Osmoregulation and branchial plasticity after acute freshwater transfer in red drum, Sciaenops ocellatus

Physiological responses of euryhaline marine fish to naturally-occurring hypersalinity

Hypersaline habitats are generally defined as those with salinities in excess of 40 ppt. Well-known hypersaline regions (e.g. salt and soda lakes) have a well-earned reputation for being among the most inhospitable habitats in the world, and fish endemic to these areas have been the subject of much research related to extremophile physiology. Yet, marine coastal hypersalinity is both a common occurrence and a growing consideration in many marine coastal ecosystems, in part owing to human influence (e.g. evaporation, river diversion, desalination effluent). Importantly, any increase in salinity will elevate the osmoregulatory challenges experienced by a fish, which must be overcome by increasing the capacity to imbibe and absorb water and excrete ions. While great attention has been given to dynamic osmoregulatory processes with respect to freshwater to seawater transitions, and to the extreme hypersalinity tolerance that is associated with the adoption of an osmo-conforming strategy, relatively little focus has been placed on the physiological implications of moderate hypersalinity exposures (e.g. ≤ 60 ppt). Importantly, these exposures often represent the threshold of osmoregulatory performance owing to energetic constraints on ion excretion and efficiency limitations on water absorption. This review will explore the current state of knowledge with respect to hypersalinity exposure in euryhaline fishes, while placing a particular focus on the physiological constraints, plasticity and downstream implications of long-term exposure to moderate hypersalinity.

Quantifying euryhaline histories in red drum Sciaenops ocellatus: otolith chemistry and muscle isotope ratios

The combined use of otolith chemistry and tissue isotopes has the potential to reveal movements, habitat associations and food web interactions at a variety of spatial and temporal scales. Here, a combination of otolith Ba:Ca life-history transects with muscle tissue δ13C and δ15N values has been used to assess habitat use and oligohaline residence in red drum Sciaenops ocellatus in subtropical estuaries in the north-western Gulf of Mexico. Tissue isotopes were distinct among capture locations, particularly between bays with differing proximities to freshwater inflow sources. Otolith edge Ba:Ca values and tissue δ13C values were not correlated. These results indicated that fish were neither residing in nor feeding in oligohaline waters for significant periods of time within the tissue turnover window of several months prior to capture. Nonetheless, spatial differences in tissue isotope values indicated limited mixing among bays and relatively high site fidelity during estuarine occupancy. Lifetime otolith Ba:Ca transects revealed individual variability in the magnitude of residence in oligohaline waters. Using a mean oligohaline occupancy threshold, an estimated 82% of individuals used oligohaline waters at some point in their life. Nonetheless, 66% of individuals spent <20% of their life histories in oligohaline waters, suggesting intermittent and infrequent excursions into low salinity waters. Finally, a literature survey identified 56 peer-reviewed publications using combinations of otolith chemistry and tissue stable isotope ratios with a wide range of marker pairings and study aims. The diversity of ecological questions that can be asked with the combined use of these two approaches will provide valuable insight into fish ecology.

Warming-induced "plastic floors" improve hypoxia vulnerability, not aerobic scope, in red drum (Sciaenops ocellatus)

Ocean warming is a prevailing threat to marine ectotherms. Recently the "plastic floors, concrete ceilings" hypothesis was proposed, which suggests that a warmed fish will acclimate to higher temperatures by reducing standard metabolic rate (SMR) while keeping maximum metabolic rate (MMR) stable, therefore improving aerobic scope (AS). Here we evaluated this hypothesis on red drum (Sciaenops ocellatus) while incorporating measures of hypoxia vulnerability (critical oxygen threshold; Pcrit) and mitochondrial performance. Fish were subjected to a 12-week acclimation to 20 °C or 28 °C. Respirometry was performed every 4 weeks to obtain metabolic rate and Pcrit; mitochondrial respirometry was performed on liver and heart samples at the end of the acclimation. 28 °C fish had a significantly higher SMR, MMR, and Pcrit than 20 °C controls at time 0, but SMR declined by 36.2 % over the 12-week acclimation. No change in SMR was observed in the control treatment. Contrary to expectations, SMR suppression did not improve AS relative to time 0 owing to a progressive decline in MMR over acclimation time. Pcrit decreased by 27.2 % in the warm-acclimated fishes, which resulted in temperature treatments having statistically similar values by 12-weeks. No differences in mitochondrial traits were observed in the heart - despite a Δ8 °C assay temperature - while liver respiratory and coupling control ratios were significantly improved, suggesting that mitochondrial plasticity may contribute to the reduced SMR with warming. Overall, this work suggests that warming induced metabolic suppression offsets the deleterious consequences of high oxygen demand on hypoxia vulnerability, and in so doing greatly expands the theoretical range of metabolically available habitats for red drum.

