Hypoxic tolerance in air-breathing invertebrates

Metabolic rates of different demographics in the sand fiddler crab Leptuca pugilator

Studies on animal energetics often focus on standardized metabolic rates to facilitate comparison across systems. Yet these standardized measurements often do not capture the realistic demographic and environmental variation that is common in natural settings. Rather, individuals included in these studies are often non-reproducing, uninjured, resting adults that have not recently eaten-far from a representative sample. We measured the respiratory rates of the sand fiddler crab Leptuca pugilator in air immediately after capture in the field, and compared rates between males, females of different reproductive states, and juveniles. As expected, we show that metabolic rates were influenced by body mass and activity level. We also show that being vitellogenic or gravid had only minor impacts on metabolic costs of females. Importantly, we demonstrate how considering demographics allows for the detection of phenomena that would otherwise go unnoticed. We found that field metabolic rates of L. pugilator in air are as much as an order of magnitude higher than previous standard metabolic rates measured on post-prandial, quiescent individuals. These higher rates may reflect a combination of high activity and active digestion, as fiddler crabs actively feed during low tide periods. Our results highlight the importance of considering differences in sex, life history stage, and reproductive state of organisms in fluctuating environments, such as intertidal habitats, when assessing energy expenditure.

Unveiling the submerged secrets: bumblebee queens' resilience to flooding

In a previous study, an experimental oversight led to the accumulation of water filling a container housing diapausing bumblebee queens. Surprisingly, after draining the water, queens were found to be alive. This observation raises a compelling question: can bumblebee queens endure periods of inundation while overwintering underground? To address this question, we conducted an experiment using 143 common eastern bumblebee (Bombus impatiens) queens placed in soil-filled tubes and subjected to artificially induced diapause in a refrigerated unit for 7 days. Tap water was then added to the tubes and queens (n = 21 per treatment) were either maintained underwater using a plunger-like apparatus or left to float naturally on the water's surface for varying durations (8 h, 24 h or 7 days) while remaining in overwintering conditions. Seventeen queens served as controls. After the submersion period, queens were removed from water, transferred to new tubes with soil and kept in cold storage for eight weeks. Overall, queen survival remained consistently high (89.5 ± 6.4%) across all treatments and did not differ among submersion regimes and durations. These results demonstrate the remarkable ability of diapausing B. impatiens queens to withstand submersion under water for up to one week, indicating their adaptations to survive periods of flooding in the wild.

Metabolic rates in the squareback marsh crab Armases cinereum

Metabolic rate is a basic individual metric that extends beyond the individual to link the ecology of populations, communities, and ecosystems via its role as a central component of energy budgets. Metabolic rates are often measured indirectly by quantifying respiration under simplified, standard laboratory conditions. This approach limits the application of these measurements to a small range of conditions that commonly do not reflect natural field conditions. We measured metabolic rates of the squareback marsh crab Armases cinereum under field conditions. Previous work highlights that movement of individual A. cinereum, especially females, provides a potential spatial subsidy of energy, as individuals consume foods in salt marshes and then transfer the resulting energy to upland forest ecosystems. We show that metabolic rate increases with size for both males and females and that metabolic rates are influenced by temperature and by whether females are vitellogenic. The metabolic rates that we measured more closely approximate field metabolic rates than standard metabolic rates and demonstrate that individual crabs experience high energy expenditures, reducing the amount of energy that may be transferred as a subsidy between marsh and forest as a result of the daily movements of individual crabs. Our measurements are therefore also a key component for the construction of an energy budget for this species.

Hypoxia tolerance, but not low pH tolerance, is associated with a latitudinal cline across populations of Tigriopus californicus

Intertidal organisms must tolerate daily fluctuations in environmental parameters, and repeated exposure to co-occurring conditions may result in tolerance to multiple stressors correlating. The intertidal copepod Tigriopus californicus experiences diurnal variation in dissolved oxygen levels and pH as the opposing processes of photosynthesis and cellular respiration lead to coordinated highs during the day and lows at night. While environmental parameters with overlapping spatial gradients frequently result in correlated traits, less attention has been given to exploring temporally correlated stressors. We investigated whether hypoxia tolerance correlates with low pH tolerance by separately testing the hypoxia and low pH stress tolerance separately of 6 genetically differentiated populations of T. californicus. We independently checked for similarities in tolerance for each of the two stressors by latitude, sex, size, and time since collection as predictors. We found that although hypoxia tolerance correlated with latitude, low pH tolerance did not, and no predictor was significant for both stressors. We concluded that temporally coordinated exposure to low pH and low oxygen did not result in populations developing equivalent tolerance for both. Although climate change alters several environmental variables simultaneously, organisms' abilities to tolerate these changes may not be similarly coupled.

