Dev-ResNet: automated developmental event detection using deep learning.

Delineating developmental events is central to experimental research using early life stages, permitting widespread identification of changes in event timing between species and environments. Yet, identifying developmental events is incredibly challenging, limiting the scale, reproducibility and throughput of using early life stages in experimental biology. We introduce Dev-ResNet, a small and efficient 3D convolutional neural network capable of detecting developmental events characterised by both spatial and temporal features, such as the onset of cardiac function and radula activity. We demonstrate the efficacy of Dev-ResNet using 10 diverse functional events throughout the embryonic development of the great pond snail, Lymnaea stagnalis. Dev-ResNet was highly effective in detecting the onset of all events, including the identification of thermally induced decoupling of event timings. Dev-ResNet has broad applicability given the ubiquity of bioimaging in developmental biology, and the transferability of deep learning, and so we provide comprehensive scripts and documentation for applying Dev-ResNet to different biological systems.

The embryonic thermal environment has positive but weak effects on thermal tolerance later in life in the aquatic invertebrate Gammarus chevreuxi.

Recent evidence suggests that the adult phenotype is influenced by temperatures experienced in early life. However, our understanding of the extent to which the embryonic environment can modulate thermal tolerance later in life is limited, owing to the paucity of studies with appropriate experimental designs to test for this form of developmental plasticity. We investigated whether the thermal environment experienced during embryonic development affects thermal limits in later life. Embryos of the estuarine amphipod Gammarus chevreuxi were incubated until hatching to 15 °C, 20 °C and 25 °C, then reared under a common temperature. Using thermal ramping assays, we determined upper thermal limits in juveniles, four weeks post-hatch. Individuals exposed to higher temperatures during embryonic development displayed greater thermal tolerance as juveniles (acclimation response ratio ≈ 0.10-0.25 for upper lethal temperature). However, we suggest that the degree of developmental plasticity observed is limited, and will provide little benefit under future climate change scenarios.

The north-south divide? Macroalgal functional trait diversity and redundancy varies with intertidal aspect.

BACKGROUND AND AIMS: Marine macroalgae ('seaweeds') are critical to coastal ecosystem structure and function, but also vulnerable to the many environmental changes associated with anthropogenic climate change (ACC). The local habitat conditions underpinning observed and predicted ACC-driven changes in intertidal macroalgal communities are complex and probably site-specific and operate in addition to more commonly reported regional factors such as sea surface temperatures. METHODS: We examined how the composition and functional trait expression of macroalgal communities in SW England varied with aspect (i.e. north-south orientation) at four sites with opposing Equator- (EF) and Pole-facing (PF) surfaces. Previous work at these sites had established that average annual (low tide) temperatures vary by 1.6 °C and that EF-surfaces experience six-fold more frequent extremes (i.e. >30 °C). KEY RESULTS: PF macroalgal communities were consistently more taxon rich; 11 taxa were unique to PF habitats, with only one restricted to EF. Likewise, functional richness and dispersion were greater on PF-surfaces (dominated by algae with traits linked to rapid resource capture and utilization, but low desiccation tolerance), although differences in both taxon and functional richness were probably driven by the fact that less diverse EF-surfaces were dominated by desiccation-tolerant fucoids. CONCLUSIONS: Although we cannot disentangle the influence of temperature variation on algal ecophysiology from the indirect effects of aspect on species interactions (niche pre-emption, competition, grazing, etc.), our study system provides an excellent model for understanding how environmental variation at local scales affects community composition and functioning. By virtue of enhanced taxonomic diversity, PF-aspects supported higher functional diversity and, consequently, greater effective functional redundancy. These differences may imbue PF-aspects with resilience against environmental perturbation, but if predicted increases in global temperatures are realized, some PF-sites may shift to a depauperate, desiccation-tolerant seaweed community with a concomitant loss of functional diversity and redundancy.

Comparative phenomics: a new approach to study heterochrony.