Enhancement of Nutrient Composition and Non-Volatile Flavor Substances in Muscle Tissue of Red Drum (Sciaenops ocellatus) Through Inland Low Salinity Saline-Alkaline Water Culture

The red drum (Sciaenops ocellatus), a globally significant marine aquaculture species, boasts formidable osmoregulatory capabilities and remarkable adaptability to low salinity, making it an ideal candidate for commercial cultivation in inland low salinity saline-alkaline waters. However, studies on the fundamental nutritional composition and flavor quality of S. ocellatus in these inland low salinity saline-alkaline waters remain unreported. This study delves into the impact of inland low salinity saline-alkaline environments on the basic nutritional components and nonvolatile flavor substances (including free amino acids and free nucleotides) in the muscle tissue of S. ocellatus. The findings reveal that redfish cultivated in these conditions exhibit a significant increase in the crude fat, ash, and protein content in their dorsal muscle tissue, coupled with a decrease in moisture content (p < 0.05), indicating an enhancement in the nutritional value of the dorsal muscle tissue. Furthermore, this cultivation environment significantly elevates the content of free amino acids in the muscle tissue (p < 0.05), particularly those contributing to umami and sweet tastes, while reducing the relative content of bitter amino acids. Although the total content of free nucleotides decreased, the equivalent umami concentration (EUC) in the muscle tissue markedly increased (p < 0.05) due to the synergistic effect of umami amino acids and flavor nucleotides, enhancing the umami taste characteristics. Therefore, inland low salinity saline-alkaline aquaculture not only elevates the nutritional value of S. ocellatus muscle tissue but also improves its umami flavor characteristics. This discovery opens new perspectives for further research into the impact of inland low salinity saline-alkaline environments on the flavor properties of marine animals.

Improved high-quality reference genome of red drum facilitates the processes of resistance-related gene exploration

Sciaenops ocellatus is among the most important artificially introduced farmed fish across 11 countries and regions. However, the frequent occurrence of extreme weather events and breeding escapes have placed great pressure on local marine biodiversity and ecosystems. We reported the de novo assembly and annotation with a contig N50 of 28.30 Mb using PacBio HiFi sequencing and Hi-C technologies, which resulted in a 283-fold increase in contig N50 length and improvement in continuity and quality in complex repetitive region for S. ocellatus compared to the previous version. In total, 257.36 Mb of repetitive sequences accounted for 35.48% of the genome, and 22,845 protein-coding genes associated with a BUSCO value of 98.32%, were identified by genome annotation. Moreover, 54 hub genes rapidly responding to hypoosmotic stress were identified by WGCNA. The high-quality chromosome-scale S. ocellatus genome and candidate resistance-related gene sets will not only provide a genomic basis for genetic improvement via molecular breeding, but will also lay an important foundation for investigating the molecular regulation of rapid responses to stress.

Hypoxia acclimation improves mitochondrial efficiency in the aerobic swimming muscle of red drum (Sciaenops ocellatus)

Environmental hypoxia (low dissolved oxygen) is a significant threat facing fishes. As fishes require oxygen to efficiently produce ATP, hypoxia can significantly limit aerobic capacity. However, some fishes show respiratory flexibility that rescues aerobic performance, including plasticity in mitochondrial performance. This plasticity may result in increased mitochondrial efficiency (e.g., less proton leak), increased oxygen storage capacity (increased myoglobin), and oxidative capacity (e.g., higher citrate synthase activity) under hypoxia. We acclimated a hypoxia-tolerant fish, red drum (Sciaenops ocellatus), to 8-days of constant hypoxia to induce a hypoxic phenotype. Fish were terminally sampled for cardiac and red muscle tissue to quantify oxidative phosphorylation, proton leak, and maximum respiration in tissue from both hypoxia-acclimated and control fish. Tissue was also collected to assess the plasticity of citrate synthase enzyme activity and mRNA expression for select oxygen storage and antioxidant pathway transcripts. We found that mitochondrial respiration rates were not affected by hypoxia exposure in cardiac tissue, though citrate synthase activity and myoglobin expression were higher following hypoxia acclimation. Interestingly, measures of mitochondrial efficiency in red muscle significantly improved in hypoxia-acclimated individuals. Hypoxia-acclimated fish had significantly higher OXPHOS Control Efficiency, OXPHOS Capacity and Coupling Control Ratios (i.e., LEAK/OXPHOS). There was no significant change to citrate synthase activity or myoglobin expression in red muscle. Overall, these results suggest that red muscle mitochondria of hypoxia-acclimated fish more efficiently utilize oxygen, which may explain previous reports in red drum of improved aerobic swimming performance in the absence of improved maximum metabolic rate following hypoxia acclimation.

