In fish, trace metals play a crucial role in a range of physiological processes such as cellular respiration, enzymatic activity, and lipid metabolism. These processes, together with the concerted extrusion and uptake of ions via osmoregulatory tissues such as gills, are highly dependent on environmental salinity. The effects of salinity on the regulation of trace metals, however, remain poorly understood. With an emphasis on zinc, iron and manganese, this study first assessed trace metal concentrations in muscle and gill in the euryhaline Mozambique tilapia, Oreochromis mossambicus, acclimated to either FW or SW. Concentrations of manganese were higher in gills of SW-acclimated fish. In muscle, manganese and iron contents were higher in FW-acclimated fish, whereas zinc was higher in SW-acclimated fish. We then employed RNA-Seq analyses (n = 3 biological replicates per treatment) and DESeq2 statistical modeling with Benjamini–Hochberg FDR correction to reveal differential responses in specific trace metal transporters of the solute carrier superfamily (SLC). Branchial transcripts from fish acclimated to steady-state salinities were also compared with those from fish transferred from SW to a tidal regime (TR) for 15 days. A TR experimental paradigm is characterized by alternating between FW and SW every 6 h to simulate salinity fluctuations of estuaries, a common habitat of euryhaline fish such as the Mozambique tilapia. Most differentially expressed slc genes were observed between SW and a TR. From those, there were seven transcripts of the slc30 and slc39 families, primarily associated with the transport of zinc, differentially expressed between fish reared in SW and TR. Generally, the expression of slc genes varied to a greater extent between either FW or SW phases of TR and steady-state FW or SW than between the two phases of the TR or between steady-state FW and SW. Collectively, these data show salinity-dependent changes in tissue zinc, iron, and manganese and transcriptional activation of associated trace metal transporters in gill. Notably, most transcripts became differentially expressed when fish were transferred to a TR, suggesting that the regulation of trace metal transport is sensitive to frequent changes in salinity.

Teleost fishes maintain hydromineral balance through the hormonal regulation of epithelial ion transport. In the euryhaline, and remarkably osmotolerant Mozambique tilapia (Oreochromis mossambicus), two prolactin (Prl) isoforms, Prl188 and Prl177, are released from the pituitary in response to hyposmotic stimulation to promote branchial ion absorption. Prl188 and Prl177 bind two Prl receptors (Prlrs), Prlr1 and Prlr2, which are expressed in the pituitary and key ionoregulatory organs, including the gill. To understand how Prl signaling operates across a range of salinities that reflect their scope for osmotolerance, we exposed Mozambique tilapia to conditions spanning from fresh water (FW; < 0.1‰) to triple-strength seawater (3x SW; 105‰). In the pituitary, prl188/prl177 and prlr1/prlr2 ratios decreased as salinity increased, dropping to levels in 3x SW that were less than 10% of those in FW. Branchial prlr1/prlr2 ratios also decreased with increased salinity, in parallel with effectors of branchial ion uptake, including Na+/Cl- cotransporter 2 and Na+/K+ ATPase-α1a. While the changes in ratios reflect differences in the reductions of both prl188 and prl177, they also reflect downregulation of prlr1 and upregulation of prlr2 with increasing salinity. Gene transcripts encoding mediators of ion extrusion, such as Na+/K+/2Cl- cotransporter 1a and cystic fibrosis transmembrane conductance regulator 1, were elevated by high-salinity conditions. Our findings indicate that when Mozambique tilapia are exposed to hypersaline conditions, they shift Prl signaling toward Prl177 and Prlr2 relative to Prl188 and Prlr1. In turn, the combinatorial nature of isoform-specific responses linked to Prl signaling contributes to both the euryhalinity and exceptional osmotolerance of Mozambique tilapia.