Understanding the links between development and evolution is one of the major challenges of biology. 'Heterochronies', evolutionary alterations in the timings of development are posited as a key mechanism of evolutionary change, but their quantification requires gross simplification of organismal development. Consequently, how changes in event timings influence development more broadly is poorly understood. Here, we measure organismal development as spectra of energy in pixel values of video, creating high-dimensional landscapes integrating development of all visible form and function. This approach we termed 'Energy proxy traits' (EPTs) is applied alongside previously identified heterochronies in three freshwater pulmonate molluscs (Lymnaea stagnalis, Radix balthica and Physella acuta). EPTs were calculated from time-lapse video of embryonic development to construct a continuous functional time series. High-dimensional transitions in phenotype aligned with major sequence heterochronies between species. Furthermore, differences in event timings between conspecifics were associated with changes in high-dimensional phenotypic space. We reveal EPTs as a powerful approach to considering the evolutionary importance of alterations to developmental event timings. Reimagining the phenotype as energy spectra enabled continuous quantification of developmental changes in high-dimensional phenotypic space, rather than measurement of timings of discrete events. This approach has the possibility to transform how we study heterochrony and development more generally.

Phenomics as an approach to Comparative Developmental Physiology.

The dynamic nature of developing organisms and how they function presents both opportunity and challenge to researchers, with significant advances in understanding possible by adopting innovative approaches to their empirical study. The information content of the phenotype during organismal development is arguably greater than at any other life stage, incorporating change at a broad range of temporal, spatial and functional scales and is of broad relevance to a plethora of research questions. Yet, effectively measuring organismal development, and the ontogeny of physiological regulations and functions, and their responses to the environment, remains a significant challenge. "Phenomics", a global approach to the acquisition of phenotypic data at the scale of the whole organism, is uniquely suited as an approach. In this perspective, we explore the synergies between phenomics and Comparative Developmental Physiology (CDP), a discipline of increasing relevance to understanding sensitivity to drivers of global change. We then identify how organismal development itself provides an excellent model for pushing the boundaries of phenomics, given its inherent complexity, comparably smaller size, relative to adult stages, and the applicability of embryonic development to a broad suite of research questions using a diversity of species. Collection, analysis and interpretation of whole organismal phenotypic data are the largest obstacle to capitalising on phenomics for advancing our understanding of biological systems. We suggest that phenomics within the context of developing organismal form and function could provide an effective scaffold for addressing grand challenges in CDP and phenomics.

A phenomics approach reveals interspecific differences in integrated developmental responses to chronic elevated temperatures.

Phenomics, high-dimensional organismal phenotyping, is advanced as a solution to quantifying complex developmental responses to elevated temperatures. 'Energy proxy traits' (EPTs) measure the phenotype as a spectrum of energy values across different temporal frequencies from pixel value fluctuations of video. Although they have proven effective in measuring the biology of complex and dynamic developing organisms, their utility in assessing environmental sensitivity of different species is untested. Using EPTs, we assess the relative thermal sensitivities of embryos of three species of freshwater snail with marked differences in their developmental event timings. Embryos of Lymnaea stagnalis, Radix balthica and Physella acuta were videoed hourly for the duration of their embryonic development at two temperatures: 20°C and 25°C. The video was used to calculate EPTs for the duration of their embryonic development, and during discrete physiological windows in development. Changes in energy spectra during development identified marked differences in thermal sensitivities between species, and suggest a relatively heightened sensitivity of gross rates of embryonic physiology and behaviour in embryos of R. balthica, developmental-window-specific thermal responses that reflect ontogenetic differences in observable physiologies, and temperature-induced changes in physiological event timing. EPTs enabled comparison of high-dimensional spectral phenotypes, providing a unique capability for assessing sensitivity continuously in developing individuals. Such integrative and scalable phenotyping is a prerequisite for improved understanding of the sensitivity of early life stages of different species.