Functional divergence of teleost carbonic anhydrase 4

The functional role of membrane-bound carbonic anhydrases (CAs) has been of keen interest in the past decade, and in particular, studies have linked CA in red muscle, heart, and eye to enhanced tissue oxygen extraction in bony fishes (teleosts). However, the number of purported membrane-bound CA isoforms in teleosts, combined with the imperfect system of CA isoform nomenclature, present roadblocks for ascribing physiological functions to particular CA isoforms across different teleost lineages. Here we developed an organizational framework for membrane-bound CAs in teleosts, providing the latest phylogenetic analysis of extant CA4 and CA4-like isoforms. Our data confirm that there are three distinct isoforms of CA4 (a, b, and c) that are conserved across major teleost lineages, with the exception of CA4c gene being lost in salmonids. Tissue distribution analyses suggest CA4a functions in oxygen delivery across teleost lineages, while CA4b may be specialized for renal acid-base balance and ion regulation. This work provides an important foundation for researchers to elucidate the functional significance of CA4 isoforms in fishes.

The role of carbonic anhydrase-mediated tissue oxygen extraction in a marine teleost acclimated to hypoxia

With the growing prevalence of hypoxia (O2 levels ≤2 mg l−1) in aquatic and marine ecosystems, there is increasing interest in the adaptive mechanisms fish may employ to better their performance in stressful environments. Here, we investigated the contribution of a proposed strategy for enhancing tissue O2 extraction – plasma-accessible carbonic anhydrase (CA-IV) – under hypoxia in a species of estuarine fish (red drum, Sciaenops ocellatus) that thrives in fluctuating habitats. We predicted that hypoxia-acclimated fish would increase the prevalence of CA-IV in aerobically demanding tissues to confer more efficient tissue O2 extraction. Furthermore, we predicted the phenotypic changes to tissue O2 extraction that occur with hypoxia acclimation may improve respiratory and swim performance under 100% O2 conditions (i.e. normoxia) when compared with performance in fish that have not been acclimated to hypoxia. Interestingly, there were no significant differences in relative CA-IV mRNA expression, protein abundance or enzyme activity between the two treatments, suggesting CA-IV function is maintained under hypoxia. Likewise, respiratory performance of hypoxia-acclimated fish was similar to that of control fish when tested in normoxia. Critical swim speed (Ucrit) was significantly higher in hypoxia-acclimated fish but translated to marginal ecological benefits with an increase of ∼0.3 body lengths per second. Instead, hypoxia-acclimated fish may have relied more heavily on anaerobic metabolism during their swim trials, utilizing burst swimming 1.5 times longer than control fish. While the maintenance of CA-IV may still be an important contributor for hypoxia tolerance, our evidence suggests hypoxia-acclimated red drum are using other mechanisms to cope in an O2-depleted environment.

Early life-stage Deepwater Horizon crude oil exposure induces latent osmoregulatory defects in larval red drum (Sciaenops ocellatus)

Crude oil is known to induce developmental defects in teleost fish exposed during early-life stages (ELSs). A recent study has demonstrated that zebrafish (Danio rerio) larvae acutely exposed to Deepwater Horizon (DHW) crude oil showed transcriptional changes in key genes involved in early kidney (pronephros) development and function, which were coupled with pronephric morphological defects. Given the osmoregulatory importance of the kidney, it is unknown whether ELS effects arising from short-term crude exposures result in long-term osmoregulatory defects, particularly within estuarine fishes likely exposed to DWH oil following the spill. To address this knowledge gap, an acute 72 h exposure to red drum (Sciaenops ocellatus) larvae was performed using high-energy water-accommodated fractions (HEWAFs) of DWH weathered oil to analyze transcriptional changes in genes involved in pronephros development and function by quantitative PCR. To test the latent effects of oil exposure on osmoregulation ability, red drum larvae were first exposed to HEWAF for 24 h. Larvae were then reared in clean seawater for two weeks and a 96 h acute osmotic challenge test was performed by exposing the fish to waters with varying salinities. Latent effects of ELS crude oil exposure on osmoregulation were assessed by quantifying survival during the acute osmotic challenge test and analyzing transcriptional changes at 14 dpf. Results demonstrated that ELS crude oil exposure reduced survival of red drum larvae when challenged in hypoosmotic waters and that latent transcriptional changes in some target pronephric genes were evident, indicating that an affected kidney likely contributed to the increased mortality.

