Embryonic critical windows: changes in incubation temperature alter survival, hatchling phenotype, and cost of development in lake whitefish (Coregonus clupeaformis)

Effects of low temperature on growth and metabolism of larval green sturgeon (Acipenser medirostris) across early ontogeny

Southern Distinct Population Segment (sDPS) green sturgeon spawn solely in one stretch of the Sacramento River in California. Management of this spawning habitat is complicated by cold water temperature requirements for the conservation of winter-run Chinook salmon. This study assessed whether low incubation and rearing temperatures resulted in carryover effects across embryo to early juvenile life stages on scaling relationships in growth and metabolism in northern DPS green sturgeon used as a proxy for sDPS green sturgeon. Fish were incubated and reared at 11 °C and 15 °C, with a subset experiencing a reciprocal temperature transfer post-hatch, to assess recovery from cold incubation or to simulate a cold-water dam release which would chill rearing larvae. Growth and metabolic rate of embryos and larvae were measured to 118 days post hatch. Reciprocal temperature transfers revealed a greater effect of low temperature exposure during larval rearing rather than during egg incubation. While 11 °C eggs hatched at a smaller length, log-transformed length-weight relationships showed that these differences in developmental trajectory dissipated as individuals achieved juvenile morphology. However, considerable size-at-age differences persisted between rearing temperatures, with 15 °C fish requiring 60 days post-hatch to achieve 1 g in mass, whereas 11 °C fish required 120 days to achieve 1 g, resulting in fish of the same age at the completion of the experiment with a ca. 37-fold difference in weight. Consequently, our study suggests that cold rearing temperatures have far more consequential downstream effects than cold embryo incubation temperatures. Growth delays from 11 °C rearing temperatures would greatly increase the period of vulnerability to predation in larval green sturgeon. The scaling relationship between log-transformed whole-body metabolism and mass exhibited a steeper slope and thus an increased oxygen requirement with size in 11 °C reared fish, potentially indicating an energetically unsustainable situation. Understanding how cold temperatures affect green sturgeon ontogeny is necessary to refine our larval recruitment estimations for this threatened species.

Embryonic and larval development of yellow perch (Perca flavescens) and its sensitivity to incubation temperature

The ontogenetic development in teleost fish is sensitive to temperature, and the developmental rate has a direct relationship with the environmental temperature within a species' thermal tolerance limit. Temperature determines time to and survival at hatching. Yellow perch is a North American species of ecological and commercial importance, and its phenology is vulnerable to climate change. The embryonic development of yellow perch was comparable to closely related members of the family Percidae. Developmental progression was fastest at 18°C and slowest at 12°C, with medial progression at 15°C. Time to hatch and swim-up, feeding onset, and exogenous feeding phases were different across all incubation temperatures regardless of a gradual post-hatch warming of the 12 and 15°C groups to a common garden temperature of 18°C. Incubation temperature may lower the rate of survival to hatch at 15°C and had complex impacts on developmental abnormalities. Temperature had significant effects on the development rate, time of hatch, survival, and incidence of developmental abnormalities. Early ontogenetic thermal history in ectotherms is an important factor determining phenotypic variation. It will be important to link the thermally induced changes in development described here to the physiological and morphological differences and to link the developmental abnormalities to functional performance.

Effect of elevated embryonic incubation temperature on the temperature preference of juvenile lake (Coregonus clupeaformis) and round whitefish (Prosopium cylindraceum)

Anthropogenic impacts can lead to increased temperatures in freshwater environments through thermal effluent and climate change. Thermal preference of aquatic organisms can be modulated by abiotic and biotic factors including environmental temperature. Whether increased temperature during embryogenesis can lead to long-term alterations in thermal preference has not been explicitly tested in native freshwater species. Lake (Coregonus clupeaformis) and round (Prosopium cylindraceum) whitefish were incubated at natural and elevated temperatures until hatching, following which, all groups were moved to common garden conditions (15°C) during the post-hatching stage. Temperature preference was determined at 8 months (Lake whitefish only) and 12 months of age (both species) using a shuttle box system. Round whitefish preferred a cooler temperature when incubated at 2 and 6°C compared with 0.5°C. Lake whitefish had similar temperature preferences regardless of age, weight and incubation temperature. These results reveal that temperature preference in freshwater fish can be programmed during early development, and that round whitefish may be more sensitive to incubation temperature. This study highlights the effects that small increases in temperature caused by anthropogenic impacts may have on cold-adapted freshwater fish.

Intergenerational plasticity to cycling high temperature and hypoxia affects offspring stress responsiveness and tolerance in zebrafish

Predicted climate change-induced increases in heat waves and hypoxic events will have profound effects on fishes, yet the capacity of parents to alter offspring phenotype via non-genetic inheritance and buffer against these combined stressors is not clear. This study tested how prolonged adult zebrafish exposure to combined diel cycles of thermal stress and hypoxia affect offspring early survival and development, parental investment of cortisol and heat shock proteins (HSPs), larval offspring stress responses, and both parental and offspring heat and hypoxia tolerance. Parental exposure to the combined stressor did not affect fecundity, but increased mortality, produced smaller embryos and delayed hatching. The combined treatment also reduced maternal deposition of cortisol and increased embryo hsf1, hsp70a, HSP70, hsp90aa and HSP90 levels. In larvae, basal cortisol levels did not differ between treatments, but acute exposure to combined heat stress and hypoxia increased cortisol levels in control larvae with no effect on larvae from exposed parents. In contrast, whereas larval basal hsf1, hsp70a and hsp90aa levels differed between parental treatments, the combined acute stressor elicited similar transcriptional responses across treatments. Moreover, the combined acute stressor only induced a marked increase in HSP47 levels in the larvae derived from exposed parents. Finally, combined hypoxia and elevated temperatures increased both thermal and hypoxia tolerance in adults and conferred an increase in offspring thermal but not hypoxia tolerance. These results demonstrate that intergenerational acclimation to combined thermal stress and hypoxia elicit complex carryover effects on stress responsiveness and offspring tolerance with potential consequences for resilience.

