Thermal limits of burrowing capacity are linked to oxygen availability and size in the Antarctic clam Laternula elliptica

Physiological and molecular effects of contaminants of emerging concerns of micro and nano-size in aquatic metazoans: overview and current gaps in Antarctic species

Although Antarctica is the most isolated continent on Earth, its remote location does not protect it from the impacts of human activities. Antarctic metazoans such as filter-feeding invertebrates are a crucial component of the Antarctic benthos. They play a key role in the benthic-pelagic carbon flux in coastal areas by filtering particles and planktonic organisms from the sediment-water interface. Due to their peculiar ecological niche, these organisms can be considered a wasp-waist in the ecosystem, making them highly sensitive to marine pollution. Recently, anthropogenic particles such as micro-nanoplastics and manufactured nanoparticles (MNP) have been classified as contaminants of emerging concern (CEC) due to their small size range, which also overlaps with the preferred particle size ingested by aquatic metazoans. Indeed, it has been demonstrated that some species such as Antarctic krill can ingest, transform, and release MNPs, making them newly bioavailable for other Antarctic filter-feeding organisms. Similarly, the production and use of anthropogenic MNP are rapidly increasing, leading to a growing presence of materials, such as nano-sized metal-oxides, in the environment. For these reasons, it is important to provide evidence of the adverse effects of such emerging contaminants at sub-lethal concentrations in environmental risk assessments. These contaminants may cause cascade effects with consequences not only on individuals but also at the community and ecosystem levels. In this review, we discuss the state-of-the-art knowledge on the physiological and molecular effects of anthropogenic MNP in Antarctic aquatic metazoans. We further highlight the importance of identifying early biomarkers using sessile metazoans as sentinels of environmental health.

Archival records of the Antarctic clam shells from Marian Cove, King George Island suggest a protective mechanism against ocean acidification

Continuous emissions of anthropogenic CO2 are changing the atmospheric and oceanic environment. Although some species may have compensatory mechanisms to acclimatize or adapt to the changing environment, most marine organisms are negatively influenced by climate change. In this study, we aimed to understand the compensatory mechanisms of the Antarctic clam, Laternula elliptica, to climate-related stressors by using archived shells from 1995 to 2018. Principal component analysis revealed that seawater pCO2 and salinity in the Antarctic Ocean, which have increased since the 2000's, are the most influential factors on the characteristics of the shell. The periostracum thickness ratio and nitrogen on the outermost surface have increased, and the dissolution area (%) has decreased. Furthermore, the calcium content and mechanical properties of the shells have not changed. The results suggest that L. elliptica retains the mechanism of protecting the shell from high pCO2 by thickening the periostracum as a phenotype plasticity.

Reconsidering the Oxygen-Temperature Hypothesis of Polar Gigantism: Successes, Failures, and Nuance

"Polar gigantism" describes a biogeographic pattern in which many ectotherms in polar seas are larger than their warmer-water relatives. Although many mechanisms have been proposed, one idea-the oxygen-temperature hypothesis-has received significant attention because it emerges from basic biophysical principles and is appealingly straightforward and testable. Low temperatures depress metabolic demand for oxygen more than supply of oxygen from the environment to the organism. This creates a greater ratio of oxygen supply to demand, releasing polar organisms from oxygen-based constraints on body size. Here we review evidence for and against the oxygen-temperature hypothesis. Some data suggest that larger-bodied taxa live closer to an oxygen limit, or that rising temperatures can challenge oxygen delivery systems; other data provide no evidence for interactions between body size, temperature, and oxygen sufficiency. We propose that these findings can be partially reconciled by recognizing that the oxygen-temperature hypothesis focuses primarily on passive movement of oxygen, implicitly ignoring other important processes including ventilation of respiratory surfaces or internal transport of oxygen by distribution systems. Thus, the hypothesis may apply most meaningfully to organisms with poorly developed physiological systems (eggs, embryos, egg masses, juveniles, or adults without mechanisms for ventilating internal or external surfaces). Finally, most tests of the oxygen-temperature hypothesis have involved short-term experiments. Many organisms can mount effective responses to physiological challenges over short time periods; however, the energetic cost of doing so may have impacts that appear only in the longer term. We therefore advocate a renewed focus on long-term studies of oxygen-temperature interactions.

Climate impacts on organisms, ecosystems and human societies: integrating OCLTT into a wider context

Physiological studies contribute to a cause and effect understanding of ecological patterns under climate change and identify the scope and limits of adaptation. Across most habitats, this requires analyzing organism responses to warming, which can be modified by other drivers such as acidification and oxygen loss in aquatic environments or excess humidity or drought on land. Experimental findings support the hypothesis that the width and temperature range of thermal performance curves relate to biogeographical range. Current warming causes range shifts, hypothesized to include constraints in aerobic power budget which in turn are elicited by limitations in oxygen supply capacity in relation to demand. Different metabolic scopes involved may set the borders of both the fundamental niche (at standard metabolic rate) and the realized niche (at routine rate). Relative scopes for aerobic performance also set the capacity of species to interact with others at the ecosystem level. Niche limits and widths are shifting and probably interdependent across life stages, with young adults being least thermally vulnerable. The principles of thermal tolerance and performance may also apply to endotherms including humans, their habitat and human society. Overall, phylogenetically based comparisons would need to consider the life cycle of species as well as organism functional properties across climate zones and time scales. This Review concludes with a perspective on how mechanism-based understanding allows scrutinizing often simplified modeling approaches projecting future climate impacts and risks for aquatic and terrestrial ecosystems. It also emphasizes the usefulness of a consensus-building process among experimentalists for better recognition in the climate debate.