Osmoregulatory plasticity during hypersaline acclimation in red drum, Sciaenops ocellatus

Prolonged drought and freshwater diversion are making periods of hypersalinity more common in coastal ecosystems. This is especially true in the Laguna Madre system along the Texas coast where salinities can exceed 60 g/kg. As such, the ability to tolerate hypersalinity is critical to the success of endemic species, such as the commercially important red drum (Sciaenops ocellatus). This study evaluated acclimation of red drum to hypersalinity (60 g/kg) using a direct transfer protocol. Hypersalinity exposure resulted in significant impacts on plasma osmolality and muscle water in the first 24 h, but returned to control values coincident with a significant increase in intestinal water volume. Hypersalinity acclimation resulted in significant branchial and intestinal plasticity. The gill showed significant elevated nka α1a, nkcc1 and vha (B subunit) mRNA abundance, as well as NKA enzyme activity. The posterior intestine showed a stronger plasticity signal than the anterior intestine, which included a 12-fold increase in nkcc2 mRNA abundance and significant increases in NKA and VHA enzyme activity. These changes were corroborated by a significant threefold increase in bumetanide-sensitive absorptive short circuit current. These data suggest that the dynamic regulation of NKCC2-mediated intestinal water absorption is an important compliment to HCO₃^--mediated water absorption during hypersalinity exposure and acclimation.

The effects of acute crude oil exposure on growth and competition in red drum, Sciaenops ocellatus

Crude oil is a well-known toxicant that reduces cardiorespiratory performance in acutely exposed fishes. While toxic effects can manifest in death in severe cases, the ecological consequences of sub-lethal exposure remain uncertain. This study investigated the impact of crude oil exposure on long-term social competition, growth, and metabolic performance in a coastal species, the red drum (Sciaenops ocellatus). Fish were acutely exposed to either control or one of two environmentally relevant oil concentrations and reared together in groups of 15 (5 from each exposure concentration) for eight weeks under resource-rich or resource-limited scenarios. Relative to controls, a 41.3% and 45.9% reduction in the specific growth rate was-observed following exposure to 25.3 and 53.4 μg l^-1 ΣPAH respectively under resource-limited conditions. These fish were subsequently sampled for metabolic performance and common indicators of social subordination including reduced glucocorticoid receptors in the gill and caudal fin damage. The reduction in specific growth rate coincided with a 15.1% and 17.3% reduction in standard metabolic rate; however, maximum metabolic rate and aerobic scope were unaffected. Additionally, measures of social subordination showed no differences between oil-exposed and control fish. These results reinforce the hypothesis that acute oil exposure can have prolonged sub-lethal effects that compromise the ability of exposed individuals to perform effectively in their environment, including gathering and/or metabolizing food. Furthermore, this work highlights the premise that oil spills can be more detrimental in already at-risk ecosystems.

Mechanisms of acid-base regulation following respiratory alkalosis in red drum (Sciaenops ocellatus)

Respiratory acidosis and subsequent metabolic compensation are well-studied processes in fish exposed to elevated CO₂ (hypercapnia). Yet, such exposures in the marine environment are invariably accompanied by a return of environmental CO₂ to atmospheric baselines. This understudied phenomenon has the potential to cause a respiratory alkalosis that would necessitate base excretion. Here we sought to explore this question and the associated physiological mechanisms that may accompany base excretions using the red drum (Sciaenops ocellatus). As expected, when high pCO₂ (15,000 μatm CO₂) acclimated red drum were transferred to normal pCO₂, their net H⁺ excretion shifted from positive (0.157 ± 0.044 μmol g^-1 h^-1) to negative (-0.606 ± 0.116 μmol g^-1 h^-1) in the 2 h post-transfer period. Net H⁺ excretion returned to control rates during the 3 to 24 h flux period. Gene expression and enzyme activity assays demonstrated that while the acidosis resulted in significant changes in several relevant transporters, no significant changes accompanied the alkalosis phase. Confocal microscopy was used to assess alkalosis-stimulated translocation of V-type H⁺ ATPase to the basolateral membrane previously seen in other marine species; however, no apparent translocation was observed. Overall, these data demonstrate that fluctuations in environmental CO₂ result in both acidic and alkalotic respiratory disturbances; however, red drum maintain sufficient regulatory capacity to accommodate base excretion. Furthermore, this work does not support a role for basolateral VHA translocation in metabolic compensation from a systemic alkalosis in teleosts.