Metabolic rates of the Asian shore crab Hemigrapsus sanguineus in air as a function of body size, location, and injury

Rapid warming in the Gulf of Maine may influence the success or invasiveness of the Asian shore crab, Hemigrapsus sanguineus. To better predict the effects of climate change on this invasive species, it is necessary to measure its energy dynamics under a range of conditions. However, previous research has only focused on the metabolism of this intertidal species in water. We sampled adult crabs from three different sites and measured their metabolic rates in the air. We show that metabolic rate increases with body mass and the number of missing limbs, but decreases with the number of regenerating limbs, possibly reflecting the timing of energy allocation to limb regeneration. Importantly, metabolic rates measured here in the air are ~4× higher than metabolic rates previously measured for this species in water. Our results provide baseline measurements of aerial metabolic rates across body sizes, which may be affected by climate change. With a better understanding of respiration in H. sanguineus, we can make more informed predictions about the combined effects of climate change and invasive species on the northeast coasts of North America.

Size constrains oxygen delivery capacity within but not between bumble bee castes

Bumble bees are eusocial, with distinct worker and queen castes that vary strikingly in size and life-history. The smaller workers rely on energetically-demanding foraging flights to collect resources for rearing brood. Queens can be 3 to 4 times larger than workers, flying only for short periods in fall and again in spring after overwintering underground. These differences between castes in size and life history may be reflected in hypoxia tolerance. When oxygen demand exceeds supply, oxygen delivery to the tissues can be compromised. Previous work revealed hypermetric scaling of tracheal system volume of worker bumble bees (Bombus impatiens); larger workers had much larger tracheal volumes, likely to facilitate oxygen delivery over longer distances. Despite their much larger size, queens had relatively small tracheal volumes, potentially limiting their ability to deliver oxygen and reducing their ability to respond to hypoxia. However, these morphological measurements only indirectly point to differences in respiratory capacity. To directly assess size- and caste-related differences in tolerance to low oxygen, we measured critical PO₂ (P_crit; the ambient oxygen level below which metabolism cannot be maintained) during both rest and flight of worker and queen bumble bees. Queens and workers had similar P_crit values during both rest and flight. However, during flight in oxygen levels near the P_crit, mass-specific metabolic rates declined precipitously with mass both across and within castes, suggesting strong size limitations on oxygen delivery, but only during extreme conditions, when demand is high and supply is low. Together, these data suggest that the comparatively small tracheal systems of queen bumble bees do not limit their ability to deliver oxygen except in extreme conditions; they pay little cost for filling body space with eggs rather than tracheal structures.

Metabolomics of anoxia tolerance in Drosophila melanogaster: evidence against substrate limitation and for roles of protective metabolites and paralytic hypometabolism

Animals vary tremendously in their capacities to survive anoxia, and the mechanisms responsible are poorly understood. Adult Drosophila melanogaster are rapidly paralyzed and survive up to 12 h of anoxia, whereas larvae vigorously attempt escape but then die if anoxia exceeds 2 h. Here we use nuclear magnetic resonance methods to compare the metabolome of larvae and adult D. melanogaster under normoxic conditions and after various anoxic durations up to 1 h. Glucose increased during anoxia in both larvae and adults, so anoxic death by carbohydrate limitation is unlikely for either stage. Lactate and alanine were the primary anaerobic end products in both adults and larvae. During the first 30 min of anoxia, larvae accumulated anaerobic end products (predominately lactate) at a higher rate, suggesting that larvae may experience greater initial acid-base disruption during anoxic exposures. Adult Drosophila did not possess higher levels of putative protective metabolites; however, these increased during anoxia in adults and decreased in larvae. Metabolites that decreased during anoxia in larvae included mannitol, xylitol, glycerol, betaine, serine, and tyrosine, perhaps due to use as fuels, antioxidants, or binding to denatured proteins. Adults showed significant increases in glycine, taurine, and the polyols glycerol, mannitol, and xylitol, suggesting that adults upregulate protective metabolites to prevent damage. Our results suggest that lower initial metabolic demand due to paralytic hypometabolism and capacities to upregulate protective metabolites may assist the better anoxia tolerance of adult Drosophila.

Vanishing benefits - The loss of actinobacterial symbionts at elevated temperatures

Only a few insect species are known to engage in symbiotic associations with antibiotic-producing Actinobacteria and profit from this kind of protection against pathogens. However, it still remains elusive how widespread the symbiotic interactions with Actinobacteria in other organisms are and how these partnerships benefit the hosts in terms of the growth and survival. We characterized a drastic temperature-induced change in the occurrence of Actinobacteria in the gut of the terrestrial isopod Porcellio scaber reared under two different temperature (15 °C and 22 °C) and oxygen conditions (10% and 22% O₂) using 16S rRNA gene sequencing. We show that the relative abundance of actinobacterial gut symbionts correlates with increased host growth at lower temperature. Actinobacterial symbionts were almost completely absent at 22 °C under both high and low oxygen conditions. In addition, we identified members of nearly half of the known actinobacterial families in the isopod microbiome, and most of these include members that are known to produce antibiotics. Our study suggests that hosting diverse actinobacterial symbionts may provide conditions favorable for host growth. These findings show how a temperature-driven decline in microbiome diversity may cause a loss of beneficial functions with negative effects on ectotherms.

