Effects of salinity on upstream-migrating, spawning sea lamprey, Petromyzon marinus

Genetic Identification of Lamprey Genera and Anadromous Ecotypes in Watersheds of the Northeastern Pacific Ocean

Nonparasitic, nonmigratory Western Brook Lamprey (WBL; Lampetra ayresii), and parasitic, anadromous Western River Lamprey (WRL; L. ayresii) are sympatric lampreys that likely represent different life history variations of a single species. Novel genetic tools are critical for differentiating WBL and WRL, whose larvae preclude morphological identification (ID) and will enable comprehensive assessment of imperiled native lampreys of the Northeastern Pacific (including WBL, WRL, and Pacific Lamprey, Entosphenus tridentatus). We developed 47 candidate single nucleotide polymorphism (SNP) markers using whole genome resequencing of WBL (N = 24) and WRL (N = 15) from Ksi Ts'oohl Ts'ap Creek (Nass River, British Columbia, Canada) which are likely ecotypes distinguished by few divergent SNPs across multiple chromosomes. We used five novel candidate SNPs to perform genetic ID of WBL and WRL ecotypes in collections of mixed native lampreys from lower Columbia River tributaries (N = 1474), Ksi Ts'oohl Ts'ap Creek (N = 352), and ocean phase WRL from the Georgia Basin (Salish Sea, British Columbia, Canada; N = 91). Two previously published SNPs were used to ID genera, Entosphenus versus Lampetra. Morphological ID utilized photographs collected from a subset of genotyped lampreys, and high concordance was demonstrated between ID methods for genera (99%) and Lampetra ecotypes (> 98%). We characterized spatial and temporal composition of lamprey genera and ecotypes surveyed across NE Pacific tributaries under the expectation these compositions would be similar across nearby sites and across years at the same site. Proportions of lamprey genera were highly variable within regions and across years; however, Lampetra ecotypic proportions were spatially and temporally stable. WRL were rare in lower Columbia tributaries (~1% average rate among Lampetra) and common further north (> 40% of Lampetra). Genetic ID methods are powerful monitoring tools that create the novel ability to ascertain genera and ecotypes regardless of life stage, while increasing the efficiency of surveys by eliminating time-intensive morphological data collection.

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.

Varying Selection Pressure for a Na+ Sensing Site in Epithelial Na+ Channel Subunits Reflect Divergent Roles in Na+ Homeostasis

The epithelial Na+ channel (ENaC) emerged early in vertebrates and has played a role in Na+ and fluid homeostasis throughout vertebrate evolution. We previously showed that proteolytic activation of the channel evolved at the water-to-land transition of vertebrates. Sensitivity to extracellular Na+, known as Na+ self-inhibition, reduces ENaC function when Na+ concentrations are high and is a distinctive feature of the channel. A fourth ENaC subunit, δ, emerged in jawed fishes from an α subunit gene duplication. Here, we analyzed 849 α and δ subunit sequences and found that a key Asp in a postulated Na+ binding site was nearly always present in the α subunit, but frequently lost in the δ subunit (e.g. human). Analysis of site evolution and codon substitution rates provide evidence that the ancestral α subunit had the site and that purifying selection for the site relaxed in the δ subunit after its divergence from the α subunit, coinciding with a loss of δ subunit expression in renal tissues. We also show that the proposed Na+ binding site in the α subunit is a bona fide site by conferring novel function to channels comprising human δ subunits. Together, our findings provide evidence that ENaC Na+ self-inhibition improves fitness through its role in Na+ homeostasis in vertebrates.

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Effects of the environment on the evolution of the vertebrate urinary tract

Evolution of the vertebrate urinary system occurs in response to numerous selective pressures, which have been incompletely characterized. Developing research into urinary evolution led to the occurrence of clinical applications and insights in paediatric urology, reproductive medicine, urolithiasis and other domains. Each nephron segment and urinary organ has functions that can be contextualized within an evolutionary framework. For example, the structure and function of the glomerulus and proximal tubule are highly conserved, enabling blood cells and proteins to be retained, and facilitating the elimination of oceanic Ca+ and Mg+. Urea emerged as an osmotic mediator during evolution, as cells of large organisms required increased precision in the internal regulation of salinity and solutes. As the first vertebrates moved from water to land, acid-base regulation was shifted from gills to skin and kidneys in amphibians. In reptiles and birds, solute regulation no longer occurred through the skin but through nasal salt glands and post-renally, within the cloaca and the rectum. In placental mammals, nasal salt glands are absent and the rectum and urinary tracts became separate, which limited post-renal urine concentration and led to the necessity of a kidney capable of high urine concentration. Considering the evolutionary and environmental selective pressures that have contributed to renal evolution can help to gain an increased understanding of renal physiology.

