulf killifish, Fundulus grandis

Estuaries routinely receive discharges of contaminants of emerging concern from urban regions. Within these dynamic estuarine systems, salinity and pH can vary across spatial and temporal scales. Our previous research identified bioaccumulation of the calcium channel blocker diltiazem and the antihistamine diphenhydramine in several species of fish residing in multiple urban estuaries along the Gulf of Mexico in Texas, where field-measured observations of diltiazem in fish plasma exceeded human therapeutic plasma doses. However, there remains a limited understanding of pharmaceutical bioaccumulation in estuarine environments. Here, we examined the influence of pH and salinity on bioconcentration of three pharmaceuticals in the Gulf killifish, Fundulus grandis. F. grandis were exposed to low levels of the ionizable pharmaceuticals carbamazepine, diltiazem, and diphenhydramine at two salinities (5 ppt, 20 ppt) and two pH levels (6.7, 8.3). pH influenced bioconcentration of select weak base pharmaceuticals, while salinity did not, suggesting that intestinal uptake via drinking does not appear to be a major exposure route of these pharmaceuticals in killifish. Compared to our previous pH dependent uptake observations with diphenhydramine in the fathead minnow model, killifish apparent volume of distribution values were markedly lower than fatheads, though killifish bioconcentration factors were similar at high pH and four fold higher at low pH than freshwater fish. Advancing an understanding of environmental gradient influences on pharmacokinetics among fish is necessary to improve bioaccumulation assessments and interpretation of toxicological observations for ionizable contaminants.

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.

Dynamics of Gene Expression Responses for Ion Transport Proteins and Aquaporins in the Gill of a Euryhaline Pupfish during Freshwater and High-Salinity Acclimation

Pupfishes (genus Cyprinodon) evolved some of the broadest salinity tolerances of teleost fishes, with some taxa surviving in conditions from freshwater to nearly 160 ppt. In this study, we examined transcriptional dynamics of ion transporters and aquaporins in the gill of the desert Amargosa pupfish (Cyprinodon nevadensis amargosae) during rapid salinity change. Pupfish acclimated to 7.5 ppt were exposed to freshwater (0.3 ppt), seawater (35 ppt), or hypersaline (55 ppt) conditions over 4 h and sampled at these salinities over 14 d. Plasma osmolality and Cl^- concentration became elevated 8 h after the start of exposure to 35 or 55 ppt but returned to baseline levels after 14 d. Osmolality recovery was paralleled by increased gill Na⁺/K⁺-ATPase activity and higher relative levels of messenger RNAs (mRNAs) encoding cystic fibrosis transmembrane conductance regulator (cftr) and Na⁺/K⁺/2Cl^- cotransporter-1 (nkcc1). Transcripts encoding one Na⁺-HCO₃^- cotransporter-1 isoform (nbce1.1) also increased in the gills at higher salinities, while a second isoform (nbce1.2) increased expression in freshwater. Pupfish in freshwater also had lower osmolality and elevated gill mRNAs for Na⁺/H⁺ exchanger isoform-2a (nhe2a) and V-type H⁺-ATPase within 8 h, followed by increases in Na⁺/H⁺ exchanger-3 (nhe3), carbonic anhydrase 2 (ca2), and aquaporin-3 (aqp3) within 1 d. Gill mRNAs for Na⁺/Cl^- cotransporter-2 (ncc2) also were elevated 14 d after exposure to 0.3 ppt. These results offer insights into how coordinated transcriptional responses for ion transporters in the gill facilitate reestablishment of osmotic homeostasis after changes in environmental salinity and provide evidence that the teleost gill expresses two Na⁺-HCO₃^- cotransporter-1 isoforms with different roles in freshwater and seawater acclimation.

