The chondrichthyan glucagon-like peptide 3 regulates hepatic ketone metabolism in the Pacific spiny dogfish Squalus suckleyi.

Chondrichthyans have a novel proglucagon-derived peptide, glucagon-like peptide (GLP)-3, in addition to GLP-1 and GLP-2 that occur in other vertebrates. Given that the GLPs are important regulators of metabolic homeostasis across vertebrates, we sought to investigate whether GLP-3 displays functional actions on metabolism within a representative chondrichthyan, the Pacific spiny dogfish Squalus suckleyi. There were no observed effects of GLP-3 perfusion (10 nM for 15 min) on the rate of glucose or oleic acid acquisition at the level of the spiral valve nor were there any measured effects on intermediary metabolism within this tissue. Despite no effects on apparent glucose transport or glycolysis in the liver, a significant alteration to ketone metabolism occurred. Firstly, ketone flux through the perfused liver switched from a net endogenous production to consumption following hormone application. Accompanying this change, significant increases in mRNA transcript abundance of putative ketone transporters and in the activity of ß-hydroxybutyrate dehydrogenase (a key enzyme regulating ketone flux in the liver) were observed. Overall, while these results show effects on hepatic metabolism, the physiological actions of GLP are distinct between this chondrichthyan and those of GLP-1 on teleost fishes. Whether this is the result of the particular metabolic dependency on ketone bodies in chondrichthyans or a differential function of a novel GLP remains to be fully elucidated.

Understanding olfactory and behavioural responses to dietary cues in age-1 lake sturgeon Acipenser fulvescens.

Detection of environmental cues is essential for all vertebrates and is typically established by the olfactory epithelium and olfactory sensory neurons (OSNs). In fishes, microvillous and ciliated OSNs are the principal types, typically detecting amino acids and bile salts, respectively. Activation of OSN receptors by specific ligands initiate downstream signal processing often leading to behavioural responses. In this study we used electrophysiological and behavioural techniques to evaluate olfactory detection and behaviour in juvenile lake sturgeon Acipenser fulvescens in response to hatchery- and natural dietary cues. We hypothesized that electro-olfactogram (EOG) and behavioural responses would be dependent on diet type. We predicted that inhibition of the phospholipase C/inositol 1,4,5-triphosphate (PLC/IP3) secondary transduction pathway would reduce EOG responses to dietary cues and, inhibition of the adenylyl cyclase/adenosine 3,5-cyclic monophosphate (cAMP) pathway, would have no effect. Furthermore, we predicted a strong EOG response would be manifested in a change in behaviour. We observed that both the PLC/IP3 and cAMP pathways were significantly involved in the detection of dietary cues. However, EOG responses did not manifest to behavioural responses, although the foraging activity to the hatchery cue was significantly greater compared to the control. Our results support the notion that lake sturgeon raised in a hatchery and fed a commercial pelleted diet may become accustomed to it prior to release into the wild. Further, this study suggests that, in conservation aquaculture settings, lake sturgeon should be exposed to natural dietary cues prior to release as one strategy to promote food recognition.

Comparison of metabolic rate between two genetically distinct populations of lake sturgeon.

Environmental temperatures differ across latitudes in the temperate zone, with relatively lower summer and fall temperatures in the north leading to a shorter growing season prior to winter. As an adaptive response, during early life stages, fish in northern latitudes may grow faster than their conspecifics in southern latitudes, which potentially manifests as different allometric relationships between body mass and metabolic rate. In the present study, we examined if population or year class had an effect on the variation of metabolic rate and metabolic scaling of age-0 lake sturgeon (Acipenser fulvescens) by examining these traits in both a northern (Nelson River) and a southern (Winnipeg River) population. We compiled 6 years of data that used intermittent flow respirometry to measure metabolic rate within the first year of life for developing sturgeon that were raised in the same environment at 16°C. We then used a Bayesian modeling approach to examine the impacts of population and year class on metabolic rate and mass-scaling of metabolic rate. Despite previous reports of genetic differences between populations, our results showed that there were no significant differences in standard metabolic rate, routine metabolic rate, maximum metabolic rate, and metabolic scaling between the two geographically separated populations at a temperature of 16°C. Our analysis implied that the lack of metabolic differences between populations could be due to family effects/parental contribution, or the rearing temperature used in the study. The present research provided insights for conservation and reintroduction strategies for these populations of lake sturgeon, which are endangered or threatened across most of their natural range.

