Urea based osmoregulation and endocrine control in elasmobranch fish with special reference to euryhalinity

Practical application of bicultural stocking and its effects on fish growth, blood biochemistry, and solid waste elimination in recirculating aquaculture system

Traditional monoculture systems often face challenges related to solid waste production and nutrient use. This study addresses these concerns by investigating the impact of bicultural farming of burbot (Lota lota) and rainbow trout (Oncorhynchus mykiss) in a recirculating aquaculture system (RAS) and the subsequent alteration of production of solid waste, growth and physiological status. The rationale behind incorporating burbot lies in its potential as a supplementary species to improve overall system efficiency and sustainability. The experimental groups in triplicate represented the different ratios of rainbow trout (T) and burbot (B) in the stock: T70/B30, T85/B15 and T100/B0. Burbot, although not assessed in monoculture due to its limited commercial significance, was incorporated into the study as a supplementary species, expected to enhance the nutrient utilisation through its bottom-feeding behaviour and anticipated consumption of solid waste produced by trout. After 77 days of culture, the survival rates of trout remained consistent across experimental groups, averaging over 99%, while burbot exhibited comparable survival rates despite lower cumulative survival, averaging 88%. Feed conversion ratios showed no significant differences between the groups, indicating consistent feed utilisation. A significantly higher specific growth rate (SGR) in trout was observed in group T70/B30. The SGR values of burbot were marginally low and without significant differences between groups. Among biochemical markers of blood plasma, phosphorus concentrations were significantly higher in group T70/B30 for both trout and burbot, suggesting better access to the feed for both species. Elevated antioxidant activity and evidence of oxidative stress were found for both species in intestinal tissue. The presence of burbot in stock significantly affected the production of suspended solids per gram of applied feed. Therefore, group T100/B0 demonstrated the highest solid production at multiple time points, suggesting a relationship between burbot presence and the production of suspended solids. Starting at the 9-week, the trout monoculture group exhibited significantly higher phosphorus levels in sediments compared to group T70/B30, emphasising the role of burbot in shaping sediment nutrient dynamics in RAS, such as supplementary cleaning fish. After 11 weeks, group T100/B0 displayed significantly higher values of produced suspended solids and their composition: DM, organic DM, and phosphorus content per gram of feed applied. These results provide evidence of the influence of burbot on suspended solids production and characteristics. In conclusion, this study indicates the positive effects of burbot presence on solid production and sediment nutrient composition.

The SLC6A18 Transporter Is Most Likely a Na-Dependent Glycine/Urea Antiporter Responsible for Urea Secretion in the Proximal Straight Tubule: Influence of This Urea Secretion on Glomerular Filtration Rate

BACKGROUND

Urea is the major end-product of protein metabolism in mammals. In carnivores and omnivores, a large load of urea is excreted daily in urine, with a concentration that is 30-100 times above that in plasma. This is important for the sake of water economy. Too little attention has been given to the existence of energy-dependent urea transport that plays an important role in this concentrating activity.

SUMMARY

This review first presents functional evidence for an energy-dependent urea secretion that occurs exclusively in the straight part of the proximal tubule (PST). Second, it proposes a candidate transmembrane transporter responsible for this urea secretion in the PST. SLC6A18 is expressed exclusively in the PST and has been identified as a glycine transporter, based on findings in SLC6A18 knockout mice. We propose that it is actually a glycine/urea antiport, secreting urea into the lumen in exchange for glycine and Na. Glycine is most likely recycled back into the cell via a transporter located in the brush border. Urea secretion in the PST modifies the composition of the tubular fluid in the thick ascending limb and, thus, contributes, indirectly, to influence the "signal" at the macula densa that plays a crucial role in the regulation of the glomerular filtration rate (GFR) by the tubulo-glomerular feedback.

KEY MESSAGES

Taking into account this secondary active secretion of urea in the mammalian kidney provides a new understanding of the influence of protein intake on GFR, of the regulation of urea excretion, and of the urine-concentrating mechanism.

Fish gill chemosensing: knowledge gaps and inconsistencies

In this review, we explore the inconsistencies in the data and gaps in our knowledge that exist in what is currently known regarding gill chemosensors which drive the cardiorespiratory reflexes in fish. Although putative serotonergic neuroepithelial cells (NEC) dominate the literature, it is clear that other neurotransmitters are involved (adrenaline, noradrenaline, acetylcholine, purines, and dopamine). And although we assume that these agents act on neurons synapsing with the NECs or in the afferent or efferent limbs of the paths between chemosensors and central integration sites, this process remains elusive and may explain current discrepancies or species differences in the literature. To date it has been impossible to link the distribution of NECs to species sensitivity to different stimuli or fish lifestyles and while the gills have been shown to be the primary sensing site for respiratory gases, the location (gills, oro-branchial cavity or elsewhere) and orientation (external/water or internal/blood sensing) of the NECs are highly variable between species of water and air breathing fish. Much of what has been described so far comes from studies of hypoxic responses in fish, however, changes in CO2, ammonia and lactate have all been shown to elicit cardio-respiratory responses and all have been suggested to arise from stimulation of gill NECs. Our view of the role of NECs is broadening as we begin to understand the polymodal nature of these cells. We begin by presenting the fundamental picture of gill chemosensing that has developed, followed by some key unanswered questions about gill chemosensing in general.

Free amino acids in response to salinity changes in fishes: relationships to osmoregulation

Free amino acids (FAAs) are believed to play important roles in osmoregulation and buffer capacity in some aquatic animals, such as fishes. However, the potential roles of FAAs have not been systematically summarized and characterized until now. In the present study, the meta-analysis was conducted to investigate the relationships between FAAs and environmental salinities. Twenty published documents were included, accounting for 106 study cases. The effect sizes of total free amino acids (TFAAs), total essential amino acids (TEAAs), and total non-essential amino acids (TNEAAs) to salinity increase were calculated and determined by the restricted maximum likelihood (REML) method. It clearly showed that the elevated salinities significantly induced the contents of TFAAs, TEAAs, and TNEAAs at the ratio of 36%, 27%, and 29%, respectively. Faced to the salinity changes, the contents of FAAs in fishes under freshwater and seawater varied significantly, while the individuals under brackish water displayed relatively constant contents of FAAs. When salinity elevated, the contents of 17 amino acids in muscles significantly increased, suggesting the important roles of FAA metabolism in osmoregulation in fishes. The results also indicated that the effect sizes of TFAAs were positively related to the rates of salinity increases, and exhibited a significant quadratic linear relationship with temperatures. Additionally, the contents of FAAs also showed positive correlation with osmotic pressure, concentrations of plasma Na+, Cl-, and urea, implying their potential roles of FAAs in osmoregulation in fishes. These findings suggested that elevated salinities greatly induced the contents of FAAs in fishes, making a great contribution to maintaining the homeostasis of fishes in response to environmental salinity changes.

The Epidermal Microbiome Within an Aggregation of Leopard Sharks (Triakis semifasciata) Has Taxonomic Flexibility with Gene Functional Stability Across Three Time-points

The epidermis of Chondrichthyan fishes consists of dermal denticles with production of minimal but protein-rich mucus that collectively, influence the attachment and biofilm development of microbes, facilitating a unique epidermal microbiome. Here, we use metagenomics to provide the taxonomic and functional characterization of the epidermal microbiome of the Triakis semifasciata (leopard shark) at three time-points collected across 4 years to identify links between microbial groups and host metabolism. Our aims include (1) describing the variation of microbiome taxa over time and identifying recurrent microbiome members (present across all time-points); (2) investigating the relationship between the recurrent and flexible taxa (those which are not found consistently across time-points); (3) describing the functional compositions of the microbiome which may suggest links with the host metabolism; and (4) identifying whether metabolic processes are shared across microbial genera or are unique to specific taxa. Microbial members of the microbiome showed high similarity between all individuals (Bray-Curtis similarity index = 82.7, where 0 = no overlap, 100 = total overlap) with the relative abundance of those members varying across sampling time-points, suggesting flexibility of taxa in the microbiome. One hundred and eighty-eight genera were identified as recurrent, including Pseudomonas, Erythrobacter, Alcanivorax, Marinobacter, and Sphingopxis being consistently abundant across time-points, while Limnobacter and Xyella exhibited switching patterns with high relative abundance in 2013, Sphingobium and Sphingomona in 2015, and Altermonas, Leeuwenhoekiella, Gramella, and Maribacter in 2017. Of the 188 genera identified as recurrent, the top 19 relatively abundant genera formed three recurrent groups. The microbiome also displayed high functional similarity between individuals (Bray-Curtis similarity index = 97.6) with gene function composition remaining consistent across all time-points. These results show that while the presence of microbial genera exhibits consistency across time-points, their abundances do fluctuate. Microbial functions however remain stable across time-points; thus, we suggest the leopard shark microbiomes exhibit functional redundancy. We show coexistence of microbes hosted in elasmobranch microbiomes that encode genes involved in utilizing nitrogen, but not fixing nitrogen, degrading urea, and resistant to heavy metal.