Objective:Euryhaline fishes can inhabit salinities ranging from fresh water (FW) to seawater (SW), in part because their endocrine system aligns the ion-transporting capacities of branchial ionocytes with the external environment. Ionocytes also directly sense extracellular osmolality and adjust their functions accordingly; therefore, this study investigated the interplay between osmotic conditions and osmoregulatory hormones in Mozambique tilapia (Oreochromis mossambicus) to further elucidate the basis of euryhalinity. We sought to determine whether prolactin (Prl) supports FW acclimation by counteracting osmotic and hormonal signals that initiate branchial responses to SW environments. Methods:We first combined hypophysectomy, hormone replacement, RNA-Seq, and qPCR to identify cystic fibrosis transmembrane conductance regulator (cftr), osmotic stress transcription factor 1 (ostf1), and serum- and glucocorticoid-inducible kinase 1 (sgk1) as Prl-repressed genes. We then leveraged a series of in vivo and in vitro experimental paradigms to characterize their regulation by environmental salinity, Prl, cortisol, and extracellular osmolality. Results:Our findings indicate that 1) Prl's capacity to antagonize cortisol-stimulated cftr expression depends on extracellular osmotic conditions; 2) Prl and cortisol play opposing roles in regulating branchial ostf1 expression; 3) Sgk1 is expressed in 'SW-type' ionocytes, where interactions among Prl, cortisol, and osmotic conditions influence its expression; and 4) Prl promotes the expression of Na+/Cl- cotransporter 2 and Clc family Cl- channel 2c, as shown previously. Conclusions:The combined actions of osmotic stimuli, Prl, and cortisol shape the branchial expression of cftr, ostf1, and sgk1. While hyperosmotic extracellular conditions and cortisol promote the activation of these genes during SW acclimation, Prl supports FW adaptation by suppressing these genes and promoting processes underlying active ion uptake. Thus, euryhaline tilapia adjust the ion-transporting activity of their branchial ionocytes to meet environmental demands by integrating multiple regulatory cues.

In response to changes in environmental salinity, euryhaline fishes mobilize energy to support the active transport of ions across osmoregulatory epithelia. Glycogen synthase (GS) and glycogen phosphorylase (GP) are key controllers of carbohydrate metabolism due to their roles in promoting glycogenesis and glycogenolysis, respectively. However, the coordinated responses of GS, GP and glucose transporters (GLUTs) in the liver and gill to changes in salinity remain unresolved. In this study, we initially subjected Mozambique tilapia (Oreochromis mossambicus) to unidirectional transfers, either from fresh water (FW) to seawater (SW) or from SW to FW. We then transferred FW- and SW-acclimated tilapia to a tidal regime (TR) where salinity alternated between FW and SW every 6 h. Our goal was to characterize how carbohydrate metabolism is affected by unidirectional and tidal changes in salinity. Whether fish were transferred from SW to FW in a unidirectional manner or to a TR, glucose levels increased after transfer to FW or during the FW phase of the TR (TF). Conversely, hepatic glycogen levels were higher in fish in SW and the SW phase of the TR (TS) than in TF. In both FW and TF, branchial expression of the gill-specific isoform of GP (gpg) was downregulated, while gs was upregulated. Branchial gpg was upregulated in fish unidirectionally transferred from FW to SW or sampled during TS. Hepatic gp and gs expression increased following transfer from SW to FW. Thus, we consistently found that increases in salinity promoted branchial glycogen breakdown, while decreases in salinity led to hyperglycaemia. Moreover, while branchial glucose transporter 1 (glut 1) expression was downregulated after transfer from FW to SW, glut1 was transiently upregulated in the liver. In both liver and gill, glut1 expression was higher in fish in TF compared to TS. Gill filament explants incubated with cortisol exhibited reduced glut1 expression regardless of medium osmolality. Our collective data indicate that salinity differentially regulates hepatic and branchial carbohydrate metabolism.

To maintain hydromineral balance during transitions between different environmental salinities, euryhaline teleosts adjust the permeability characteristics of their surface epithelia to align with ambient conditions. Tight junctions (TJs), which form the apical-lateral barriers between epithelial cells, control the paracellular movement of solutes and water. Consequently, euryhaline species must efficiently reorganize branchial TJs when acclimating to freshwater (FW) and seawater (SW) environments. However, little is known about how TJs are regulated under tidally changing salinities. In the current study, we employed RNA-Seq to identify several branchial TJ transcripts that are differentially expressed in Mozambique tilapia (Oreochromis mossambicus) acclimated to FW, SW, or a tidal regime (TR). The most abundant and differentially expressed TJ transcripts, which included claudins (cldn-4 like, cldn7b, cldn23a, and cldn-like ZF-A89), occludins (oclna and oclnb), and TJ proteins (tjp1a and tjp3), were further investigated by qPCR. Generally, the expression of TJ transcripts varied to a greater extent between the FW and SW phases of the TR (denoted TF and TS, respectively) than between steady-state FW and SW conditions. Furthermore, TJ transcripts were usually upregulated in TF compared to TS or SW controls. In contrast, the gene expression of branchial ion transporters did not change as markedly under a TR. Together, these results suggest that TJs play a crucial role in regulating the permeability of branchial epithelia by preventing ion loss in hyposmotic conditions, particularly during short-term tidally changing salinities.