Physiological responses to acute warming at the agitation temperature in a temperate shark.

Thermal tolerance and associated mechanisms are often tested via the critical thermal maximum (CTmax). The agitation temperature is a recently described thermal limit in fishes that has received little mechanistic evaluation. The present study used a temperate elasmobranch fish to test the hypothesis that this thermal tolerance trait is partially set by the onset of declining cardiorespiratory performance and the cellular stress response. Pacific spiny dogfish (Squalus suckleyi) were screened for cardiorespiratory and whole-organism thermal limits to test for associations between thermal performance and tolerance. Then, biochemical markers of secondary stress, aerobic and anaerobic enzyme activities, and molecular markers of cellular stress were determined for various tissues at the agitation temperature and secondary stress markers were determined at CTmax. In dogfish, the agitation temperature was characterised by increased turning activity within experimental chambers and was equal to the temperature at which dogfish exhibited maximum heart rate. Citrate synthase activity increased at the agitation temperature in white muscle relative to unmanipulated dogfish. Furthermore, lactate dehydrogenase activity and accumulated lactate in the plasma and muscle were not affected by acute warming. Cellular stress was apparent in hypothalamus, gill filament and ventricle, denoted by elevated transcript abundance of the stress response gene hsp70 but not the oxygen homeostasis gene hif1α. Conversely, CTmax was characterised by metabolic acidosis driven by anaerobic lactate production, signifying an increased reliance on anaerobic metabolism between the agitation temperature and CTmax. Together, these data provide partial support for our hypothesis, in that cellular stress, but not declining thermal performance, occurred at the agitation temperature.

An adapted liver perfusion in a shark species, Squalus suckleyi: investigation of energy mobilization.

The liver is an essential energy storage organ in vertebrates. In teleosts and elasmobranchs, previous studies examining hepatic energy balance have used isolated hepatocytes. Although these studies have been informative, the high-fat content in the elasmobranch liver limits isolation of hepatocytes and therefore the utility of this method to understand hepatic metabolic processes. In the present study, we developed an in situ liver perfusion in the North Pacific spiny dogfish Squalus suckleyi. Perfusions were conducted by cannulating the hepatic portal vein (inflowing cannulation) and the sinus venosus through the heart (outflowing cannulation). Changes in major elasmobranch metabolites (glucose and 3-hydroxybutarate [3-HB]) were determined by the arterial (inflow)-venous (outflow) difference in metabolite concentration. Liver preparations were considered viable due to consistent oxygen consumption over 3 h and the maintenance of predictable vasoconstriction following administration of homologous 10-7 M angiotensin II (ANG II). Removal and reintroduction from the perfusate of metabolites showed endogenous 3-HB production in the isolated perfused livers but did not affect glucose balance. However, the arterial-venous difference of both metabolites did not change following perfusion with heterologous insulin and homologous glucagon, which may be due to the glucose intolerant nature of elasmobranchs. Ultimately, we show the viability of this perfusion for the investigation of hepatic energy mobilization in sharks.NEW & NOTEWORTHY We describe a viable liver perfusion in a shark species for the first time as determined by oxygen consumption and hormone-mediated changes in hemodynamics (angiotensin II, ANG II). In addition, removal of major energy metabolites confirms hepatic ketone [3-hydroxybutyrate (3-HB)] production by an elasmobranch liver. Perfusion with heterologous insulin and homologous glucagon did not cause changes in glucose balance, however, possibly demonstrating differences in glucose metabolism in this taxon as compared with more derived vertebrates.

Tissue-specific transcriptomes reveal potential mechanisms of microbiome heterogeneity in an ancient fish.