Transcriptomic response to three osmotic stresses in gills of hybrid tilapia (Oreochromis mossambicus female × O. urolepis hornorum male)

Background

Osmotic stress is a widespread phenomenon in aquatic animal. The ability to cope with salinity stress and alkaline stress is quite important for the survival of aquatic species under natural conditions. Tilapia is an important commercial euryhaline fish species. What’s more tilapia is a good experimental material for osmotic stress regulation research, but the molecular regulation mechanism underlying different osmotic pressure of tilapia is still unexplored.

Results

To elucidate the osmoregulation strategy behind its hyper salinity, alkalinity and salinity-alkalinity stress of tilapia, the transcriptomes of gills in hybrid tilapia (Oreochromis mossambicus ♀ × O. urolepis hornorum ♂) under salinity stress (S: 25‰), alkalinity stress(A: 4‰) and salinity-alkalinity stress (SA: S: 15‰, A: 4‰) were sequenced using deep-sequencing platform Illumina/HiSeq-2000 and differential expression genes (DEGs) were identified. A total of 1958, 1472 and 1315 upregulated and 1824, 1940 and 1735 downregulated genes (P-value < 0.05) were identified in the salt stress, alkali stress and saline-alkali stress groups, respectively, compared with those in the control group. Furthermore, Kyoto Encyclopedia of Genes and Genomes pathway analyses were conducted in the significant different expression genes. In all significant DEGs, some of the typical genes involved in osmoregulation, including carbonic anhydrase (CA), calcium/calmodulin-dependent protein kinase (CaM kinase) II (CAMK2), aquaporin-1(AQP1), sodium bicarbonate cotransporter (SLC4A4/NBC1), chloride channel 2(CLCN2), sodium/potassium/chloride transporter (SLC12A2 / NKCC1) and other osmoregulation genes were also identified. RNA-seq results were validated with quantitative real-time PCR (qPCR), the 17 random selected genes showed a consistent direction in both RNA-Seq and qPCR analysis, demonstrated that the results of RNA-seq were reliable.

Conclusions

The present results would be helpful to elucidate the osmoregulation mechanism of aquatic animals adapting to saline-alkali challenge. This study provides a global overview of gene expression patterns and pathways that related to osmoregulation in hybrid tilapia, and could contribute to a better understanding of the molecular regulation mechanism in different osmotic stresses.

Red drum Sciaenops ocellatus growth and expression of bile salt-dependent lipase in response to increasing dietary lipid supplementation

Sciaenops ocellatus has a long history in aquaculture and many difficulties associated with its commercial culture have been addressed and successfully resolved; nevertheless, further research in lipid nutrition could address more comprehensive questions on the way these nutrients are utilized. The purpose of this study was to evaluate S. ocellatus growth and lipase gene expression in response to increasing dietary lipid supplementation. Four experimental diets were formulated to provide 3, 10, 16, or 23% lipid using menhaden fish oil. Twenty juveniles (mean initial weight 2.3 ± 0.1 g) were stocked per aquaria in a recirculating system; each diet was assigned to three aquaria and fed to fish for 6 weeks. At the end of the study, fish fed 3% of dietary lipid were significantly (P < 0.0001) smaller and showed significantly lower feed efficiency, condition factor, hepatosomatic index, and intraperitoneal fat than fish fed the other diets, but no differences were observed among fish fed 10, 16, or 23% lipid. A straight broken-line regression model for thermal growth coefficient provided an estimated value of 9.4% of dietary lipid as the optimal inclusion level. The bile salt-dependent lipase (BSDL) of red drum was 80.3 kDa. Relative gene expression of BSDL was significantly higher (P = 0.0007) in fish fed 10% lipid, with no differences among the other dietary treatments. Results provided could help monitor the metabolic status of farmed fish and contribute to optimize diet formulations based on maximum gene expression of BSDL for supplementation of dietary lipid.