Recent increases in demand for sustainable aquaculture have been accompanied by a rising interest in its practice, including the continued development of inland recirculating aquaculture systems (RAS). The transfer of knowledge necessary to optimize learning outcomes for aquaculture students and practitioners relies on accessibility to versatile state-of-the art facilities. A new land-based aquaculture facility called the Tuahine Aquaculture Research and Education Center (TAREC) was built at the University of Hawaiʻi at Mānoa and designed to integrate research, education, and extension activities. With two RAS and a compact footprint, TAREC was designed to hold freshwater (FW), seawater (SW), or both FW and SW aquatic organisms simultaneously, including the option for modifications along a salinity gradient under experimental conditions. An account of the requirements and specifications for building the system is provided along with an initial trial comparing growth rates of the euryhaline Mozambique tilapia, Oreochromis mossambicus , reared in FW and artificial seawater (ASW) when stocked at ~0.6 kg/m3. Over three months, fish reared in ASW grew faster and had lower feed conversion ratios than those in FW. These findings not only validate salinity-dependent growth trials in RAS but also demonstrate TAREC’s capacity as a versatile experimental facility for conducting fundamental and applied research and training for the next generation of aquaculture researchers and practitioners.

In the state of Hawai'i, there is rising interest in the culture of native sea cucumbers, such as Stichopus horrens; however, current production is hindered by a limited understanding of their reproductive biology. Originally described in starfish, relaxin-like gonad stimulating peptide (RGP) has been characterized and used to induce oocyte maturation and spawning in sea cucumbers. For the first time using S. horrens specifically, we identified the RGP (Sh-RGP) coding sequence through de novo transcriptomics, synthesized the mature peptide, and investigated its role in inducing oocyte maturation in vitro, and spawning in vivo. The Sh-RGP precursor gene encodes a single mature peptide composed of two amino acid chains. The B- and A-chains contain two and four cysteine residues, respectively, resulting in an intra-chain linkage on the A-chain and two disulfide connections between the two chains. A synthetic Sh-RGP was produced from solid-phase peptide synthesis, bonded, and isolated by reverse-phase HPLC. A tissue distribution analysis revealed that shrgp mRNA expression is highest (P<0.001) in the neural ring. In vitro, Sh-RGP (0.1-10 μM) induced germinal vesicle breakdown in oocytes incubated with gonadal tissue in time and concentration-dependent fashions. When injected with Sh-RGP (∼4.5 μg/kg body weight), all mature male and female sea cucumbers spawned within 39 min of treatment. Overall, these results indicate that Sh-RGP induces oocyte maturation and spawning and provide a framework for integrating transcriptomics and functional assays to elucidate and validate endogenous species-specific reproductive hormones that can be used to address barriers in sea cucumber aquaculture.

The life history, distribution and diversity of fishes are largely influenced by environmental salinity. Changes in salinity affect a range of physiological processes including metabolism, nutrition, reproduction and growth. Therefore, fish can be conditioned to environmental parameters most suitable for production, where distinct traits are optimised through species-specific manipulation of salinities. The primary purpose of this review is to summarise the existing literature on the salinity tolerance of aquacultured fish. The various experimental approaches for determining salinity tolerance are compared, along with summarised information for key species employed in aquaculture, including their native distributions, life history stage and optimal salinity for survival and growth. The implications for production were assessed by considering the effects of salinity on growth, reproduction, management, disease mitigation and marketability.

With the expanding global population, interest has increased in the sustainable aquaculture development of indigenous fishes. In Hawai'i, the brassy chub, Kyphosus vaigiensis, has drawn interest as a candidate species for production. However, little is known about its resilience in aquaculture settings where fish are exposed to acute, husbandry-related stressors. To address the response of brassy chub to handling stress, we characterized the effects of simulated netting and confinement stress on physiological parameters indicative of the alarm and resistance phases of the stress response over a 24-h time course. Specifically, we measured plasma osmolality and glucose, mucous cortisol, hepatic activity of the antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GPX), and hepatic lipid peroxidation (LPO). Plasma osmolality in stressed fish was increased relative to controls at 1 and 6 h. Mucous cortisol levels were transiently increased in stressed fish at 1 h following handling stress. In stressed fish, plasma glucose was generally increased from 1 h to 24 h relative to controls. By contrast, SOD activity decreased in stressed fish at 6 and 12 h, returning to control levels at 24 h; GPX activity and LPO were unaffected by the handling stressor. The transient and modest responses of mucous cortisol and antioxidant enzyme activity indicate the recovery of brassy chub from handling stress. Moreover, the sustained increase in plasma glucose and lack of change in LPO suggest mobilization of energy and long-term resiliency toward handling stressors.