The lake sturgeon (Acipenser fulvescens) is an ancient, octoploid fish faced with conservation challenges across its range in North America, but a lack of genomic resources has hindered molecular research in the species. To support such research, we created a transcriptomic database from 13 tissues: brain, esophagus, gill, head kidney, heart, white muscle, liver, glandular stomach, muscular stomach, anterior intestine, pyloric cecum, spiral valve and rectum. The transcriptomes for each tissue were sequenced and assembled individually from a mean of 98.3 million (±38.9 million SD) reads each. In addition, an overall transcriptome was assembled and annotated with all data used for each tissue-specific transcriptome. All assembled transcriptomes and their annotations were made publicly available as a scientific resource. The non-gut transcriptomes provide important resources for many research avenues. However, we focused our analysis on messenger ribonucleic acid (mRNA) observations in the gut because the gut represents a compartmentalized organ system with compartmentalized functions, and seven of the sequenced tissues were from each of these portions. These gut-specific analyses were used to probe evidence of microbiome regulation by studying heterogeneity in microbial genes and genera identified from mRNA annotations. Gene set enrichment analyses were used to reveal the presence of photoperiod and circadian-related transcripts in the pyloric cecum, which may support periodicity in lake sturgeon digestion. Similar analyses were used to identify different types of innate immune regulation across the gut, while analyses of unique transcripts annotated to microbes revealed heterogeneous genera and genes among different gut tissues. The present results provide a scientific resource and information about the mechanisms of compartmentalized function across gut tissues in a phylogenetically ancient vertebrate. Database URL: https://figshare.com/projects/Lake_Sturgeon_Transcriptomes/133143.

In situ method for the determination of nutrient acquisition and its hormonal regulation in the spiral valve of two chondrichthyan fishes.

Chondrichthyans play an important role in nutrient cycling of many marine ecosystems, yet little is known about their nutritional physiology particularly relating to nutrient acquisition in the spiral valve intestine. This unique organ poses challenges for examining nutrient transport physiology using traditional reductionist methods owing to its scroll-like morphology. Thus, we established a method for the characterization of nutrient uptake rates in two representative chondrichthyans, the Pacific spiny dogfish (Squalus suckleyi) and the Pacific spotted ratfish (Hydrolagus colliei). We validated a dual-cannulation method wherein perfusate was circulated through the vasculature of the spiral valve via the anterior and posterior intestinal arteries, and [14C]glucose or [3H]oleic acid was accumulated from the static spiral valve lumen into the anterior and posterior intestinal veins. Radiotracer accumulated at a stable rate in the venous effluent in comparison with measures of mucosal disappearance. Interestingly, similar anterior and posterior glucose uptake was observed in dogfish, yet significantly more oleic acid was accumulated in the posterior veins of ratfish. Further validation of the preparation in dogfish demonstrated sodium dependence of glucose transport as well as an effect of bovine insulin administration to the arterial circulation. Each of these manipulations resulted in significant differences in glucose handling between the anterior and posterior veins, suggestive of heretofore unknown heterogenous functions along the intestine. This preparation demonstrates a new and reliable method for the measurement of nutrient acquisition and regulation thereof in a unique digestive organ. Furthermore, it presents avenues for investigation of differential functional along the spiral valve.NEW & NOTEWORTHY We describe a novel dual cannulation method for investigating radiolabeled nutrient uptake from a unique organ, the spiral valve. Furthermore, we identify functional differences in nutrient transport along the length of the spiral valve which consists of a homogenous gross morphology. Finally, this method reveals a useful way in which to manipulate the arterial supply to better understand postprandial physiology as it varies with metabolites and endocrine factors.

Using 15N to determine the metabolic fate of dietary nitrogen in North Pacific spiny dogfish (Squalus acanthias suckleyi).

Marine elasmobranchs are ureosmotic, retaining large concentrations of urea to balance their internal osmotic pressure with that of the external marine environment. The synthesis of urea requires the intake of exogenous nitrogen to maintain whole-body nitrogen balance and satisfy obligatory osmoregulatory and somatic processes. We hypothesized that dietary nitrogen may be directed toward the synthesis of specific nitrogenous molecules in post-fed animals; specifically, we predicted the preferential accumulation and retention of labelled nitrogen would be directed towards the synthesis of urea necessary for osmoregulatory purposes. North Pacific spiny dogfish (Squalus acanthias suckleyi) were fed a single meal of 7 mmol l-1 15NH4Cl in a 2% ration by body mass of herring slurry via gavage. Dietary labelled nitrogen was tracked from ingestion to tissue incorporation and the subsequent synthesis of nitrogenous compounds (urea, glutamine, bulk amino acids, protein) in the intestinal spiral valve, plasma, liver and muscle. Within 20 h post-feeding, we found labelled nitrogen was incorporated into all tissues examined. The highest δ15N values were seen in the anterior region of the spiral valve at 20 h post-feeding, suggesting this region was particularly important in assimilating the dietary labelled nitrogen. In all tissues examined, enrichment of the nitrogenous compounds was sustained throughout the 168 h experimental period, highlighting the ability of these animals to retain and use dietary nitrogen for both osmoregulatory and somatic processes.