Biochemical composition and adenylate energy charge shifts in longfin yellowtail (Seriola rivoliana) embryos during development under different temperatures

The longfin yellowtail Seriola rivoliana is an emerging species for aquaculture diversification worldwide and production relies on fertilized eggs from captive broodstock. Temperature is the main factor that influences the developmental process and success during fish ontogeny. However, the effects of temperature on the utilization of the main biochemical reserves and bioenergetics are scarcely investigated in fish, whereas protein, lipid and carbohydrate metabolism have critical roles in maintaining cellular energy homeostasis. In this context, we aimed to evaluate metabolic fuels (protein, lipids, triacylglicerides, carbohydrates), adenylic nucleotides and derivates (ATP, ADP, AMP, IMP), and the adenylate energy charge (AEC) during embryogenesis and in hatched larvae in S. rivoliana at different temperatures. For this purpose, fertilized eggs were incubated at six constant (20, 22, 24, 26, 28 and 30 °C) and two oscillating (21⇄29 °C) temperatures. Biochemical analyses were made at blastula, optic vesicles, neurula, prehatch and hatch periods. Results indicated that the developmental period had a major influence on the biochemical composition at any temperature regime tested during the incubation. Protein content decreased only at hatching mainly due to the loss of the chorion, total lipids tended to increase at the neurula period and variations in carbohydrates depended on the particular spawn analyzed. Triacylglicerides were a critical egg fuel during hatching. The high AEC during embryogenesis and even in hatched larvae suggested an optimal energy balance regulation. The lack of critical biochemical changes from different temperature regimes during embryo development confirmed that this species exhibits a high adaptive capacity in response to constant and fluctuating temperatures. However, the timing of hatching was the most critical period of development, where biochemical components and energy utilization significantly changed. The oscillating temperatures tested may have physiological advantages without detrimental energetic effects that will require further research on larval quality after hatching.

Intestinal metabolomics of juvenile lenok (Brachymystax lenok) in response to heat stress

Changes in the metabolic profile within the intestine of lenok (Brachymystax lenok) when challenged to acute and lethal heat stress (HS) are studied using no-target HPLC-MS/MS metabonomic analysis. A total of 51 differentially expressed metabolites (VIP > 1, P < 0.05) were identified in response to HS, and 34 occurred in the positive ion mode and 17 in negative ion mode, respectively. After heat stress, changes in metabolites related to glycolysis (i.e., alpha-D-glucose, stachyose, and L-lactate) were identified. The metabolites (acetyl carnitine, palmitoylcarnitine, carnitine, and erucic acid) related to fatty acid β-oxidation accumulated significantly, and many amino acids (L-tryptophan, D-proline, L-leucine, L-phenylalanine, L-aspartate, L-tyrosine, L-methionine, L-histidine, and L-glutamine) were significantly decreased in HS-treated lenok. The mitochondrial β-oxidation pathway might be inhibited, while severe heat stress might activate the anaerobic glycolysis and catabolism of amino acid for energy expenditure. Oxidative damage in HS-treated lenok was indicated by the decreased glycerophospholipid metabolites (i.e., glycerophosphocholine, 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphoethanolamine, 1-palmitoyl-sn-glycero-3-phosphocholine, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine, and 1, 2-dioleoyl-sn-glycero-3-phosphatidylcholine) and the increased oxylipin production (12-HETE and 9R, 10S-EpOME). The minor oxidative pathways (omega-oxidation and peroxisomal beta-oxidation) were likely to be induced in HS-treated lenok.

Hypoxia- and hyperoxia-related gene expression dynamics during developmental critical windows of the tropical gar Atractosteus tropicus

Aquatic hypoxia is both a naturally-occurring and anthropogenically-generated event. Fish species have evolved different adaptations to cope with hypoxic environments, including gill modifications and air breathing. However, little is known about the molecular mechanisms involved in the respiration of embryonic and larval fishes during critical windows of development. We assessed expression of the genes hif-1α, fih-1, nhe1, epo, gr and il8 using the developing tropical gar as a piscine model during three developmental periods (fertilization to hatch, 1 to 6 days post hatch (dph) and 7 to 12 dph) when exposed to normoxia (~7.43 mg/L DO), hypoxia (~2.5 mg/L DO) or hyperoxia (~9.15 mg/L DO). All genes had higher expression when fish were exposed to either hypoxia or hyperoxia during the first two developmental periods. However, fish continuously exposed to hypoxia had increased expression of the six genes by hatching and 6 dph, and by 12 dph only hif-1α still had increased expression. The middle developmental period was the most hypoxia-sensitive, coinciding with several changes in physiology and morphology. The oldest larvae were the most resilient to gene expression change, with little variation in expression of the six genes compared. This study is the first to relate the molecular response of an air-breathing fish to oxygen availability to developmental critical windows and contributes to our understanding of some molecular responses of developing fish to changes in oxygen availability.