Comparative transcriptome analyses of adzuki bean weevil (Callosobruchus chinensis) response to hypoxia and hypoxia/hypercapnia

Stored product insects show high adaption to hypoxia and hypercapnia, but the underlying mechanism is still unclear. Herein, a comparative transcriptome on 4th adzuki bean weevil (Callosobruchus chinensis) instar larvae was studied to clarify the response mechanisms to hypoxia (HA) and hypoxia/hypercapnia (HHA) using NextSeq500 RNA-Seq. Transcript profiling showed a significant difference in HA or HHA exposure both quantitatively and qualitatively. Compared with control, 631 and 253 genes were significantly changed in HHA and HA, respectively. Comparing HHA with HA, 1135 differentially expressed genes (DEGs) were identified. The addition of hypercapnia made a complex alteration on the hypoxia response of bean weevil transcriptome, carbohydrate, energy, lipid and amino acid metabolism were the most highly enriched pathways for genes significantly changed. In addition, some biological processes that were not significantly enriched but important were also discussed, such as immune system and signal transduction. Most of the DEGs related to metabolism both in HHA and HA were up-regulated, while the DEGs related to the immune system, stress response or signal transduction were significantly down-regulated or suppressed. This research reveals a comparatively full-scale result in adzuki bean weevil hypoxia and hypoxia/hypercapnia tolerance mechanism at transcription level, which might provide new insights into the genomic research of this species.

Hyperthermal-driven mass extinctions: killing models during the Permian-Triassic mass extinction

Many mass extinctions of life in the sea and on land have been attributed to geologically rapid heating, and in the case of the Permian-Triassic and others, driven by large igneous province volcanism. The Siberian Traps eruptions raised ambient temperatures to 35-40°C. A key question is how massive eruptions during these events, and others, could have killed life in the sea and on land; proposed killers are reviewed here. In the oceans, benthos and plankton were killed by anoxia-euxinia and lethal heating, respectively, and the habitable depth zone was massively reduced. On land, the combination of extreme heating and drought reduced the habitable land area, and acid rain stripped forests and soils. Physiological experiments show that some animals can adapt to temperature rises of a few degrees, and that some can survive short episodes of increases of 10°C. However, most plants and animals suffer major physiological damage at temperatures of 35-40°C. Studies of the effects of extreme physical conditions on modern organisms, as well as assumptions about rates of environmental change, give direct evidence of likely killing effects deriving from hyperthermals of the past.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

Food consumption in ground beetles is limited under hypoxic conditions in response to ad libitum feeding, but not restricted feeding

Habitats on land with low oxygen availability provide unique niches inhabited by numerous species. The occupation of such hypoxic niches by animals is hypothesized to come at a cost linked to the limitations of aerobic metabolism and thus energy budget but may also provide benefits through physical protection from predators and parasitoids or reduced competition for food. We investigated the effects of hypoxic conditions on standard metabolic rate (SMR) and specific dynamic action (SDA) in male Carabus nemoralis. SMR and SDA were determined under three manipulated oxygen availabilities: 7, 14 and 21% O₂ and two feeding regimes: limited or ad libitum food consumption. In both hypoxic conditions, C. nemoralis was able to maintain SMR at levels similar to those in normoxia. When the meal size was limited, SDA duration did not differ among the oxygen availability conditions, but SDA was smaller under hypoxic conditions than at normoxic levels. The relative cost of digestion was significantly higher in normoxia than in hypoxia, but it did not affect net energy intake. In contrast, when offered a large meal to simulate ad libitum food conditions, beetles reduced their food consumption and net energy gain by 30% under hypoxia. Oxygen availability may influence the consumed prey size: the hypoxic condition did not limit net energy gain when the beetles fed on a small meal but did when they fed on a large meal. The results indicate that meal size is an important variable in determining differences in physiological costs and whole animal energy budgets at different concentrations of environmental oxygen levels.

Packing and Postirradiation Handling of the Anastrepha ludens (Diptera: Tephritidae) Tapachula-7 Genetic Sexing Strain: Combined Effects of Hypoxia, Pupal Size, and Temperature on Adult Quality

The production of genetic sexing strains (GSS) of tephritid flies for sterile insect technique (SIT) programs convey the need to determine new conditions for packing and shipment since these flies are more susceptible to stressors than standard bisexual strains. We studied the effect of hypoxia, pupae size, and temperature on the new GSS Tapachula-7 of Anastrepha ludens flies (Diptera: Tephritidae). In one experiment, we tested the interaction size hypoxia using three pupae sizes, 6 (11.6 ± 1.1 mg), 7 (15.3 ± 1.5 mg), and 8 (17.9 ± 1.3 mg) (95% of produced pupae exhibit these categories of size), and four hypoxia periods, 12, 24, 36, 48 h and a control. In a second experiment, we tested two periods of hypoxia (24 and 48 h) and four temperatures: 15, 20, 25, and 30°C and a control (without hypoxia at laboratory temperature). Our results showed that the emergence and percent of fliers from the pupae exposed to hypoxia were adversely affected; however, emergence was higher in pupae of size 7. Treatment for 12 and 24 h hypoxia led to a higher number of fliers. In the case of the interaction of hypoxia and temperature, it was observed that those flies that emerged from the pupae exposed to hypoxia at 15 and 20°C exhibited quality control parameters similar to those that were not exposed to hypoxia. We discuss our results on the basis of the metabolic response to these factors and its application in the SIT programs.