The influence of glacial melt and retreat on the nutritional condition of the bivalve Nuculana inaequisculpta (Protobranchia: Nuculanidae) in the West Antarctic Peninsula

Due to climate change, numerous ice bodies have been lost in the West Antarctic Peninsula (WAP). As a consequence, deglaciation is expected to impact the marine environment and its biota at physiological and ecosystem levels. Nuculana inaequisculpta is a marine bivalve widely distributed around Antarctica that plays an important role for ecosystem functioning. Considering that N. inaequisculpta inhabits coastal areas under effect of glacial melt and retreat, impacts on its nutritional condition are expected due to alterations on its physiology and food availability. To test this hypothesis, biochemical composition (lipids, proteins, and fatty acids) and energy content were measured in individuals of N. inaequisculpta collected in a fjord at different distances to the retreating glacier in the WAP. Oceanographic parameters of the top and bottom-water layers (temperature, salinity, dissolved oxygen, and chlorophyll-a) were measured to investigate how the environment changes along the fjord. Results showed that surface oceanographic parameters displayed a lower temperature and dissolved oxygen, but a higher salinity and chlorophyll-a content at nearest compared to farthest sites to the glacier. In contrast, a lower temperature and chlorophyll-a, and a higher salinity and dissolved oxygen was measured in the bottom-water layer toward the glacier. N. inaequisculpta had a higher amount of lipids (17.42 ± 3.24 vs. 12.16 ± 3.46%), protein (24.34 ± 6.12 vs. 21.05 ± 2.46%) and energy content (50.57 ± 6.97 J vs. 39.14 ± 5.80 J) in the farthest compared to the nearest site to the glacier. No differences were found in total fatty acids among all sites. It seems likely that lower individual fitness related to proximity to the glacier would not be related to nutritional quality of sediment food, but rather to food quantity.

Insights into high-pressure acclimation: comparative transcriptome analysis of sea cucumber Apostichopus japonicus at different hydrostatic pressure exposures

Background

Global climate change is predicted to force the bathymetric migrations of shallow-water marine invertebrates. Hydrostatic pressure is proposed to be one of the major environmental factors limiting the vertical distribution of extant marine invertebrates. However, the high-pressure acclimation mechanisms are not yet fully understood.

Results

In this study, the shallow-water sea cucumber Apostichopus japonicus was incubated at 15 and 25 MPa at 15 °C for 24 h, and subjected to comparative transcriptome analysis. Nine samples were sequenced and assembled into 553,507 unigenes with a N50 length of 1204 bp. Three groups of differentially expressed genes (DEGs) were identified according to their gene expression patterns, including 38 linearly related DEGs whose expression patterns were linearly correlated with hydrostatic pressure, 244 pressure-sensitive DEGs which were up-regulated at both 15 and 25 MPa, and 257 high-pressure-induced DEGs which were up-regulated at 25 MPa but not up-regulated at 15 MPa.

Conclusions

Our results indicated that the genes and biological processes involving high-pressure acclimation are similar to those related to deep-sea adaptation. In addition to representative biological processes involving deep-sea adaptation (such as antioxidation, immune response, genetic information processing, and DNA repair), two biological processes, namely, ubiquitination and endocytosis, which can collaborate with each other and regulate the elimination of misfolded proteins, also responded to high-pressure exposure in our study. The up-regulation of these two processes suggested that high hydrostatic pressure would lead to the increase of misfolded protein synthesis, and this may result in the death of shallow-water sea cucumber under high-pressure exposure.

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'.

Polar gigantism and the oxygen-temperature hypothesis: a test of upper thermal limits to body size in Antarctic pycnogonids

The extreme and constant cold of the Southern Ocean has led to many unusual features of the Antarctic fauna. One of these, polar gigantism, is thought to have arisen from a combination of cold-driven low metabolic rates and high oxygen availability in the polar oceans (the 'oxygen-temperature hypothesis'). If the oxygen-temperature hypothesis indeed underlies polar gigantism, then polar giants may be particularly susceptible to warming temperatures. We tested the effects of temperature on performance using two genera of giant Antarctic sea spiders (Pycnogonida), Colossendeis and Ammothea, across a range of body sizes. We tested performance at four temperatures spanning ambient (-1.8°C) to 9°C. Individuals from both genera were highly sensitive to elevated temperature, but we found no evidence that large-bodied pycnogonids were more affected by elevated temperatures than small individuals; thus, these results do not support the predictions of the oxygen-temperature hypothesis. When we compared two species, Colossendeis megalonyx and Ammothea glacialis, C. megalonyx maintained performance at considerably higher temperatures. Analysis of the cuticle showed that as body size increases, porosity increases as well, especially in C. megalonyx, which may compensate for the increasing metabolic demand and longer diffusion distances of larger animals by facilitating diffusive oxygen supply.