Effect of temperature on the development of deformities during the embryonic stages of Charonia seguenzae (Aradas & Benoit, 1870)

The Mediterranean Triton Charonia seguenzae (Aradas and Benoit, 1870) is an endangered marine gastropod. Re-establishment of C. seguenzae populations in the depleted habitats requires knowledge of its biology and breeding in captivity. Deformities have a huge impact on offspring survival and quality. Temperature has been recorded to affect the development of deformities in marine gastropods. The present study aims to identify the stage of development at which deformities occur, under four temperature regimes (17, 23, 26 and 29 °C). At the stages of trochophore, veliger and free veliger larvae, three capsules that were acclimated at the examined temperatures at the stage of morula were collected, opened and 50 larvae per capsule sampled. Deformities were observed at every examined developmental stage under all tested temperatures. The lower rate of deformities at every stage occurred at 23 °C. The higher tested temperature (29 °C) was lethal and at the lower tested temperature (17 °C) almost every specimen was deformed (96.66%) at eclosion. The effect of acclimation at four developmental stages (morula, trochophore, shell formation and veliger) on the development of deformities at the free veligers of Charonia seguenzae, was studied under three temperature conditions (17, 26 and 29 °C). At eclosion, three capsules were collected, opened and 50 larvae per capsule were sampled. The acclimation at morula and trochophore larva stages led to the higher rates of deformities at eclosion. The size of the free veliger larvae was also affected by temperature with maximum size at eclosion observed at 23 °C. Charonia seguenzae's embryos tolerate elevated temperatures within environmental limits (26 °C) but near future global warming will probably pose a threat to their survival. The free veliger larvae survival at the environmental minimum is related to the time window of the acclimation, since Triton's embryos are more vulnerable to temperature alterations during the early developmental stages.

The heat shock response shows plasticity in embryonic lake whitefish (Coregonus clupeaformis) exposed to repeated thermal stress

We examined the impact of repeated thermal stress on the heat shock response (HSR) of thermally sensitive lake whitefish (Coregonus clupeaformis) embryos. Our treatments were designed to mimic temperature fluctuations in the vicinity of industrial thermal effluents. Embryos were either maintained at control temperatures (3 ^oC) or exposed to a repeated thermal stress (TS) of 3 or 6 ^oC above control temperature every 3 or 6 days throughout embryonic development. At 82 days post-fertilisation, repeated TS treatments were stopped and embryos received either a high level TS of 12, 15, or 18 ^oC above ambient temperature for 1 or 4 h, or no additional TS. These treatments were carried out after a 6 h recovery from the last repeated TS. Embryos in the no repeated TS group responded, as expected, with increases in hsp70 mRNA in response to 12, 15 and 18 ^oC high-level TS. However, exposure to repeated TS of 3 or 6 ⁰C every 6 days also resulted in a significant upregulation of hsp70 mRNA relative to the controls. Importantly, these repeated TS events and the associated elevations in hsp70 attenuated the upregulation of hsp70 in response to a 1 h, high-level TS of 12 ^oC above ambient, but not to either longer (4 h) or higher (15 or 18 ^oC) TS events. Conversely, hsp90α mRNA levels were not consistently elevated in the no repeated TS groups exposed to high-level TS. In some instances, hsp90α levels appeared to decrease in embryos exposed to repeated TS followed by a high-level TS. The observed attenuation of the HSR in lake whitefish embryos demonstrates that embryos of this species have plasticity in their HSR and repeated TS may protect against high-level TS, but the response differs based on repeated TS treatment, high-level TS temperature and duration, and the gene of interest.

Survival, Growth, and Development in the Early Stages of the Tropical Gar Atractosteus tropicus: Developmental Critical Windows and the Influence of Temperature, Salinity, and Oxygen Availability

Alterations in fish developmental trajectories occur in response to genetic and environmental changes, especially during sensitive periods of development (critical windows). Embryos and larvae of Atractosteus tropicus were used as a model to study fish survival, growth, and development as a function of temperature (28 °C control, 33 °C, and 36 °C), salinity (0.0 ppt control, 4.0 ppt, and 6.0 ppt), and air saturation (control ~95% air saturation, hypoxia ~30% air saturation, and hyperoxia ~117% air saturation) during three developmental periods: (1) fertilization to hatch, (2) day 1 to day 6 post hatch (dph), and (3) 7 to 12 dph. Elevated temperature, hypoxia, and hyperoxia decreased survival during incubation, and salinity at 2 and 3 dph. Growth increased in embryos incubated at elevated temperature, at higher salinity, and in hyperoxia but decreased in hypoxia. Changes in development occurred as alterations in the timing of hatching, yolk depletion, acceptance of exogenous feeding, free swimming, and snout shape change, especially at high temperature and hypoxia. Our results suggest identifiable critical windows of development in the early ontogeny of A. tropicus and contribute to the knowledge of fish larval ecology and the interactions of individuals × stressors × time of exposure.

Differential sensitivity to warming and hypoxia during development and long-term effects of developmental exposure in early life stage Chinook salmon

Warming and hypoxia are two stressors commonly found within natural salmon redds that are likely to co-occur. Warming and hypoxia can interact physiologically, but their combined effects during fish development remain poorly studied, particularly stage-specific effects and potential carry-over effects. To test the impacts of warm water temperature and hypoxia as individual and combined developmental stressors, late fall-run Chinook salmon embryos were reared in 10 treatments from fertilization through hatching with two temperatures [10°C (ambient) and 14°C (warm)], two dissolved oxygen saturation levels [normoxia (100% air saturation, 10.4-11.4 mg O₂/l) and hypoxia (50% saturation, 5.5 mg O₂/l)] and three exposure times (early [eyed stage], late [silver-eyed stage] and chronic [fertilization through hatching]). After hatching, all treatments were transferred to control conditions (10°C and 100% air saturation) through the fry stage. To study stage-specific effects of stressor exposure we measured routine metabolic rate (RMR) at two embryonic stages, hatching success and growth. To evaluate carry-over effects, where conditions during one life stage influence performance in a later stage, RMR of all treatments was measured in control conditions at two post-hatch stages and acute stress tolerance was measured at the fry stage. We found evidence of stage-specific effects of both stressors during exposure and carry-over effects on physiological performance. Both individual stressors affected RMR, growth and developmental rate while multiple stressors late in development reduced hatching success. RMR post-hatch showed persistent effects of embryonic stressor exposure that may underlie differences observed in developmental timing and acute stress tolerance. The responses to stressors that varied by stage during development suggest that stage-specific management efforts could support salmon embryo survival. The persistent carry-over effects also indicate that considering sub-lethal effects of developmental stressor exposure may be important to understanding how climate change influences the performance of salmon across life stages.