Phenomics enables measurement of complex responses of developing animals to global environmental drivers.

Phenomics offers technological advances for high-dimensional phenotyping, facilitating rapid, high-throughput assessment of physiological performance and has proven invaluable in global research challenges including drug discovery and food security. However, this rapidly growing discipline has remained largely inaccessible to the increasingly urgent challenge of assessing organismal functional sensitivity to global change drivers. Here, we investigate the response of an ecologically important marine invertebrate to multiple environmental drivers using Energy Proxy Traits (EPTs), a new approach for measuring complex phenotypes captured on video as a spectrum of energy levels across different temporal frequencies in fluctuating pixel values. We imaged three developmental stages of the common prawn Palaemon serratus at different salinities and temperatures, and measured EPTs and heart rate, a major proxy of physiological performance in ectotherms present across stages. Significant interactions were detected between temperature, developmental stage and salinity in frequency-specific energy levels. Despite cardiac activity being a significant contributor to the EPT spectra, treatment interactions were different from those observed on EPTs, highlighting additional phenotypic drivers of EPTs. Elevated temperature resulted in a shift of the EPT spectra towards higher frequency signals, indicating a reallocation of resources within the phenome. Using a non-linear dimensionality reduction, we interrogated the responses of EPT spectra in high-dimensional space. We discovered complex developmental-stage specific sensitivities, highlighting both the complexity of phenotypic responses, and the limits of using univariate approaches with pre-selected traits to assess responses to multiple global environmental drivers. EPTs are a high-dimensional, transferrable method of phenotyping, and are therefore highly relevant to addressing the current limitations of traditional methods of phenotyping applied to assessing biological sensitivity to drivers of global change. We predict that EPTs will become an important tool for indiscriminate phenotyping, transferrable between species, developmental stages and experimental designs.

Effect of Animal Stocking Density and Habitat Enrichment on Survival and Vitality of Wild Green Shore Crabs, Carcinus maenas, Maintained in the Laboratory.

The wide geographic distribution, large size and ease of capture has led to decapod crustaceans being used extensively in laboratory experiments. Recently in the United Kingdom decapod crustaceans were listed as sentient beings, resulting in their inclusion in animal care protocols. Ironically, little is known about how captive conditions affect the survival and general condition of wild decapod crustaceans. We used the green shore crab, Carcinus maenas, to investigate the effects of stocking density and shelter on survival and vitality indices during a 6 month period in the laboratory. Neither stocking density nor the presence of shelter affected survival. Stocking density also had no effect on the vitality indices (limb loss, claw strength, BRIX, righting time, leg flare and retraction). The presence of shelter did affect the number of limbs lost and the leg retraction response, but had no effect on the other vitality indices. All vitality indices changed, and mortality increased over time, independent of treatment: this became most apparent after 8 to 11 weeks storage in the laboratory. This decline in condition may have been due to repeated handling of the crabs, rather than the stocking conditions. In support of this, untracked, non-handled (control) individuals sustained a 4% mortality rate compared with 67% mortality in experimental crabs during the 6 month period. Although simple experimental monitoring of crabs with biweekly vitality tests only produced transient short-term stress events, the repeated handling over time apparently led to a cumulative stress and a deterioration in animal health. Bringing wild crustaceans into the laboratory and holding them, even with modest experimental manipulation, may result in high mortality rates. Researchers and animal care committees need to be aware that wild captive invertebrates will respond very differently to laboratory-bred vertebrates, and plan experiments accordingly.

HeartCV: a tool for transferrable, automated measurement of heart rate and heart rate variability in transparent animals.