Segment-Dependent Gene Expression Profiling of the Cartilaginous Fish Nephron Using Laser Microdissection for Functional Characterization of Nephron at Segment Levels

For adaptation to a high salinity marine environment, cartilaginous fishes have evolved a ureosmotic strategy. They have a highly elaborate "four-loop nephron" in the kidney, which is considered to be important for reabsorption of urea from the glomerular filtrate to maintain a high concentration of urea in the body. However, the function and regulation, generally, of the "four-loop nephron" are still largely unknown due to the complicated configuration of the nephron and its many subdivided segments. Laser microdissection (LMD) followed by RNA-sequencing (RNA-seq) analysis is a powerful technique to obtain segment-dependent gene expression profiles. In the present study, using the kidney of cloudy catshark, Scyliorhinus torazame, we tested several formaldehyde-free and formaldehyde-based fixatives to optimize the fixation methods. Fixation by 1% neutral buffered formalin for 15 min resulted in sufficient RNA and structural integrities, which allowed LMD clipping of specific nephron segments and subsequent RNA-seq analysis. RNA-seq from the LMD samples of the second-loop, the fourth-loop, and the five tubular segments in the bundle zone revealed a number of specific membrane transporter genes that can characterize each segment. Among them, we examined expressions of the Na ⁺ -coupled cotransporters abundantly expressed in the second loop samples. Although the proximal II segment of the second loop is known for the elimination of excess solutes, the present results imply that the PII segment is also crucial for reabsorption of valuable solutes. Looking ahead to future studies, the segment-dependent gene expression profiling will be a powerful technique for unraveling the renal mechanisms and regulation in euryhaline elasmobranchs.

Molecular and morphological investigations on the renal mechanisms enabling euryhalinity of red stingray Hemitrygon akajei

Most cartilaginous fishes live in seawater (SW), but a few exceptional elasmobranchs (sharks and rays) are euryhaline and can acclimate to freshwater (FW) environments. The plasma of elasmobranchs is high in NaCl and urea concentrations, which constrains osmotic water loss. However, these euryhaline elasmobranchs maintain high levels of plasma NaCl and urea even when acclimating to low salinity, resulting in a strong osmotic gradient from external environment to body fluid. The kidney consequently produces a large volume of dilute urine to cope with the water influx. In the present study, we investigated the molecular mechanisms of dilute urine production in the kidney of Japanese red stingray, Hemitrygon akajei, transferred from SW to low-salinity environments. We showed that red stingray maintained high plasma NaCl and urea levels by reabsorbing more osmolytes in the kidney when transferred to low salinity. RNA-seq and qPCR analyses were conducted to identify genes involved in NaCl and urea reabsorption under the low-salinity conditions, and the upregulated gene expressions of Na⁺-K⁺-Cl^- cotransporter 2 (nkcc2) and Na⁺/K⁺-ATPase (nka) were found in the FW-acclimated individuals. These upregulations occurred in the early distal tubule (EDT) in the bundle zone of the kidney, which coils around the proximal and collecting tubules to form the highly convoluted structure of batoid nephron. Considering the previously proposed model for urea reabsorption, the upregulation of nkcc2 and nka not only causes the reabsorption of NaCl in the EDT, but potentially also supports enhanced urea reabsorption and eventually the production of dilute urine in FW-acclimated individuals. We propose advantageous characteristics of the batoid-type nephron that facilitate acclimation to a wide range of salinities, which might have allowed the batoids to expand their habitats.

Blood biochemical variables, antioxidative status, and histological features of intestinal, gill, and liver tissues of African catfish (Clarias gariepinus) exposed to high salinity and high-temperature stress

African catfish is a freshwater species with a high ability to resist brackish water conditions, but heat stress may impair the health status of fish. Thus, the impact of varying levels of water salinity (0, 4, 8, and 12 ppt) was investigated on the growth performance, survival rate, and blood biochemistry of African catfish (average weight: 180.58 ± 2.8 g and average length: 38 ± 1.2 cm) for 4 weeks; then, fish were stressed with high temperature (32 °C) for 72 h. The growth performance and survival rate were markedly higher in fish reared in 0, 4, and 8 ppt than fish in 12 ppt (p < 0.05). Before heat stress, the superoxide dismutase (SOD), catalase (CAT), glutathione (GSH) activities, and malondialdehyde (MDA) levels were markedly increased in fish stressed with 12-ppt salinity (p < 0.05). After heat stress, all groups showed a marked increased SOD, CAT, GSH, and MDA levels than fish before heat stress in the same manner (p < 0.05). Furthermore, fish in the 12 ppt group showed severe intestinal, gill, and liver histological features. The levels of blood glucose and cortisol were markedly increased in fish exposed with 8 and 12 ppt than 0 ppt gradually either before or after heat stress (p < 0.05). The highest values of ALT, AST, urea, creatinine, and the lowest total protein, albumin, and globulin were observed in fish reared in 12 ppt. Significant salinity and heat stress interactions were seen on the ALT, AST, urea, creatinine, total protein, albumin, and globulin values (p < 0.05). The integrated multi-biomarker response (IBR) results showed marked differences among the groups and increased gradually before and after heat stress, with the highest IBR in 12 ppt. In conclusion, growing African catfish in high salinity (12 ppt) hampered the growth performance and health status while the heat stress improved the antioxidative status vis-a-vis increased lipid peroxidation along with higher stress-related markers in expressed both blood and tissue.

The gut microbiome may influence post-prandial nitrogen handling in an elasmobranch, the Pacific spiny dogfish (Squalus suckleyi)

Nitrogen recycling through the gut microbiome is an important mechanism used throughout vertebrates to reclaim valuable nitrogen trapped in urea. Evidence suggests it may be especially important in nitrogen limited animals, yet little is known about its role in marine elasmobranchs, which are said to be severely nitrogen limited. In the present study we used antibiotics to deplete the gut microbiome of Pacific spiny dogfish and assessed the role of the microbiome in nitrogen handling in both fed and fasted states. In fed animals, antibiotic treatment eliminated the activity of the microbial enzyme urease and reduced cellulase activity by 78%. This reduction in microbial enzyme activity resulted in significantly lower plasma urea levels which then trended upward as urea excretion rates decreased. Ammonia excretion rates were also significantly lower in antibiotic treated fish compared to the control fed. Finally, antibiotic treated fed individuals lost an average of 7.4% of their body mass while the fed controls lost only 1.8% of their body mass. Nitrogen handling in fasted animals was not significantly impacted by a reduction in microbial activity. These results suggest that compromising the gut microbiome significantly influences post-prandial nitrogen handling in spiny dogfish, and that the recycling of urea‑nitrogen may be vital to maintaining nitrogen balance in these fish.

Bacterial community dynamics during embryonic development of the little skate (Leucoraja erinacea)

Background

Microbial transmission from parent to offspring is hypothesized to be widespread in vertebrates. However, evidence for this is limited as many evolutionarily important clades remain unexamined. There is currently no data on the microbiota associated with any Chondrichthyan species during embryonic development, despite the global distribution, ecological importance, and phylogenetic position of this clade. In this study, we take the first steps towards filling this gap by investigating the microbiota associated with embryonic development in the little skate, Leucoraja erinacea, a common North Atlantic species and popular system for chondrichthyan biology.

Methods

To assess the potential for bacterial transmission in an oviparous chondrichthyan, we used 16S rRNA amplicon sequencing to characterize the microbial communities associated with the skin, gill, and egg capsule of the little skate, at six points during ontogeny. Community composition was analyzed using the QIIME2 pipeline and microbial continuity between stages was tracked using FEAST.