In estuarine environments, euryhaline fish maintain a narrow range of internal osmolality despite daily changes in environmental salinity that can range from fresh water (FW) to seawater (SW). The capacity of euryhaline fish to maintain homeostasis in a range of environmental salinities is primarily facilitated by the neuroendocrine system. One such system, the hypothalamic-pituitary-interrenal (HPI) axis, culminates in the release of corticosteroids such as cortisol into circulation. Cortisol functions as both a mineralocorticoid and glucocorticoid in fish because of its roles in osmoregulation and metabolism, respectively. The gill, a key site for osmoregulation, and the liver, the primary storage site for glucose, are known targets of cortisol’s actions during salinity stress. While cortisol facilitates acclimation to SW environments, less is known on its role during FW adaptation. In this study, we characterized the responses of plasma cortisol, mRNA expression of pituitary pro-opiomelanocortin (pomc), and mRNA expression of liver and gill corticosteroid receptors (gr1, gr2, and mr) in the euryhaline Mozambique tilapia (Oreochromis mossambicus) under salinity challenges. Specifically, tilapia were subjected to salinity transfer regimes from steady-state FW to SW, SW to FW (experiment 1) or steady state FW or SW to tidal regimen (TR, experiment 2). In experiment 1, fish were sampled at 0 h, 6 h, 1, 2, and 7 d post transfer; while in experiment 2, fish were sampled at day 0 and day 15. We found a rise in pituitary pomc expression and plasma cortisol following transfer to SW while branchial corticosteroid receptors were immediately downregulated after transfer to FW. Moreover, branchial expression of corticosteroid receptors changed with each salinity phase of the TR, suggesting rapid environmental modulation of corticosteorid action. Together, these results support the role of the HPI-axis in promoting salinity acclimation, including in dynamically-changing environments.

The almost limitless complexity of biology has led to two general approaches to understanding biological phenomena. One approach is dominated by reductionism in which high-level phenomena of whole systems are viewed as emerging from relatively simple and generally understood interactions at a substantially lower level. Although this approach is theoretically general, it can become intractable in practice when attempting to simultaneously explain a wide range of systems. A second approach is for specialists to investigate biological phenomena within one of many different hierarchical levels of description that are separated to decouple from concerns at other levels. Although this approach reduces the explanatory burden on specialists that operate within each level, it also reduces integration from insights gained at other levels. Thus, as beneficial as these approaches have been, they limit the scope and integration of knowledge across scales of biological organization to the detriment of a truly synoptic view of life. The challenge is to find a theoretical and experimental framework that facilitates a broader understanding of the hierarchy of life—providing permeability for the exchange of ideas among disciplinary specialists without discounting the peculiarities that have come to define those disciplines. For this purpose, coarse-grained, scale-invariant properties, and resources need to be identified that describe the characteristic features of a living system at all spatiotemporal scales. The approach will be aided by a common vernacular that underscores the realities of biological connections across a wide range of scales. Therefore, in this vision paper, we propose a conceptual approach based on four identified resources—energy, conductance, storage, and information (ECSI)—to reintegrate biological studies with the aim of unifying life sciences under resource limitations. We argue that no functional description of a living system is complete without accounting for at least all four of these resources. Thus, making these resources explicit will help to identify commonalities to aid in transdisciplinary discourse as well as opportunities for integrating among the differently scoped areas of specialized inquiry. The proposed conceptual framework for living systems should be valid across all scales and may uncover potential limitations of existing hypotheses and help researchers develop new hypotheses addressing fundamental processes of life without having to resort to reductionism.