Effect of increased embryonic temperature during developmental windows on survival, morphology and oxygen consumption of rainbow trout (Oncorhynchus mykiss)

Temperature is a crucial environmental factor that influences physiological functions in fishes, and increased temperature during development can shape an organism's phenotype. An active line of inquiry in comparative developmental physiology is whether short-term exposure to thermal changes have lasting phenotypic effects. This is the first study to apply a developmental 3-dimensional critical window experimental design for a vertebrate, using time, temperature, and phenotypic response (i.e., variables measured). Rainbow trout (Oncorhynchus mykiss) are an anadromous species for which resident populations occupy freshwater environments that are likely impacted by variable and rising temperatures, particularly during embryonic development. To assess thermal effects on fish development, we examined trout hatchling phenotypes following rearing in constant temperatures (5, 10, 15 and 17.5 °C) and following exposure to increased temperature above 5 °C during specific developmental windows. Time to 50% hatch, hatchling mass and body length showed general trends of decreasing with increasing constant temperature, and survival was highest in constant 10 °C incubation. Thermally shifting embryos into 17.5 °C during gastrulation and organogenesis reduced survival at hatch compared to 10 °C, and exposure to 15 and 17.5 °C only late in development produced lighter and shorter hatchlings. Oxygen consumption rate (V̇o₂) at organogenesis differed between embryos incubated constantly in increased temperature or exposed only during organogenesis, but generally we found limited temperature effects on V̇o₂ that may be due to high data variability. Collectively, these results suggest that survival of rainbow trout hatchlings is most sensitive to 17.5 °C exposure during gastrulation and organogenesis, while warm water exposure later in development has greater impacts on morphology. Thus, trait-specific critical windows of thermal sensitivity exist for rainbow trout embryos that alter the hatching phenotype.

Modifying effects of a cobble substrate on thermal environments and implications for embryonic development in lake whitefish (Coregonus clupeaformis)

A laboratory flume was constructed to examine substrate effects on aquatic development. The flume was designed as a once-through system with a submerged cobble-filled corebox. Lake whitefish (Coregonus clupeaformis) embryos and temperature probes were deployed at multiple sites within the cobble and in the open water channel. Embryos were incubated in the flume for two different experimental periods: one to examine substrate impacts during natural lake cooling (37 days: 5 December 2016 to 10 January 2017) and the second to investigate substrate effects while administering a twice weekly 1 h heat shock (51 days: 11 January to 2 March 2017). During incubation, no significant difference was found in the average temperature between locations; however, temperatures were more stable within the cobble. Following both incubation periods, embryos retrieved from the cobble were significantly smaller in both dry mass and body length by up to 20%. These results demonstrate differences between embryos submerged in a cobble substrate and in the open water column, highlighting the need to consider the physical influences from the incubation environment when assessing development effects as part of any scientific study or environmental assessment.

The impacts of seasonal variation on the immune status of Nile tilapia larvae and their response to different immunostimulants feed additives

Few data are available on the thermal tolerance of Nile tilapia fish larvae in relation to their immune status and survival. The aims of this work were to evaluate the immune status of one day old Nile tilapia (Oreochromis niloticus) larval stage collected at the beginning (March), middle (August) and at the end (October) of hatching season through morphometric assessment of the larvae parameters including yolk sac diameter, body length and width as well as the expression of some immune-related genes (rag, sacs and tlr), inflammatory (il1b and il8) and stress related genes (hsp27, hsp70). Also, to compare the effect of three different immunostimulants (β-glucan, Vitamin C, and methionine/lysine amino acids mix) on the expression of the studied genes at two variant temperatures (23 ± 1 °C and 30 ± 1 °C) in experimental study for 21 days. The immune status of Nile tilapia is affected by thermal fluctuation throughout the hatching season reflected by altered yolk sac size, length, and expression of the immune and stress related genes of the larvae, the best performances was observed at the beginning of the hatching season (March). High temperature (30 °C) suppress immune and stress responses throughout downregulation of all the genes under study, mask any effects for the immunostimulants, increased mortality in fish larvae suggesting narrow thermal tolerance range for the larvae compared with the adult fish. We recommend the use of amino acid mix as immunostimulant for Nile tilapia larvae, it reduces the mortality percentage and improve cellular response. Also, the use of β-glucan should be prohibited during this developmental stage of larvae, it induced the highest mortality percentage.

The effects of temperature and dissolved oxygen on the growth, survival and oxidative capacity of newly hatched hybrid yellow catfish larvae (Tachysurus fulvidraco♀ × Pseudobagrus vachellii♂)

Demand for yellow catfish fry, an economically important farmed fish in China, has increased dramatically. Newly hatched larvae are highly sensitive to changes in environmental conditions, with water temperature (T) and dissolved oxygen (DO) being two important factors that affect their early development. We investigate optimal T (between 19.0 and 33.0 °C) and DO (between 2.0 and 12.0 mg L^-1) concentrations on growth and antioxidant enzyme activity of newly hatched hybrid yellow catfish larvae (Tachysurus fulvidraco × Pseudobagrus vachellii) using a central composite design. We use a response surface method to optimize the response variables for survival (S) and growth, and the reduction of oxidative stress, over a 50-day experimental duration. T has a significant effect on specific growth rate (SGR), hepatic malondialdehyde (MDA) content, and superoxide dismutase (SOD) and catalase (CAT) activities (P < 0.05). DO concentration has a significant effect on SGR, S, hepatic MDA content, and SOD and CAT activities (P < 0.05). T and DO also have significant second order effects on SGR, S, SOD, and CAT activities (P < 0.05). Increased DO at low T stimulates SOD and CAT activities and alleviates oxidative damage. Adjusted R² values for SGR, S, CAT, SOD, and MDA models are 0.734, 0.937, 0.916, 0.894 and 0.826, respectively. A combination of 26.8 °C and 7.3 mg L^-1 represents optimal rearing conditions, in that larval growth and antioxidant ability is improved. Results show that T and DO during larviculture of yellow catfish have important implications for aquaculture.