Profiling microRNA expression in Atlantic killifish (Fundulus heteroclitus) gill and responses to arsenic and hyperosmotic stress

The Atlantic killifish (Fundulus heteroclitus), native to estuarine areas of the Atlantic coast of the United States, has become a valuable ecotoxicological model as a result of its ability to acclimate to rapid environmental changes and adapt to polluted habitats. MicroRNAs (miRNAs) are highly conserved small RNAs that regulate gene expression and play critical roles in stress responses in a variety of organisms. Global miRNA expression in killifish and the potential roles miRNA have in environmental acclimation have yet to be characterized. Accordingly, we profiled miRNA expression in killifish gill for the first time and identified a small group of highly expressed, well-conserved miRNAs as well as 16 novel miRNAs not yet identified in other organisms. Killifish respond to large fluctuations in salinity with rapid changes in gene expression and protein trafficking to maintain osmotic balance, followed by a secondary phase of gene and protein expression changes that enable remodeling of the gills. Arsenic, a major environmental toxicant, was previously shown to inhibit gene expression responses in killifish gill, as well the ability of killifish to acclimate to a rapid increase in salinity. Thus, we examined the individual and combined effects of salinity and arsenic on miRNA expression in killifish gill. Using small RNA sequencing, we identified 270 miRNAs expressed in killifish, and found that miR-135b was differentially expressed in response to arsenic and at 24 h following transfer to salt water. Predicted targets of miR-135b are involved in ion transport, cell motility and migration, GTPase mediated signal transduction and organelle assembly. Consistent with previous studies of these two environmental stressors, we found a significant interaction (i.e., arsenic dependent salinity effect), whereby killifish exposed to arsenic exhibited an opposite response in miR-135b expression at 24 h post hyperosmotic challenge compared to controls. By examining mRNA expression of predicted miRNA targets during salinity acclimation and arsenic exposure, we found that miR-135b targets were significantly more likely to decrease during salinity acclimation than non-targets. Our identification of a significant interaction effect of arsenic and salinity on miR-135b expression supports the hypothesis that arsenic alters upstream regulators of stress response networks, which may adversely affect the killifish response to osmotic stress. The characterization of miRNAs in this ecotoxicological model will be a valuable resource for future studies investigating the role of miRNAs in response to environmental stress.

Mechanisms of Ca2+ uptake in freshwater and seawater-acclimated killifish, Fundulus heteroclitus, and their response to acute salinity transfer

Killifish (Fundulus heteroclitus) has been extensively used as a model for ion regulation by euryhaline fishes. Na⁺ and Cl^- dynamics have been well studied in killifish, but few studies have addressed that of Ca²⁺. Therefore, this study aimed to characterize Ca²⁺ fluxes in freshwater (FW) and seawater (SW)-acclimated killifish, their response to salinity transfer, and to elucidate the mechanisms of Ca²⁺ influx in FW and SW. SW killifish displayed a significantly higher Ca²⁺ influx rate than that of FW fish, while Ca²⁺ efflux rates were comparable in both salinities. Ca²⁺ influx was saturable in FW (K_m = 78 ± 19 µmol/L; J_max = 53 ± 3 nmol/g/h) and influx by SW killifish was linear up to 7 mmol/L Ca²⁺. In SW-acclimated fish, 36% of Ca²⁺ influx was attributed to "intestinal Ca²⁺ intake", likely caused by drinking, whereas intestinal Ca²⁺ intake in FW contributed to < 2% of total. Throughout the study, results suggested that "cation competition" in SW modulates Ca²⁺ influx. Therefore, we hypothesized that SW-acclimated fish actually have a higher affinity Ca²⁺ influx system than FW-acclimated fish but that it is competitively inhibited by competing SW cations. In agreement with this cation competition hypothesis, we demonstrated for the first time that "extra-intestinal" Ca²⁺ influx was inhibited by Mg²⁺ in both FW and SW-acclimated killifish. Following acute salinity transfer, extra-intestinal Ca²⁺ influx was rapidly regulated within 12-24 h, similar to Na⁺ and Cl^-. Ca²⁺ influx in FW was inhibited by La³⁺, an epithelial Ca²⁺ channel blocker, whereas La³⁺ had no significant effect in SW.