In euryhaline fish, prolactin (Prl) plays a key role in freshwater acclimation. Prl release in the rostral pars distalis (RPD) of the pituitary is directly stimulated by a fall in extracellular osmolality. Recently, we identified several putative transcription factor modules (TFM) predicted to bind to the promoter regions of the two prl isoforms in Mozambique tilapia, Oreochromis mossambicus. We characterized the effects of extracellular osmolality on the activation of these TFMs from RPDs, in vivo and in vitro. OCT1_PIT1 01, CEBP_CEBP 01 and BRNF_RXRF 01 were significantly activated in freshwater (FW) - acclimated tilapia RPDs while SORY_PAX3 02 and SP1F_SP1F 06, SP1F_SP1F 09 were significantly activated in seawater (SW)-counterparts. Short-term incubation of SW-acclimated tilapia RPDs in hyposmotic media (280 mOsm/kg) resulted in activation of CAAT_AP1F 01, OCT1_CEBP 01, AP1F_SMAD 01, GATA_SP1F 01, SORY_PAX6 01 and CREB_EBOX 02, EBOX_AP2F 01, EBOX_MITF 01 while hyperosmotic media (420 mOsm/kg) activated SORY_PAX3 02 and AP1F_SMAD 01 in FW-tilapia. Short-term incubation of dispersed Prl cells from FW- acclimated fish exposed to hyperosmotic conditions decreased pou1f1, pou2f1b, stat3, stat1a and ap1b1 expression, while pou1f1, pou2f1b, and stat3 were inversely related to osmolality in their SW-counterparts. Further, in Prl cells of SW- tilapia, creb3l1 was suppressed in hyposmotic media. Collectively, our results indicate that multiple TFMs are involved in regulating prl transcription at different acclimation salinities and, together, they modulate responses of Prl cells to changes in extracellular osmolality. These responses reflect the complexity of osmosensitive molecular regulation of the osmoreceptive Prl cell of a euryhaline teleost.

In euryhaline fish, prolactin (Prl) plays an essential role in freshwater (FW) acclimation. In the euryhaline and eurythermal Mozambique tilapia, Oreochromis mossambicus, Prl cells are model osmoreceptors, recently described to be thermosensitive. To investigate the effects of temperature on osmoreception, we incubated Prl cells of tilapia acclimated to either FW or seawater (SW) in different temperature (20, 26 and 32°C) and osmolality (280, 330 and 420 mOsm/kg) combinations for 6 h. Release of both Prl isoforms, Prl188 and Prl177, increased in hyposmotic media and were further augmented with a rise in temperature. Hyposmotically-induced release of Prl188 was inhibited at 20°C. In SW fish, mRNA expression of prl188 and prl177 showed direct and inverse relationships with temperature, respectively. In SW-acclimated tilapia Prl cells incubated in hyperosmotic media, Prl receptors, prlr1 and prlr2, and the stretch-activated Ca2+ channel, trpv4, were inhibited at 32°C, suggesting the presence of a cellular mechanism to compensate for elevated Prl release. Transcription factors, pou1f1, pou2f1b, creb3l1, cebpb, stat3, stat1a and nfat1c, known to regulate prl188 and prl177, were also downregulated at 32°C. Our findings provide evidence that osmoreception is modulated by temperature, and that both thermal and osmotic responses vary with acclimation salinity.

Prolactin (PRL) cells within the rostral pars distalis (RPD) of euryhaline and eurythermal Mozambique tilapia, Oreochromis mossambicus, rapidly respond to a hyposmotic stimulus by releasing two distinct PRL isoforms, PRL188 and PRL177. Here, we describe how environmentally relevant temperature changes affected mRNA levels of prl188 and prl177 and the release of immunoreactive prolactins from RPDs and dispersed PRL cells. When applied under isosmotic conditions (330 mosmol/kgH2O), a 6°C rise in temperature stimulated the release of PRL188 and PRL177 from both RPDs and dispersed PRL cells under perifusion. When exposed to this same change in temperature, ∼50% of dispersed PRL cells gradually increased in volume by ∼8%, a response partially inhibited by the water channel blocker, mercuric chloride. Following their response to increased temperature, PRL cells remained responsive to a hyposmotic stimulus (280 mosmol/kgH2O). The mRNA expression of transient potential vanilloid 4, a Ca2+-channel involved in hyposmotically induced PRL release, was elevated in response to a rise in temperature in dispersed PRL cells and RPDs at 6 and 24 h, respectively; prl188 and prl177 mRNAs were unaffected. Our findings indicate that thermosensitive PRL release is mediated, at least partially, through a cell-volume-dependent pathway similar to how osmoreceptive PRL release is achieved.