Investigating nitrogen movement in North Pacific spiny dogfish (Squalus acanthias suckleyi), with focus on UT, Rhp2, and Rhbg mRNA abundance.

For ureosmotic marine elasmobranchs, the acquisition and retention of nitrogen is critical for the synthesis of urea. To better understand whole-body nitrogen homeostasis, we investigated mechanisms of nitrogen trafficking in North Pacific spiny dogfish (Squalus acanthias suckleyi). We hypothesized that the presence of nitrogen within the spiral valve lumen would affect both the transport of nitrogen and the mRNA abundance of a urea transporter (UT) and two ammonia transport proteins (Rhp2, Rhbg) within the intestinal epithelium. The in vitro preincubation of intestinal tissues in NH4Cl, intended to simulate dietary nitrogen availability, showed that increased ammonia concentrations did not significantly stimulate the net uptake of total urea or total methylamine. We also examined the mRNA abundance of UT, Rhp2, and Rhbg in the gills, kidney, liver, and spiral valve of fasted, fed, excess urea fed, and antibiotic-treated dogfish. After fasting, hepatic UT mRNA abundance was significantly lower, and Rhp2 mRNA in the gills was significantly higher than the other treatments. Feeding significantly increased Rhp2 mRNA levels in the kidney and mid spiral valve region. Both excess urea and antibiotics significantly reduced Rhbg mRNA levels along all three spiral valve regions. The antibiotic treatment also significantly diminished UT mRNA abundance levels in the anterior and mid spiral valve, and Rhbg mRNA levels in the kidney. In our study, no single treatment had significantly greater influence on the overall transcript abundance of the three transport proteins compared to another treatment, demonstrating the dynamic nature of nitrogen balance in these ancient fish.

Elevated temperatures reduce population-specific transcriptional plasticity in developing lake sturgeon (Acipenser fulvescens).

Rising mean and variance in temperatures elevates threats to endangered freshwater species such as lake sturgeon, Acipenser fulvescens. Previous research demonstrated that higher temperatures during development result in physiological consequences for lake sturgeon populations throughout Manitoba, Canada, with alteration of metabolic rate, thermal tolerance, transcriptional responses, growth and mortality. We acclimated lake sturgeon (30-60 days post fertilization, a period of high mortality) from northern and southern populations (56°02'46.5″N, 96°54'18.6″W and 50°17'52″N, 95°32'51″W, respectively, separated by approximately 650 km) within Manitoba to current (summer highs of 20-23°C) and future projected (+2-3°C) environmental temperatures of 16, 20 and 24°C for 30 days, and we measured gill transcriptional responses using RNAseq. Transcripts revealed SNPs consistent with genetically distinct populations and transcriptional responses altered by acclimation temperature. There were a higher number of differentially expressed transcripts observed in the southern, compared to the northern, population as temperatures increased, indicating enhanced transcriptional plasticity. Both lake sturgeon populations responded to elevated acclimation temperatures by downregulating the transcription of genes involved in protein synthesis and energy production. Furthermore, there were population-specific thresholds for the downregulation of processes promoting transcriptional plasticity as well as mitochondrial function as the northern population showed decreases at 20°C, while this capacity was not diminished until 24°C in the southern population. These transcriptional responses highlight the molecular impacts of increasing temperatures for divergent lake sturgeon populations during vulnerable developmental periods and the critical influence of transcriptome plasticity on acclimation capacity.

Elevated temperatures dampen the innate immune capacity of developing lake sturgeon (Acipenser fulvescens).