Integrative Population and Physiological Genomics Reveals Mechanisms of Adaptation in Killifish

Adaptive divergence between marine and freshwater (FW) environments is important in generating phyletic diversity within fishes, but the genetic basis of this process remains poorly understood. Genome selection scans can identify adaptive loci, but incomplete knowledge of genotype-phenotype connections makes interpreting their significance difficult. In contrast, association mapping (genome-wide association mapping [GWAS], random forest [RF] analyses) links genotype to phenotype, but offer limited insight into the evolutionary forces shaping variation. Here, we combined GWAS, RF, and selection scans to identify loci important in adaptation to FW environments. We utilized FW-native and brackish water (BW)-native populations of Atlantic killifish (Fundulus heteroclitus) as well as a naturally admixed population between the two. We measured morphology and multiple physiological traits that differ between populations and may contribute to osmotic adaptation (salinity tolerance, hypoxia tolerance, metabolic rate, body shape) and used a reduced representation approach for genome-wide genotyping. Our results show patterns of population divergence in physiological capabilities that are consistent with local adaptation. Population genomic scans between BW-native and FW-native populations identified genomic regions evolving by natural selection, whereas association mapping revealed loci that contribute to variation for each trait. There was substantial overlap in the genomic regions putatively under selection and loci associated with phenotypic traits, particularly for salinity tolerance, suggesting that these regions and genes are important for adaptive divergence between BW and FW environments. Together, these data provide insight into the mechanisms that enable diversification of fishes across osmotic boundaries.

The effects of acute transfer to freshwater on ion transporters of the pharyngeal cavity in European seabass (Dicentrarchus labrax)

Gene expression of key ion transporters (the Na⁺/K⁺-ATPase NKA, the Na⁺, K⁺-2Cl^- cotransporter NKCC1, and CFTR) in the gills, opercular inner epithelium, and pseudobranch of European seabass juveniles (Dicentrarchus labrax) were studied after acute transfer up to 4 days from seawater (SW) to freshwater (FW). The functional remodeling of these organs was also studied. Handling stress (SW to SW transfer) rapidly induced a transcript level decrease for the three ion transporters in the gills and operculum. NKA and CFTR relative expression level were stable, but in the pseudobranch, NKCC1 transcript levels increased (up to 2.4-fold). Transfer to FW induced even more organ-specific responses. In the gills, a 1.8-fold increase for NKA transcript levels occurs within 4 days post transfer with also a general decrease for CFTR and NKCC1. In the operculum, transcript levels are only slightly modified. In the pseudobranch, there is a transient NKCC1 increase followed by 0.6-fold decrease and 0.8-fold CFTR decrease. FW transfer also induced a density decrease for the opercular ionocytes and goblet cells. Therefore, gills and operculum display similar trends in SW-fish but have different responses in FW-transferred fish. Also, the pseudobranch presents contrasting response both in SW and in FW, most probably due to the high density of a cell type that is morphologically and functionally different compared to the typical gill-type ionocyte. This pseudobranch-type ionocyte could be involved in blood acid-base regulation masking a minor osmotic regulatory capacity of this organ compared to the gills.

Evaluation of Reference Genes to Analyze Gene Expression in Silverside Odontesthes humensis Under Different Environmental Conditions

Some mammalian reference genes, which are widely used to normalize the qRT-PCR, could not be used for this purpose due to its high expression variation. The normalization with false reference genes leads to misinterpretation of results. The silversides (Odontesthes spp.) has been used as models for evolutionary, osmoregulatory and environmental pollution studies but, up to now, there are no studies about reference genes in any Odontesthes species. Furthermore, many studies on silversides have used reference genes without previous validations. Thus, present study aimed to was to clone and sequence potential reference genes, thereby identifying the best ones in Odontesthes humensis considering different tissues, ages and conditions. For this purpose, animals belonging to three ages (adults, juveniles, and immature) were exposed to control, Roundup®, and seawater treatments for 24 h. Blood samples were subjected to flow-cytometry and other collected tissues to RNA extraction; cDNA synthesis; molecular cloning; DNA sequencing; and qRT-PCR. The candidate genes tested included 18s, actb, ef1a, eif3g, gapdh, h3a, atp1a, and tuba. Gene expression results were analyzed using five algorithms that ranked the candidate genes. The flow-cytometry data showed that the environmental challenges could trigger a systemic response in the treated fish. Even during this systemic physiological disorder, the consensus analysis of gene expression revealed h3a to be the most stable gene expression when only the treatments were considered. On the other hand, tuba was the least stable gene in the control and gapdh was the least stable in both Roundup® and seawater groups. In conclusion, the consensus analyses of different tissues, ages, and treatments groups revealed that h3a is the most stable gene whereas gapdh and tuba are the least stable genes, even being considered two constitutive genes.