Immediate and Persistent Effects of Temperature on Oxygen Consumption and Thermal Tolerance in Embryos and Larvae of the Baja California Chorus Frog, Pseudacris hypochondriaca

The developmental environment has significant immediate effects on phenotypes, but it may also persistently or permanently shape phenotypes across life history. This study examined how developmental temperature influenced embryonic and larval phenotypes of Baja California chorus frog (Pseudacris hypochondriaca), an abundant amphibian in southern California and northern Baja California. We collected egg clutches from native ponds in northern San Diego County within 24 h of fertilization, and clutches were separated and distributed between constant temperatures of 10, 15, 20, and 25°C for incubation. Oxygen consumption rate (V˙O₂), developmental stage, and embryo and yolk masses were measured throughout development. Time to 50% hatch, survival at 50% hatch, and hatch duration were determined. Development rate was strongly affected by temperature, with warmer temperatures reducing time to hatch and hatch duration. Survival to hatch was high across all temperatures, >90%. Mass-specific V˙O₂ of embryos either remained constant or increased throughout development, and by hatching energy demand was significantly increased at higher temperatures. There were limited temperature effects on growth, with embryo and yolk dry mass similar between temperatures throughout embryonic development. To examine long-term effects of embryonic temperature, we reared hatchlings from each temperature until onset of larval feeding. Once feeding, larvae were acclimated to 20 or 25°C (>2 weeks). Following acclimation to 20 or 25°C, we measured larval mass-specific V˙O₂ and critical thermal maximum (CTMax) at a common developmental stage (Gosner stages 32–36, “hindlimb toe differentiation”). Embryonic temperature had persistent effects on larval V˙O₂ and CTMax, with warmer temperatures generally resulting in similar or higher V˙O₂ and CTMax. This partially supported a “warmer is better” effect of embryonic incubation temperature. These results suggest that in a thermally robust amphibian species, temperature may program the phenotype during early development to construct traits in thermal tolerance and energy use that may persist. Overall, P. hypochondriaca displays a thermally robust phenotype, and it is possible that amphibians that possess a wider range of phenotypic plasticity will be relatively more successful mitigating effects of climate change.

Hypoxia alters the expression of hif-1a mRNA and downstream HIF-1 response genes in embryonic and larval lake whitefish (Coregonus clupeaformis)

Lake whitefish (Coregonus clupeaformis) embryos and larvae were exposed to hypoxia at different developmental ages to determine when the cellular response to hypoxia could be initiated. mRNA levels of hypoxia-inducible factor 1α (hif-1α), hsp70, and several HIF-1 target genes were quantified in embryos at 21, 38, 63, 83- and 103-days post fertilisation (dpf) and in larvae at 1, 2, 3- and 4-weeks post hatch (wph) following a 6-hour hypoxia exposure. hsp70 mRNA levels were increased in response to hypoxia at all embryonic ages. By comparison, the first observed change in hif-1α mRNA in response to hypoxia was at 38 dpf, where it was down-regulated from high basal levels, with this response persisting through to 83 dpf. Interestingly, this decrease in hif-1α mRNA coincided with increases in the mRNA levels of the HIF-1 target genes: vegfa (vascular endothelial growth factor A), igfbp1 (insulin-like growth factor binding protein 1), ldha (lactate dehydrogenase a), gapdh (glyceraldehyde-3-phosphate dehydrogenase) and epo (erythropoietin) at select ages. Collectively, this suggests a possible HIF-1-mediated response to hypoxia despite a decrease in hif-1α mRNA. Coinciding with a decrease in basal levels, increases in hif-1α were measured in response to hypoxia at 103 dpf and in larval fish at 1, 2 and 3 wph but there were no consistent increases in HIF-1 target genes at these ages. Overall, our findings indicate that lake whitefish can mount a response to hypoxia early in embryogenesis which may mitigate some of the damaging effects of exposure to low oxygen levels at these critical life history stages.

Linking life-history theory and metabolic theory explains the offspring size-temperature relationship

Temperature often affects maternal investment in offspring. Across and within species, mothers in colder environments generally produce larger offspring than mothers in warmer environments, but the underlying drivers of this relationship remain unresolved. We formally evaluated the ubiquity of the temperature-offspring size relationship and found strong support for a negative relationship across a wide variety of ectotherms. We then tested an explanation for this relationship that formally links life-history and metabolic theories. We estimated the costs of development across temperatures using a series of laboratory experiments on model organisms, and a meta-analysis across 72 species of ectotherms spanning five phyla. We found that both metabolic and developmental rates increase with temperature, but developmental rate is more temperature sensitive than metabolic rate, such that the overall costs of development decrease with temperature. Hence, within a species' natural temperature range, development at relatively cooler temperatures requires mothers to produce larger, better provisioned offspring.

Impacts of temperature, morpholine, and chronic radiation on the embryonic development of round whitefish (Prosopium cylindraceum)

During incubation, round whitefish embryos may experience fluctuating or elevated temperatures from natural (e.g., seasonal temperature changes) and/or anthropogenic sources. Anthropogenic sources like once-through cooling discharges from nuclear power plants can also expose embryos to chemicals (e.g., morpholine) and/or radiation. To examine the effects of these potential stressors on embryogenesis, round whitefish were incubated under fluctuating or constant temperatures, with morpholine or ¹³⁷ Cs gamma rays. We report the percentage of prehatch and posthatch mortality, developmental rate, hatch dynamics, and morphometrics at 4 development stages. Embryos reared at constant temperatures had delayed developmental stage onset and median hatch, higher mortality at constant 8 °C, and lower mortality at ≤5 °C, compared with embryos reared under seasonal temperature regimes. Embryos incubated with ≥500 mg L^-1 morpholine (>200× regulatory limits) had advanced hatch, reduced body size, and increased prehatch (100% at 1000 mg L^-1 ) and posthatch (≈95% at 500 mg L^-1 ) mortality compared with controls. Relative to controls, embryos irradiated with ≥0.16 mGy/d had larger body mass early in development, and all irradiated embryos had decreased posthatch mortality; the lowest dose was >300× discharge limits. Our study suggests that fluctuating or elevated temperatures and high-dose morpholine can alter development rate, hatch dynamics, and growth, and/or increase mortality compared with embryos reared at constant temperatures of ≤5 °C; conversely, low-dose irradiation had transient developmental effects but may benefit early posthatch survival. Environ Toxicol Chem 2018;37:2593-2608. © 2018 SETAC.