Heart function is a key component of whole-organismal physiology. Bioimaging is commonly, but not exclusively, used for quantifying heart function in transparent individuals, including early developmental stages of aquatic animals, many of which are transparent. However, a central limitation of many imaging-related methods is the lack of transferability between species, life-history stages and experimental approaches. Furthermore, locating the heart in mobile individuals remains challenging. Here, we present HeartCV: an open-source Python package for automated measurement of heart rate and heart rate variability that integrates automated localization and is transferrable across a wide range of species. We demonstrate the efficacy of HeartCV by comparing its outputs with measurements made manually for a number of very different species with contrasting heart morphologies. Lastly, we demonstrate the applicability of the software to different experimental approaches and to different dataset types, such as those corresponding to longitudinal studies.

Consequences of thermal plasticity for hypoxic performance in coastal amphipods.

Physiological plasticity may confer an ability to deal with the effect of rapid climate change on aquatic ectotherms. However, plasticity induced by one stressor may only be adaptive in situ if it generates cross-tolerance to other stressors. Understanding the consequences of thermal acclimation on hypoxia thresholds is vital to understanding future climate-driven hypoxia. We tested if thermal acclimation benefits hypoxic performance in four closely-related amphipod species. The effects of thermal acclimation (7 days at 10 or 20 °C) on routine metabolic rate (RMR) and critical oxygen tensions (Pcrit) were determined at a standardised test temperature (20 °C). Gammarus chevreuxi and Echinogammarus marinus displayed increased Pcrit with acute warming but warm acclimation negated this increase. Pcrit of Gammarus duebeni was thermally insensitive. Gammarus zaddachi displayed increased Pcrit upon acute warming but little change via acclimation. Cross-tolerance between thermal plasticity and hypoxia may improve performance for some, but not all, species under future environmental change.

Spectral phenotyping of embryonic development reveals integrative thermodynamic responses.

BACKGROUND: Energy proxy traits (EPTs) are a novel approach to high dimensional organismal phenotyping that quantify the spectrum of energy levels within different temporal frequencies associated with mean pixel value fluctuations from video. They offer significant potential in addressing the phenotyping bottleneck in biology and are effective at identifying lethal endpoints and measuring specific functional traits, but the extent to which they might contribute additional understanding of the phenotype remains unknown. Consequently, here we test the biological significance of EPTs and their responses relative to fundamental thermodynamic principles. We achieve this using the entire embryonic development of Radix balthica, a freshwater pond snail, at different temperatures (20, 25 & 30 °C) and comparing responses against predictions from Arrhenius' equation (Q10 = 2). RESULTS: We find that EPTs are thermally sensitive and their spectra of frequency response enable effective high-dimensional treatment clustering throughout organismal development. Temperature-specific deviation in EPTs from thermodynamic predictions were evident and indicative of physiological mitigation, although they differed markedly in their responses from manual measures. The EPT spectrum was effective in capturing aspects of the phenotype predictive of biological outcomes, and suggest that EPTs themselves may reflect levels of energy turnover. CONCLUSIONS: Whole-organismal biology is incredibly complex, and this contributes to the challenge of developing universal phenotyping approaches. Here, we demonstrate the biological relevance of a new holistic approach to phenotyping that is not constrained by preconceived notions of biological importance. Furthermore, we find that EPTs are an effective approach to measuring even the most dynamic life history stages.

Transcriptional frontloading contributes to cross-tolerance between stressors.

The adaptive value of phenotypic plasticity for performance under single stressors is well documented. However, plasticity may only truly be adaptive in the natural multifactorial environment if it confers resilience to stressors of a different nature, a phenomenon known as cross-tolerance. An understanding of the mechanistic basis of cross-tolerance is essential to aid prediction of species resilience to future environmental change. Here, we identified mechanisms underpinning cross-tolerance between two stressors predicted to increasingly challenge aquatic ecosystems under climate change, chronic warming and hypoxia, in an ecologically-important aquatic invertebrate. Warm acclimation improved hypoxic performance through an adaptive hypometabolic strategy and changes in the expression of hundreds of genes that are important in the response to hypoxia. These 'frontloaded' genes showed a reduced reaction to hypoxia in the warm acclimated compared to the cold acclimated group. Frontloaded genes included stress indicators, immune response and protein synthesis genes that are protective at the cellular level. We conclude that increased constitutive gene expression as a result of warm acclimation reduced the requirement for inducible stress responses to hypoxia. We propose that transcriptional frontloading contributes to cross-tolerance between stressors and may promote fitness of organisms in environments increasingly challenged by multiple anthropogenic threats.