Intestinal Na+, K+, 2Cl- cotransporter 2 plays a crucial role in hyperosmotic transitions of a euryhaline teleost

Euryhaline fishes, such as the red drum (Sciaenops ocellatus), must quickly transition between hyperosmotic and hypoosmotic physiological strategies. When freshwater individuals transition to seawater they are exposed to increased diffusive water loss and ion gain. To maintain osmoregulatory balance these animals must drink and absorb seawater through the intestine, followed by ion excretion at the gills. The ability of fishes to transition between strategies can limit the magnitude of osmotic shock that can be tolerated. Here, we demonstrate that red drum can tolerate direct transfer from freshwater to full‐strength seawater with marginal impacts on osmotic balance, as indicated by plasma and muscle ion concentration, as well as muscle water. Seawater transition is concurrent with a significant increase in intestinal fluid volume. Typical patterns of osmoregulatory plasticity were observed in the gill with increased expression of nkcc1 and cftr. Expression changes in the anterior intestine were observed after 24 h for nkcc2 with smaller and later responses observed for slc26a3, slc26a6, and nbc. Immunofluorescence staining demonstrated similar patterns of NKCC localization in freshwater and seawater intestines; however, reduced basolateral staining of V‐type ATPase was observed in seawater. Electrophysiological preparations demonstrated that seawater fish had increased absorptive current in the anterior intestine, which was significantly reduced in the presence of 10 μmol/L bumetanide. Overall, these results suggest that nkcc2 plays a crucial role during hyperosmotic transitions, and may be a more important complement to the well‐known bicarbonate secretion pathway than generally considered.

RNA sequencing analysis of transcriptional change in the freshwater mussel Elliptio complanata after environmentally relevant sodium chloride exposure

To identify potential biomarkers of salt stress in a freshwater sentinel species, we examined transcriptional responses of the common mussel Elliptio complanata to controlled sodium chloride (NaCl) exposures. Ribonucleic acid sequencing (RNA-Seq) of mantle tissue identified 481 transcripts differentially expressed in adult mussels exposed to 2 ppt NaCl (1.2 ppt chloride) for 7 d, of which 290 had nonoverlapping intervals. Differentially expressed gene categories included ion and transmembrane transport, oxidoreductase activity, maintenance of protein folding, and amino acid metabolism. The rate-limiting enzyme for synthesis of taurine, an amino acid frequently linked to osmotic stress in aquatic species, was upregulated, as was the transmembrane ion pump sodium/potassium adenosine 5'-triphosphatase. These patterns confirm a primary transcriptional response to the experimental dose, albeit likely overlapping with nonspecific secondary stress responses. Substantial involvement of the heat shock protein 70 chaperone family and the water-transporting aquaporin family was not detected, however, in contrast to some studies in other bivalves. A subset of the most significantly regulated genes was confirmed by quantitative polymerase chain reaction in an independent sample. Cluster analysis showed separation of mussels exposed to 2 ppt NaCl from control mussels in multivariate space, but mussels exposed to 1 ppt NaCl were largely indistinguishable from controls. Transcriptome-scale analysis of salt exposure under laboratory conditions efficiently identified candidate biomarkers for further functional analysis and field validation. Environ Toxicol Chem 2017;36:2352-2366. © Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Na+, K+-ATPase β1 subunit associates with α1 subunit modulating a "higher-NKA-in-hyposmotic media" response in gills of euryhaline milkfish, Chanos chanos