Chronic exposure to high temperatures may leave freshwater fishes vulnerable to opportunistic pathogens, particularly during early life stages. Lake sturgeon, Acipenser fulvescens, populations within the northern expanse of their range in Manitoba, Canada, may be susceptible to high temperature stress and pathogenic infection. We acclimated developing lake sturgeon for 22 days to two ecologically relevant, summer temperatures (16 and 20°C). Individuals from both acclimation treatments were then exposed to 0, 30 and 60 µg ml-1 bacterial lipopolysaccharides (endotoxins), as an immune stimulus, for 48 h and sampled 4 and 48 h during trial exposures and following a 7 day recovery period. We then measured whole-body transcriptional (mRNA) responses involved in the innate immune, stress and fatty acid responses following acute exposure to the bacterial endotoxins. Data revealed that overall levels of mRNA transcript abundance were higher in 20°C-reared sturgeon under control conditions. However, following exposure to a bacterial stimulus, lake sturgeon acclimated to 16°C produced a more robust and persistent transcriptional response with higher mRNA transcript abundance across innate immune, stress and fatty acid responses than their 20°C-acclimated counterparts. Additional whole-animal performance metrics (critical thermal maximum, metabolic rate, cortisol concentration and whole-body and mucosal lysozyme activity) demonstrated acclimation-specific responses, indicating compromised metabolic, stress and enzymatic capacity following the initiation of immune-related responses. Our study showed that acclimation to 20°C during early development impaired the immune capacity of developing lake sturgeon as well as the activation of molecular pathways involved in the immune, stress and fatty acid responses. The present study highlights the effects of ecologically relevant, chronic thermal stress on seasonal pathogen susceptibility in this endangered species.

Nitrogen transporters along the intestinal spiral valve of cloudy catshark (Scyliorhinus torazame): Rhp2, Rhbg, UT.

As part of their osmoregulatory strategy, marine elasmobranchs retain large quantities of urea to balance the osmotic pressure of the marine environment. The main source of nitrogen used to synthesize urea comes from the digestion and absorption of food across the gastrointestinal tract. In this study we investigated possible mechanisms of nitrogen movement across the spiral valve of the cloudy catshark (Scyliorhinus torazame) through the molecular identification of two Rhesus glycoprotein ammonia transporters (Rhp2 and Rhbg) and a urea transporter (UT). We used immunohistochemistry to determine the cellular localizations of Rhp2 and UT. Within the spiral valve, Rhp2 was expressed along the apical brush border membrane, and UT was expressed along the basolateral membrane and the blood vessels. The mRNA abundance of Rhp2 was significantly higher in all regions of the spiral valve of fasted catsharks compared to fed catsharks. The mRNA abundance of UT was significantly higher in the anterior spiral valve of fasted catsharks compared to fed. The mRNA transcript of four ornithine urea cycle (OUC) enzymes were detected along the length of the spiral valve and in the renal tissue, indicating the synthesis of urea via the OUC occurs in these tissues. The presence of Rhp2, Rhbg, and UT along the length of the spiral valve highlights the importance of ammonia and urea movement across the intestinal tissues, and increases our understanding of the mechanisms involved in maintaining whole-body nitrogen homeostasis in the cloudy catshark.

Spatiotemporal variation of ringed seal blubber cortisol levels in the Canadian Arctic.

Climate change in the Arctic has widespread and complex effects on the health of animals and their populations. We used radioimmunoassay to measure blubber cortisol in ringed seals (Pusa hispida) sampled in Ulukhaktok, Inuvialuit Settlement Region, Northwest Territories, Canada (spring, 2002, 2004-2005, 2007-2012) and Arviat, Nunavut, Canada (autumn, 2003-2012) to examine chronic stress relative to biology (age, sex, length), body condition (blubber depth), and diet (δ13C, δ15N, and isotopic niche size). Ulukhaktok ringed seals had higher cortisol concentrations overall (0.46 ± 0.04 ng/g) than Arviat ringed seals (0.36 ± 0.03 ng/g), and these higher concentrations were associated with higher muscle δ15N and lower blubber thickness. In contrast, blubber cortisol concentrations for Arviat ringed seals decreased with blubber depth and increased with age, though testing of age effects individually suggests that age-related patterns are weak. Annual mean cortisol concentration increased from 2003 to 2012 in Arviat ringed seals, but low sample sizes precluded analysis of annual patterns for Ulukhaktok ringed seals. The trend of increased cortisol over time in Arviat ringed seals suggests that they might be experiencing greater chronic stress over time, which could have implications for numerous population health metrics including reproduction and pup recruitment.

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 urea­nitrogen may be vital to maintaining nitrogen balance in these fish.