Do aquatic ectotherms perform better under hypoxia after warm acclimation?

Aquatic animals increasingly encounter environmental hypoxia due to climate-related warming and/or eutrophication. Although acute warming typically reduces performance under hypoxia, the ability of organisms to modulate hypoxic performance via thermal acclimation is less understood. Here, we review the literature and ask whether hypoxic performance of aquatic ectotherms improves following warm acclimation. Interpretation of thermal acclimation effects is limited by reliance on data from experiments that are not designed to directly test for beneficial or detrimental effects on hypoxic performance. Most studies have tested hypoxic responses exclusively at test temperatures matching organisms' acclimation temperatures, precluding the possibility of distinguishing between acclimation and acute thermal effects. Only a few studies have applied appropriate methodology to identify beneficial thermal acclimation effects on hypoxic performance, i.e. acclimation to different temperatures prior to determining hypoxic responses at standardised test temperatures. These studies reveal that acute warming predominantly impairs hypoxic performance, whereas warm acclimation tends to be either beneficial or have no effect. If this generalises, we predict that warm-acclimated individuals in some species should outperform non-acclimated individuals under hypoxia. However, acclimation seems to only partially offset acute warming effects; therefore, aquatic ectotherms will probably display overall reduced hypoxic performance in the long term. Drawing on the appropriate methodology, future studies can quantify the ability of organisms to modulate hypoxic performance via (reversible) thermal acclimation and unravel the underlying mechanisms. Testing whether developmental acclimation and multigenerational effects allow for a more complete compensation is essential to allow us to predict species' resilience to chronically warmer, hypoxic environments.

Both maternal and embryonic exposure to mild hypoxia influence embryonic development of the intertidal gastropod Littorina littorea.

There is growing evidence that maternal exposure to environmental stressors can alter offspring phenotype and increase fitness. Here, we investigate the relative and combined effects of maternal and developmental exposure to mild hypoxia (65 and 74% air saturation, respectively) on the growth and development of embryos of the marine gastropod Littorina littorea Differences in embryo morphological traits were driven by the developmental environment, whereas the maternal environment and interactive effects of maternal and developmental environment were the main driver of differences in the timing of developmental events. While developmental exposure to mild hypoxia significantly increased the area of an important respiratory organ, the velum, it significantly delayed hatching of veliger larvae and reduced their size at hatching and overall survival. Maternal exposure had a significant effect on these traits, and interacted with developmental exposure to influence the time of appearance of morphological characters, suggesting that both are important in affecting developmental trajectories. A comparison between embryos that successfully hatched and those that died in mild hypoxia revealed that survivors exhibited hypertrophy in the velum and associated pre-oral cilia, suggesting that these traits are linked with survival in low-oxygen environments. We conclude that both maternal and developmental environments shape offspring phenotype in a species with a complex developmental life history, and that plasticity in embryo morphology arising from exposure to even small reductions in oxygen tensions affects the hatching success of these embryos.

Universal metabolic constraints shape the evolutionary ecology of diving in animals.