Comparative transcriptome analysis of Eogammarus possjeticus at different hydrostatic pressure and temperature exposures

Hydrostatic pressure is an important environmental factor affecting the vertical distribution of marine organisms. Laboratory-based studies have shown that many extant shallow-water marine benthic invertebrates can tolerate hydrostatic pressure outside their known natural distributions. However, only a few studies have focused on the molecular mechanisms of pressure acclimatisation. In the present work, we examined the pressure tolerance of the shallow-water amphipod Eogammarus possjeticus at various temperatures (5, 10, 15, and 20 °C) and hydrostatic pressures (0.1–30 MPa) for 16 h. Six of these experimental groups were used for transcriptome analysis. We found that 100% of E. possjeticus survived under 20 MPa at all temperature conditions for 16 h. Sequence assembly resulted in 138, 304 unigenes. Results of differential expression analysis revealed that 94 well-annotated genes were up-regulated under high pressure. All these findings indicated that the pressure tolerance of E. possjeticus was related to temperature. Several biological processes including energy metabolism, antioxidation, immunity, lipid metabolism, membrane-related process, genetic information processing, and DNA repair are probably involved in the acclimatisation in deep-sea environments.

A Dynamic Energy Budget (DEB) model to describe Laternula elliptica (King, 1832) seasonal feeding and metabolism

Antarctic marine organisms are adapted to an extreme environment, characterized by a very low but stable temperature and a strong seasonality in food availability arousing from variations in day length. Ocean organisms are particularly vulnerable to global climate change with some regions being impacted by temperature increase and changes in primary production. Climate change also affects the biotic components of marine ecosystems and has an impact on the distribution and seasonal physiology of Antarctic marine organisms. Knowledge on the impact of climate change in key species is highly important because their performance affects ecosystem functioning. To predict the effects of climate change on marine ecosystems, a holistic understanding of the life history and physiology of Antarctic key species is urgently needed. DEB (Dynamic Energy Budget) theory captures the metabolic processes of an organism through its entire life cycle as a function of temperature and food availability. The DEB model is a tool that can be used to model lifetime feeding, growth, reproduction, and their responses to changes in biotic and abiotic conditions. In this study, we estimate the DEB model parameters for the bivalve Laternula elliptica using literature-extracted and field data. The DEB model we present here aims at better understanding the biology of L. elliptica and its levels of adaptation to its habitat with a special focus on food seasonality. The model parameters describe a metabolism specifically adapted to low temperatures, with a low maintenance cost and a high capacity to uptake and mobilise energy, providing this organism with a level of energetic performance matching that of related species from temperate regions. It was also found that L. elliptica has a large energy reserve that allows enduring long periods of starvation. Additionally, we applied DEB parameters to time-series data on biological traits (organism condition, gonad growth) to describe the effect of a varying environment in food and temperature on the organism condition and energy use. The DEB model developed here for L. elliptica allowed us to improve benchmark knowledge on the ecophysiology of this key species, providing new insights in the role of food availability and temperature on its life cycle and reproduction strategy.

Biodiversity in marine invertebrate responses to acute warming revealed by a comparative multi-omics approach

Understanding species' responses to environmental change underpins our abilities to make predictions on future biodiversity under any range of scenarios. In spite of the huge biodiversity in most ecosystems, a model species approach is often taken in environmental studies. To date, we still do not know how many species we need to study to input into models and inform on ecosystem-level responses to change. In this study, we tested current paradigms on factors setting thermal limits by investigating the acute warming response of six Antarctic marine invertebrates: a crustacean Paraceradocus miersi, a brachiopod Liothyrella uva, two bivalve molluscs, Laternula elliptica, Aequiyoldia eightsii, a gastropod mollusc Marseniopsis mollis and an echinoderm Cucumaria georgiana. Each species was warmed at the rate of 1 °C h^-1 and taken to the same physiological end point (just prior to heat coma). Their molecular responses were evaluated using complementary metabolomics and transcriptomics approaches with the aim of discovering the underlying mechanisms of their resilience or sensitivity to warming. The responses were species-specific; only two showed accumulation of anaerobic end products and three exhibited the classical heat shock response with expression of HSP70 transcripts. These diverse cellular measures did not directly correlate with resilience to heat stress and suggested that each species may have a different critical point of failure. Thus, one unifying molecular mechanism underpinning response to warming could not be assigned, and no overarching paradigm was supported. This biodiversity in response makes future ecosystems predictions extremely challenging, as we clearly need to develop a macrophysiology-type approach to cellular evaluations of the environmental stress response, studying a range of well-rationalized members from different community levels and of different phylogenetic origins rather than extrapolating from one or two arbitrary model species.

Effects of mechanical disturbance and salinity stress on bioenergetics and burrowing behavior of the soft shell clam Mya arenaria

Bioturbation of sediments by burrowing organisms plays a key role in the functioning of the coastal ecosystems. Burrowing is considered an energetically expensive activity, yet the energy costs of burrowing and the potential impacts of multiple stressors (such as salinity stress and wave action) on bioenergetics and burrowing performance of marine bioturbators are not well understood. We investigated the effects of mechanical disturbance and salinity stress on the burrowing behavior, aerobic capacity and energy expense of digging in a common marine bioturbator, the soft clam Mya arenaria from the Baltic Sea (control salinity 15). M. arenaria showed large individual variability in the burrowing efficiency, with an average of ∼7% of the body energy reserves used per burial. Clams with higher mitochondrial capacity and lower energy expenditure per burial showed higher endurance. Acclimation for 3-4 weeks to low (5) or fluctuating (5-15) salinity reduced the burrowing speed and the number of times the clams can re-bury but did not affect the mitochondrial capacity of the whole body or the gill. Acclimation to the fluctuating salinity shifted the predominant fuel use for burrowing from proteins to lipids. Our data indicate that the reduced burrowing performance of clams under the salinity stress is not due to the limitations of energy availability or aerobic capacity but must involve other mechanisms (such as impaired muscle performance). The reduction in the burrowing capacity of clams due to salinity stress may have important implications for survival, activity and ecological functions of the clams in shallow coastal ecosystems.