Phenotypic plasticity in gene expression and physiological response in red drum Sciaenops ocellatus exposed to a long-term freshwater environment

Red drum (Sciaenops ocellatus) is a euryhaline fish commonly found in the Gulf of Mexico and along the Atlantic coast of North America. Because of high commercial demand and its euryhaline characteristics, aquaculture of this species has diversified from marine to low-salinity aquaculture systems. In recent years, interest in the feasibility of producing red drum in inland freshwater systems has grown and this prompted us to investigate its osmoregulatory capacity after rearing for 8 months in a freshwater aquaculture system. We compared the activities of several genes and enzymes involved in the osmoregulatory process in freshwater-acclimatized (FW) and seawater (SW) red drum. The gene expression profiles were variable: the expression of genes encoding Na⁺/K⁺-ATPase (NKA) and the cystic fibrosis transmembrane regulator (CFTR) was slightly higher in SW than FW fish, while phosphoenolpyruvate carboxykinase (PEPCK) and the glucocorticoid receptor messenger RNA (mRNA) levels were higher in FW red drum. The total plasma K concentration was 60.3% lower, and gill NKA activity was 63.5% lower in FW than in SW fish. PEPCK activity was twofold higher in FW than in SW red drum. Similarly, liver glycogen was 60% higher in FW fish. In summary, both gene expression and the enzyme activity data support the phenotypic plasticity of red drum and suggest that the limited capacity for ion homeostasis observed, in particular the low plasma K concentration, was due to the composition of freshwater and does not necessarily reflect a physiological inability to osmoregulate.

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.

Acclimation to prolonged hypoxia alters hemoglobin isoform expression and increases hemoglobin oxygen affinity and aerobic performance in a marine fish

Hemoglobin (Hb) multiplicity is common in fish, yet despite its ubiquitous nature, the functional significance is unclear. Here we explore the hypothesis that Hb multiplicity plays a role in hypoxia tolerance using the red drum (Sciaenops ocellatus). Red drum is an economically and ecologically important species native to coastal regions and estuaries of the Gulf of Mexico – habitats that routinely experience pronounced hypoxic events. Using a transcriptomic approach, we demonstrate that red drum red blood cells express 7 and 5 Hbα and Hbβ isoforms, respectively. Phylogenetic analysis grouped these isoforms into distinct isoHb clades, and provided evidence of lineage specific expression of particular isoHbs. In normoxia, three isoHbs predominated (Hbα-3.1, -3.2, and Hbβ-3.1). A three-week hypoxia acclimation (48 mmHg) resulted in significant up-regulation of Hbα-2, Hbα-3.2, and Hbβ-3.1, effectively switching the predominantly expressed isoforms. Changes in subunit expression were correlated with a decrease in non-stripped hemolysate P₅₀. Similarly, hypoxia acclimation resulted in a 20% reduction in whole animal critical oxygen threshold (P_crit). Hypoxia acclimation was not associated with changes in gill morphology, hematocrit, or relative ventricular mass. Overall, these data provide support for the hypothesis that Hb isoform switching can provide a physiological benefit to counteract environmental stress in fishes.

Sustained impairment of respiratory function and swim performance following acute oil exposure in a coastal marine fish

Acute exposure to crude oil polycyclic aromatic hydrocarbons (PAH) can severely impair cardiorespiratory function and swim performance of larval fish; however, the effects of acute oil exposure on later life stages and the capacity for subsequent recovery is less clear. Red drum (Sciaenops ocellatus) is an economically important apex predator native to the Gulf of Mexico, which was directly exposed to the 2010 Deep Water Horizon (DWH) oil spill. Here we examine impact and recovery of young adult red drum from exposure to concentrations of 0, 4.1, and 12.1μgL^-1 ΣPAH₅₀ naturally weathered oil-water accommodated fractions (geometric mean), which are well within the range of concentrations measured during the DWH incident. We focused on aerobic scope (ASc), burst- and critical swimming speeds (U_burst and U_crit), cost of transport (COT), as well as the capacity to repay oxygen debt following exhaustive exercise (EPOC), which are critical parameters for success of all life stages of fishes. A 24h acute exposure to 4.1μgL^-1 ΣPAH caused a significant 9.7 and 12.6% reduction of U_burst and U_crit respectively, but no change in ASc, COT or EPOC, highlighting a decoupled effect on the respiratory and swimming systems. A higher exposure concentration, 12.1μgL^-1 ΣPAH, caused an 8.6 and 8.4% impairment of U_burst and U_crit, as well as an 18.4% reduction in ASc. These impairments persisted six weeks post-exposure, suggesting that recorded impacts are entrenched. Large predatory fishes are critically dependent on the cardiorespiratory and swimming systems for ecological fitness, and long-term impairment of performance due to acute oil exposure suggests that even acute exposure events may have long lasting impacts on the ecological fitness of affected populations.