Seascape genomics reveals limited dispersal and suggests spatially varying selection among European populations of sea lamprey (Petromyzon marinus)

Sea lamprey Petromyzon marinus is an anadromous and semelparous fish without homing behaviors. Despite being a freshwater, free-living organism for a large part of their life cycle, its adulthood is spent as a parasite of marine vertebrates. In their native European range, while it is well-established that sea lampreys comprise a single nearly-panmictic population, few studies have further explored the evolutionary history of natural populations. Here, we performed the first genome-wide characterization of sea lamprey's genetic diversity in their European natural range. The objectives were to investigate the connectivity among river basins and explore evolutionary processes mediating dispersal during the marine phase, with the sequencing of 186 individuals from 8 locations spanning the North Eastern Atlantic coast and the North Sea with double-digest RAD-sequencing, obtaining a total of 30,910 bi-allelic SNPs. Population genetic analyses reinforced the existence of a single metapopulation encompassing freshwater spawning sites within the North Eastern Atlantic and the North Sea, though the prevalence of private alleles at northern latitudes suggested some limits to the species' dispersal. Seascape genomics suggested a scenario where oxygen concentration and river runoffs impose spatially varying selection across their distribution range. Exploring associations with the abundance of potential hosts further suggested that hake and cod could also impose selective pressures, although the nature of such putative biotic interactions was unresolved. Overall, the identification of adaptive seascapes in a panmictic anadromous species could contribute to conservation practices by providing information for restoration activities to mitigate local extinctions on freshwater sites.

The GH/IGF axis in the sea lamprey during metamorphosis and seawater acclimation

How the growth hormone (GH)/insulin-like growth factor (IGF) system affects osmoregulation in basal vertebrates remains unknown. We examined changes in the expression of components of the GH/IGF axis and gill ion transporters during metamorphosis and following seawater (SW) exposure of sea lamprey. During metamorphosis, increases in gill nka and nkcc1 and salinity tolerance were accompanied by increases in pituitary gh, liver igf1, gill ghr and igf1, but not liver ghr. SW exposure of fully metamorphosed sea lamprey resulted in slight increases in plasma chloride concentrations after SW exposure, indicating a high level of SW tolerance, but no major changes in mRNA levels of gill ion transporters or components of the GH/IGF axis. Our results indicate that metamorphosis is a critical point in the lifecycle of sea lamprey for stimulation of the GH/IGF axis and is temporally associated with and likely promotes metamorphosis and SW tolerance.

Discovery of prolactin-like in lamprey: Role in osmoregulation and new insight into the evolution of the growth hormone/prolactin family

We used a representative of one of the oldest extant vertebrate lineages (jawless fish or agnathans) to investigate the early evolution and function of the growth hormone (GH)/prolactin (PRL) family. We identified a second member of the GH/PRL family in an agnathan, the sea lamprey (Petromyzon marinus). Structural, phylogenetic, and synteny analyses supported the identification of this hormone as prolactin-like (PRL-L), which has led to added insight into the evolution of the GH/PRL family. At least two ancestral genes were present in early vertebrates, which gave rise to distinct GH and PRL-L genes in lamprey. A series of gene duplications, gene losses, and chromosomal rearrangements account for the diversity of GH/PRL-family members in jawed vertebrates. Lamprey PRL-L is produced in the proximal pars distalis of the pituitary and is preferentially bound by the lamprey PRL receptor, whereas lamprey GH is preferentially bound by the lamprey GH receptor. Pituitary PRL-L messenger RNA (mRNA) levels were low in larvae, then increased significantly in mid-metamorphic transformers (stage 3); thereafter, levels subsided in final-stage transformers and metamorphosed juveniles. The abundance of PRL-L mRNA and immunoreactive protein increased in the pituitary of juveniles under hypoosmotic conditions, and treatment with PRL-L blocked seawater-associated inhibition of freshwater ion transporters. These findings clarify the origin and divergence of GH/PRL family genes in early vertebrates and reveal a function of PRL-L in osmoregulation of sea lamprey, comparable to a role of PRLs that is conserved in jawed vertebrates.