Establishing the thermal window for aerobic scope in New Zealand geoduck clams (Panopea zelandica)

Geoduck clams (Panopea spp.) are the longest-lived and largest deep burrowing bivalve. Their unique morphology allows them to live buried in the sediment at depths of up to 1 m. The endemic New Zealand geoduck (Panopea zelandica Quoy and Gaimard, 1835) has recently been identified as a potential species for aquaculture. However, very little is known about the biology and physiology of this entirely subtidal geoduck species. Currently, the New Zealand geoduck fishery relies entirely upon wild harvests, but farms are expected to emerge as cultivation protocols are established. A key step in the optimization of cultivation procedures is the identification of optimal temperature and food rations. One method for establishing thermal optima is to identify the temperature window that supports the widest aerobic scope: the degree to which metabolic rate can be increased to support elevated activity demands. Thus, we investigated the aerobic scope for activity at five different temperatures representative of typical environmental conditions (8, 11, 15, 19, and 23 °C) for juvenile and young adult P. zelandica. Clearance rate was also measured at all temperatures. Comparisons of aerobic scope for activity and clearance rates between size classes revealed that juvenile geoducks had a narrower thermal optimum than young adults (15-19 versus 11-19 °C, respectively). Temperatures higher than 19 °C resulted in a reduction of aerobic scope for activity and clearance rate for both juvenile and young adults, which may lead to reduced performance and elevated mortality. These findings provide the first measures of aerobic scope in P. zelandica, a key step towards a meaningful understanding of the ecophysiology of this unusual species.

Seasonal proliferation rates and the capacity to express genes involved in cell cycling and maintenance in response to seasonal and experimental food shortage in Laternula elliptica from King George Island

Melting of coastal glaciers at the West Antarctic Peninsula (WAP) causes shorter winter sea ice duration, intensified ice scouring, sediment erosion and surface freshening in summer, which alters coastal productivity and feeding conditions for the benthos. The soft shell clam Laternula elliptica is a fast growing and abundant filter feeder in coastal Antarctica and a key element for bentho-pelagic carbon recycling. Our aim was to assess the cellular growth and maintenance capacity of small and large clams during natural winter food shortage (seasonal sampling) and in response to experimental starvation exposure. We measured tissue specific proliferation rates, the expression of cell cycling genes, and the iron binding protein Le-ferritin in freshly collected specimens in spring (Nov 2008) and at the end of summer (March 2009). For the experimental approach, we focused on 14 cell cycling and metabolic genes using the same animal size groups. Mantle tissue of young bivalves was the only tissue showing accelerated proliferation in summer (1.7% of cells dividing per day in March) compared to 0.4% dividing cells in animals collected in November. In mantle, siphon and adductor muscle proliferation rates were higher in younger compared to older individuals. At transcript level, Le-cyclin D was upregulated in digestive gland of older animals collected in spring (Nov) compared to March indicating initiation of cell proliferation. Likewise, during experimental starvation Le-cyclin D expression increased in large clam digestive gland, whereas Le-cyclin D and the autophagic factor beclin1 decreased in digestive gland of smaller starved clams. The paper corroborates earlier findings of size and age dependent differences in the metabolic response and gene expression patterns in L. elliptica under energetic deprivation. Age structure of shallow water populations can potentially change due to differences in cellular response between young and old animals as environmental stress levels increase.

Age-related thermal response: the cellular resilience of juveniles

Understanding species' responses to environmental challenges is key to predicting future biodiversity. However, there is currently little data on how developmental stages affect responses and also whether universal gene biomarkers to environmental stress can be identified both within and between species. Using the Antarctic clam, Laternula elliptica, as a model species, we examined both the tissue-specific and age-related (juvenile versus mature adult) gene expression response to acute non-lethal warming (12 h at 3 °C). In general, there was a relatively muted response to this sub-lethal thermal challenge when the expression profiles of treated animals, of either age, were compared with those of 0 °C controls, with none of the "classical" stress response genes up-regulated. The expression profiles were very variable between the tissues of all animals, irrespective of age with no single transcript emerging as a universal biomarker of thermal stress. However, when the expression profiles of treated animals of the different age groups were directly compared, a very different pattern emerged. The profiles of the younger animals showed significant up-regulation of chaperone and antioxidant transcripts when compared with those of the older animals. Thus, the younger animals showed evidence of a more robust cellular response to warming. These data substantiate previous physiological analyses showing a more resilient juvenile population.