Long-term effects of temperature during early life on growth and fatty acid metabolism in age-0 Lake Sturgeon (Acipenser fulvescens).

Environmental temperature during early life may have prolonged effects on growth and fatty acid metabolism, which could strongly influence overwintering survival in the first year of life for temperate-zone fish. In the present study, we examined how temperature during early life history might influence growth performance and fatty acid metabolism in age-0 Lake Sturgeon (Acipenser fulvescens) when exposed to cold temperatures at later stages. Fish were initially at 16 °C and subsequently held at 16 °C or 20 °C for 60 days beginning at 34 days post fertilization (dpf). Then, all fish were subsequently raised at the same temperature of 16 °C until the onset of cold conditioning at 158 dpf where temperature was gradually decreased to 3.5 °C and remained there for two weeks. Samples were collected before (151 dpf) and after cold conditioning (199 dpf) to measure total length, body mass, whole body metabolic rate, fatty acid profile in phospholipids and triglycerides and mRNA expression of genes associated with fatty acid desaturation, elongation and ß-oxidation. Results revealed that before cold conditioning, total length and body mass did not differ between temperature groups, but fish raised at 20 °C showed a lower condition factor. During the cold conditioning, only fish raised at 16 °C grew significantly longer and heavier. There was no difference in metabolic rates between treatments. Significant increases in total monounsaturated fatty acids with decreases in total saturated fatty acids were identified in phospholipids and triglycerides in both temperature groups after the cold conditioning; however, the 20 °C group did not significantly increase levels of gene expression associated with fatty acid desaturation (SCD and FADS1) whereas the 16 °C group did. Our results suggest that thermal experience during early life may influence overwintering survival of age-0 Lake Sturgeon.

Sustained endocrine and exocrine function in the pancreas of the Pacific spiny dogfish post-feeding.

Secretions of the exocrine pancreas contain digestive enzymes integral to the digestive process. The Pacific spiny dogfish (Squalus suckleyi) has a discrete pancreas, divided into two lobes termed the dorsal and ventral lobes. These lobes drain into the anterior intestine via a common duct to enable digestion. Previous studies have identified that the exocrine pancreas produces (co)lipases, chymotrypsin, carboxypeptidase, and low levels of chitinases; however, investigations into other digestive enzymes are limited. We detect the presence of lipase, trypsin, and carbohydrase and show that activities are equivalent between both lobes of the pancreas. Additionally, we sought to investigate the influence of a single feeding event (2% body weight ration of herring by gavage) on enzyme activities over an extended time course (0, 20, 48, 72, 168 h) post-feeding. The results indicate that there are no differences in pancreatic tissue digestive enzyme activities between fed or fasted states. Analysis of acinar cell circumference post-feeding demonstrates a significant increase at 20 and 48 h, that returns to fasting levels by 72 h. No significant changes were observed regarding whole-tissue insulin or glucagon mRNA abundance or with glucose transporter (glut) 1 or 3. Yet, a significant and transient decrease in glut4 and sodium glucose-linked transporter mRNA abundance was found at 48 h post-feeding. We propose that the constant enzyme activity across this relatively large organ, in combination with a relatively slow rate of digestion leads to an evenly distributed, sustained release of digestive enzymes regardless of digestive state.

Effects of Dietary Shifts on Ontogenetic Development of Metabolic Rates in Age 0 Lake Sturgeon (Acipenser fulvescens).

AbstractIn many fish species, ontogenetic dietary shifts cause changes in both quantitative and qualitative intake of energy, and these transitions can act as significant bottlenecks in survival within a given year class. In the present study, we estimated routine metabolic rate (RMR) and forced maximum metabolic rate (FMR) in age 0 lake sturgeon (Acipenser fulvescens) on a weekly basis from 6 to 76 days posthatch (dph) within the same cohort of fish. We were particularly interested in the period of dietary transition from yolk to exogenous feeding between 6 and 17 dph and as the fish transitioned from an artemia-based diet to a predominantly bloodworm diet between 49 and 67 dph. Measurement of growth rate and energy density throughout indicated that there was a brief period of growth arrest during the transition from artemia to bloodworm. The highest mass-specific RMR (mg O2 kg-1 h-1) recorded throughout the first 76 d of development occurred during the yolk sac phase and during transition from artemia to bloodworm. Similarly, diet transition from artemia to bloodworm-when growth arrest was observed-increased scaled RMR (i.e., mg O2 kg-0.89 h-1), and it did not significantly differ from scaled FMR. Log-log relationships between non-mass-specific RMR or FMR (i.e., mg O2 h-1) and body mass significantly changed as the growing fish adapted to the nutritional differences of their primary diet. We demonstrate that dietary change during early ontogeny has consequences for growth that may reflect altered metabolic performance. Results have implications for understanding cohort and population dynamics during early life and effective management for conservation fish hatcheries.