Tissue and salinity specific Na+/Cl- cotransporter (NCC) orthologues involved in the adaptive osmoregulation of sea lamprey (Petromyzon marinus)

Two orthologues of the gene encoding the Na⁺-Cl⁻ cotransporter (NCC), termed ncca and nccb, were found in the sea lamprey genome. No gene encoding the Na⁺-K⁺-2Cl⁻ cotransporter 2 (nkcc2) was identified. In a phylogenetic comparison among other vertebrate NCC and NKCC sequences, the sea lamprey NCCs occupied basal positions within the NCC clades. In freshwater, ncca mRNA was found only in the gill and nccb only in the intestine, whereas both were found in the kidney. Intestinal nccb mRNA levels increased during late metamorphosis coincident with salinity tolerance. Acclimation to seawater increased nccb mRNA levels in the intestine and kidney. Electrophysiological analysis of intestinal tissue ex vivo showed this tissue was anion absorptive. After seawater acclimation, the proximal intestine became less anion absorptive, whereas the distal intestine remained unchanged. Luminal application of indapamide (an NCC inhibitor) resulted in 73% and 30% inhibition of short-circuit current (I_sc) in the proximal and distal intestine, respectively. Luminal application of bumetanide (an NKCC inhibitor) did not affect intestinal I_sc. Indapamide also inhibited intestinal water absorption. Our results indicate that NCCb is likely the key ion cotransport protein for ion uptake by the lamprey intestine that facilitates water absorption in seawater. As such, the preparatory increases in intestinal nccb mRNA levels during metamorphosis of sea lamprey are likely critical to development of whole animal salinity tolerance.

11-Deoxycortisol is a stress responsive and gluconeogenic hormone in a jawless vertebrate, the sea lamprey (Petromyzon marinus)

Although corticosteroid-mediated hepatic gluconeogenic activity in response to stress has been extensively studied in fishes and other vertebrates, there is little information on the stress response in basal vertebrates. In sea lamprey (Petromyzon marinus), a representative member of the most basal extant vertebrate group Agnatha, 11-deoxycortisol and deoxycorticosterone are the major circulating corticosteroids. The present study examined changes in circulating glucose and 11-deoxycortisol concentrations in response to a physical stressor. Furthermore, the gluconeogenic actions of 11-deoxycortisol and deoxycorticosterone were examined. Within 6 h of exposure of larval and juvenile sea lamprey to an acute handling stress, plasma 11-deoxycortisol levels increased 15- and 6-fold, respectively, and plasma glucose increased 3- and 4-fold, respectively. Radiometric receptor binding studies revealed that a corticosteroid receptor (CR) is present in the liver at lower abundance than in other tissues (gill and anterior intestine) and that the binding affinity of the liver CR was similar for 11-deoxycortisol and deoxycorticosterone. Transcriptional tissue profiles indicate a wide distribution of cr transcription, kidney-specific transcription of steroidogenic acute regulatory protein (star) and liver-specific transcription of phosphoenolpyruvate carboxykinase (pepck). Ex vivo incubation of liver tissue with 11-deoxycortisol resulted in dose-dependent increases in pepck mRNA levels. Finally, intraperitoneal administration of 11-deoxycortisol and deoxycorticosterone demonstrated that only 11-deoxycortisol resulted in an increase in plasma glucose. Together, these results provide the first direct evidence for the gluconeogenic activity of 11-deoxycortisol in an agnathan, indicating that corticosteroid regulation of plasma glucose is a basal trait among vertebrates.