Diversification, evolution and sub-functionalization of 70kDa heat-shock proteins in two sister species of antarctic krill: differences in thermal habitats, responses and implications under climate change

BACKGROUND

A comparative thermal tolerance study was undertaken on two sister species of Euphausiids (Antarctic krills) Euphausia superba and Euphausia crystallorophias. Both are essential components of the Southern Ocean ecosystem, but occupy distinct environmental geographical locations with slightly different temperature regimes. They therefore provide a useful model system for the investigation of adaptations to thermal tolerance.

METHODOLOGY/PRINCIPAL FINDING

Initial CTmax studies showed that E. superba was slightly more thermotolerant than E. crystallorophias. Five Hsp70 mRNAs were characterized from the RNAseq data of both species and subsequent expression kinetics studies revealed notable differences in induction of each of the 5 orthologues between the two species, with E. crystallorophias reacting more rapidly than E. superba. Furthermore, analyses conducted to estimate the evolutionary rates and selection strengths acting on each gene tended to support the hypothesis that diversifying selection has contributed to the diversification of this gene family, and led to the selective relaxation on the inducible C form with its possible loss of function in the two krill species.

CONCLUSIONS

The sensitivity of the epipelagic species E. crystallorophias to temperature variations and/or its adaptation to cold is enhanced when compared with its sister species, E. superba. These results indicate that ice krill could be the first of the two species to be impacted by the warming of coastal waters of the Austral ocean in the coming years due to climate change.

Explaining bathymetric diversity patterns in marine benthic invertebrates and demersal fishes: physiological contributions to adaptation of life at depth

Bathymetric biodiversity patterns of marine benthic invertebrates and demersal fishes have been identified in the extant fauna of the deep continental margins. Depth zonation is widespread and evident through a transition between shelf and slope fauna from the shelf break to 1000 m, and a transition between slope and abyssal fauna from 2000 to 3000 m; these transitions are characterised by high species turnover. A unimodal pattern of diversity with depth peaks between 1000 and 3000 m, despite the relatively low area represented by these depths. Zonation is thought to result from the colonisation of the deep sea by shallow-water organisms following multiple mass extinction events throughout the Phanerozoic. The effects of low temperature and high pressure act across hierarchical levels of biological organisation and appear sufficient to limit the distributions of such shallow-water species. Hydrostatic pressures of bathyal depths have consistently been identified experimentally as the maximum tolerated by shallow-water and upper bathyal benthic invertebrates at in situ temperatures, and adaptation appears required for passage to deeper water in both benthic invertebrates and demersal fishes. Together, this suggests that a hyperbaric and thermal physiological bottleneck at bathyal depths contributes to bathymetric zonation. The peak of the unimodal diversity-depth pattern typically occurs at these depths even though the area represented by these depths is relatively low. Although it is recognised that, over long evolutionary time scales, shallow-water diversity patterns are driven by speciation, little consideration has been given to the potential implications for species distribution patterns with depth. Molecular and morphological evidence indicates that cool bathyal waters are the primary site of adaptive radiation in the deep sea, and we hypothesise that bathymetric variation in speciation rates could drive the unimodal diversity-depth pattern over time. Thermal effects on metabolic-rate-dependent mutation and on generation times have been proposed to drive differences in speciation rates, which result in modern latitudinal biodiversity patterns over time. Clearly, this thermal mechanism alone cannot explain bathymetric patterns since temperature generally decreases with depth. We hypothesise that demonstrated physiological effects of high hydrostatic pressure and low temperature at bathyal depths, acting on shallow-water taxa invading the deep sea, may invoke a stress-evolution mechanism by increasing mutagenic activity in germ cells, by inactivating canalisation during embryonic or larval development, by releasing hidden variation or mutagenic activity, or by activating or releasing transposable elements in larvae or adults. In this scenario, increased variation at a physiological bottleneck at bathyal depths results in elevated speciation rate. Adaptation that increases tolerance to high hydrostatic pressure and low temperature allows colonisation of abyssal depths and reduces the stress-evolution response, consequently returning speciation of deeper taxa to the background rate. Over time this mechanism could contribute to the unimodal diversity-depth pattern.

Age-dependent expression of stress and antimicrobial genes in the hemocytes and siphon tissue of the Antarctic bivalve, Laternula elliptica, exposed to injury and starvation

Increasing temperatures and glacier melting at the Western Antarctic Peninsula (WAP) are causing rapid changes in shallow coastal and shelf systems. Climate change-related rising water temperatures, enhanced ice scouring, as well as coastal sediment runoff, in combination with changing feeding conditions and microbial community composition, will affect all elements of the nearshore benthic ecosystem, a major component of which is the Antarctic soft-shelled clam Laternula elliptica. A 454-based RNA sequencing was carried out on tissues and hemocytes of L. elliptica, resulting in 42,525 contigs, of which 48 % was assigned putative functions. Changes in the expression of putative stress response genes were then investigated in hemocytes and siphon tissue of young and old animals subjected to starvation and injury experiments in order to investigate their response to sedimentation (food dilution and starvation) and iceberg scouring (injury). Analysis of antioxidant defense (Le-SOD and Le-catalase), wound repair (Le-TIMP and Le-chitinase), and stress and immune response (Le-HSP70, Le-actin, and Le-theromacin) genes revealed that most transcripts were more clearly affected by injury rather than starvation. The upregulation of these genes was particularly high in the hemocytes of young, fed individuals after acute injury. Only minor changes in expression were detected in young animals under the selected starvation conditions and in older individuals. The stress response of L. elliptica thus depends on the nature of the environmental cue and on age. This has consequences for future population predictions as the environmental changes at the WAP will differentially impact L. elliptica age classes and is bound to alter population structure.