Diving as a lifestyle has evolved on multiple occasions when air-breathing terrestrial animals invaded the aquatic realm, and diving performance shapes the ecology and behaviour of all air-breathing aquatic taxa, from small insects to great whales. Using the largest dataset yet assembled, we show that maximum dive duration increases predictably with body mass in both ectotherms and endotherms. Compared to endotherms, ectotherms can remain submerged for longer, but the mass scaling relationship for dive duration is much steeper in endotherms than in ectotherms. These differences in diving allometry can be fully explained by inherent differences between the two groups in their metabolic rate and how metabolism scales with body mass and temperature. Therefore, we suggest that similar constraints on oxygen storage and usage have shaped the evolutionary ecology of diving in all air-breathing animals, irrespective of their evolutionary history and metabolic mode. The steeper scaling relationship between body mass and dive duration in endotherms not only helps explain why the largest extant vertebrate divers are endothermic rather than ectothermic, but also fits well with the emerging consensus that large extinct tetrapod divers (e.g. plesiosaurs, ichthyosaurs and mosasaurs) were endothermic.

The effects of elevated temperature and PCO2 on the energetics and haemolymph pH homeostasis of juveniles of the European lobster, Homarus gammarus.

Regulation of extracellular acid-base balance, while maintaining energy metabolism, is recognised as an important aspect when defining an organism's sensitivity to environmental changes. This study investigated the haemolymph buffering capacity and energy metabolism (oxygen consumption, haemolymph [l-lactate] and [protein]) in early benthic juveniles (carapace length <40 mm) of the European lobster, Homarus gammarus, exposed to elevated temperature and PCO2 At 13°C, H. gammarus juveniles were able to fully compensate for acid-base disturbances caused by the exposure to elevated seawater PCO2  at levels associated with ocean acidification and carbon dioxide capture and storage (CCS) leakage scenarios, via haemolymph [HCO3-] regulation. However, metabolic rate remained constant and food consumption decreased under elevated PCO2 , indicating reduced energy availability. Juveniles at 17°C showed no ability to actively compensate haemolymph pH, resulting in decreased haemolymph pH particularly under CCS conditions. Early benthic juvenile lobsters at 17°C were not able to increase energy intake to offset increased energy demand and therefore appear to be unable to respond to acid-base disturbances due to increased PCO2 at elevated temperature. Analysis of haemolymph metabolites suggests that, even under control conditions, juveniles were energetically limited. They exhibited high haemolymph [l-lactate], indicating recourse to anaerobic metabolism. Low haemolymph [protein] was linked to minimal non-bicarbonate buffering and reduced oxygen transport capacity. We discuss these results in the context of potential impacts of ongoing ocean change and CCS leakage scenarios on the development of juvenile H. gammarus and future lobster populations and stocks.

Exoskeleton dissolution with mechanoreceptor damage in larval Dungeness crab related to severity of present-day ocean acidification vertical gradients.

Ocean acidification (OA) along the US West Coast is intensifying faster than observed in the global ocean. This is particularly true in nearshore regions (<200 m) that experience a lower buffering capacity while at the same time providing important habitats for ecologically and economically significant species. While the literature on the effects of OA from laboratory experiments is voluminous, there is little understanding of present-day OA in-situ effects on marine life. Dungeness crab (Metacarcinus magister) is perennially one of the most valuable commercial and recreational fisheries. We focused on establishing OA-related vulnerability of larval crustacean based on mineralogical and elemental carapace to external and internal carapace dissolution by using a combination of different methods ranging from scanning electron microscopy, energy dispersive X-ray spectroscopy, elemental mapping and X-ray diffraction. By integrating carapace features with the chemical observations and biogeochemical model hindcast, we identify the occurrence of external carapace dissolution related to the steepest Ω calcite gradients (∆Ωcal,60) in the water column. Dissolution features are observed across the carapace, pereopods (legs), and around the calcified areas surrounding neuritic canals of mechanoreceptors. The carapace dissolution is the most extensive in the coastal habitats under prolonged (1-month) long exposure, as demonstrated by the use of the model hindcast. Such dissolution has a potential to destabilize mechanoreceptors with important sensory and behavioral functions, a pathway of sensitivity to OA. Carapace dissolution is negatively related to crab larval width, demonstrating a basis for energetic trade-offs. Using a retrospective prediction from a regression models, we estimate an 8.3% increase in external carapace dissolution over the last two decades and identified a set of affected OA-related sublethal pathways to inform future risk assessment studies of Dungeness crabs.