Results

We identify site-specific and stage-specific microbiota dominated by the bacterial phyla Proteobacteria and Bacteroidetes. This composition is similar to, but distinct from, that of previously published data on the adult microbiota of other chondrichthyan species. Our data reveal that the skate egg capsule harbors a highly diverse bacterial community–particularly on the internal surface of the capsule–and facilitates intergenerational microbial transfer to the offspring. Embryonic skin and external gill tissues host similar bacterial communities; the skin and gill communities later diverge as the internal gills and skin denticles develop.

Conclusions

Our study is the first exploration of the chondrichthyan microbiota throughout ontogeny and provides the first evidence of vertical transmission in this group.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-021-00136-x.

Determining the appropriate pretreatment procedures and the utility of liver tissue for bulk stable isotope (δ13 C and δ15 N) studies in sharks

Stable-isotope analysis (SIA) provides a valuable tool to address complex questions pertaining to elasmobranch ecology. Liver, a metabolically active, high turnover tissue (~166 days for 95% turnover), has the potential to reveal novel insights into recent feeding/movement behaviours of this diverse group. To date, limited work has used this tissue, but ecological application of SIA in liver requires consideration of tissue preparation techniques given the potential for high concentrations of urea and lipid that could bias δ¹³ C and δ¹⁵ N values (i.e., result in artificially lower δ¹³ C and δ¹⁵ N values). Here we investigated the effectiveness of (a) deionized water washing (WW) for urea removal from liver tissue and (b) chloroform-methanol for extraction of lipids from this lipid rich tissue. We then (a) established C:N thresholds for deriving ecologically relevant liver isotopic values given complications of removing all lipid and (b) undertook a preliminary comparison of δ¹³ C values between tissue pairs (muscle and liver) to test if observed isotopic differences correlated with known movement behaviour. Tests were conducted on four large shark species: the dusky (DUS, Carcharhinus obscurus), sand tiger (RAG, Carcharias taurus), scalloped hammerhead (SCA, Sphyrna lewini) and white shark (GRE, Carcharodon carcharias). There was no significant difference in δ¹⁵ N values between lipid-extracted (LE) liver and lipid-extracted/water washed (WW) treatments, however, WW resulted in significant increases in %N, δ¹³ C and %C. Following lipid extraction (repeated three times), some samples were still biased by lipids. Our species-specific "C:N thresholds" provide a method to derive ecologically viable isotope data given the complexities of this lipid rich tissue (C:N thresholds of 4.0, 3.6, 4.7 and 3.9 for DUS, RAG, SCA and GRE liver_LEWW tissue, respectively). The preliminary comparison of C:N threshold corrected liver and muscle δ¹³ C values corresponded with movement/habitat behaviours for each shark; minor differences in δ¹³ C values were observed for known regional movements of DUS and RAG (δ¹³ C_Diffs = 0.24 ± 0.99‰ and 0.57 ± 0.38‰, respectively), while SCA and GRE showed greater differences (1.24 ± 0.63‰ and 1.08 ± 0.71‰, respectively) correlated to large-scale movements between temperate/tropical and pelagic/coastal environments. These data provide an approach for the successful application of liver δ¹³ C and δ¹⁵ N values to examine elasmobranch ecology.

The Evolution of Oxytocin and Vasotocin Receptor Genes in Jawed Vertebrates: A Clear Case for Gene Duplications Through Ancestral Whole-Genome Duplications

The neuronal and neuroendocrine peptides oxytocin (OT) and vasotocin (VT), including vasopressins, have six cognate receptors encoded by six receptor subtype genes in jawed vertebrates. The peptides elicit a broad range of responses that are specifically mediated by the receptor subtypes including neuronal functions regulating behavior and hormonal actions on reproduction and water/electrolyte balance. Previously, we have demonstrated that these six receptor subtype genes, which we designated VTR1A, VTR1B, OTR, VTR2A, VTR2B and VTR2C, arose from a syntenic ancestral gene pair, one VTR1/OTR ancestor and one VTR2 ancestor, through the early vertebrate whole-genome duplications (WGD) called 1R and 2R. This was supported by both phylogenetic and chromosomal conserved synteny data. More recently, other studies have focused on confounding factors, such as the OTR/VTR orthologs in cyclostomes, to question this scenario for the origin of the OTR/VTR gene family; proposing instead less parsimonious interpretations involving only one WGD followed by complex series of chromosomal or segmental duplications. Here, we have updated the phylogeny of the OTR/VTR gene family, including a larger number of vertebrate species, and revisited seven representative neighboring gene families from our previous conserved synteny analyses, adding chromosomal information from newer high-coverage genome assemblies from species that occupy key phylogenetic positions: the polypteriform fish reedfish (Erpetoichthys calabaricus), the cartilaginous fish thorny skate (Amblyraja radiata) and a more recent high-quality assembly of the Western clawed frog (Xenopus tropicalis) genome. Our analyses once again add strong support for four-fold symmetry, i.e., chromosome quadruplication in the same time window as the WGD events early in vertebrate evolution, prior to the jawed vertebrate radiation. Thus, the evolution of the OTR/VTR gene family can be most parsimoniously explained by two WGD events giving rise to the six ancestral genes, followed by differential gene losses of VTR2 genes in different lineages. We also argue for more coherence and clarity in the nomenclature of OT/VT receptors, based on the most parsimonious scenario.

Impacts of low salinity exposure and antibiotic application on gut transport activity in the Pacific spiny dogfish, Squalus acanthias suckleyi

The role of the marine elasmobranch gastrointestinal tract in nitrogen-recycling and osmotic homeostasis has become increasingly apparent, with the gut microbial community likely playing a significant role converting urea, an important osmolyte in elasmobranchs, into ammonia. The Pacific spiny dogfish can experience and tolerate reduced environmental salinities, yet how this environmental challenge may affect the microbiome, and consequently nitrogen transport across the gut, is as of yet unknown. In the present study, excised gut sac preparations were made from dogfish acclimated to the following: full-strength seawater (C), low salinity for 7 days (LS), and after acute transfer of LS-acclimated fish to full-strength SW for 6 h (AT). Significantly reduced microbial derived urease activity was observed in the mucosal saline of gut sac preparations from the LS (by 81%) and AT (by 89%) treatments relative to the C treatment. Microbial derived cellulase activity from mucosal saline samples tended to follow similar patterns. To further ensure an effective decrease in the spiral valve microbial population, an antibiotic cocktail was applied to the mucosal saline used for in vitro measurements of ion, water, and nitrogen flux in these gut sac preparations. This caused a further 57-61% decrease in the mucosal saline urease activity of the C and LS treatments. Overall, we observed relatively little flux across the stomach for all measured parameters aside from water movement, which switched from a net efflux in control fish to a net influx in acutely transferred fish, indicative of drinking. While no significant differences were observed in terms of nitrogen flux (urea or ammonia), we tended to see the accumulation of ammonia in the spiral valve lumen and a switch from efflux to influx of urea in control versus acutely transferred fish. The increased ammonia production likely occurs as a result of heightened metabolism in a challenging environment, while the retention and acquisition of urea is suggestive of nitrogen scavenging under nitrogen-limiting conditions.

