Fritzie T. Celino-Brady

Euryhaline fishes tolerate wide variations in salinity through complex physiological mechanisms. Some species inhabit estuaries, where they encounter frequently changing salinities that challenge homeostasis. Located at a critical interface with the aquatic environment, the gills simultaneously support hydromineral balance and mount immune responses against environmental pathogens. In this study, Mozambique tilapia (Oreochromis mossambicus) were exposed to a tidal regime (TR) that entailed oscillations between freshwater (FW) and seawater (SW) conditions every 6 h. We observed branchial interlamellar thickening in fish exposed to a TR, suggesting activation of immune, stress, hyperplasia, and cellular differentiation responses. RNA-Seq analysis was performed to probe gene expression patterns underlying these apparent responses. Comparative analysis between salinity groups highlighted enriched biological processes and signaling pathways related to immune regulation, cellular stress responses, hyperplasia/proliferation, and epithelial-mesenchymal transition (EMT). The greatest number of differentially expressed genes was found when comparing fish held in a TR with those held continuously in SW. The most abundant and differentially expressed transcripts associated with immune and stress responses (ccl20, il1b, gsto1, gstr, and hsp40) and with hyperplasia and EMT (pcna, npcc, mki67, fndc7a, and ets1) were selected for qPCR analysis. While il1b, gstr, gsto1, hsp40, and pcna were differentially expressed between the FW and SW phases of the TR, ccl20, nppc, mki67, fndc7a, and ets1 were downregulated in TR compared with SW. Collectively, these results indicate that tilapia thicken their branchial epithelium, exhibit immune and stress responses, and activate cellular proliferation and differentiation pathways to sustain osmoregulatory balance under dynamic tidal conditions.

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 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.

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 combinations of temperatures (20, 26 and 32 °C) and osmolalities (280, 330 and 420 mOsm/kg) 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, but not Prl₁₇₇, was suppressed at 20 °C. In SW fish, mRNA expression of prl₁₈₈ increased with rising temperatures at lower osmolalities, while and prl₁₇₇ decreased at 32 °C and higher osmolalities. In Prl cells of SW-acclimated tilapia incubated in hyperosmotic media, the expressions of Prl receptors, prlr1 and prlr2, and the stretch-activated Ca²⁺ channel, trpv4,decreased 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.