Corticosteroid control of Na+/K+-ATPase in the intestine of the sea lamprey (Petromyzon marinus)

Anadromous sea lamprey (Petromyzon marinus) larvae undergo a months-long true metamorphosis during which they develop seawater (SW) tolerance prior to downstream migration and SW entry. We have previously shown that intestinal Na⁺/K⁺-ATPase (NKA) activity increases during metamorphosis and is critical to the osmoregulatory function of the intestine in SW. The present study investigated the role of 11-deoxycortisol (S) in controlling NKA in the anterior (AI) and posterior (PI) intestine during sea lamprey metamorphosis. In a tissue profile, nka mRNA and protein were most abundant in the gill, kidney, and AI. During metamorphosis, AI nka mRNA increased 10-fold, whereas PI nka mRNA did not change. Specific corticosteroid receptors were found in the AI, which had a higher binding affinity for S compared to 11-deoxycorticosterone (DOC). In vivo administration of S in mid-metamorphic lamprey upregulated NKA activity 3-fold in the AI and PI, whereas administration of DOC did not affect intestinal NKA activity. During a 24 h SW challenge test, dehydration of white muscle moisture was rescued by prior treatment with S, which was associated with increased intestinal nka mRNA and NKA activity. These results indicate that intestinal osmoregulation in sea lamprey is a target for control by S during metamorphosis and the development of SW tolerance.

Ecological adaptation in Atlantic herring is associated with large shifts in allele frequencies at hundreds of loci

Atlantic herring is widespread in North Atlantic and adjacent waters and is one of the most abundant vertebrates on earth. This species is well suited to explore genetic adaptation due to minute genetic differentiation at selectively neutral loci. Here, we report hundreds of loci underlying ecological adaptation to different geographic areas and spawning conditions. Four of these represent megabase inversions confirmed by long read sequencing. The genetic architecture underlying ecological adaptation in herring deviates from expectation under a classical infinitesimal model for complex traits because of large shifts in allele frequencies at hundreds of loci under selection.

Functional re-organization of the gills of metamorphosing sea lamprey (Petromyzon marinus): preparation for a blood diet and the freshwater to seawater transition

Sea lamprey (Petromyzon marinus) begin life as filter-feeding larvae (ammocoetes) before undergoing a complex metamorphosis into parasitic juveniles, which migrate to the sea where they feed on the blood of large-bodied fishes. The greater protein intake during this phase results in marked increases in the production of nitrogenous wastes (N-waste), which are excreted primarily via the gills. However, it is unknown how gill structure and function change during metamorphosis and how it is related to modes of ammonia excretion, nor do we have a good understanding of how the sea lamprey's transition from fresh water (FW) to sea water (SW) affects patterns and mechanisms of N-waste excretion in relation to ionoregulation. Using immunohistochemistry, we related changes in the gill structure of larval, metamorphosing, and juvenile sea lampreys to their patterns of ammonia excretion (J_amm) and urea excretion (J_urea) in FW, and following FW to artificial seawater (ASW) transfer. Rates of J_amm and J_urea were low in larval sea lamprey and increased in feeding juvenile, parasitic sea lamprey. In freshwater-dwelling ammocoetes, immunohistochemical analysis revealed that Rhesus glycoprotein C-like protein (Rhcg-like) was diffusely distributed on the lamellar epithelium, but following metamorphosis, Rhcg-like protein was restricted to SW mitochondrion-rich cells (MRCs; ionocytes) between the gill lamellae. Notably, these interlamellar Rhcg-like proteins co-localized with Na⁺/K⁺-ATPase (NKA), which increased in expression and activity by almost tenfold during metamorphosis. The distribution of V-type H⁺-ATPase (V-ATPase) on the lamellae decreased following metamorphosis, indicating it may have a more important role in acid-base regulation and Na⁺ uptake in FW, compared to SW. We conclude that the re-organization of the sea lamprey gill during metamorphosis not only plays a critical role in allowing them to cope with greater salinity following the FW-SW transition, but that it simultaneously reflects fundamental changes in methods used to excrete ammonia.