Daily, repeating fluctuations in embryonic incubation temperature alter metabolism and growth of Lake whitefish (Coregonus clupeaformis)

Lake whitefish (Coregonus clupeaformis) utilize overwintering embryonic development (up to 180 days), and such stenothermic, cold-water embryos may be particularly susceptible to thermal shifts. We incubated whitefish embryos in temperature treatments that were constant temperature (2.0 ± 0.1 °C, 5.0 ± 0.1 °C, and 8.0 ± 0.1 °C; mean ± SD) or variable temperature (VT, mean = 5.0 ± 0.3 °C). In the VT, a daily 2 °C temperature change followed a continuous pattern throughout development: 2-4-6-8-6-4-2 °C. Hatchling survival proportion from fertilization to hatch was significantly impacted by incubation temperature (P < 0.001): 2 °C (0.88 ± 0.01) and 5 °C (0.91 ± 0.01) showed higher survival than both the VT (0.83 ± 0.02) and 8 °C groups (0.15 ± 0.06), which were statistically distinct from each other. Time to hatch (dpf) was significantly different across all treatments (P < 0.001): 8 °C (68 ± 2 dpf), VT (111 ± 4 dpf), 5 °C (116 ± 4 dpf), 2 °C (170 ± 3 dpf). Likewise, hatchling yolk-free dry mass (mg) and total body length (mm) were significantly different across all treatments (P < 0.001): 8 °C (0.66 ± 0.08 mg; 11.1 ± 0.08 mm), VT (0.97 ± 0.06 mg; 11.7 ± 0.05 mm), 5 °C (1.07 ± 0.03 mg; 12.0 ± 0.02 mm), 2 °C (1.36 ± 0.04 mg; 12.8 ± 0.05 mm). Oxygen consumption rate (V̇o₂) was significantly affected by the interaction between treatment and measurement temperature (P < 0.001). Hatchling VT whitefish showed mean V̇o₂ that was higher compared to the 2 °C group measured at 2 °C, and lower compared to the 2 °C and 5 °C group measured at 8 °C. This study demonstrates that the VT incubation treatment produced fewer (increased mortality), smaller embryos that hatched earlier than 2 °C and 5 °C embryos. The plasticity of V̇o₂ for this stenothermic-incubating fish species under variable incubation conditions reveals a metabolic cost to cycling thermal incubation conditions.

Impacts of environmental matching on the routine metabolic rate and mass of native and mixed-ancestry brook trout (Salvelinus fontinalis) fry

The environment an organism experiences during early development can impact its physiology and survival later in life. The objective of this study was to determine if temperatures experienced at embryonic life stages of brook trout (Salvelinus fontinalis) affected mass and routine metabolic rate (RMR) of a subsequent life stage (free-swimming fry). As part of this, we assessed the contributions and importance of hierarchical levels of biological organization [ancestral type (native vs. hatchery-introgressed), population, and family] to variability in mass and RMR of fry. As embryos and alevin, individuals were reared at either natural environmental (5°C) or elevated (9°C) temperatures and then acclimated to either matched or mismatched temperature treatments once yolk sacs were resorbed. Mass differences among fry were strongly influenced by population of origin as well as initial rearing and final acclimation temperatures. Variation in mass-adjusted RMR of fry was also strongly accounted for by source population, acclimation temperature, and individual mass. A significant interaction between population RMR and final acclimation temperature indicated that not all brook trout populations responded the same way to temperature changes. In contrast to expectations, the highest ancestry category (native vs. introgressed) did not significantly influence mass or mass-adjusted RMR.

Metabolic rates of embryos and alevin from a cold-adapted salmonid differ with temperature, population and family of origin: implications for coping with climate change

Early developmental stages of cold-adapted ectotherms such as brook trout (Salvelinus fontinalis) are at higher risk of mortality with increasing water temperatures. To determine the amount of variation present in early life, which may allow for potential adaptation to increasing temperature, we examined the routine metabolic rates (RMR) of wild-origin brook trout embryos and alevins reared at normal (5°C) and elevated (9°C) temperatures. The experiment was structured to examine variation in RMR within and among several levels of biological organization (family, population and ancestral type (native vs. mixed ancestry)). As expected, family and temperature variables were most important for predicting RMR and body mass, although population-level differences also existed when family was excluded for more detailed analysis. Additionally, body mass strongly influenced RMR at all life stages except for eyed embryos. When family identity was removed from the analysis, population became the most significant variable. Variation in RMR and mass within and among populations may indicate existing adaptive potential within and among brook trout populations to respond to predicted warming under climate change scenarios.