Quantifying susceptibility of marine invertebrate biocomposites to dissolution in reduced pH.

Ocean acidification threatens many ecologically and economically important marine calcifiers. The increase in shell dissolution under the resulting reduced pH is an important and increasingly recognized threat. The biocomposites that make up calcified hardparts have a range of taxon-specific compositions and microstructures, and it is evident that these may influence susceptibilities to dissolution. Here, we show how dissolution (thickness loss), under both ambient and predicted end-century pH (approx. 7.6), varies between seven different bivalve molluscs and one crustacean biocomposite and investigate how this relates to details of their microstructure and composition. Over 100 days, the dissolution of all microstructures was greater under the lower pH in the end-century conditions. Dissolution of lobster cuticle was greater than that of any bivalve microstructure, despite its calcite mineralogy, showing the importance of other microstructural characteristics besides carbonate polymorph. Organic content had the strongest positive correlation with dissolution when all microstructures were considered, and together with Mg/Ca ratio, explained 80-90% of the variance in dissolution. Organic content, Mg/Ca ratio, crystal density and mineralogy were all required to explain the maximum variance in dissolution within only bivalve microstructures, but still only explained 50-60% of the variation in dissolution.

Moderate reductions in dissolved oxygen may compromise performance in an ecologically-important estuarine invertebrate.

Coastal ecosystems, including estuaries, are increasingly pressured by expanding hypoxic regions as a result of human activities such as increased release of nutrients and global warming. Hypoxia is often defined as oxygen concentrations below 2 mL O2 L-1. However, taxa vary markedly in their sensitivity to hypoxia and can be affected by a broad spectrum of low oxygen levels. To better understand how reduced oxygen availability impacts physiological and molecular processes in invertebrates, we investigated responses of an estuarine amphipod to an ecologically-relevant level of moderate hypoxia (~2.6 mL O2 L-1) or severe hypoxia (~1.3 mL O2 L-1). Moderate hypoxia elicited a reduction in aerobic scope, and widespread changes to gene expression, including upregulation of metabolic genes and stress proteins. Under severe hypoxia, a marked hyperventilatory response associated with maintenance of aerobic performance was accompanied by a muted transcriptional response. This included a return of metabolic genes to baseline levels of expression and downregulation of transcripts involved in protein synthesis, most of which indicate recourse to hypometabolism and/or physiological impairment. We conclude that adverse ecological effects may occur under moderate hypoxia through compromised individual performance and, therefore, even modest declines in future oxygen levels may pose a significant challenge to coastal ecosystems.

Will giant polar amphipods be first to fare badly in an oxygen-poor ocean? Testing hypotheses linking oxygen to body size.

It has been suggested that giant Antarctic marine invertebrates will be particularly vulnerable to declining O2 levels as our ocean warms in line with current climate change predictions. Our study provides some support for this oxygen limitation hypothesis, with larger body sizes being generally more sensitive to O2 reductions than smaller body sizes. However, it also suggests that the overall picture is a little more complex. We tested predictions from three different, but overlapping, O2-related hypotheses accounting for gigantism, using four Antarctic amphipod species encompassing a wide range of body sizes. We found a significant effect of body size, but also of species, in their respiratory responses to acutely declining O2 tensions. The more active lifestyle of intermediate-sized Prostebbingia brevicornis was supported by a better respiratory performance than predicted by the oxygen limitation hypothesis alone, but consistent with the symmorphosis hypothesis. We suggest that giant polar amphipods are likely to be some of the first to fare badly in an O2-poor ocean. However, the products of past evolutionary innovation, such as respiratory pigments that enhance O2-transport and novel gas exchange structures, may in some species offset any respiratory disadvantages of either large or small body size. This article is part of the theme issue 'Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen'.