Evolution of epithelial sodium channels: current concepts and hypotheses

The conquest of freshwater and terrestrial habitats was a key event during vertebrate evolution. Occupation of low-salinity and dry environments required significant osmoregulatory adaptations enabling stable ion and water homeostasis. Sodium is one of the most important ions within the extracellular liquid of vertebrates, and molecular machinery for urinary reabsorption of this electrolyte is critical for the maintenance of body osmoregulation. Key ion channels involved in the fine-tuning of sodium homeostasis in tetrapod vertebrates are epithelial sodium channels (ENaCs), which allow the selective influx of sodium ions across the apical membrane of epithelial cells lining the distal nephron or the colon. Furthermore, ENaC-mediated sodium absorption across tetrapod lung epithelia is crucial for the control of liquid volumes lining the pulmonary surfaces. ENaCs are vertebrate-specific members of the degenerin/ENaC family of cation channels; however, there is limited knowledge on the evolution of ENaC within this ion channel family. This review outlines current concepts and hypotheses on ENaC phylogeny and discusses the emergence of regulation-defining sequence motifs in the context of osmoregulatory adaptations during tetrapod terrestrialization. In light of the distinct regulation and expression of ENaC isoforms in tetrapod vertebrates, we discuss the potential significance of ENaC orthologs in osmoregulation of fishes as well as the putative fates of atypical channel isoforms in mammals. We hypothesize that ancestral proton-sensitive ENaC orthologs might have aided the osmoregulatory adaptation to freshwater environments whereas channel regulation by proteases evolved as a molecular adaptation to lung liquid homeostasis in terrestrial tetrapods.

Best practices for non-lethal blood sampling of fish via the caudal vasculature

Blood sampling through the caudal vasculature is a widely used technique in fish biology for investigating organismal health and physiology. In live fishes, it can provide a quick, easy and relatively non-invasive method for obtaining a blood sample (cf. cannulation and cardiac puncture). Here, a general set of recommendations are provided for optimizing the blood sampling protocol that reflects best practices in animal welfare and sample integrity. This includes selecting appropriate use of anaesthetics for blood sampling as well as restraint techniques for situations where sedation is not used. In addition, ideal sampling environments where the fish can freely ventilate and strategies for minimizing handling time are discussed. This study summarizes the techniques used for extracting blood from the caudal vasculature in live fishes, highlighting the phlebotomy itself, the timing of sampling events and acceptable blood sample volumes. This study further discuss considerations for selecting appropriate physiological metrics when sampling in the caudal region and the potential benefits that this technique provides with respect to long-term biological assessments. Although general guidelines for blood sampling are provided here, it should be recognized that contextual considerations (e.g., taxonomic diversity, legal matters, environmental constraints) may influence the approach to blood sampling. Overall, it can be concluded that when done properly, blood sampling live fishes through the caudal vasculature is quick, efficient and minimally invasive, thus promoting conditions where live release of focal animals is possible.

Pseudobranch mimics gill in expressing Na+K+-ATPase 1 α-subunit and carbonic anhydrase in concert with H+-ATPase in adult hilsa (Tenualosa ilisha) during river migration

In hilsa (Tenualosa ilisha), pseudobranch comprises a row of parallel filaments bear numerous leaf-like lamellae arranged on both sides throughout its length. The purpose of this study was to elucidate involvement of pseudobranchial Na⁺, K⁺-ATPase (NKA) 1 α-subunit, and carbonic anhydrase (CA) in concert with H⁺-ATPase (HAT) compared to their branchial counterparts in freshwater acclimation of hilsa during spawning migration from off-shore of the Bay of Bengal to the Bhagirathi-Hooghly zones of the Ganga river system in India. Adult hilsa fish were collected from seawater (SW), freshwater 1 (FW1), and freshwater 2 (FW2) locations, where the salinity level was 26-28‰, 1-5‰, and 0-0.04‰, respectively. Hilsa migrating through freshwater showed a consistent decrease in the plasma osmolality, sodium (Na⁺) and chloride (Cl^-) ion levels indicates unstable ionic homeostasis. The mRNA expression and activity of NKA 1 α-subunit in pseudobranch as well as in true gills declined with the migration to upstream locations. The pseudobranchial CA activity almost mirrors its branchial counterpart most notably while hilsa entered the freshwater zone, in the upstream river suggesting its diverse role in hypo-osmotic regulatory acclimation. Nevertheless, the H⁺-ATPase activity of both the tissues increased with the freshwater entry and remained similar during up-river movement into the freshwater environment. The results confirm that the pseudobranchial NKA 1 α-subunit mRNA expression and activity mimic its branchial counterpart in the process of ionoregulatory acclimation during migration through salt barriers. Also, the increase in the activities of pseudobranchial and branchial CA in concert with H⁺-ATPase (HAT) during freshwater acclimation of hilsa suggests their critical involvement in ion uptake.