Critical developmental windows for morphology and hematology revealed by intermittent and continuous hypoxic incubation in embryos of quail (Coturnix coturnix)

Hypoxia during embryonic growth in embryos is frequently a powerful determinant of development, but at least in avian embryos the effects appear to show considerable intra- and inter-specific variation. We hypothesized that some of this variation may arise from different protocols that may or may not result in exposure during the embryo’s critical window for hypoxic effects. To test this hypothesis, quail embryos (Coturnix coturnix) in the intact egg were exposed to hypoxia (~15% O₂) during “early” (Day 0 through Day 5, abbreviated as D0-D5), “middle” (D6-D10) or “late” (D11-D15) incubation or for their entire 16–18 day incubation (“continuous hypoxia”) to determine critical windows for viability and growth. Viability, body mass, beak and toe length, heart mass, and hematology (hematocrit and hemoglobin concentration) were measured on D5, D10, D15 and at hatching typically between D16 and D18 Viability rate was ~50–70% immediately following the exposure period in the early, middle and late hypoxic groups, but viability improved in the early and late groups once normoxia was restored. Middle hypoxia groups showed continuing low viability, suggesting a critical period from D6-D10 for embryo viability. The continuous hypoxia group experienced viability reaching <10% after D15. Hypoxia, especially during late and continuous hypoxia, also inhibited growth of body, beak and toe when measured at D15. Full recovery to normal body mass upon hatching occurred in all other groups except for continuous hypoxia. Contrary to previous avian studies, heart mass, hematocrit and hemoglobin concentration were not altered by any hypoxic incubation pattern. Although hypoxia can inhibit embryo viability and organ growth during most incubation periods, the greatest effects result from continuous or middle incubation hypoxic exposure. Hypoxic inhibition of growth can subsequently be “repaired” by catch-up growth if a final period of normoxic development is available. Collectively, these data indicate a critical developmental window for hypoxia susceptibility during the mid-embryonic period of development.

Is There a Trade-Off between Radiation-Stimulated Growth and Metabolic Efficiency?

Beneficial protective effects may result from an adaptive respose to low dose radiation exposure. However, such benefits must be accompanied by some form of cost because the responsible biological mechanisms are not normally maintained in an upregulated state. It has been suggested that stimulation of adaptive response mechanisms could be metabolically costly, or that the adaptive response could come at a sacrifice to other physiological processes. We exposed developing lake whitefish embryos to a fractionated regime of gamma radiation (662 keV; 0.3 Gy min^-1) to determine whether radiation-stimulated growth was accompanied by a trade-off in metabolic efficiency. Developing embryos were exposed at the eyed stage to different radiation doses delivered in four fractions, ranging from 15 mGy to 8 Gy per fraction, with a 14 day separation between dose fractions. Dry weight and standard length measurements were taken 2-5 weeks after delivery of the final radiation exposure and yolk conversion efficiency was estimated by comparing the unpreserved dry weight of the yolk to the unpreserved yolk-free dry weight of the embryos and normalizing for size-related differences in somatic maintenance. Our results show that the irradiated embryos were 8-10% heavier than the controls but yolk conversion efficiency was slightly improved. This finding demonstrates that stimulated growth in developing lake whitefish embryos is not "paid for" by a trade-off in the efficiency of yolk conversion.

Development of the embryonic heat shock response and the impact of repeated thermal stress in early stage lake whitefish (Coregonus clupeaformis) embryos

Lake whitefish (Coregonus clupeaformis) embryos were exposed to thermal stress (TS) at different developmental stages to determine when the heat shock response (HSR) can be initiated and if it is altered by exposure to repeated TS. First, embryos were subject to one of three different TS temperatures (6, 9, or 12°C above control) at 4 points in development (21, 38, 60 and 70 days post-fertilisation (dpf)) for 2h followed by a 2h recovery to understand the ontogeny of the HSR. A second experiment explored the effects of repeated TS on the HSR in embryos from 15 to 75 dpf. Embryos were subjected to one of two TS regimes; +6°C TS for 1h every 6 days or +9°C TS for 1h every 6 days. Following a 2h recovery, a subset of embryos was sampled. Our results show that embryos could initiate a HSR via upregulation of heat shock protein 70 (hsp70) mRNA at all developmental ages studied, but that this response varied with age and was only observed with a TS of +9 or +12°C. In comparison, when embryos received multiple TS treatments, hsp70 was not induced in response to the 1h TS and 2h recovery, and a downregulation was observed at 39 dpf. Downregulation of hsp47 and hsp90α mRNA was also observed in early age embryos. Collectively, these data suggest that embryos are capable of initiating a HSR at early age and throughout embryogenesis, but that repeated TS can alter the HSR, and may result in either reduced responsiveness or a downregulation of inducible hsps. Our findings warrant further investigation into both the short- and long-term effects of repeated TS on lake whitefish development.

The REPAIR Project: Examining the Biological Impacts of Sub-Background Radiation Exposure within SNOLAB, a Deep Underground Laboratory

Considerable attention has been given to understanding the biological effects of low-dose ionizing radiation exposure at levels slightly above background. However, relatively few studies have been performed to examine the inverse, where natural background radiation is removed. The limited available data suggest that organisms exposed to sub-background radiation environments undergo reduced growth and an impaired capacity to repair genetic damage. Shielding from background radiation is inherently difficult due to high-energy cosmic radiation. SNOLAB, located in Sudbury, Ontario, Canada, is a unique facility for examining the effects of sub-background radiation exposure. Originally constructed for astroparticle physics research, the laboratory is located within an active nickel mine at a depth of over 2,000 m. The rock overburden provides shielding equivalent to 6,000 m of water, thereby almost completely eliminating cosmic radiation. Additional features of the facility help to reduce radiological contamination from the surrounding rock. We are currently establishing a biological research program within SNOLAB: Researching the Effects of the Presence and Absence of Ionizing Radiation (REPAIR project). We hypothesize that natural background radiation is essential for life and maintains genomic stability, and that prolonged exposure to sub-background radiation environments will be detrimental to biological systems. Using a combination of whole organism and cell culture model systems, the effects of exposure to a sub-background environment will be examined on growth and development, as well as markers of genomic damage, DNA repair capacity and oxidative stress. The results of this research will provide further insight into the biological effects of low-dose radiation exposure as well as elucidate some of the processes that may drive evolution and selection in living systems. This Radiation Research focus issue contains reviews and original articles, which relate to the presence or absence of low-dose ionizing radiation exposure.