Energy and nitrogenous waste from glutamate/glutamine catabolism facilitates acute osmotic adjustment in non-neuroectodermal branchial cells

Maintenance of homeostasis is one of the most important physiological responses for animals upon osmotic perturbations. Ionocytes of branchial epithelia are the major cell types responsible for active ion transport, which is mediated by energy-consuming ion pumps (e.g., Na⁺-K⁺-ATPase, NKA) and secondary active transporters. Consequently, in addition to osmolyte adjustments, sufficient and immediate energy replenishment is essenttableial for acclimation to osmotic changes. In this study, we propose that glutamate/glutamine catabolism and trans-epithelial transport of nitrogenous waste may aid euryhaline teleosts Japanese medaka (Oryzias latipes) during acclimation to osmotic changes. Glutamate family amino acid contents in gills were increased by hyperosmotic challenge along an acclimation period of 72 hours. This change in amino acids was accompanied by a stimulation of putative glutamate/glutamine transporters (Eaats, Sat) and synthesis enzymes (Gls, Glul) that participate in regulating glutamate/glutamine cycling in branchial epithelia during acclimation to hyperosmotic conditions. In situ hybridization of glutaminase and glutamine synthetase in combination with immunocytochemistry demonstrate a partial colocalization of olgls1a and olgls2 but not olglul with Na⁺/K⁺-ATPase-rich ionocytes. Also for the glutamate and glutamine transporters colocalization with ionocytes was found for oleaat1, oleaat3, and olslc38a4, but not oleaat2. Morpholino knock-down of Sat decreased Na⁺ flux from the larval epithelium, demonstrating the importance of glutamate/glutamine transport in osmotic regulation. In addition to its role as an energy substrate, glutamate deamination produces NH₄⁺, which may contribute to osmolyte production; genes encoding components of the urea production cycle, including carbamoyl phosphate synthetase (CPS) and ornithine transcarbamylase (OTC), were upregulated under hyperosmotic challenges. Based on these findings the present work demonstrates that the glutamate/glutamine cycle and subsequent transepithelial transport of nitrogenous waste in branchial epithelia represents an essential component for the maintenance of ionic homeostasis under a hyperosmotic challenge.

Comprehensive analysis of genes contributing to euryhalinity in the bull shark, Carcharhinus leucas; Na+-Cl- co-transporter is one of the key renal factors upregulated in acclimation to low-salinity environment

Most cartilaginous fishes live principally in seawater (SW) environments, but a limited number of species including the bull shark, Carcharhinus leucas, inhabit both SW and freshwater (FW) environments during their life cycle. Euryhaline elasmobranchs maintain high internal urea and ion levels even in FW environments, but little is known about the osmoregulatory mechanisms that enable them to maintain internal homeostasis in hypoosmotic environments. In the present study, we focused on the kidney because this is the only organ that can excrete excess water from the body in a hypoosmotic environment. We conducted a transfer experiment of bull sharks from SW to FW and performed differential gene expression analysis between the two conditions using RNA-sequencing. A search for genes upregulated in the FW-acclimated bull shark kidney indicated that the expression of the Na⁺-Cl^- cotransporter (NCC; Slc12a3) was 10 times higher in the FW-acclimated sharks compared with that in SW sharks. In the kidney, apically located NCC was observed in the late distal tubule and in the anterior half of the collecting tubule, where basolateral Na⁺/K⁺-ATPase was also expressed, implying that these segments contribute to NaCl reabsorption from the filtrate for diluting the urine. This expression pattern was not observed in the houndshark, Triakis scyllium, which had been transferred to 30% SW; this species cannot survive in FW environments. The salinity transfer experiment combined with a comprehensive gene screening approach demonstrates that NCC is a key renal protein that contributes to the remarkable euryhaline ability of the bull shark.

Effects of pre-treatments on bulk stable isotope ratios in fish samples: A cautionary note for studies comparisons

RATIONALE

Stable isotope analysis (SIA) has revolutionised ecological studies over the past thirty years. One of the major fields where SIA is applied in the marine environment is related to the definition of ecosystem structure and function. With marine top predators such as sharks, SIA is a method of choice because tissue samples can be collected without the sacrifice of the animal. In elasmobranch research, the influence of compounds such as urea, trimethylamine oxide and lipids must be considered when using stable isotopes as ecological markers. Currently, a range of pre-treatments are used to chemically remove these molecules prior to SIA.

METHODS

This study investigated the impact of eleven commonly used pre-treatments on carbon and nitrogen contents and C:N atomic ratio, as well as carbon and nitrogen SI ratios in elasmobranch tissues and its prey, measured by isotope ratio mass spectrometry. Three tissues were tested: blood and muscle of the ragged-tooth shark Carcharias taurus, and muscle of one teleost species, the Cape knifejaw Oplegnathus conwayi.

RESULTS

Compared with untreated samples, no trend or generalisation could be highlighted with the influence of pre-treatments being species-, tissue- and chemical-element-dependent. For the δ¹³ C and δ¹⁵ N values, differences among pre-treatments were as high as 3‰, therefore potentially leading to erroneous ecological interpretation.

CONCLUSIONS

The chemical properties of compounds (e.g. urea, lipids) combined with the polarity of solutions (e.g. water, solvents) explained a large part of these observations. This study highlights that pre-treatments need to be considered especially when comparing carbon and nitrogen stable isotope ratios between studies. The results of this study provide a call to all stable isotope researchers to make a concerted effort to standardise pre-treatment methods. This is crucial as global reviews are becoming increasingly more informative.

A fragment activity assay reveals the key residues of TBC1D15 GTPase-activating protein (GAP) in Chiloscyllium plagiosum

BACKGROUND

GTPase-activating proteins (GAPs) with a TBC (Tre-2/Bub2/Cdc16) domain architecture serve as negative regulators of Rab GTPases. The related crystal structure has been studied and reported by other members of our research group in 2017 (Chen et al. in Protein Sci 26(4):834-846, 2017). The protein crystal structure and sequencing data accession numbers in Protein structure database (PDB) are 5TUB (Shark TBC1D15 GAP) and 5TUC (Sus TBC1D15 GAP), respectively. In this paper, we analyzed the Rab-GAP specificity of TBC1D15 in the evolution and influence of key amino acid residue mutations on Rab-GAP activity.

RESULTS

Sequence alignment showed that five arginine residues of the TBC1D15-GAP domain are conserved among the species Sus/Mus/Homo but have been replaced by glycine or lysine in Shark. A fragment activity assay was conducted by altering the five residues of Shark TBC1D15-GAP to arginine, and the corresponding arginine in TBC1D15 GAP domains from Sus and Homo species were mutated to resemble Shark TBC1D15-GAP. Our data revealed that the residues of G28, K45, K119, K122 and K221 in the Shark TBC1D15-GAP domain had a key role in determining the specificity for Rab7 and Rab11. Mutation of the five residues significantly altered the Shark TBC1D15-GAP activity.

CONCLUSIONS

These results revealed that the substrate specificity of TBC1D15 has had different mechanisms across the evolution of species from lower-cartilaginous fish to higher mammals. Collectively, the data support a different mechanism of Shark TBC1D15-GAP in substrate selection, which provides a new idea for the development of Marine drugs.

Alanine, proline and urea are major organic osmolytes in the snail Theodoxus fluviatilis under hyperosmotic stress

Hyperosmotic stress may result in osmotic volume loss from the body to the environment in animals that cannot control the water permeability of their integument. Euryhaline animals (which have a wide tolerance range of environmental salinities) have generally evolved the ability to counteract cell volume shrinkage by accumulating inorganic and organic osmolytes within their cells to balance internal and external osmolalities. Molluscs use very different combinations of amino acids and amino acid derivatives to achieve this goal. Theodoxus fluviatilis is a neritid gastropod that is distributed not only in limnic habitats in Europe but also in brackish waters (e.g. along the shoreline of the Baltic Sea). Animals from brackish sites survive better in high salinities than animals from freshwater locations. The results of the present study indicate that these differences in salinity tolerance cannot be explained by differences in the general ability to accumulate amino acids as organic osmolytes. Although there may be differences in the metabolic pathways involved in osmolyte accumulation in foot muscle tissue, the two groups of animals accumulate amino acid mixtures equally well when stepwise acclimated to their respective maximum tolerable salinity for extended periods. Among these amino acids, alanine and proline, as well as the osmolyte urea, hold a special importance for cell volume preservation in T. fluviatilis under hyperosmotic stress. It is possible that the accumulation of various amino acids during hyperosmotic stress occurs via hydrolysis of storage proteins, while alanine and proline are probably newly synthesised under conditions of hyperosmotic stress in the animals.