Claudins of sea lamprey (Petromyzon marinus) - organ-specific expression and transcriptional responses to water of varying ion content

The role of lamprey epithelium tight junctions (TJs) in the regulation of salt and water balance is poorly understood. This study reported on claudin (Cldn) TJ protein transcripts of pre-metamorphic larval and post-metamorphic juvenile sea lamprey (Petromyzon marinus) and the transcriptional response of genes encoding Cldns to changed environmental ion levels. Transcripts encoding Cldn-3b, -4, -5, -10, -14, -18 and -19 were identified, and mRNA expression profiles revealed the organ-specific presence of cldn-5 and -14, broad expression of cldn-3b, -4, -10, -18 and -19 and spatial differences in the mRNA abundance of cldn-4, -3b and -14 along the ammocoete intestine. Expression profiles were qualitatively similar in ammocoetes and juvenile fishes. Transcript abundance of genes encoding Cldns in osmoregulatory organs (gill, kidney, intestine and skin) was subsequently investigated after exposure of ammocoetes to ion-poor water (IPW) and juveniles to hyperosmotic conditions [60% sea water (SW)]. IPW-acclimated ammocoetes increased mRNA abundance of nearly all cldns in the gill. Simultaneously, cldn-10 abundance increased in the skin, whereas cldn-4, -14 and -18 decreased in the kidney. Ammocoete cldn mRNA abundance in the intestine was altered in a region-specific manner. In contrast, cldn transcript abundance was mostly downregulated in osmoregulatory organs of juvenile fish acclimated to SW - cldn-3b, -10 and -19 in the gill; cldn-3b, -4, -10 and -19 in the skin; cldn-3b in the kidney; and cldn-3b and -14 in the intestine. Data support the idea that Cldn TJ proteins play an important role in the osmoregulatory physiology of pre- and post-metamorphic sea lamprey and that Cldn participation can occur across organs, in an organ-specific manner, as well as differ spatially within organs, which contributes to the regulation of salt and water balance in these fishes.

Osmoregulatory role of the intestine in the sea lamprey (Petromyzon marinus)

Lampreys are the most basal vertebrates with an osmoregulatory strategy. Previous research has established that the salinity tolerance of sea lamprey increases dramatically during metamorphosis, but underlying changes in the gut have not been examined. In the present work, we examined changes in intestinal function during metamorphosis and seawater exposure of sea lamprey (Petromyzon marinus). Fully metamorphosed juvenile sea lamprey had 100% survival after direct exposure to 35 parts per thousand seawater (SW) and only slight elevations in plasma chloride (Cl^-) levels. Drinking rates of sea lamprey juveniles in seawater were 26-fold higher than juveniles in freshwater (FW). Na⁺-K⁺-ATPase (NKA) activity in the anterior and posterior intestine increased 12- and 3-fold, respectively, during metamorphosis, whereas esophageal NKA activity was lower than in the intestine and did not change with development. Acclimation to SW significantly enhanced NKA activity in the posterior intestine but did not significantly change NKA activity in the anterior intestine, which remained higher than that in the posterior intestine. Intestinal Cl^- and water uptake, which were observed in ex vivo preparations of anterior and posterior intestine under both symmetric and asymmetric conditions, were higher in juveniles than in larvae and were similar in magnitude of those of teleost fish. Inhibition of NKA by ouabain in ex vivo preparations inhibited intestinal water absorption by 64%. Our results indicate drinking and intestinal ion and water absorption are important to osmoregulation in SW and that preparatory increases in intestinal NKA activity are important to the development of salinity tolerance that occurs during sea lamprey metamorphosis.

Functional characterization and osmoregulatory role of the Na+-K+-2Cl- cotransporter in the gill of sea lamprey (Petromyzon marinus), a basal vertebrate