Developmental plasticity and stability in the tracheal networks supplying Drosophila flight muscle in response to rearing oxygen level

While it is clear that the insect tracheal system can respond in a compensatory manner to both hypoxia and hyperoxia, there is substantial variation in how different parts of the system respond. However, the response of tracheal structures, from the tracheoles to the largest tracheal trunks, have not been studied within one species. In this study, we examined the effect of larval/pupal rearing in hypoxia, normoxia, and hyperoxia (10, 21 or 40kPa oxygen) on body size and the tracheal supply to the flight muscles of Drosophila melanogaster, using synchrotron radiation micro-computed tomography (SR-µCT) to assess flight muscle volumes and the major tracheal trunks, and confocal microscopy to assess the tracheoles. Hypoxic rearing decreased thorax length whereas hyperoxic-rearing decreased flight muscle volumes, suggestive of negative effects of both extremes. Tomography at the broad organismal scale revealed no evidence for enlargement of the major tracheae in response to lower rearing oxygen levels, although tracheal size scaled with muscle volume. However, using confocal imaging, we found a strong inverse relationship between tracheole density within the flight muscles and rearing oxygen level, and shorter tracheolar branch lengths in hypoxic-reared animals. Although prior studies of larger tracheae in other insects indicate that axial diffusing capacity should be constant with sequential generations of branching, this pattern was not found in the fine tracheolar networks, perhaps due to the increasing importance of radial diffusion in this regime. Overall, D. melanogaster responded to rearing oxygen level with compensatory morphological changes in the small tracheae and tracheoles, but retained stability in most of the other structural components of the tracheal supply to the flight muscles.

Biohybrid actuators for robotics: A review of devices actuated by living cells

Actuation is essential for artificial machines to interact with their surrounding environment and to accomplish the functions for which they are designed. Over the past few decades, there has been considerable progress in developing new actuation technologies. However, controlled motion still represents a considerable bottleneck for many applications and hampers the development of advanced robots, especially at small length scales. Nature has solved this problem using molecular motors that, through living cells, are assembled into multiscale ensembles with integrated control systems. These systems can scale force production from piconewtons up to kilonewtons. By leveraging the performance of living cells and tissues and directly interfacing them with artificial components, it should be possible to exploit the intricacy and metabolic efficiency of biological actuation within artificial machines. We provide a survey of important advances in this biohybrid actuation paradigm.

Erythropoietin-Mediated Neuroprotection in Insects Suggests a Prevertebrate Evolution of Erythropoietin-Like Signaling

The cytokine erythropoietin (Epo) mediates protective and regenerative functions in mammalian nervous systems via activation of poorly characterized receptors that differ from the "classical" homodimeric Epo receptor expressed on erythroid progenitor cells. Epo genes have been identified in vertebrate species ranging from human to fish, suggesting that Epo signaling evolved earlier than the vertebrate lineage. Studies on insects (Locusta migratoria, Chorthippus biguttulus, Tribolium castaneum) revealed Epo-mediated neuroprotection and neuroregeneration. Recombinant human Epo (rhEpo) prevents apoptosis by binding to a janus kinase-associated receptor, stimulation of STAT transcription factors, and generation of factors that prevent the activation of proapoptotic caspases. Insect neurons were also protected by a neuroprotective but nonerythropoietic Epo splice variant, suggesting similarity with mammalian neuroprotective but not with homodimeric "classical" Epo receptors. Additionally, rhEpo promotes the regeneration of neurites in primary cultured insect brain neurons and after nerve crush in an in vivo preparation. In contrast to neuroprotective and regenerative effects shared with mammalian species, no evidence for a role of Epo signaling in the regulation of neuro- or gliogenesis was found in insects. Similar structural and functional characteristics of the Epo binding receptors, partly shared transduction pathways that prevent apoptosis and the functional implication in neuroprotective and neuroregenerative processes in both mammalian and insect species, suggest that Epo-like signaling was already established in their last common ancestor. Originally functioning as a tissue-protective response to unfavorable physiological situations, cell injury, and pathogen invasion, Epo was later adapted as a humoral regulator of erythropoiesis in the vertebrate lineage.

Octopamine stabilizes conduction reliability of an unmyelinated axon during hypoxic stress

Mechanisms that could mitigate the effects of hypoxia on neuronal signaling are incompletely understood. We show that axonal performance of a locust visual interneuron varied depending on oxygen availability. To induce hypoxia, tracheae supplying the thoracic nervous system were surgically lesioned and action potentials in the axon of the descending contralateral movement detector (DCMD) neuron passing through this region were monitored extracellularly. The conduction velocity and fidelity of action potentials decreased throughout a 45-min experiment in hypoxic preparations, whereas conduction reliability remained constant when the tracheae were left intact. The reduction in conduction velocity was exacerbated for action potentials firing at high instantaneous frequencies. Bath application of octopamine mitigated the loss of conduction velocity and fidelity. Action potential conduction was more vulnerable in portions of the axon passing through the mesothoracic ganglion than in the connectives between ganglia, indicating that hypoxic modulation of the extracellular environment of the neuropil has an important role to play. In intact locusts, octopamine and its antagonist, epinastine, had effects on the entry to, and recovery from, anoxic coma consistent with octopamine increasing overall neural performance during hypoxia. These effects could have functional relevance for the animal during periods of environmental or activity-induced hypoxia.