Variations in salinity are among the main physical parameters that drive the capacity of fish to survive and thrive in a range of environments. Acclimation to changes in environmental salinity is regulated by the interplay of local osmotically-induced cellular responses and the systemic regulation by the neuroendocrine system, which together, direct ion extrusion or uptake responses via the gill and other osmoregulatory tissues. Many euryhaline teleost fish are native to waters in which salinity varies tidally between that of fresh water (FW) and seawater (SW), such as estuaries. The physiology of salinity acclimation of euryhaline teleosts has been studied extensively in steady-state salinities, and after one-way transfers between steady-state salinities. Fewer studies, however, have addressed salinity regimes that reflect or simulate the continuous dynamic changes by which euryhaline fishes may be subject to in a native environment. Experimentally, the simulation of a changing environment was obtained by rearing fish in salinities that changed between those of FW and SW every six hours, in a tidal regime (TR). An overview of the main differences in the regulation of transcripts involved in ion balance between euryhaline Mozambique tilapia, Oreochromis mossambicus, responding to TR and those acclimating to steady-state salinities is provided. Transcripts analyzed include branchial Na+/K+-ATPase (nka), Na+, Cl- co-transporter (ncc), Na+/H+ antiporter 3 (nhe3), 2Cl- co-transporter (nkcc), cystic fibrosis transmembrane conductance regulator (cftr), and aquaporin 3 (aqp3). In particular, both cftrand aqp3were highly sensitive to changes in salinity in fish acclimated to a TR, indicating indispensable roles in rapidly maintaining hydromineral balance. The pituitary hormones, prolactin and growth hormone, known to play osmoregulatory roles in Mozambique tilapia, were also compared along with their receptors between TR and steady-state salinity paradigms. Specifically, through the dynamic changes in hormone receptor transcription observed in the TR rearing paradigm, the endocrine control of osmoregulatory outcomes appears shifted from systemic to local regulation at the level of target tissues. Together, these studies indicate that fish are able to compensate for broad and frequent changes in external salinity while keeping osmoregulatory parameters within a narrow range.

Across the vertebrate lineage, sexual dimorphism in body size is a common phenomenon that results from trade-offs between growth and reproduction. To address how key hormones that regulate growth and reproduction interact in teleost fishes, we studied Mozambique tilapia (Oreochromis mossambicus) to determine whether the activities of luteinizing hormone (Lh) are modulated by growth hormone (Gh), and conversely, whether targets of Gh are affected by the presence of Lh. In particular, we examined how gonadal morphology and specific gene transcripts responded to ovine GH (oGH) and/or LH (oLH) in hypophysectomized male and female tilapia. Hypophysectomized females exhibited a diminished gonadosomatic index (GSI) concomitant with ovarian follicular atresia. The combination of oGH and oLH restored GSI and ovarian morphology to conditions observed in sham-operated controls. A similar pattern was observed for GSI in males. In control fish, gonadal gh receptor (ghr2) and estrogen receptor β (erβ) expression was higher in females versus males. A combination of oGH and oLH restored erβ and arβ in females. In males, testicular insulin-like growth factor 3 (igf3) expression was reduced following hypophysectomy and subsequently restored to control levels by either oGH or oLH. By contrast, the combination of both hormones was required to recover ovarian igf3 expression in females. In muscle, ghr2 expression was more responsive to oGH in males versus females. In the liver of hypophysectomized males, igf2 expression was diminished by both oGH and oLH; there was no effect of hypophysectomy, oGH, or oLH on igf2 expression in females. Collectively, our results indicate that gene transcripts associated with growth and reproduction exhibit sex-specific responses to oGH and oLH. These responses reflect, at least in part, how hormones mediate trade-offs between growth and reproduction, and thus sexual dimorphism, in teleost fishes.