Hypoxia impacts large adults first: consequences in a warming world

Future oceans are predicted to contain less oxygen than at present. This is because oxygen is less soluble in warmer water and predicted stratification will reduce mixing. Hypoxia in marine environments is thus likely to become more widespread in marine environments and understanding species-responses is important to predicting future impacts on biodiversity. This study used a tractable model, the Antarctic clam, Laternula elliptica, which can live for 36 years, and has a well-characterized ecology and physiology to understand responses to hypoxia and how the effect varied with age. Younger animals had a higher condition index, higher adenylate energy charge and transcriptional profiling indicated that they were physically active in their response to hypoxia, whereas older animals were more sedentary, with higher levels of oxidative damage and apoptosis in the gills. These effects could be attributed, in part, to age-related tissue scaling; older animals had proportionally less contractile muscle mass and smaller gills and foot compared with younger animals, with consequential effects on the whole-animal physiological response. The data here emphasize the importance of including age effects, as large mature individuals appear to be less able to resist hypoxic conditions and this is the size range that is the major contributor to future generations. Thus, the increased prevalence of hypoxia in future oceans may have marked effects on benthic organisms' abilities to persist and this is especially so for long-lived species when predicting responses to environmental perturbation.

Juveniles Are More Resistant to Warming than Adults in 4 Species of Antarctic Marine Invertebrates

Juvenile stages are often thought to be less resistant to thermal challenges than adults, yet few studies make direct comparisons using the same methods between different life history stages. We tested the resilience of juvenile stages compared to adults in 4 species of Antarctic marine invertebrate over 3 different rates of experimental warming. The species used represent 3 phyla and 4 classes, and were the soft-shelled clam Laternula elliptica, the sea cucumber Cucumaria georgiana, the sea urchin Sterechinus neumayeri, and the seastar Odontaster validus. All four species are widely distributed, locally abundant to very abundant and are amongst the most important in the ecosystem for their roles. At the slowest rate of warming used (1°C 3 days⁻¹) juveniles survived to higher temperatures than adults in all species studied. At the intermediate rate (1°C day⁻¹) juveniles performed better in 3 of the 4 species, with no difference in the 4^th, and at the fastest rate of warming (1°C h⁻¹) L. elliptica adults survived to higher temperatures than juveniles, but in C. georgiana juveniles survived to higher temperatures than adults and there were no differences in the other species. Oxygen limitation may explain the better performance of juveniles at the slower rates of warming, whereas the loss of difference between juveniles and adults at the fastest rate of warming suggests another mechanism sets the temperature limit here.

Iceberg scour and shell damage in the Antarctic bivalve Laternula elliptica

We document differences in shell damage and shell thickness in a bivalve mollusc (Laternula elliptica) from seven sites around Antarctica with differing exposures to ice movement. These range from 60% of the sea bed impacted by ice per year (Hangar Cove, Antarctic Peninsula) to those protected by virtually permanent sea ice cover (McMurdo Sound). Patterns of shell damage consistent with blunt force trauma were observed in populations where ice scour frequently occurs; damage repair frequencies and the thickness of shells correlated positively with the frequency of iceberg scour at the different sites with the highest repair rates and thicker shells at Hangar Cove (74.2% of animals damaged) compared to the other less impacted sites (less than 10% at McMurdo Sound). Genetic analysis of population structure using Amplified Fragment Length Polymorphisms (AFLPs) revealed no genetic differences between the two sites showing the greatest difference in shell morphology and repair rates. Taken together, our results suggest that L. elliptica exhibits considerable phenotypic plasticity in response to geographic variation in physical disturbance.

Why might they be giants? Towards an understanding of polar gigantism

Beginning with the earliest expeditions to the poles, over 100 years ago, scientists have compiled an impressive list of polar taxa whose body sizes are unusually large. This phenomenon has become known as ‘polar gigantism’. In the intervening years, biologists have proposed a multitude of hypotheses to explain polar gigantism. These hypotheses run the gamut from invoking release from physical and physiological constraints, to systematic changes in developmental trajectories, to community-level outcomes of broader ecological and evolutionary processes. Here we review polar gigantism and emphasize two main problems. The first is to determine the true strength and generality of this pattern: how prevalent is polar gigantism across taxonomic units? Despite many published descriptions of polar giants, we still have a poor grasp of whether these species are unusual outliers or represent more systematic shifts in distributions of body size. Indeed, current data indicate that some groups show gigantism at the poles whereas others show nanism. The second problem is to identify underlying mechanisms or processes that could drive taxa, or even just allow them, to evolve especially large body size. The contenders are diverse and no clear winner has yet emerged. Distinguishing among the contenders will require better sampling of taxa in both temperate and polar waters and sustained efforts by comparative physiologists and evolutionary ecologists in a strongly comparative framework.