Temperature and the Ventilatory Response to Hypoxia in Gromphadorhina portentosa (Blattodea: Blaberidae)

In general, insects respond to hypoxia by increasing ventilation frequency, as seen in most other animals. Higher body temperatures usually also increase ventilation rates, likely due to increases in metabolic rates. In ectothermic air-breathing vertebrates, body temperatures and hypoxia tend to interact significantly, with an increasing responsiveness of ventilation to hypoxia at higher temperatures. Here, we tested whether the same is true in insects, using the Madagascar hissing cockroach, Gromphadorhina portentosa (Schaum) (Blattodea: Blaberidae). We equilibrated individuals to a temperature (beginning at 20 °C), and animals were exposed to step-wise decreases in PO2 (21, 15, 10, and 5 kPa, in that order), and we measured ventilation frequencies from videotapes of abdominal pumping after 15 min of exposure to the test oxygen level. We then raised the temperature by 5 °C, and the protocol was repeated, with tests run at 20, 25, 30, and 35 °C. The 20 °C animals had high initial ventilation rates, possibly due to handling stress, so these animals were excluded from subsequent analyses. Across all temperatures, ventilation increased in hypoxia, but only significantly at 5 kPa PO2 Surprisingly, there was no significant interaction between temperature and oxygen, and no significant effect of temperature on ventilation frequency from 25 to 35 °C. Plausibly, the rise in metabolic rates at higher temperatures in insects is made possible by increasing other aspects of gas exchange, such as decreasing internal PO2, or increases in tidal volume, spiracular opening (duration or amount), or removal of fluid from the tracheoles.

Hypoxia and hypercarbia in endophagous insects: Larval position in the plant gas exchange network is key

Gas composition is an important component of any micro-environment. Insects, as the vast majority of living organisms, depend on O2 and CO2 concentrations in the air they breathe. Low O2 (hypoxia), and high CO2 (hypercarbia) levels can have a dramatic effect. For phytophagous insects that live within plant tissues (endophagous lifestyle), gas is exchanged between ambient air and the atmosphere within the insect habitat. The insect larva contributes to the modification of this environment by expiring CO2. Yet, knowledge on the gas exchange network in endophagous insects remains sparse. Our study identified mechanisms that modulate gas composition in the habitat of endophagous insects. Our aim was to show that the mere position of the insect larva within plant tissues could be used as a proxy for estimating risk of occurrence of hypoxia and hypercarbia, despite the widely diverse life history traits of these organisms. We developed a conceptual framework for a gas diffusion network determining gas composition in endophagous insect habitats. We applied this framework to mines, galls and insect tunnels (borers) by integrating the numerous obstacles along O2 and CO2 pathways. The nature and the direction of gas transfers depended on the physical structure of the insect habitat, the photosynthesis activity as well as stomatal behavior in plant tissues. We identified the insect larva position within the gas diffusion network as a predictor of risk exposure to hypoxia and hypercarbia. We ranked endophagous insect habitats in terms of risk of exposure to hypoxia and/or hypercarbia, from the more to the less risky as cambium mines>borer tunnels≫galls>bark mines>mines in aquatic plants>upper and lower surface mines. Furthermore, we showed that the photosynthetically active tissues likely assimilate larval CO2 produced. In addition, temperature of the microhabitat and atmospheric CO2 alter gas composition in the insect habitat. We predict that (i) hypoxia indirectly favors the evolution of cold-tolerant gallers, which do not perform well at high temperatures, and (ii) normoxia (ambient O2 level) in mines allows miners to develop at high temperatures. Little is known, however, about physiological and morphological adaptations to hypoxia and hypercarbia in endophagous insects. Endophagy strongly constrains the diffusion processes with cascading consequences on the evolutionary ecology of endophagous insects.

Extended hypoxia in the alfalfa leafcutting bee, Megachile rotundata, increases survival but causes sub-lethal effects

Many insects are tolerant of hypoxic conditions, but survival may come at a cost to long-term health. The alfalfa leaf-cutting bee, Megachile rotundata, develops in brood cells inside natural cavities, and may be exposed to hypoxic conditions for extended periods of time. Whether M. rotundata is tolerant of hypoxia, and whether exposure results in sub-lethal effects, has never been investigated. Overwintering M. rotundata prepupae were exposed to 10%, 13%, 17%, 21% and 24% O2 for 11 months. Once adults emerged, five indicators of quality - emergence weight, body size, feeding activity, flight performance, and adult longevity, - were measured to determine whether adult bees that survived past exposure to hypoxia were competent pollinators. M. rotundata prepupae are tolerant of hypoxic condition and have higher survival rates in hypoxia, than in normoxia. Under hypoxia, adult emergence rates did not decrease over the 11 months of the experiment. In contrast, bees reared in normoxia had decreased emergence rates by 8 months, and were dead by 11 months. M. rotundata prepupae exposed to extended hypoxic conditions had similar emergence weight, head width, and cross-thorax distance compared to bees reared in standard 21% oxygen. Despite no significant morphological differences, hypoxia-exposed bees had lower feeding rates and shorter adult lifespans. Hypoxia may play a role in post-diapause physiology of M. rotundata, with prepupae showing better survival under hypoxic conditions. Extended exposure to hypoxia, while not fatal, causes sub-lethal effects in feeding rates and longevity in the adults, indicating that hypoxia tolerance comes at a cost.