Multi-tissue RNA-seq and transcriptome characterisation of the spiny dogfish shark (Squalus acanthias) provides a molecular tool for biological research and reveals new genes involved in osmoregulation

The spiny dogfish shark (Squalus acanthias) is one of the most commonly used cartilaginous fishes in biological research, especially in the fields of nitrogen metabolism, ion transporters and osmoregulation. Nonetheless, transcriptomic data for this organism is scarce. In the present study, a multi-tissue RNA-seq experiment and de novo transcriptome assembly was performed in four different spiny dogfish tissues (brain, liver, kidney and ovary), providing an annotated sequence resource. The characterization of the transcriptome greatly increases the scarce sequence information for shark species. Reads were assembled with the Trinity de novo assembler both within each tissue and across all tissues combined resulting in 362,690 transcripts in the combined assembly which represent 289,515 Trinity genes. BUSCO analysis determined a level of 87% completeness for the combined transcriptome. In total, 123,110 proteins were predicted of which 78,679 and 83,164 had significant hits against the SwissProt and Uniref90 protein databases, respectively. Additionally, 61,215 proteins aligned to known protein domains, 7,208 carried a signal peptide and 15,971 possessed at least one transmembrane region. Based on the annotation, 81,582 transcripts were assigned to gene ontology terms and 42,078 belong to known clusters of orthologous groups (eggNOG). To demonstrate the value of our molecular resource, we show that the improved transcriptome data enhances the current possibilities of osmoregulation research in spiny dogfish by utilizing the novel gene and protein annotations to investigate a set of genes involved in urea synthesis and urea, ammonia and water transport, all of them crucial in osmoregulation. We describe the presence of different gene copies and isoforms of key enzymes involved in this process, including arginases and transporters of urea and ammonia, for which sequence information is currently absent in the databases for this model species. The transcriptome assemblies and the derived annotations generated in this study will support the ongoing research for this particular animal model and provides a new molecular tool to assist biological research in cartilaginous fishes.

Stable Isotope Applications for Understanding Shark Ecology in the Northeast Pacific Ocean

Stable isotopes are used to address a wide range of ecological questions and can help researchers and managers better understand the movement and trophic ecology of sharks. Here, we review how shark studies from the Northeast Pacific Ocean (NEP) have employed stable isotopes to estimate trophic level and diet composition and infer movement and habitat-use patterns. To date, the number of NEP shark studies that have used stable isotopes is limited, suggesting that the approach is underutilized. To aid shark researchers in understanding the strengths and limitations of the approach, we provide a brief overview of carbon and nitrogen stable isotope trophic discrimination properties (e.g., change in δ¹⁵N between predator and prey), tissue sample preparation methods specific to elasmobranchs, and methodological considerations for the estimation of trophic level and diet composition. We suggest that stable isotopes are a potentially powerful tool for addressing basic questions about shark ecology and are perhaps most valuable when combined and analysed with other data types (e.g., stomach contents, tagging data, or other intrinsic biogeochemical markers).

Differential expression and miRNA regulation of the GSTP1 gene in the regenerating liver of Chiloscyllium plagiosum

Liver regeneration is a complicated process, and understanding the regulatory mechanism will be helpful in the treatment of diseases associated with liver. In this study, the one-third liver resection model was established in Chiloscyllium plagiosum, and the whole transcriptome of the C. plagiosum was generated using the Illumina-Solexa sequencing platform. Differentially expressed genes were analyzed using bioinformatics methods and verified using quantitative real-time PCR (qRT-PCR). Using miRanda and TargetScan, we screened the microRNA library for miRNAs that target the glutathione S-transferase P1(GSTP1) gene. Dual-luciferase reporter assays were used to confirm binding between the miRNA and GSTP1. Finally, we used western blotting analysis to determine expression of the GSTP1 protein. As a result, 65,356 unigenes were obtained in normal and damaged liver tissues, with mean length of 955 bp. A total of 359 differentially expressed genes were acquired; 217 of which were upregulated, and 142 were downregulated, including the GSTP1 gene, following liver resection. The presence of the GSTP1 protein in C. plagiosum was shown for the first time. Luciferase reporter assay revealed that GSTP1 messenger RNA was targeted by ipu-miR-143. The discovery and differential expression analysis of GSTP1 in C. plagiosum will be a valuable resource to explain the molecular mechanism of GSTP1 regulation of liver repair.

Distribution and dynamics of branchial ionocytes in houndshark reared in full-strength and diluted seawater environments

In teleost fishes, it is well-established that the gill serves as an important ionoregulatory organ in addition to its primary function of respiratory gas exchange. In elasmobranchs, however, the ionoregulatory function of the gills is still incompletely understood. Although two types of ionocytes, Na(+)/K(+)-ATPase (NKA)-rich (type-A) cell and vacuolar-type H(+)-ATPase (V-ATPase)-rich (type-B) cell, have been found in elasmobranch fishes, these cells were considered to function primarily in acid-base regulation. In the present study, we examined ion-transporting proteins expressed in ionocytes of Japanese-banded houndshark, Triakis scyllium, reared in full-strength seawater (SW) and transferred to diluted (30%) SW. In addition to the upregulation of NKA and Na(+)/H(+) exchanger type 3 (NHE3) mRNAs in the type-A ionocytes, we found that Na(+), Cl(-) cotransporter (NCC, Slc12a3) is expressed in a subpopulation of the type-B ionocytes, and that the expression level of NCC mRNA was enhanced in houndsharks transferred to a low-salinity environment. These results suggest that elasmobranch gill ionocytes contribute to NaCl uptake in addition to the already described function of acid-base regulation, and that NCC is most probably one of the key molecules for hyper-osmoregulatory function of elasmobranch gills. The existence of two types of ionocytes (NHE3- and NCC-expressing cells) that are responsible for NaCl absorption seems to be a common feature in both teleosts and elasmobranchs for adaptation to a low salinity environment. A possible driving mechanism for NCC in type-B ionocytes is discussed.

Physiological responses to hypersalinity correspond to nursery ground usage in two inshore shark species (Mustelus antarcticus & Galeorhinus galeus)

Shark nurseries are susceptible to environmental fluctuations in salinity because of their shallow, coastal nature; however, the physiological impacts on resident elasmobranchs are largely unknown. Gummy (Mustelus antarcticus) and school sharks (Galeorhinus galeus) use the same Tasmanian estuary as a nursery ground; however, each species has distinct distribution patterns that are coincident with changes in local environmental conditions, such as increases in salinity. We hypothesized that these differences were directly related to differential physiological tolerances to high salinity. To test this hypothesis, we exposed wild, juvenile school and gummy sharks to an environmentally-relevant hypersaline (120% SW) event for 48 h. Metabolic rate decreased 20-35% in both species, and gill Na+/K+ ATPase activity was maintained in gummy sharks but decreased 37% in school sharks. We measured plasma ions (Na+, K+, Cl−), and osmolytes (urea and trimethylamine oxide (TMAO)), and observed a 33% increase in plasma Na+ in gummy sharks with hyperosmotic exposure, while school sharks displayed a typical ureosmotic increase in plasma urea (∼20%). With elevated salinity, gill TMAO concentration increased by 42% in school sharks and by 30% in gummy sharks. Indicators of cellular stress (heat shock proteins HSP70, 90, 110, and ubiquitin) significantly increased in gill, and white muscle in both a species- and tissue- specific manner. Overall, gummy sharks exhibited greater osmotic perturbation and ionic dysregulation and a larger cellular stress response compared to school sharks. Our findings provide physiological correlates to the observed distribution, and movement of these shark species in their critical nursery grounds.

Effects of short-term hyper- and hypo-osmotic exposure on the osmoregulatory strategy of unfed North Pacific spiny dogfish (Squalus suckleyi)

The North Pacific spiny dogfish (Squalus suckleyi) is a partially euryhaline species of elasmobranch that often enter estuaries where they experience relatively large fluctuations in environmental salinity that can affect plasma osmolality. Previous studies have investigated the effects of altered salinity on elasmobranchs over the long term, but fewer studies have conducted time courses to investigate how rapidly they can adapt to such changes. In this study, we exposed unfed (no exogenous source of nitrogen or TMAO) spiny dogfish to hyper- and hypo-osmotic conditions and measured plasma and tissue osmolytes, nitrogen excretion, and changes in enzyme activity and mRNA levels in the rectal gland over 24h. It was shown that plasma osmolality changes to approximately match the ambient seawater within 18-24h. In the hypersaline environment, significant increases in urea, sodium, and chloride were observed, whereas in the hyposaline environment, only significant decreases in TMAO and sodium were observed. Both urea and ammonia excretion increased at low salinities suggesting a reduction in urea retention and possibly urea production. qPCR and enzyme activity data for Na(+)/K(+)-ATPase did not support the idea of rectal gland activation following exposure to increased salinities. Therefore, we suggest that the rectal gland may not be a quantitatively important aspect of the dogfish osmoregulatory strategy during changes in environmental salinity, or it may be active only in the very early stages (i.e., less than 6h) of responses to altered salinity.