The euryhaline milkfish (Chanos chanos) is a popular aquaculture species that can be cultured in fresh water, brackish water, or seawater in Southeast Asia. In gills of the milkfish, Na⁺, K⁺-ATPase (i.e., NKA; sodium pump) responds to salinity challenges including changes in mRNA abundance, protein amount, and activity. The functional pump is composed of a heterodimeric protein complex composed of α- and β-subunits. Among the NKA genes, α1-β1 isozyme comprises the major form of NKA subunits in mammalian osmoregulatory organs; however, most studies on fish gills have focused on the α1 subunit and did not verify the α1-β1 isozyme. Based on the sequenced milkfish transcriptome, an NKA β1 subunit gene was identified that had the highest amino acid homology to β233, a NKA β1 subunit paralog originally identified in the eel. Despite this high level of homology to β233, phylogenetic analysis and the fact that only a single NKA β1 subunit gene exists in the milkfish suggest that the milkfish gene should be referred to as the NKA β1 subunit gene. The results of accurate domain prediction of the β1 subunit, co-localization of α1 and β1 subunits in epithelial ionocytes, and co-immunoprecipitation of α1 and β1 subunits, indicated the formation of a α1-β1 complex in milkfish gills. Moreover, when transferred to hyposmotic media (fresh water) from seawater, parallel increases in branchial mRNA and protein expression of NKA α1 and β1 subunits suggested their roles in hypo-osmoregulation of euryhaline milkfish. This study molecularly characterized the NKA β1 subunit and provided the first evidence for an NKA α1-β1 association in gill ionocytes of euryhaline teleosts.

RNA-Seq analysis of salinity stress-responsive transcriptome in the liver of spotted sea bass (Lateolabrax maculatus)

Salinity is one of the most prominent abiotic factors, which greatly influence reproduction, development, growth, physiological and metabolic activities of fishes. Spotted sea bass (Lateolabrax maculatus), as a euryhaline marine teleost, has extraordinary ability to deal with a wide range of salinity changes. However, this species is devoid of genomic resources, and no study has been conducted at the transcriptomic level to determine genes responsible for salinity regulation, which impedes the understanding of the fundamental mechanism conferring tolerance to salinity fluctuations. Liver, as the major metabolic organ, is the key source supplying energy for iono- and osmoregulation in fish, however, little attention has been paid to its salinity-related functions but which should not be ignored. In this study, we perform RNA-Seq analysis to identify genes involved in salinity adaptation and osmoregulation in liver of spotted sea bass, generating from the fishes exposed to low and high salinity water (5 vs 30ppt). After de novo assembly, annotation and differential gene expression analysis, a total of 455 genes were differentially expressed, including 184 up-regulated and 271 down-regulated transcripts in low salinity-acclimated fish group compared with that in high salinity-acclimated group. A number of genes with a potential role in salinity adaptation for spotted sea bass were classified into five functional categories based on the gene ontology (GO) and enrichment analysis, which include genes involved in metabolites and ion transporters, energy metabolism, signal transduction, immune response and structure reorganization. The candidate genes identified in L. maculates liver provide valuable information to explore new pathways related to fish salinity and osmotic regulation. Besides, the transcriptomic sequencing data supplies significant resources for identification of novel genes and further studying biological questions in spotted sea bass.

A Stenohaline Medaka, Oryzias woworae, Increases Expression of Gill Na(+), K(+)-ATPase and Na(+), K(+), 2Cl(-) Cotransporter 1 to Tolerate Osmotic Stress

The present study aimed to evaluate the osmoregulatory mechanism of Daisy's medaka, O. woworae,as well as demonstrate the major factors affecting the hypo-osmoregulatory characteristics of euryhaline and stenohaline medaka. The medaka phylogenetic tree indicates that Daisy's medaka belongs to the celebensis species group. The salinity tolerance of Daisy's medaka was assessed. Our findings revealed that 20‰ (hypertonic) saltwater (SW) was lethal to Daisy's medaka. However, 62.5% of individuals survived 10‰ (isotonic) SW with pre-acclimation to 5‰ SW for one week. This transfer regime, "Experimental (Exp.) 10‰ SW", was used in the following experiments. After 10‰ SW-transfer, the plasma osmolality of Daisy's medaka significantly increased. The protein abundance and distribution of branchial Na(+), K(+)-ATPase (NKA) and Na(+), K(+), 2Cl(-) cotransporter 1 (NKCC1) were also examined after transfer to 10‰ SW for one week. Gill NKA activity increased significantly after transfer to 10‰ SW. Meanwhile, elevation of gill NKA αα-subunit protein-abundance was found in the 10‰ SW-acclimated fish. In gill cross-sections, more and larger NKA-immunoreactive (NKA-IR) cells were observed in the Exp. 10‰ SW medaka. The relative abundance of branchial NKCC1 protein increased significantly after transfer to 10‰ SW. NKCC1 was distributed in the basolateral membrane of NKA-IR cells of the Exp. 10‰ SW group. Furthermore, a higher abundance of NKCC1 protein was found in the gill homogenates of the euryhaline medaka, O. dancena, than in that of the stenohaline medaka, O. woworae.