The present study provides molecular and functional characterization of Na⁺-K⁺-2Cl^- cotransporter (NKCC1/Slc12a2) in the gills of sea lamprey (Petromyzon marinus), the most basal extant vertebrate with an osmoregulatory strategy. We report the full-length peptide sequence for the lamprey Na-K-Cl cotransporter 1 (NKCC1), which we show groups strongly with and occupies a basal position among other vertebrate NKCC1 sequences. In postmetamorphic juvenile lamprey, nkcc1 mRNA was present in many tissues but was fivefold higher in the gill than any other examined tissue, and NKCC1 protein was only detected in the gill. Gill mRNA and protein abundances of NKCC1 and Na⁺-K⁺-ATPase (NKA/Atp1a1) were significantly upregulated (20- to 200-fold) during late metamorphosis in fresh water, coinciding with the development of salinity tolerance, and were upregulated an additional twofold after acclimation to seawater (SW). Immunohistochemistry revealed that NKCC1 in the gill is found in filamental ionocytes coexpressing NKA, which develop during metamorphosis in preparation for SW entry. Lamprey treated with bumetanide, a widely used pharmacological inhibitor of NKCC1, exhibited higher plasma Cl^- and osmolality as well as reduced muscle water content after 24 h in SW; there were no effects of bumetanide in freshwater-acclimated lamprey. This work provides the first functional characterization of NKCC1 as a mechanism for branchial salt secretion in lampreys, providing evidence that this mode of Cl^- secretion has been present among vertebrates for ~550 million years.

Alteration in branchial NKA and NKCC ion-transporter expression and ionocyte distribution in adult hilsa during up-river migration

Hilsa (Tenualosa ilisha) is a clupeid that migrates from the off-shore area through the freshwater river for spawning. The purpose of this study was to investigate the involvement of branchial Na⁺/K⁺-ATPase (NKA) and Na⁺/K⁺/2Cl^- cotransporter (NKCC) in maintaining ionic homeostasis in hilsa while moving across the salt barriers. Hilsa, migrating through marine and brackish waters, did not show any significant decline in NKA activity, plasma osmolality, and plasma ionic concentration. In contrast, all the parameters declined significantly, after the fish reached in freshwater zone of the river. Immunoblotting with NKA α antibody recognized two bands in gill homogenates. The intensity of the higher molecular NKA band decreased, while the other band subsequently increased accompanying the movement of hilsa from marine water (MW) to freshwater. Nevertheless, total NKA expression in marine water did not change prior to freshwater entry. NKCC expression was down-regulated in gill, parallel with NKA activity, as the fish approached to the freshwater stretch of river. The NKA α-1 and NKCC1 protein abundance decreased in freshwater individuals by 40% and 31%, respectively, compared to MW. NKA and NKCC1 were explicitly localized to branchial ionocytes and immunoreactive signal appeared throughout the cytoplasm except for the nucleus and the most apical region indicates a basolateral/tubular distribution. Immunoreactive ionocytes were distributed on the filaments and lamellae; lamellar ionocytes were more in number irrespective of habitat salinity. The decrease in salinity caused a slight reduction in ionocyte number, but not in size and the underlying distribution pattern did not alter. The overall results support previously proposed models that both the ion transporters are involved in maintaining ionic homeostasis and lamellar ionocytes may have the function in hypo-osmoregulation in migrating hilsa, unlike other anadromous teleosts.

Effect of dendritic organ ligation on striped eel catfish Plotosus lineatus osmoregulation

Unique amongst the teleost, Plotosidae catfish possess a dendritic organ (DO) as a purported salt secreting organ, whereas other marine teleosts rely on their gill ionocytes for active NaCl excretion. To address the role of the DO in ionregulation, ligation experiments were conducted in brackish water (BW) 3‰ and seawater (SW) 34‰ acclimated Plotosus lineatus and compared to sham operated fish. Ligation in SW resulted in an osmoregulatory impairment in blood (elevated ions and hematocrit) and muscle (dehydration). However, SW ligation did not elicit compensatory changes in gill or kidney Na⁺/K⁺-ATPase (NKA) activity and/or protein expression while a decrease in anterior intestine and increased in posterior intestine were observed but this was not reflected at the protein level. Following ligation in SW, protein levels of carbonic anhydrase (CA) and V-ATPase B subunit (VHAB) were higher in kidney but either lower (CA) or unchanged (VHAB) in other tissues. Taken together, the osmotic disturbance in ligated SW fish indicates the central role of the DO in salt secretion and the absence of a compensatory response from the gill.