Water transport and functional dynamics of aquaporins in osmoregulatory organs of fishes

Aquaporins play distinct roles for water transport in fishes as they do in mammals-both at the cellular, organ, and organismal levels. However, with over 32,000 known species of fishes inhabiting almost every aquatic environment, from tidal pools, small mountain streams, to the oceans and extreme salty desert lakes, the challenge to obtain consensus as well as specific knowledge about aquaporin physiology in these vertebrate clades is overwhelming. Because the integumental surfaces of these animals are in intimate contact with the surrounding milieu, passive water loss and uptake represent two of the major osmoregulatory challenges that need compensation. However, neither obligatory nor regulatory water transport nor their mechanisms have been elucidated to the same degree as, for example, ion transport in fishes. Currently fewer than 60 papers address fish aquaporins. Most of these papers identify "what is present" and describe tissue expression patterns in various teleosts. The agnathans, chondrichthyans, and functionality of fish aquaporins generally have received little attention. This review emphasizes the functional physiology of aquaporins in fishes, focusing on transepithelial water transport in osmoregulatory organs in euryhaline species - primarily teleosts, but covering other taxonomic groups as well. Most current knowledge comes from teleosts, and there is a strong need for related information on older fish clades. Our survey aims to stimulate new, original research in this area and to bring together new collaborations across disciplines.

Chaperone roles for TMAO and HSP70 during hyposmotic stress in the spiny dogfish shark (Squalus acanthias)

Salinity decreases are experienced by many marine elasmobranchs. To understand how these fishes cope with hyposmotic stress on a cellular level, we used the spiny dogfish shark (Squalus acanthias) as a model to test whether a reciprocal relationship exists between the cell's two primary protein protection mechanisms, the chemical (e.g., trimethylamine oxide, TMAO) and molecular (e.g., heat shock protein 70, HSP70) chaperone systems. This relationship is interesting given that many elasmobranchs are expected to gain water and lose osmolytes, chemical chaperones, and ions as they osmoconform to new, lowered salinity. Dogfish were cannulated for repeated blood sampling and exposed to 70% seawater (SW) for 48 h. These hyposmotic conditions had no effect on red blood cell (RBC) and white muscle TMAO concentrations, and did not result in HSP70 induction or signs of protein damage (i.e., increased ubiquitin), suggesting that TMAO levels were sufficiently protective in these tissues. However, in the gill, we observed a significant decrease in TMAO concentration and a significant induction of HSP70 as well as signs of protein damage. In the face of this cellular stress response, gill Na(+)/K(+)-ATPase (NKA) activity significantly increased during hyposmotic conditions, as expected. We suggest that this functional preservation in the gill is partly the result of HSP70 induction with lowered salinity. We conclude a reciprocal relationship between TMAO and HSP70 in the gills of dogfish as a result of in vivo hyposmotic stress. When osmotically induced protein damage surpasses the protective capacity of remaining TMAO, HSP70 is induced to preserve tissue and organismal function.

Osmoregulation by juvenile brown-banded bamboo sharks, Chiloscyllium punctatum, in hypo- and hyper-saline waters

While there is a considerable body of work describing osmoregulation by elasmobranchs in brackish and saltwater, far fewer studies have investigated osmoregulation in hypersaline waters. We examined osmo- and ionoregulatory function and plasticity in juvenile brown-banded bamboo sharks, Chiloscyllium punctatum, exposed to three experimental salinities (25, 34 and 40‰) for two weeks. C. punctatum inhabits sheltered coastal areas and bays which can naturally become hypersaline as a consequence of evaporation of water but can also become hyposaline during flood events. We hypothesised that C. punctatum would demonstrate a phenotypically plastic osmoregulatory physiology. Plasma osmolality, urea, Na(+) and Cl(-) levels increased significantly with increasing environmental salinity. Rectal gland and branchial sodium-potassium ATPase (NKA) activities were unaffected by salinity. Using immunohistochemistry and Western Blotting we found evidence for the presence of the key ion-regulatory proteins vacuolar H(+)-ATPase (VHA), pendrin (Cl(-)/HCO₃(-) co-transporter) and the Na(+)-H(+) exchanger isoform 3 (NHE3) in discrete cells within the branchial epithelia. These results indicate that C. punctatum is a partially euryhaline elasmobranch able to maintain osmo- and ionoregulatory function between environmental salinities of 25‰ and 40‰. As suggested for other elasmobranchs, the gills of C. punctatum likely play a limited role in maintaining Na(+) homeostasis over the salinity range studied, but may play an important role in acid-base balance.

Morphological and functional characteristics of the kidney of cartilaginous fishes: with special reference to urea reabsorption

For adaptation to high-salinity marine environments, cartilaginous fishes (sharks, skates, rays, and chimaeras) adopt a unique urea-based osmoregulation strategy. Their kidneys reabsorb nearly all filtered urea from the primary urine, and this is an essential component of urea retention in their body fluid. Anatomical investigations have revealed the extraordinarily elaborate nephron system in the kidney of cartilaginous fishes, e.g., the four-loop configuration of each nephron, the occurrence of distinct sinus and bundle zones, and the sac-like peritubular sheath in the bundle zone, in which the nephron segments are arranged in a countercurrent fashion. These anatomical and morphological characteristics have been considered to be important for urea reabsorption; however, a mechanism for urea reabsorption is still largely unknown. This review focuses on recent progress in the identification and mapping of various pumps, channels, and transporters on the nephron segments in the kidney of cartilaginous fishes. The molecules include urea transporters, Na(+)/K(+)-ATPase, Na(+)-K(+)-Cl(-) cotransporters, and aquaporins, which most probably all contribute to the urea reabsorption process. Although research is still in progress, a possible model for urea reabsorption in the kidney of cartilaginous fishes is discussed based on the anatomical features of nephron segments and vascular systems and on the results of molecular mapping. The molecular anatomical approach thus provides a powerful tool for understanding the physiological processes that take place in the highly elaborate kidney of cartilaginous fishes.

Does ammonia trigger hyperventilation in the elasmobranch, Squalus acanthias suckleyi?

We examined the ventilatory response of the spiny dogfish, to elevated internal or environmental ammonia. Sharks were injected via arterial catheters with ammonia solutions or their Na salt equivalents sufficient to increase plasma total ammonia concentration [TAmm]a by 3-5 fold from 145±21μM to 447±150μM using NH4HCO3 and a maximum of 766±100μM using (NH4)2SO4. (NH4)2SO4 caused a small increase in ventilation frequency (+14%) and a large increase in amplitude (+69%), while Na2SO4 did not. However, CO2 partial pressure (PaCO2) also increased and arterial pHa and plasma bicarbonate concentration ([HCO3(-)]a) decreased. NH4HCO3 caused a smaller increase in plasma ammonia resulting in a smaller but significant, short lived increases in ventilation frequency (+6%) and amplitude (36%), together with a rise in PaCO2 and [HCO3(-)]a. Injection with NaHCO3 which increased pHa and [HCO3(-)]a did not change ventilation. Plasma ammonia concentration correlated significantly with ventilation amplitude, while ventilation frequency showed a (negative) correlation with pHa. Exposure to high environmental ammonia (1500μM NH4HCO3) did not induce changes in ventilation until plasma [TAmm]a increased and ventilation amplitude (but not frequency) increased in parallel. We conclude that internal ammonia stimulates ventilation in spiny dogfish, especially amplitude or stroke volume, while environmental ammonia only stimulates ventilation after ammonia diffuses into the bloodstream.