Individual to community-level faunal responses to environmental change from a marine fossil record of Early Miocene global warming

Modern climate change has a strong potential to shift earth systems and biological communities into novel states that have no present-day analog, leaving ecologists with no observational basis to predict the likely biotic effects. Fossil records contain long time-series of past environmental changes outside the range of modern observation, which are vital for predicting future ecological responses, and are capable of (a) providing detailed information on rates of ecological change, (b) illuminating the environmental drivers of those changes, and (c) recording the effects of environmental change on individual physiological rates. Outcrops of Early Miocene Newport Member of the Astoria Formation (Oregon) provide one such time series. This record of benthic foraminiferal and molluscan community change from continental shelf depths spans a past interval environmental change (≈ 20.3-16.7 mya) during which the region warmed 2.1-4.5°C, surface productivity and benthic organic carbon flux increased, and benthic oxygenation decreased, perhaps driven by intensified upwelling as on the modern Oregon coast. The Newport Member record shows that (a) ecological responses to natural environmental change can be abrupt, (b) productivity can be the primary driver of faunal change during global warming, (c) molluscs had a threshold response to productivity change while foraminifera changed gradually, and (d) changes in bivalve body size and growth rates parallel changes in taxonomic composition at the community level, indicating that, either directly or indirectly through some other biological parameter, the physiological tolerances of species do influence community change. Ecological studies in modern and fossil records that consider multiple ecological levels, environmental parameters, and taxonomic groups can provide critical information for predicting future ecological change and evaluating species vulnerability.

Upper temperature limits of tropical marine ectotherms: global warming implications

Animal physiology, ecology and evolution are affected by temperature and it is expected that community structure will be strongly influenced by global warming. This is particularly relevant in the tropics, where organisms are already living close to their upper temperature limits and hence are highly vulnerable to rising temperature. Here we present data on upper temperature limits of 34 tropical marine ectotherm species from seven phyla living in intertidal and subtidal habitats. Short term thermal tolerances and vertical distributions were correlated, i.e., upper shore animals have higher thermal tolerance than lower shore and subtidal animals; however, animals, despite their respective tidal height, were susceptible to the same temperature in the long term. When temperatures were raised by 1°C hour(-1), the upper lethal temperature range of intertidal ectotherms was 41-52°C, but this range was narrower and reduced to 37-41°C in subtidal animals. The rate of temperature change, however, affected intertidal and subtidal animals differently. In chronic heating experiments when temperature was raised weekly or monthly instead of every hour, upper temperature limits of subtidal species decreased from 40°C to 35.4°C, while the decrease was more than 10°C in high shore organisms. Hence in the long term, activity and survival of tropical marine organisms could be compromised just 2-3°C above present seawater temperatures. Differences between animals from environments that experience different levels of temperature variability suggest that the physiological mechanisms underlying thermal sensitivity may vary at different rates of warming.

Limits to diffusive O2 transport: flow, form, and function in nudibranch egg masses from temperate and polar regions

Background

Many aquatic animals enclose embryos in gelatinous masses, and these embryos rely on diffusion to supply oxygen. Mass structure plays an important role in limiting or facilitating O₂ supply, but external factors such as temperature and photosynthesis can play important roles as well. Other external factors are less well understood.

Methodology/Principal Findings

We first explored the effects of water flow on O₂ levels inside nudibranch embryo masses and compared the effects of flow on masses from temperate and polar regions. Water flow (still vs. vigorously bubbled) had a strong effect on central O₂ levels in all masses; in still water, masses were considerably more hypoxic than in bubbled water. This effect was stronger in temperate than in polar masses, likely due to the increased metabolic demand and O₂ consumption of temperate masses. Second, we made what are to our knowledge the first measurements of O₂ in invertebrate masses in the field. Consistent with laboratory experiments, O₂ in Antarctic masses was high in masses in situ, suggesting that boundary-layer effects do not substantially limit O₂ supply to polar embryos in the field.

Conclusions/Significance

All else being equal, boundary layers are more likely to depress O₂ in masses in temperate or tropical regions; thus, selection on parents to choose high-flow sites for mass deposition is likely greater in warm water. Because of the large number of variables affecting diffusive O₂ supply to embryos in their natural environment, field observations are necessary to test hypotheses generated from laboratory experiments and mathematical modeling.

Insights into shell deposition in the Antarctic bivalve Laternula elliptica: gene discovery in the mantle transcriptome using 454 pyrosequencing

Background

The Antarctic clam, Laternula elliptica, is an infaunal stenothermal bivalve mollusc with a circumpolar distribution. It plays a significant role in bentho-pelagic coupling and hence has been proposed as a sentinel species for climate change monitoring. Previous studies have shown that this mollusc displays a high level of plasticity with regard to shell deposition and damage repair against a background of genetic homogeneity. The Southern Ocean has amongst the lowest present-day CaCO₃ saturation rate of any ocean region, and is predicted to be among the first to become undersaturated under current ocean acidification scenarios. Hence, this species presents as an ideal candidate for studies into the processes of calcium regulation and shell deposition in our changing ocean environments.