Carbon dioxide induced plasticity of branchial acid-base pathways in an estuarine teleost

Anthropogenic CO₂ is expected to drive ocean pCO₂ above 1,000 μatm by 2100 – inducing respiratory acidosis in fish that must be corrected through branchial ion transport. This study examined the time course and plasticity of branchial metabolic compensation in response to varying levels of CO₂ in an estuarine fish, the red drum, which regularly encounters elevated CO₂ and may therefore have intrinsic resilience. Under control conditions fish exhibited net base excretion; however, CO₂ exposure resulted in a dose dependent increase in acid excretion during the initial 2 h. This returned to baseline levels during the second 2 h interval for exposures up to 5,000 μatm, but remained elevated for exposures above 15,000 μatm. Plasticity was assessed via gene expression in three CO₂ treatments: environmentally realistic 1,000 and 6,000 μatm exposures, and a proof-of-principle 30,000 μatm exposure. Few differences were observed at 1,000 or 6,000 μatm; however, 30,000 μatm stimulated widespread up-regulation. Translocation of V-type ATPase after 1 h of exposure to 30,000 μatm was also assessed; however, no evidence of translocation was found. These results indicate that red drum can quickly compensate to environmentally relevant acid-base disturbances using baseline cellular machinery, yet are capable of plasticity in response to extreme acid-base challenges.

Cardiac function and survival are affected by crude oil in larval red drum, Sciaenops ocellatus

Following exposure to weathered and non-weathered oil, lethal and sub-lethal impacts on red drum larvae were assessed using survival, morphological, and cardiotoxicity assays. The LC50 for red drum ranged from 14.6 (10.3-20.9) to 21.3 (19.1-23.8) μgl^-1 ΣPAH with no effect of exposure timing during the pre-hatch window or oil weathering. Similarly, morphological deformities showed dose responses in the low ppb range. Cardiac output showed similar sensitivity resulting in a major 70% reduction after exposure to 2.6μgl^-1 ΣPAH. This cardiac failure was driven by reduced stroke volume rather than bradycardia, meaning that in some species, cardiac function is more sensitive than previously thought. After the Deepwater Horizon oil spill, much of this type of work has primarily focused on pelagic species with little known about fast developing estuarine species. These results demonstrate similarity sensitivity of the red drum as their pelagic counter parts, and more importantly, that cardiac function is dramatically reduced in concert with pericardial edema.

Effects of salinity acclimation and eyestalk ablation on Na(+), K(+), 2Cl(-) cotransporter gene expression in the gill of Portunus trituberculatus:a molecular correlate for salt-tolerant trait

The Na(+), K(+), 2Cl(-) cotransporter (NKCC) is an important gene in ion transport. In order to elucidate its function, and regulatory mechanisms, in salinity acclimation, the complete cDNA sequence of NKCC (4218 bp) from Portunus trituberculatus (PtNKCC) was first cloned and characterized. It was found to encode 1055 amino acids containing conserved AA-permease and SLC12 motifs. Results show that PtNKCC is expressed to the greatest extent in gills. High salinity stress exposure led to significant increases (9.6-fold) of PtNKCC mRNA expression in the gills 12 h after treatment, declining to less than the levels seen in the control group between 48 and 72 h. During low salinity stress, expression levels of PtNKCC in gills were found to be upregulated at each sampling time, reaching their peak after 6 h (a 12.4-fold increase). Eyestalk ablation also triggered an 11.3-fold increase in PtNKCC mRNA, while re-injection with eyestalk homogenates significantly reduced the expression of PtNKCC mRNA. Four single nucleotide polymorphisms (SNPs) were detected in the PtNKCC open reading frame, and one SNP was associated with salt tolerance. Our results indicate that PtNKCC plays an important role in the salinity acclimation of P. trituberculatus, while there may be a compound present in the XOSG that inhibits the expression of PtNKCC.