Transcriptome profiling reveals higher vertebrate orthologous of intra-cytoplasmic pattern recognition receptors in grey bamboo shark

From an immunologist perspective, sharks are an important group of jawed cartilaginous fishes and survey of the public database revealed a great gap in availability of large-scale sequence data for the group of Chondrichthyans the elasmobranchs. In an attempt to bridge this deficit we generated the transcriptome from the spleen and kidney tissues (a total of 1,606,172 transcripts) of the shark, Chiloscyllium griseum using the Illumina HiSeq2000 platform. With a cut off of > = 300 bp and an expression value of >1RPKM we used 43,385 transcripts for BLASTX analysis which revealed 17,548 transcripts matching to the NCBI nr database with an E-value of < = 10(-5) and similarity score of 40%. The longest transcript was 16,974 bases with matched to HECT domain containing E3 ubiqutin protein ligase. MEGAN4 annotation pipeline revealed immune and signalling pathways including cell adhesion molecules, cytokine-cytokine receptor interaction, T-cell receptor signalling pathway and chemokine signaling pathway to be highly expressed in spleen, while different metabolism pathways such as amino acid metabolism, carbohydrate metabolism, lipid metabolism and xenobiotic biodegradation were highly expressed in kidney. Few of the candidate genes were selected to analyze their expression levels in various tissues by real-time PCR and also localization of a receptor by in-situ PCR to validate the prediction. We also predicted the domains structures of some of the identified pattern recognition receptors, their phylogenetic relationship with lower and higher vertebrates and the complete downstream signaling mediators of classical dsRNA signaling pathway. The generated transcriptome will be a valuable resource to further genetic and genomic research in elasmobranchs.

Amines and amine-related compounds in surface waters: a review of sources, concentrations and aquatic toxicity

This review compiles available information on the concentrations, sources, fate and toxicity of amines and amine-related compounds in surface waters, including rivers, lakes, reservoirs, wetlands and seawater. There is a strong need for this information, especially given the emergence of amine-based post-combustion CO2 capture technologies, which may represent a new and significant source of amines to the environment. We identify a broad range of anthropogenic and natural sources of amines, nitrosamines and nitramines to the aquatic environment, and identify some key fate and degradation pathways of these compounds. There were very few data available on amines in surface waters, with reported concentrations often below detection and only rarely exceeding 10 μg/L. Reported concentrations for seawater and reservoirs were below detection or very low, while for lakes and rivers, concentrations spanned several orders of magnitude. The most prevalent and commonly detected amines were methylamine (MA), dimethylamine (DMA), ethylamine (EA), diethylamine (DEA) and monoethanolamine (MEAT). The paucity of data may reflect the analytical challenges posed by determination of amines in complex environmental matrices at ambient levels. We provide an overview of available aquatic toxicological data for amines and conclude that at current environmental concentrations, amines are not likely to be of toxicological concern to the aquatic environment, however, the potential for amines to act as precursors in the formation of nitrosamines and nitramines may represent a risk of contamination of drinking water supplies by these often carcinogenic compounds. More research on the prevalence and toxicity of amines, nitrosamines and nitramines in natural waters is necessary before the environmental impact of new point sources from carbon capture facilities can be adequately quantified.

Time course of the acute response of the North Pacific spiny dogfish shark (Squalus suckleyi) to low salinity

Dogfish are considered stenohaline sharks but are known to briefly enter estuaries. The acute response of North Pacific spiny dogfish (Squalus suckleyi) to lowered salinity was tested by exposing sharks to 21‰ salinity for 48 h. Temporal trends in blood pH, plasma osmolality, CO2, HCO3(-), Na(+), Cl(-), K(+), and urea concentrations, and in the rates of urea efflux and O2 consumption, were quantified. The rate of O2 consumption exhibited cyclic variation and was significantly depressed by lowered salinity. After 9 h, plasma [Cl(-)] stabilized at 9% below initial levels, while plasma [Na(+)] decreased by more than 20% within the first 12 h. Plasma [urea] dropped by 15% between 4 and 6 h, and continued to decrease. The rate of urea efflux increased over time, peaking after 36 h at 72% above the initial rate. Free-swimming sharks subjected to the same salinity challenge survived over 96 h and differed from cannulated sharks with respect to patterns of Na(+) and urea homeostasis. This high-resolution study reveals that dogfish exposed to 21‰ salinity can maintain homeostasis of Cl(-) and pH, but Na(+) and urea continue to be lost, likely accounting for the inability of the dogfish to fully acclimate to reduced salinity.

Active urea transport in lower vertebrates and mammals

Some unicellular organisms can take up urea from the surrounding fluids by an uphill pumping mechanism. Several active (energy-dependent) urea transporters (AUTs) have been cloned in these organisms. Functional studies show that active urea transport also occurs in elasmobranchs, amphibians, and mammals. In the two former groups, active urea transport may serve to conserve urea in body fluids in order to balance external high ambient osmolarity or prevent desiccation. In mammals, active urea transport may be associated with the need to either store and/or reuse nitrogen in the case of low nitrogen supply, or to excrete nitrogen efficiently in the case of excess nitrogen intake. There are probably two different families of AUTs, one with a high capacity able to establish only a relatively modest transepithelial concentration difference (renal tubule of some frogs, pars recta of the mammalian kidney, early inner medullary collecting duct in some mammals eating protein-poor diets) and others with a low capacity but able to maintain a high transepithelial concentration difference that has been created by another mechanism or in another organ (elasmobranch gills, ventral skin of some toads, and maybe mammalian urinary bladder). Functional characterization of these transporters shows that some are coupled to sodium (symports or antiports) while others are sodium-independent. In humans, only one genetic anomaly, with a mild phenotype (familial azotemia), is suspected to concern one of these transporters. In spite of abundant functional evidence for such transporters in higher organisms, none have been molecularly identified yet.

Physiological effects of waterborne lead exposure in spiny dogfish (Squalus acanthias)

To broaden our knowledge about the toxicity of metals in marine elasmobranchs, cannulated spiny dogfish (Squalus acanthias) were exposed to 20 μM and 100 μM lead (Pb). Since we wanted to focus on sub lethal ion-osmoregulatory and respiratory disturbances, arterial blood samples were analysed for pH(a), PaO(2), haematocrit and total CO(2) values at several time points. Plasma was used to determine urea, TMAO, lactate and ion concentrations. After 96 h, Pb concentrations were determined in a number of tissues, such as gill, rectal gland, skin and liver. To further investigate ion and osmoregulation, Na(+)/K(+)-ATPase activities in gill and rectal gland were analysed as well as rates of ammonia and urea excretion. Additionally, we studied the energy reserves in muscle and liver. Pb strongly accumulated in gills and especially in skin. Lower accumulation rates occurred in gut, kidney and rectal gland. A clear disturbance in acid-base status was observed after one day of exposure indicating a transient period of hyperventilation. The increase in pH(a) was temporary at 20 μM, but persisted at 100 μM. After 2 days, plasma Na and Cl concentrations were reduced compared to controls at 100 μM Pb and urea excretion rates were elevated. Pb caused impaired Na(+)/K(+)-ATPase activity in gills, but not in rectal gland. We conclude that spiny dogfish experienced relatively low ion-osmoregulatory and respiratory distress when exposed to lead, particularly when compared to effects of other metals such as silver. These elasmobranchs appear to be able to minimize the disturbance and maintain physiological homeostasis during an acute Pb exposure.

The gallbladder of the electric ray Torpedo marmorata Risso displays excrescent cholecystocytes with merocrine and apocrine-like secretions

The gallbladder of Torpedo marmorata exhibits a mucosal surface layer of simple columnar epithelium with very tall cholecystocytes. The apical domain of each cell has few microvilli, but many mucous vesicles that are secreted by exocytosis at the cell apices. The apical regions may also elongate and undergo self-excision while shedding mucus and cell debris into the gallbladder lumen in a manner similar to that described in mammals as a result of sex steroid treatment to induce gallstones and to that found in the cholecystitis associated with cholelithiasis. Numerous small mitochondria, spherical to elongated, are distributed throughout the cells, while the nuclei are often located in the lower third of each cell. In the lower part of the cholecystocytes, large and very densely contrasted lysosomes can be found. All cells are tightly joined by junctional complexes, including long, highly contrasted desmosomes. The fibromuscular layer is made of a loose stroma with a limited muscular component and a poor blood supply. Large diameter blood vessels can only be found in the subserosal layer. It is hypothesized that the obligatorily carnivorous diet of this ureotelic fish has resulted in the evolution of a gallbladder ultrastructure resembling that found in cholecystitis but without the associated cholelithiasis.