Osmoregulation in the Plotosidae Catfish: Role of the Salt Secreting Dendritic Organ

Unlike other marine teleosts, the Plotosidae catfishes reportedly have an extra-branchial salt secreting dendritic organ (DO). Salinity acclimation [brackishwater (BW) 3aaa, seawater (SWcontrol) 34aaa, and hypersaline water (HSW) 60aaa] for 14 days was used to investigate the osmoregulatory abilities of Plotosus lineatus through measurements of blood chemistry, muscle water content (MWC), Na⁺/K⁺-ATPase (NKA) specific activity and ion transporter expression in gills, DO, kidney and intestine. Ion transporter expression was determined using immunoblotting, immunohistochemistry (IHC) and quantitative polymerase chain reaction (qPCR). HSW elevated mortality, plasma osmolality and ions, and hematocrit, and decreased MWC indicating an osmoregulatory challenge. NKA specific activity and protein levels were significantly higher in DO compared to gill, kidney and intestine at all salinities. NKA specific activity increased in kidney and posterior intestine with HSW but only kidney showed correspondingly higher NKA α-subunit protein levels. Since DO mass was greater in HSW, the total amount of DO NKA activity expressed per gram fish was greater indicating higher overall capacity. Gill NKA and V-ATPase protein levels were greater with HSW acclimation but this was not reflected in NKA activity, mRNA or ionocyte abundance. BW acclimation resulted in lower NKA activity in gill, kidney and DO. Cl^- levels were better regulated and the resulting strong ion ratio in BW suggests a metabolic acidosis. Elevated DO heat shock protein 70 levels in HSW fish indicate a cellular stress. Strong NKA and NKCC1 (Na⁺:K⁺:2Cl^- cotransporter1) co-localization was observed in DO parenchymal cells, which was rare in gill ionocytes. NKCC1 immunoblot expression was only detected in DO, which was highest at HSW. Cystic fibrosis transmembrane regulator Cl^- channel (CFTR) localize apically to DO NKA immunoreactive cells. Taken together, the demonstration of high NKA activity in DO coexpressed with NKCC1 and CFTR indicates the presence of the conserved secondary active Cl^- secretion mechanism found in other ion transporting epithelia suggesting a convergent evolution with other vertebrate salt secreting organs. However, the significant osmoregulatory challenge of HSW indicates that the DO may be of limited use under more extreme salinity conditions in contrast to the gill based ionoregulatory strategy of marine teleosts.

An optimal stopping approach for onset of fish migration

Comprehending life history of migratory fish, onset of migration in particular, is a key biological and ecological research topic that still has not been clarified. In this paper, we propose a simple mathematical model for the onset of fish migration in the context of a stochastic optimal stopping theory, which is a new attempt to our knowledge. Finding the criteria of the onset of migration reduces to solving a variational inequality of a degenerate elliptic type. As a first step of the new mathematical modeling, mathematical and numerical analyses with particular emphasis on whether the model is consistent with the past observation results of fish migration are examined, demonstrating reasonable agreement between the theory and observation results. The present mathematical model thus potentially serves as a simple basis for analyzing onset of fish migration.

Conservation physiology of marine fishes: state of the art and prospects for policy

The state of the art of research on the environmental physiology of marine fishes is reviewed from the perspective of how it can contribute to conservation of biodiversity and fishery resources. A major constraint to application of physiological knowledge for conservation of marine fishes is the limited knowledge base; international collaboration is needed to study the environmental physiology of a wider range of species. Multifactorial field and laboratory studies on biomarkers hold promise to relate ecophysiology directly to habitat quality and population status. The 'Fry paradigm' could have broad applications for conservation physiology research if it provides a universal mechanism to link physiological function with ecological performance and population dynamics of fishes, through effects of abiotic conditions on aerobic metabolic scope. The available data indicate, however, that the paradigm is not universal, so further research is required on a wide diversity of species. Fish physiologists should interact closely with researchers developing ecological models, in order to investigate how integrating physiological information improves confidence in projecting effects of global change; for example, with mechanistic models that define habitat suitability based upon potential for aerobic scope or outputs of a dynamic energy budget. One major challenge to upscaling from physiology of individuals to the level of species and communities is incorporating intraspecific variation, which could be a crucial component of species' resilience to global change. Understanding what fishes do in the wild is also a challenge, but techniques of biotelemetry and biologging are providing novel information towards effective conservation. Overall, fish physiologists must strive to render research outputs more applicable to management and decision-making. There are various potential avenues for information flow, in the shorter term directly through biomarker studies and in the longer term by collaborating with modellers and fishery biologists.