Sublethal Cd-induced cellular damage and metabolic changes in the freshwater crab Sinopotamon henanense

To explore whether sublethal cadmium (Cd) exposure causes branchial cellular damages and affects the metabolic activity in brachyuran crustaceans, the freshwater crab Sinopotamon henanense was exposed to 0.71, 1.43, and 2.86 mg/L Cd(2+) for 3 weeks. Gill morphology, metabolic activity (activities of isocitrate dehydrogenase (IDH), cytochrome c oxidase (CCO) and lactate dehydrogenase (LDH), mRNA expression of CCO active subunit 1 (cco-1) and ldh, as well as ATP levels) in crab muscle were investigated. The results showed that sublethal Cd exposure caused profound morphological damages in the gills. The branchial epithelial cells were disorganized and vacuolized. Ultrastructurally, a decrease in number of apical microvilli, vacuolized mitochondria, and condensed chromatin were observed in gill epithelial cells. Correspondingly, the Cd exposure also induced downregulations of cco-1 and ldh mRNA expression and reduced activities of IDH, CCO, and LDH, in accordance with the lower ATP level in crab muscle. These results led to the conclusion that gill damage caused by sublethal Cd exposure could lead to an impairment of oxygen uptake of S. henanense, and the inhibition of metabolic activity decreases the oxygen demand of the crab and assists them to survive under the condition of lower oxygen availability. These effects add to our understanding on toxic effects of Cd and survival management of S. henanense subchronically exposed to sublethal Cd.

Climate change and ocean acidification-interactions with aquatic toxicology

The possibilities for interactions between toxicants and ocean acidification are reviewed from two angles. First, it is considered how toxicant responses may affect ocean acidification by influencing the carbon dioxide balance. Second, it is introduced, how the possible changes in environmental conditions (temperature, pH and oxygenation), expected to be associated with climate change and ocean acidification, may interact with the toxicant responses of organisms, especially fish. One significant weakness in available data is that toxicological research has seldom been connected with ecological and physiological/biochemical research evaluating the responses of organisms to temperature, pH or oxygenation changes occurring in the natural environment. As a result, although there are significant potential interactions between toxicants and natural environmental responses pertaining to climate change and ocean acidification, it is very poorly known if such interactions actually occur, and can be behind the observed disturbances in the function and distribution of organisms in our seas.

Molecular and functional characterization of hemocyanin of the giant African millipede, Archispirostreptus gigas

In contrast to other terrestrial arthropods where gaseous O2 that fuels aerobic metabolism diffuses to the tissues in tracheal tubes, and most other metazoans where O2 is transported to tissues by circulating respiratory proteins, the myriapods (millipedes and centipedes) strikingly have tracheal systems as well as circulating hemocyanin (Hc). In order to elucidate the evolutionary origin and biological significance of millipede Hc we report the molecular structure (subunit composition and amino acid sequence) of multimeric (36-mer) Hc from the forest-floor dwelling giant African millipede Archispirostreptus gigas and its allosteric oxygen binding properties under various physico-chemical conditions. A. gigas Hc consists of only a single subunit type with differential glycosylation. Phylogenic analysis reveals that millipede Hc is a sister group to centipede HcA, which supports an early divergence of distinct Hc subunits in myriapods and an ancient origin of multimeric Hcs. A. gigas Hc binds O2 with a high affinity and shows a strong normal Bohr effect. O2 binding is moreover modulated by Ca2+ ions, which increase the O2 affinity of the Hc in the T (tense; deoxygenated) as well as the R (relaxed; oxygenated) states, and by (L)-lactate, which modulates Hc-O2 affinity by changing the allosteric equilibrium constant, L. Cooperativity in O2-binding at half O2-saturation (n50) is pH-dependent and maximal at pH ~7.4 and the number of interacting O2 binding sites (q) is markedly increased by binding Ca2+. The data is discussed in the light of the role of mutually supplementary roles of Hc and the tracheal system for tissue O2 supply.

Body size is not critical for critical PO2 in scarabaeid and tenebrionid beetles

Constraints on oxygen delivery potentially limit animal body size. Because diffusion rates are highly distance dependent, and because tracheal length increases with size, gas exchange was traditionally thought to be more difficult for larger insects. As yet the effect of body size on critical oxygen partial pressure (Pcrit) has not been measured for any clade of insect species for which there are interspecific data on tracheal scaling. We addressed this deficiency by measuring Pcrit over a 4150-fold mass range (ratio of largest to smallest species mean) of two families of Coleoptera (Tenebrionidae and Scarabaeidae). We exposed adult beetles to progressively lower oxygen levels and measured their ability to maintain CO2 release rates. Absolute metabolic rates increased hypometrically with beetle body mass (M) at both normoxic (M0.748) and hypoxic (M0.846) conditions. Pcrit, however, was independent of body size. Maximum overall conductances for oxygen from air to mitochondria (GO2,max) matched metabolic rates as insects became larger, likely enabling the similar Pcrit values observed in large and small beetles. These data suggest that current atmospheric oxygen levels do not limit body size of insects because of limitations on gas exchange. However, increasing relative investment in the tracheal system in larger insects may produce trade-offs or meet spatial limits that constrain insect size.