Body fluid osmolytes and urea and ammonia flux in the colon of two chondrichthyan fishes, the ratfish, Hydrolagus colliei, and spiny dogfish, Squalus acanthias

The present study has examined the role of the colon in regulating ammonia and urea nitrogen balance in two species of chondrichthyans, the ratfish, Hydrolagus colliei (a holocephalan) and the spiny dogfish, Squalus acanthias (an elasmobranch). Stripped colonic tissue from both the dogfish and ratfish was mounted in an Ussing chamber and in both species bi-directional urea flux was found to be negligible. Urea uptake by the mucosa and serosa of the isolated colonic epithelium through accumulation of (14)C-urea was determined to be 2.8 and 6.2 fold greater in the mucosa of the dogfish compared to the serosa of the dogfish and the mucosa of the ratfish respectively. Furthermore, there was no difference between serosal and mucosal accumulation of (14)C-urea in the ratfish. Through the addition of 2mM NH(4)Cl to the mucosal side of each preparation the potential for ammonia flux was also examined. This was again found to be negligible in both species suggesting that the colon is an extremely tight epithelium to the movement of both urea and ammonia. Plasma, chyme and bile fluid samples were also taken from the agastric ratfish and were compared with solute concentrations of equivalent body fluids in the dogfish. Finally molecular analysis revealed expression of 3 isoforms of the urea transport protein (UT) and an ammonia transport protein (Rhbg) in the gill, intestine, kidney and colon of the ratfish. Partial nucleotide sequences of the UT-1, 2 and 3 isoforms in the ratfish had 95, 95 and 92% identity to the equivalent UT isoforms recently identified in another holocephalan, the elephantfish, Callorhinchus milii. Finally, the nucleotide sequence of the Rhbg identified in the ratfish had 73% identity to the Rhbg protein recently identified in the little skate, Leucoraja erinacea.

Branchial osmoregulation in the euryhaline bull shark, Carcharhinus leucas: a molecular analysis of ion transporters

Bull sharks, Carcharhinus leucas, are one of only a few species of elasmobranchs that live in both marine and freshwater environments. Osmoregulation in euryhaline elasmobranchs is achieved through the control and integration of various organs (kidney, rectal gland and liver) in response to changes in environmental salinity. However, little is known regarding the mechanisms of ion transport in the gills of euryhaline elasmobranchs and how they are affected by osmoregulatory challenges. This study was conducted to gain insight into the branchial ion and acid-base regulatory mechanisms of C. leucas by identifying putative ion transporters and determining whether their expression is influenced by environmental salinity. We hypothesised that expression levels of the Na+/K+-ATPase (NKA) pump, Na+/H+ exchanger 3 (NHE3), vacuolar-type H+-ATPase (VHA) and anion exchanger pendrin (PDN) would be upregulated in freshwater (FW) C. leucas. Immunohistochemistry was used to localise all four ion transporters in gills of bull sharks captured in both FW and estuarine/seawater (EST/SW) environments. NHE3 immunoreactivity occurred in the apical region of cells with basolateral NKA expression whereas PDN was apically expressed in cells that also exhibited basolateral VHA immunoreactivity. In accordance with our hypotheses, quantitative real-time PCR showed that the mRNA expression of NHE3 and NKA was significantly upregulated in gills of FW-captured C. leucas relative to EST/SW-captured animals. These data suggest that NHE3 and NKA together may be important in mediating branchial Na+ uptake in freshwater environments, whereas PDN and VHA might contribute to Cl-/HCO3- transport in marine and freshwater bull shark gills.

A role for inositol monophosphatase 1 (IMPA1) in salinity adaptation in the euryhaline eel (Anguilla anguilla)

This study investigated the expression and tissue distribution of inositol monophosphatase (IMPA1) and characterized its role in salinity adaptation in the eel. The coding sequence of eel IMPA1 was determined and confirmed to be orthologous to the mammalian gene/enzyme by phylogenetic analysis and structural modeling. Quantitative real-time PCR and Western blot techniques indicated up to 17-fold increases in mRNA expression and 2-fold increases in protein abundance in major osmoregulatory tissues following transfer of fish to seawater (SW). This was accompanied by up to 5-fold increases in enzyme activity, and 1.8- and 3-fold increases in inositol contents within the gill and kidney, respectively. Immunohistological studies revealed that IMPA1 protein expression predominated in SW-acclimated fish within basal epithelial/epidermal layers of the gill, esophagus, intestine, skin, and fins. SW transfer also induced a 10-fold increase in inositol content in the fin. IMPA1 immunoreactivity was also identified in chondrocytes within the cartilagenous matrix of the gills and fins, as well as in clusters of interstitial cells surrounding the kidney tubules. The observed increases in expression of IMPA1 highlight a protective role for inositol within various eel tissues following SW acclimation. This constitutes an adaptive mechanism in teleost fish naturally exposed to hypertonic environments.

Hematological and plasma biochemical parameters of aquarium-maintained cownose rays

There is generally a dearth of information involving reference ranges of health variables for numerous elasmobranch species commonly housed in zoos and aquaria; thus, extrapolation from a few existing elasmobranch studies is commonly used to assess health parameters in these species. The primary objective of this study was to establish baseline complete blood cell count and plasma chemistry reference ranges for captive individuals of the cownose ray Rhinoptera bonasus, an elasmobranch that is widely displayed in zoos and aquaria worldwide. This study was conducted using 18 adult cownose rays currently housed at the Mystic Aquarium and Institute for Exploration in Connecticut. Median blood and plasma values were 550 cells/microL for total white blood cell count; 511,250 cells/microL for total red blood cell count; 31% for packed cell volume; 5.8 g/L for total solids; 2.85 g/dL for total protein; 33 units (U)/L for aspartate aminotransferase; 34 U/L for alkaline phosphatase; 0.2 mg/dL for total bilirubin; 1,155 mg/dL for urea nitrogen; 0.1 mg/dL for creatinine; 0.6 g/dL for albumin; 2.15 g/dL for globulins; 144 mg/dL for cholesterol; 157 mg/dL for triglycerides; 45 mg/dL for glucose; 16.9 mg/dL for calcium; 5.8 mg/dL for phosphorus; 294 mmol/L for sodium; 1.55 mmol/L for potassium; and 270 mmol/L for chloride. Gamma glutamyl transpeptidase, amylase, creatine kinase, and lactate dehydrogenase were below the detectable limits for several samples. In nine samples, alanine aminotransferase levels were below the instrument range (< 4 U/L). No significant differences between sexes were detected. The reference ranges reported in this study should provide a useful guide for routine health monitoring of captive cownose rays.

Proteomic and physiological responses of leopard sharks (Triakis semifasciata) to salinity change

Partially euryhaline elasmobranchs may tolerate physiologically challenging, variable salinity conditions in estuaries as a trade-off to reduce predation risk or to gain access to abundant food resources. To further understand these trade-offs and to evaluate the underlying mechanisms, we examined the responses of juvenile leopard sharks to salinity changes using a suite of measurements at multiple organizational levels: gill and rectal gland proteomes (using 2-D gel electrophoresis and tandem mass spectrometry), tissue biochemistry (Na+/K+-ATPase, caspase 3/7 and chymotrypsin-like proteasome activities), organismal physiology (hematology, plasma composition, muscle moisture) and individual behavior. Our proteomics results reveal coordinated molecular responses to low salinity – several of which are common to both rectal gland and gill – including changes in amino acid and inositol (i.e. osmolyte) metabolism, energy metabolism and proteins related to transcription, translation and protein degradation. Overall, leopard sharks employ a strategy of maintaining plasma urea, ion concentrations and Na+/K+-ATPase activities in the short-term, possibly because they rarely spend extended periods in low salinity conditions in the wild, but the sharks osmoconform to the surrounding conditions by 3 weeks. We found no evidence of apoptosis at the time points tested, while both tissues exhibited proteomic changes related to the cytoskeleton, suggesting that leopard sharks remodel existing osmoregulatory epithelial cells and activate physiological acclimatory responses to solve the problems posed by low salinity exposure. The behavioral measurements reveal increased activity in the lowest salinity in the short-term, while activity decreased in the lowest salinity in the long-term. Our data suggest that physiological/behavioral trade-offs are involved in using estuarine habitats, and pathway modeling implicates tumor necrosis factor α (TNFα) as a key node of the elasmobranch hyposmotic response network.