The effects of population and thermal acclimation on the growth, condition and cold responsive mRNA expression of age-0 lake sturgeon (Acipenser fulvescens).

In Manitoba, Canada, wild lake sturgeon (Acipenser fulvescens) populations exist along a latitudinal gradient and are reared in hatcheries to bolster threatened populations. We reared two populations of lake sturgeon, one from each of the northern and southern ends of Manitoba and examined the effects of typical hatchery temperatures (16°C) as well as 60-day acclimation to elevated rearing temperatures (20°C) on mortality, growth and condition throughout early development. Additionally, we examined the cold shock response, which may be induced during stocking, through the hepatic mRNA expression of genes involved in the response to cold stress and homeoviscous adaptation (HSP70, HSP90a, HSP90b, CIRP and SCD). Sturgeon were sampled after 1 day and 1 week following stocking into temperatures of 8, 6 and 4°C in a controlled laboratory environment. The southern population showed lower condition and higher mortality during early life than the northern population while increased rearing temperature impacted the growth and condition of developing northern sturgeon. During the cold shock, HSP70 and HSP90a mRNA expression increased in all sturgeon treatments as stocking temperature decreased, with higher expression observed in the southern population. Expression of HSP90b, CIRP and SCD increased as stocking temperature decreased in northern sturgeon with early acclimation to 20°C. Correlation analyses indicated the strongest molecular relationships were in the expression of HSP90b, CIRP and SCD, across all treatments, with a correlation between HSP90b and body condition in northern sturgeon with early acclimation to 20°C. Together, these observations highlight the importance of population and rearing environment throughout early development and on later cellular responses induced by cold stocking temperatures.

Simulated heatwave and fishing stressors alter corticosteroid and energy balance in neonate blacktip reef sharks, Carcharhinus melanopterus.

The increasing frequency and duration of marine heatwaves attributed to climate change threatens coastal elasmobranchs and may exacerbate existing anthropogenic stressors. While the elasmobranch stress response has been well studied, the role of the unique corticosteroid-1α-hydroxycorticosterone (1α-OHB)-in energy balance is not understood. Therefore, 1α-OHB's utility as a stress biomarker in elasmobranch conservation physiology is equivocal. Here, we analyse the roles of corticosteroids, 1α-OHB and corticosterone, and metabolites, glucose and 3-hydroxybutyrate (3-HB), in response to stress in a protected tropical shark species, the blacktip reef shark (Carcharhinus melanopterus). Wild-caught neonates were exposed to ambient (27°C) or heatwave conditions (29°C) and subsequently a simulated fishing stressor (1 min air exposure). Blood samples were taken prior to temperature exposure, prior to air exposure, and 30 min, 1 h, 24 h, and 48 h post-air exposure at treatment temperatures. Plasma 1α-OHB was elevated for 48 h in 27°C-exposed sharks but declined over time in 29°C-exposed sharks. Plasma 1α-OHB was not correlated with either metabolite. Plasma glucose was higher and plasma 3-HB was lower in 29°C-exposed sharks. In a separate experiment, blood samples were collected from both neonate and adult sharks immediately following capture and again 5 min later, and analysed for corticosteroids and metabolites. Plasma 1α-OHB increased in neonates within 5 min, but neonates displayed lower plasma 1α-OHB and higher glucose concentrations than adults. We conclude that 1α-OHB does not serve as a classic glucocorticoid role in C. melanopterus under these stressors. Furthermore, we show for the first time, ontogenetic differences in plasma 1α-OHB. Ultimately, our findings provide insights into hormonal control of energy mobilization during stress in C. melanopterus, particularly during simulated heatwave conditions, which seem to alter both endocrine and energy mobilization. Further work is needed to determine the utility of 1α-OHB as a biomarker for the mobilization of energy during a stress event in elasmobranchs.