Inhibition of oxygen sensors as a therapeutic strategy for ischaemic and inflammatory disease

Cells in the human body need oxygen to function and survive, and severe deprivation of oxygen, as occurs in ischaemic heart disease and stroke, is a major cause of mortality. Nevertheless, other organisms, such as the fossorial mole rat or diving seals, have acquired the ability to survive in conditions of limited oxygen supply. Hypoxia tolerance also allows the heart to survive chronic oxygen shortage, and ischaemic preconditioning protects tissues against lethal hypoxia. The recent discovery of a new family of oxygen sensors--including prolyl hydroxylase domain-containing proteins 1-3 (PHD1-3)--has yielded exciting novel insights into how cells sense oxygen and keep oxygen supply and consumption in balance. Advances in understanding of the role of these oxygen sensors in hypoxia tolerance, ischaemic preconditioning and inflammation are creating new opportunities for pharmacological interventions for ischaemic and inflammatory diseases.

Investigating onychophoran gas exchange and water balance as a means to inform current controversies in arthropod physiology

Several controversies currently dominate the fields of arthropod metabolic rate, gas exchange and water balance, including the extent to which modulation of gas exchange reduces water loss, the origins of discontinuous gas exchange,the relationship between metabolic rate and life-history strategies, and the causes of Palaeozoic gigantism. In all of these areas, repeated calls have been made for the investigation of groups that might most inform the debates,especially of taxa in key phylogenetic positions. Here we respond to this call by investigating metabolic rate, respiratory water loss and critical oxygen partial pressure (Pc) in the onychophoran Peripatopsis capensis, a member of a group basal to the arthropods, and by synthesizing the available data on the Onychophora. The rate of carbon dioxide release (V̇CO2) at 20°C in P. capensis is 0.043 ml CO2 h–1, in keeping with other onychophoran species; suggesting that low metabolic rates in some arthropod groups are derived. Continuous gas exchange suggests that more complex gas exchange patterns are also derived. Total water loss in P. capensis is 57 mg H2O h–1 at 20°C,similar to modern estimates for another onychophoran species. High relative respiratory water loss rates (∼34%; estimated using a regression technique) suggest that the basal condition in arthropods may be a high respiratory water loss rate. Relatively high Pc values(5–10% O2) suggest that substantial safety margins in insects are also a derived condition. Curling behaviour in P. capensisappears to be a strategy to lower energetic costs when resting, and the concomitant depression of water loss is a proximate consequence of this behaviour.

Mitochondrially encoded cysteine predicts animal lifespan

The role of genetic factors in the determination of lifespan is undisputed. However, numerous successful efforts to identify individual genetic modulators of longevity have not yielded yet a quantitative measure to estimate the lifespan of a species from scratch, merely based on its genomic constitution. Here, we report on a meta-examination of genome sequences from 248 animal species with known maximum lifespan, including mammals, birds, fish, insects, and helminths. Our analysis reveals that the frequency with which cysteine is encoded by mitochondrial DNA is a specific and phylogenetically ubiquitous molecular indicator of aerobic longevity: long-lived species synthesize respiratory chain complexes which are depleted of cysteine. Cysteine depletion was also found on a proteome-wide scale in aerobic versus anaerobic bacteria, archaea, and unicellular eukaryotes; in mitochondrial versus hydrogenosomal sequences; and in the mitochondria of free-living, aerobic versus anaerobic-parasitic worms. The association of longevity with mitochondrial cysteine depletion persisted after correction for body mass and phylogenetic interdependence, but it was uncoupled in helminthic species with predominantly anaerobic lifestyle. We conclude that protein-coding genes on mitochondrial DNA constitute a quantitative trait locus for aerobic longevity, wherein the oxidation of mitochondrially translated cysteine mediates the coupling of trait and locus. These results provide distinct support for the free radical theory of aging.

Effects of temperature and oxygen availability on water loss and carbon dioxide release in two sympatric saproxylic invertebrates

Water loss and VCO(2) relative to temperature and oxygen tension was investigated in a log-dwelling onychophoran (Euperipatoides rowelli) and a sympatric, un-described millipede species using flow-through respirometry. Onychophorans possess a tracheal system featuring permanently open spiracles. Total body water loss was consistently very high in E. rowelli and there was a positive correlation with increasing temperature. CO(2) output was continuous, increasing with higher temperatures and decreasing under lower oxygen concentrations. The millipede which has occludible spiracles also showed continuous gas exchange; however water loss was up to an order of magnitude lower than E. rowelli. An ability to survive under hypoxia is apparent for both species and corresponds with reports of hypoxic conditions within rotting logs. The rotting log habitat common to both taxa is characterized by high relative humidity and typically cool temperatures that approach 0 degrees C at night in winter. Consequently, dispersal through the higher temperatures and lower humidity of the exposed and dry understorey between suitable habitat may be hazardous for E. rowelli due to high desiccation susceptibility.