Sexual dimorphism in body size is a common phenomenon among vertebrates, including teleost fishes. The hormonal underpinnings of this phenomenon, however, have not been fully clarified and may be associated with interactions between the endocrine factors that control growth and reproduction. Given their well-described patterns of sexually dimorphic growth, we used Mozambique tilapia (Oreochromis mossambicus) as a model to examine how the activities of luteinizing hormone (LH) are modulated by growth hormone (GH), and conversely, how LH affects targets of GH. We hypothesized that GH would induce factors within the GH/insulin-like growth factor (GH/IGF) axis that favor growth, while LH would attenuate growth-promoting factors and stimulate factors that favor reproduction. We analyzed gonad morphology and gene expression in hypophysectomized tilapia injected with ovine GH (oGH) and LH (oLH) either alone or in combination. Gonadosomatic index (GSI) was reduced in females following hypophysectomy; the combined administration of oGH and oLH restored GSI to control levels. Hypophysectomy regressed the ovary and the combined treatment of oGH and oLH restored ovarian structure. Gonadal gh receptor (ghr2) and estrogen receptor β (erβ) were higher in females, whereas igf1, igf3, erα, and androgen receptor ß (arß) were higher in males of control fish. The inhibitory effects of hypophysectomy were most pronounced on igf3transcripts, and these levels were restored by both oGH and oLH alone, or in combination, in males. In females, the combination of both hormones was required to recover gonadal igf3 levels. A hypophysectomy-induced decrease in arβ was restored by a combination of oGH and oLH in females, while there were no effects of oGH or oLH on arβ in males. In muscle, ghr2 was more responsive to oGH in males, while igf2 was more responsive to oLH in females. oGH and oLH both inhibited a hypophysectomy-induced increase in hepatic igf2 in males. Our results indicate that genes associated with growth and reproduction exhibit sexually dimorphic responses to GH and LH in tilapia, and therefore, provide insight into the endocrine bases of sexually dimorphic growth in fishes.

Salinity is one of the main physical properties that govern the distribution of fishes across aquatic habitats. In order to maintain their body fluids near osmotic set points in the face of salinity changes, euryhaline fishes rely upon tissue-level osmotically-induced responses and systemic endocrine signaling to direct adaptive ion-transport processes in the gill and other critical osmoregulatory organs. Some euryhaline teleosts inhabit tidally influenced waters such as estuaries where salinity can vary between fresh water (FW) and seawater (SW). The physiological adaptations that underlie euryhalinity in teleosts have been traditionally identified in fish held under steady-state conditions or following unidirectional transfers between FW and SW. Far fewer studies have employed salinity regimes that simulate the tidal cycles that some euryhaline fishes may experience in their native habitats. With an emphasis on prolactin (Prl) signaling and branchial ionocytes, this mini-review contrasts the physiological responses between euryhaline fish responding to tidal versus unidirectional changes in salinity. Three patterns that emerged from studying Mozambique tilapia (Oreochromis mossambicus) subjected to tidally-changing salinities include, 1) fish can compensate for continuous and marked changes in external salinity to maintain osmoregulatory parameters within narrow ranges, 2) tilapia maintain branchial ionocyte populations in a fashion similar to SW-acclimated fish, and 3) there is a shift from systemic to local modulation of Prl signaling.

Euryhaline fishes maintain hydromineral balance in a broad range of environmental salinities via the activities of multiple osmoregulatory organs, namely the gill, gastrointestinal tract, skin, kidney, and urinary bladder. Teleosts residing in freshwater (FW) environments are faced with the diffusive loss of ions and the osmotic gain of water, and, therefore, the kidney and urinary bladder reabsorb Na+ and Cl- to support the production of dilute urine. Nonetheless, the regulated pathways for Na+ and Cl- transport by euryhaline fishes, especially in the urinary bladder, have not been fully resolved. Here, we first investigated the ultrastructure of epithelial cells within the urinary bladder of FW-acclimated Mozambique tilapia (Oreochromis mossambicus) by electron microscopy. We then investigated whether tilapia employ Na+/Cl- cotransporter 1 (Ncc1) and Clc family Cl- channel 2c (Clc2c) for the reabsorption of Na+ and Cl- by the kidney and urinary bladder. We hypothesized that levels of their associated gene transcripts vary inversely with environmental salinity. In whole kidney and urinary bladder homogenates, ncc1 and clc2c mRNA levels were markedly higher in steady-state FW- versus SW (seawater)-acclimated tilapia. Following transfer from SW to FW, ncc1 and clc2c in both the kidney and urinary bladder were elevated within 48 h. A concomitant increase in branchial ncc2, and decreases in Na+/K+/2Cl-cotransporter 1a (nkcc1a) and cystic fibrosis transmembrane regulator 1 (cftr1) levels indicated a transition from Na+ and Cl- secretion to absorption by the gills in parallel with the identified renal and urinary bladder responses to FW transfer. Our findings suggest that Ncc1 and Clc2c contribute to the functional plasticity of the kidney and urinary bladder in tilapia.