Molecular characterization and expression of Na+/K+-ATPase α1 isoforms in the European sea bass Dicentrarchus labrax osmoregulatory tissues following salinity transfer

The Na⁺/K⁺-ATPase (NKA) is considered as the main pump involved in active ion transport. In the European sea bass, Dicentrarchus labrax, we found two genes encoding for the alpha 1 subunit isoforms (NKA α1a and NKA α1b). NKA α1a and NKA α1b isoform amino acid (aa) sequences were compared through phylogeny and regarding key functional motifs between salmonids and other acanthomorph species. Analysis of aa sequences of both isoforms revealed a high degree of conservation across teleosts. The expression pattern of both nka α1a and nka α1b was measured in the gill, kidney and posterior intestine of fish in seawater (SW) and transferred to fresh water (FW) at different exposure times. Nka α1a was more expressed than nka α1b whatever the condition and the tissue analyzed. After long-term salinity acclimation (2.5 years) either in FW or SW, transcript levels of nka α1a were higher in the kidney followed by the posterior intestine and the gill. Compared to SW conditions, expression of nka α1a in FW was significantly increased or decreased, respectively, in gill and posterior intestine. In contrast, branchial nka α1b was significantly decreased in FW-acclimated fish. Short-term FW acclimation seems to rapidly increase nka α1a transcript levels in the kidney unlike in gill tissues where different gene expression levels are detected only after long-term acclimation.

Inbreeding effects on gene-specific DNA methylation among tissues of Chinook salmon

Inbreeding depression is the loss of fitness resulting from the mating of genetically related individuals. Traditionally, the study of inbreeding depression focused on genetic effects, although recent research has identified DNA methylation as also having a role in inbreeding effects. Since inbreeding depression and DNA methylation change with age and environmental stress, DNA methylation is a likely candidate for the regulation of genes associated with inbreeding depression. Here, we use a targeted, multigene approach to assess methylation at 22 growth-, metabolic-, immune- and stress-related genes. We developed PCR-based DNA methylation assays to test the effects of intense inbreeding on intragenic gene-specific methylation in inbred and outbred Chinook salmon. Inbred fish had altered methylation at three genes, CK-1, GTIIBS and hsp70, suggesting that methylation changes associated with inbreeding depression are targeted to specific genes and are not whole-genome effects. While we did not find a significant inbreeding by age interaction, we found that DNA methylation generally increases with age, although methylation decreased with age in five genes, CK-1, IFN-ɣ, HNRNPL, hsc71 and FSHb, potentially due to environmental context and sexual maturation. As expected, we found methylation patterns differed among tissue types, highlighting the need for careful selection of target tissue for methylation studies. This study provides insight into the role of epigenetic effects on ageing, environmental response and tissue function in Chinook salmon and shows that methylation is a targeted and regulated cellular process. We provide the first evidence of epigenetically based inbreeding depression in vertebrates.

Hypersalinity drives physiological and morphological changes in Limia perugiae (Poeciliidae)

A fundamental question in biology is how an organism's morphology and physiology are shaped by its environment. Here, we evaluate the effects of a hypersaline environment on the morphology and physiology of a population of livebearing fish in the genus Limia (Poeciliidae). We sampled from two populations of Limia perugiae (one freshwater and one hypersaline) in the southwest Dominican Republic. We evaluated relative abundance of osmoregulatory proteins using western blot analyses and used a geometric morphometric approach to evaluate fine-scale changes to size and shape. Our data show that gill tissue isolated from hypersaline fish contained approximately two and a half times higher expression of Na(+)/K(+) ATPase proteins. We also show evidence for mitochondrial changes within the gills, with eight times more complex I and four times higher expression of ATP synthase within the gill tissue from the hypersaline population. The energetic consequences to Limia living in saline and hypersaline environments may be a driver for phenotypic diversity, reducing the overall body size and changing the relative size and shape of the head, as well as impeding the growth of secondary sex features among the males.