The mangrove ant, Camponotus anderseni, switches to anaerobic respiration in response to elevated CO2 levels

The small tree-living mangrove ant Camponotus anderseni is remarkably adapted for surviving tidal inundation. By blocking the nest entrance with a soldier's head, water intrusion into the nest cavity can be effectively prevented, but lack of gas-exchange caused extremely high concentrations of CO(2)(>30%) and very low O(2) concentrations (<1%). The O(2) uptake in experiments with CO(2) absorption showed a linear decrease until about 4%, whereas the O(2) uptake in chambers without absorbent showed a decrease with a different pattern, consisting of three parts. The first component of this decrease is a linear decrease to about 18%, which is the normal O(2) concentration in open natural nests. The second phase is an exponential decrease continuing to about 4% O(2), showing that the CO(2) concentrations have influence on the O(2) uptake. The final component is also exponential, but with a much smaller slope. The respiratory quotient (RQ) was 0.92 until CO(2) concentration increased to about 15-17%, and after that it showed a strong increase, which is due to the initiation of anaerobic respiration. Anaerobic respiration has not been demonstrated for social insects before, but it is not surprising that it is found in this ant species, which lives in the extreme conditions of a hollow twig in an inundated mangrove.

Limitation of size by hypoxia in the fruit fly Drosophila melanogaster

The size of an organism is of fundamental importance in all biological processes. It dictates many of the critical interactions and physical factors that delimit the envelope within which an organism can grow. We investigated the effects of reduced oxygen on size and development in the fruit fly Drosophila melanogaster, and showed that limiting the oxygen in the environment limits both whole animal and cell size. When oxygen levels were reduced from 20% in nitrogen to 15%, 10% and 7.5%, there was a linear decrease in both male and female mass. Both cell size and cell number decreased in low oxygen, but changes in cell size accounted for a larger proportion of the overall change in fly size. Cell numbers decreased by a maximum of 11% between flies reared in 20% oxygen and those reared in 7.5% oxygen, whereas cell surface area decreased by 17%. Low oxygen levels increased development time and mortality, but reduced fecundity. Reducing the level of oxygen available significantly slowed development times, with flies reared in 10% oxygen emerging on average 1.5 days later than those in 20% oxygen. The effect of oxygen on size is reversible during embryonic and larval development up to the pupal stage, when final size is set.

Sensing and responding to hypoxia via HIF in model invertebrates

This past decade has brought considerable progress towards elucidating the molecular mechanisms of oxygen sensing pathways by which mammalian cells are able to detect and adjust, or succumb, to hypoxia. In contrast, far less is known about the protein and DNA constituents that endow many invertebrate species to withstand and recover from even more severe and prolonged O2 limitations. In spite of these differences in hypoxia tolerance, inadequacy in oxygen supply is, from mammals to insects to nematodes, signaled onto the DNA level predominantly by hypoxia-inducible factors (HIFs). Across the animal kingdom, HIF accumulates in hypoxic, but not normoxic, cells and functions in a remarkably conserved pathway. Using crustacean (Daphnia magna) and insect (Drosophila melanogaster) models, work by us and others has implicated HIF in restoring O2 delivery via stimulated hemoglobin synthesis (Daphnia) or tracheal remodeling (Drosophila). HIF is essential for these arthropods to adapt and survive during moderate O2 limitations. A similar life-preserving role for HIF-signaling in hypoxic, but not anoxic, environments had previously been established for another stress-tolerant invertebrate model, the nematode Caenorhabditis elegans. Exploring regulations of oxygen-dependent Daphnia and Drosophila genes in cell culture and in vivo have furthermore aided in uncovering novel HIF-targeting mechanisms that might operate to fine-tune the activity of this transcription factor under steadily hypoxic, rather than changing, oxygen tensions. We conclude our review with yet another addition to the growing list of HIF's many functions: the control of cellular growth during fly development.

Oxygen partial pressure effects on metabolic rate and behavior of tethered flying locusts

Resting insects are extremely tolerant of hypoxia. However, oxygen requirements increase dramatically during flight. Does the critical atmospheric P (O)(2) (P(c)) increase strongly during flight, or does increased tracheal conductance allow even flying insects to possess large safety margins for oxygen delivery? We tested the effect of P(O)(2) on resting and flying CO(2) emission, as well as on flight behavior and vertical force production in flying locusts, Schistocerca americana. The P(c) for CO(2) emission of resting animals was less than 1 kPa, similar to prior studies. The P(c) for flight bout duration was between 10 and 21 kPa, the P(c) for vertical force production was between 3 and 5 kPa, and the P(c) for CO(2) emission was between 10 and 21 kPa. Our study suggests that the P(c) for steady-state oxygen consumption is between 10 and 21 kPa (much higher than for resting animals), and that tracheal oxygen stores allowed brief flights in 5 and 10 kPa P(O)(2) atmospheres to occur. Thus, P(c) values strongly increased during flight, consistent with the hypothesis that the excess oxygen delivery capacity observed in resting insects is substantially reduced during flight.