Results

454 sequencing of L. elliptica mantle tissue generated 18,290 contigs with an average size of 535 bp (ranging between 142 bp-5.591 kb). BLAST sequence similarity searching assigned putative function to 17% of the data set, with a significant proportion of these transcripts being involved in binding and potentially of a secretory nature, as defined by GO molecular function and biological process classifications. These results indicated that the mantle is a transcriptionally active tissue which is actively proliferating. All transcripts were screened against an in-house database of genes shown to be involved in extracellular matrix formation and calcium homeostasis in metazoans. Putative identifications were made for a number of classical shell deposition genes, such as tyrosinase, carbonic anhydrase and metalloprotease 1, along with novel members of the family 2 G-Protein Coupled Receptors (GPCRs). A membrane transport protein (SEC61) was also characterised and this demonstrated the utility of the clam sequence data as a resource for examining cold adapted amino acid substitutions. The sequence data contained 46,235 microsatellites and 13,084 Single Nucleotide Polymorphisms(SNPs/INDELS), providing a resource for population and also gene function studies.

Conclusions

This is the first 454 data from an Antarctic marine invertebrate. Sequencing of mantle tissue from this non-model species has considerably increased resources for the investigation of the processes of shell deposition and repair in molluscs in a changing environment. A number of promising candidate genes were identified for functional analyses, which will be the subject of further investigation in this species and also used in model-hopping experiments in more tractable and economically important model aquaculture species, such as Crassostrea gigas and Mytilus edulis.

Transcription profiling of acute temperature stress in the Antarctic plunderfish Harpagifer antarcticus

Harpagifer antarcticus (the Antarctic plunderfish), a shallow-water benthic fish distributed around the Antarctic Peninsula, is a member of the notothenioid family, one of whose adaptations to the cold waters of Antarctica has been the loss of the classic heat shock response. In order to gain a more comprehensive understanding of the effects of temperature stress on H. antarcticus, we constructed a liver cDNA library and a 10,371 feature microarray. This was hybridized with material from a time course series of animals held at 6°C for 48h. The resulting expression profiles show that this fish displays the classical vertebrate acute inflammatory response. There was also a pronounced signal for increased energy requirements via up-regulation of genes involved in the β oxidation of fatty acids and also a strong signature of response to oxidative stress. Genes in the latter category did not include the "classic" antioxidants such as glutathione S-transferase, but genes involved in the production of reducing potential in the form of NADPH, peroxisome proliferation via peroxisomal acyl co-enzyme A oxidase 1 and genes known to be up-regulated by hypoxia-inducible factor 1 (HIF1). These identifications provide clear support for oxygen being the whole animal limiting factor at least in acute short-term temperature challenges. The classical heat shock proteins were not up-regulated during this trial, although numerous clones for each were present on the gene chip, confirming the lack of this response in this species. These data significantly increase our knowledge of the cellular stress response from animals in this unique environment.

Oxygen hypothesis of polar gigantism not supported by performance of Antarctic pycnogonids in hypoxia

Compared to temperate and tropical relatives, some high-latitude marine species are large-bodied, a phenomenon known as polar gigantism. A leading hypothesis on the physiological basis of gigantism posits that, in polar water, high oxygen availability coupled to low metabolic rates relieves constraints on oxygen transport and allows the evolution of large body size. Here, we test the oxygen hypothesis using Antarctic pycnogonids, which have been evolving in very cold conditions (-1.8-0 degrees C) for several million years and contain spectacular examples of gigantism. Pycnogonids from 12 species, spanning three orders of magnitude in body mass, were collected from McMurdo Sound, Antarctica. Individual sea spiders were forced into activity and their performance was measured at different experimental levels of dissolved oxygen (DO). The oxygen hypothesis predicts that, all else being equal, large pycnogonids should perform disproportionately poorly in hypoxia, an outcome that would appear as a statistically significant interaction between body size and oxygen level. In fact, although we found large effects of DO on performance, and substantial interspecific variability in oxygen sensitivity, there was no evidence for sizexDO interactions. These data do not support the oxygen hypothesis of Antarctic pycnogonid gigantism and suggest that explanations must be sought in other ecological or evolutionary processes.

Geographical variation in thermal tolerance within Southern Ocean marine ectotherms

Latitudinal comparisons of the Southern Ocean limpet, Nacella concinna, and clam, Laternula elliptica, acclimated to 0.0 degrees C, were used to assess differences in thermal response to two regimes, 0.0, 5.1 to 10.0 degrees C and 2.5, 7.5 to 12.5 degrees C, raised at 5.0 degrees C per week. At each temperature, tissue energy status was measured through a combination of O(2) consumption, intracellular pH, cCO(2), citrate synthase (CS) activity, organic acids (succinate, acetate, propionate), adenylates (ATP, ADP, AMP, ITP, PLA (phospho-L-arginine)) and heart rate. L. elliptica from Signy (60 degrees S) and Rothera (67 degrees S), which experience a similar thermal regime (-2 to +1 degrees C) had the same lethal (7.5-10.0 degrees C), critical (5.1-7.5 degrees C) and pejus (<5.1 degrees C;=getting worse) limits with only small differences in biochemical response. N. concinna, which experiences a wider thermal regime (-2 to +15.8 degrees C), had higher lethal limits (10.0-12.5 degrees C). However, at their Northern geographic limit N. concinna, which live in a warmer environment (South Georgia, 54 degrees S), had a lower critical limit (5.1-10.0 degrees C; O(2), PLA and organic acids) than Rothera and Signy N. concinna (10.0-12.5 degrees C). This lower limit indicates that South Georgia N. concinna have different biochemical responses to temperatures close to their thermal limit, which may make them more vulnerable to future warming trends.