Silver toxicity across salinity gradients: the role of dissolved silver chloride species (AgCl x ) in Atlantic killifish (Fundulus heteroclitus) and medaka (Oryzias latipes) early life-stage toxicity

The influence of salinity on Ag toxicity was investigated in Atlantic killifish (Fundulus heteroclitus) early life-stages. Embryo mortality was significantly reduced as salinity increased and Ag(+) was converted to AgCl(solid). However, as salinity continued to rise (>5 ‰), toxicity increased to a level at least as high as observed for Ag(+) in deionized water. Rather than correlating with Ag(+), Fundulus embryo toxicity was better explained (R(2) = 0.96) by total dissolved Ag (Ag(+), AgCl2 (-), AgCl3 (2-), AgCl4 (3-)). Complementary experiments were conducted with medaka (Oryzias latipes) embryos to determine if this pattern was consistent among evolutionarily divergent euryhaline species. Contrary to Fundulus data, medaka toxicity data were best explained by Ag(+) concentrations (R(2) = 0.94), suggesting that differing ionoregulatory physiology may drive observed differences. Fundulus larvae were also tested, and toxicity did increase at higher salinities, but did not track predicted silver speciation. Alternatively, toxicity began to increase only at salinities above the isosmotic point, suggesting that shifts in osmoregulatory strategy at higher salinities might be an important factor. Na(+) dysregulation was confirmed as the mechanism of toxicity in Ag-exposed Fundulus larvae at both low and high salinities. While Ag uptake was highest at low salinities for both Fundulus embryos and larvae, uptake was not predictive of toxicity.

High levels of plasma cortisol and impaired hypoosmoregulation in a mutant medaka deficient in P450c17I

scl is a spontaneous medaka mutant deficient in P450c17I, which is required for production of sex steroids, but not of cortisol, the major role of which is osmoregulation in teleost fish. The scl mutant provides a new model to study the functions of these hormones. We first found that fish homozygous for this mutation have plasma cortisol constitutively at a high physiological level (1000 nM). Since we previously showed that this level reversed the seawater-type differentiation of the medaka gastrointestinal tract, hypoosmoregulation of the scl mutant was analyzed. Muscle water contents in freshwater were normal in scl homozygotes, but the contents were lower than those of the wild type (WT) after seawater transfer. There were no differences in gill mRNA levels of corticosteroid receptors or ion transporters between scl homozygotes and WT. In the intestine, expression of glucocorticoid receptors and Na(+)/K(+)/2Cl(-) cotransporter were induced in WT during seawater acclimation, but not in scl homozygotes. The high plasma cortisol may prevent hypoosmoregulation by inhibition of increased intestinal water absorption, essentially by the Na(+)/K(+)/2Cl(-) cotransporter, in seawater.

iTRAQ proteomic analysis of salinity acclimation proteins in the gill of tropical marbled eel (Anguilla marmorata)

Osmoregulation plays an important role in the migration process of catadromous fish. The osmoregulatory mechanisms of tropical marbled eel (Anguilla marmorata), a typical catadromous fish, did not gain sufficient attention, especially at the molecular level. In order to enrich the protein database of A. marmorata, a proteomic analysis has been carried out by iTRAQ technique. Among 1937 identified proteins in gill of marbled eel, the expression of 1560 proteins (80 %) was quantified. Compared with the protein expression level in the gill of marbled eel in freshwater (salinity of 0 ‰), 336 proteins were up-regulated and 67 proteins were down-regulated in seawater (salinity of 25 ‰); 33 proteins were up-regulated and 32 proteins were down-regulated in brackish water (salinity of 10 ‰). These up-regulated proteins including Na(+)/K(+)-ATPase, V-type proton ATPase, sodium-potassium-chloride co-transporter and heat shock protein 90 were enriched in many KEGG-annotated pathways, which are related to different functions of the gill. The up-regulated oxidative phosphorylation and seleno-compound metabolism pathways involve the synthesis and consumption of ATP, which represents extra energy consumption. Another identified pathway is the ribosome pathway in which a large number of up-regulated proteins are involved. It is also more notable that tight junction and cardiac muscle contraction pathways may have correlation with ion transport in gill cells. This is the first report describing the proteome of A. marmorata for acclimating to the change of salinity. These results provide a functional database for migratory fish and point out some possible new interactions on osmoregulation in A. marmorata.