Developmental effects of the industrial cooling water additives morpholine and sodium hypochlorite on lake whitefish (Coregonus clupeaformis)

Chemicals used in the prevention of corrosion and biofouling may be released into the environment via industrial cooling water discharges. The authors assessed the impacts of 2 commonly used chemicals, morpholine and sodium hypochlorite, on development in lake whitefish (Coregonus clupeaformis). Embryos were exposed chronically, beginning at fertilization or at the eyed stage. Acute 96-h exposures were also examined at 4 development stages. Chronic morpholine resulted in median lethal concentrations (LC50s) of 219 ± 54 mg/L when exposure began at fertilization and 674 ± 12 mg/L when exposure began at the eyed stage, suggesting that embryos are more sensitive earlier in development. Chronic morpholine exposure advanced hatching by up to 30%, and the early hatching embryos were up to 10% smaller in body length. A decrease in yolk conversion efficiency was also observed in embryos exposed to chronic morpholine concentrations of 1000 mg/L. The majority of effects from morpholine exposure manifested near hatch, possibly reflecting changes in chorion permeability at the end of embryonic development. Sodium hypochlorite only impacted survival with chronic exposure from fertilization, where the total residual chlorine LC50 was 0.52 ± 0.11 mg/L. Acute exposures to both chemicals had minimal effects up to the highest tested concentrations. Overall, the results suggest that the risk during development from exposure to morpholine and sodium hypochlorite is low under normal operating conditions. Environ Toxicol Chem 2017;36:1955-1965. © 2016 SETAC.

Oxygen Consumption is Limited at an Ecologically Relevant Rearing Temperature in Pupfish Eggs

The habitat of the critically endangered Devils Hole Pupfish, Cyprinodon diabolis is marked by constant high temperatures and low oxygen availability. In order to explore the effects of these conditions on development and recruitment of eggs in Devils Hole, we tested the effects of two ecologically relevant temperatures on the development, hatch success, and oxygen consumption of eggs from a refuge population of pupfish derived from C. diabolis and eggs from its close sister species, Cyprinodon nevadensis mionectes. We developed a simple method to measure oxygen consumption in a single egg. Parent acclimation temperature, rather than incubation temperature, was the most important factor influencing hatch success. Eggs incubated at 33°C hatched more quickly compared to those incubated at 28°C. Despite this accelerated development, larvae from both temperatures were of similar size at hatch. Unexpectedly, eggs incubated at 33°C experience lower than expected oxygen consumption rates compared to those incubated at 28°C. Oxygen consumption rates would be limited at PO₂ values that are much higher than environmental oxygen tensions. Oxygen consumption increased dramatically upon hatch, indicating that low oxygen conditions such as those present in Devils Hole may limit developing eggs.

Fine-Scale Ecological and Genetic Population Structure of Two Whitefish (Coregoninae) Species in the Vicinity of Industrial Thermal Emissions

Thermal pollution from industrial processes can have negative impacts on the spawning and development of cold-water fish. Point sources of thermal effluent may need to be managed to avoid affecting discrete populations. Correspondingly, we examined fine-scale ecological and genetic population structure of two whitefish species (Coregonus clupeaformis and Prosopium cylindraceum) on Lake Huron, Canada, in the immediate vicinity of thermal effluent from nuclear power generation. Niche metrics using δ¹³C and δ¹⁵N stable isotopes showed high levels of overlap (48.6 to 94.5%) in resource use by adult fish captured in areas affected by thermal effluent compared to nearby reference locations. Isotopic niche size, a metric of resource use diversity, was 1.3- to 2.8-fold higher than reference values in some thermally affected areas, indicative of fish mixing. Microsatellite analyses of genetic population structure (F_st, STRUCTURE and DAPC) indicated that fish captured at all locations in the vicinity of the power plant were part of a larger population extending beyond the study area. In concert, ecological and genetic markers do not support the presence of an evolutionarily significant unit in the vicinity of the power plant. Thus, future research should focus on the potential impacts of thermal emissions on development and recruitment.

Challenges and opportunities in developmental integrative physiology

This review explores challenges and opportunities in developmental physiology outlined by a symposium at the 2014 American Physiological Society Intersociety Meeting: Comparative Approaches to Grand Challenges in Physiology. Across animal taxa, adverse embryonic/fetal environmental conditions can alter morphological and physiological phenotypes in juveniles or adults, and capacities for developmental plasticity are common phenomena. Human neonates with body sizes at the extremes of perinatal growth are at an increased risk of adult disease, particularly hypertension and cardiovascular disease. There are many rewarding areas of current and future research in comparative developmental physiology. We present key mechanisms, models, and experimental designs that can be used across taxa to investigate patterns in, and implications of, the development of animal phenotypes. Intraspecific variation in the timing of developmental events can be increased through developmental plasticity (heterokairy), and could provide the raw material for selection to produce heterochrony--an evolutionary change in the timing of developmental events. Epigenetics and critical windows research recognizes that in ovo or fetal development represent a vulnerable period in the life history of an animal, when the developing organism may be unable to actively mitigate environmental perturbations. 'Critical windows' are periods of susceptibility or vulnerability to environmental or maternal challenges, periods when recovery from challenge is possible, and periods when the phenotype or epigenome has been altered. Developmental plasticity may allow survival in an altered environment, but it also has possible long-term consequences for the animal. "Catch-up growth" in humans after the critical perinatal window has closed elicits adult obesity and exacerbates a programmed hypertensive phenotype (one of many examples of "fetal programing"). Grand challenges for developmental physiology include integrating variation in developmental timing within and across generations, applying multiple stressor dosages and stressor exposure at different developmental timepoints, assessment of epigenetic and parental influences, developing new animal models and techniques, and assessing and implementing these designs and models in human health and development.