Long-term effects of altered pH and temperature on the feeding energetics of the Antarctic sea urchin, Sterechinus neumayeri

Investigating intraspecific variability in the biological responses of sea urchins (Paracentrotus lividus) to seawater acidification

Alterations in seawater chemistry posed by acidification may lead to immunological and antioxidant defence impairment in sea urchins, with differences among local populations. Here, we analyzed the effects of reduced pH on Paracentrotus lividus, with a multibiomarker approach, and the possible intraspecific variations in sea urchin responses. Two groups of animals with different ecological histories (i.e., the pattern of environmental characteristics and pressures experienced throughout the organism's lifetime) were maintained at ambient pH and pH reduced of 0.4 units for 8 months. Changes in gonadosomatic index (GSI), immunological, and oxidative stress biomarkers were assessed in coelomic fluid, gonads, and digestive tract. Animals maintained at reduced pH showed limited impact of seawater acidification compared to the ambient pH condition. However, sea urchins from the two sites were differently influenced by the seawater pH (as shown by multivariate analyses). GSI and immunological and antioxidant status were differentially modulated between the two sexes, with generally higher values in females, but differences between sexes in relation to the pH of exposure were limited. Overall, our findings highlight that the impact of environmental stressors may differ in sea urchins from different locations. This has implications for the maintenance of P. lividus wild populations under future global change scenarios.

Behavioural and physiological impacts of low salinity on the sea urchin Echinus esculentus

Reduced seawater salinity as a result of freshwater input can exert a major influence on the ecophysiology of benthic marine invertebrates, such as echinoderms. While numerous experimental studies have explored the physiological and behavioural effects of short-term, acute exposure to low salinity in echinoids, surprisingly few have investigated the consequences of chronic exposure, or compared the two. In this study, the European sea urchin, Echinus esculentus, was exposed to low salinity over the short term (11‰, 16‰, 21‰, 26‰ and 31‰ for 24 h) and longer term (21, 26 and 31‰ for 25 days). Over the short term, oxygen consumption, activity coefficient and coelomic fluid osmolality were directly correlated with reduced salinity, with 100% survival at ≥21‰ and 0% at ≤16‰. Over the longer term at 21‰ (25 days), oxygen consumption was significantly higher, feeding was significantly reduced and activity coefficient values were significantly lower than at control salinity (31‰). At 26‰, all metrics were comparable to the control by the end of the experiment, suggesting acclimation. Furthermore, beneficial functional resistance (righting ability and metabolic capacity) to acute low salinity was observed at 26‰. Osmolality values were slightly hyperosmotic to the external seawater at all acclimation salinities, while coelomocyte composition and concentration were unaffected by chronic low salinity. Overall, E. esculentus demonstrate phenotypic plasticity that enables acclimation to reduced salinity around 26‰; however, 21‰ represents a lower acclimation threshold, potentially limiting its distribution in coastal areas prone to high freshwater input.

Impacts of ocean warming on echinoderms: A meta-analysis

Rising ocean temperatures are threatening marine species and populations worldwide, and ectothermic taxa are particularly vulnerable. Echinoderms are an ecologically important phylum of marine ectotherms and shifts in their population dynamics can have profound impacts on the marine environment. The effects of warming on echinoderms are highly variable across controlled laboratory-based studies. Accordingly, synthesis of these studies will facilitate the better understanding of broad patterns in responses of echinoderms to ocean warming. Herein, a meta-analysis incorporating the results of 85 studies (710 individual responses) is presented, exploring the effects of warming on various performance predictors. The mean responses of echinoderms to all magnitudes of warming were compared across multiple biological responses, ontogenetic life stages, taxonomic classes, and regions, facilitated by multivariate linear mixed effects models. Further models were conducted, which only incorporated responses to warming greater than the projected end-of-century mean annual temperatures at the collection sites. This meta-analysis provides evidence that ocean warming will generally accelerate metabolic rate (+32%) and reduce survival (-35%) in echinoderms, and echinoderms from subtropical (-9%) and tropical (-8%) regions will be the most vulnerable. The relatively high vulnerability of echinoderm larvae to warming (-20%) indicates that this life stage may be a significant developmental bottleneck in the near-future, likely reducing successful recruitment into populations. Furthermore, asteroids appear to be the class of echinoderms that are most negatively affected by elevated temperature (-30%). When considering only responses to magnitudes of warming representative of end-of-century climate change projections, the negative impacts on asteroids, tropical species and juveniles were exacerbated (-51%, -34% and -40% respectively). The results of these analyses enable better predictions of how keystone and invasive echinoderm species may perform in a warmer ocean, and the possible consequences for populations, communities and ecosystems.

Acclimation of the Antarctic sea urchin Sterechinus neumayeri to warmer temperatures involves a modulation of cellular machinery

Global warming is threatening marine Antarctic fauna, which has evolved in isolation in a cold environment for millions of years. Facing increasing temperatures, marine Antarctic invertebrates can either tolerate or develop adaptations to these changes. On a short timescale, their survival and resistance to warming will be driven by the efficiency of their phenotypic plasticity through their capacity for acclimation. The current study aims at evaluating the capacity for acclimation of the Antarctic sea urchin Sterechinus neumayeri to predicted ocean warming scenarios (+2, RCP 2.6 and + 4 °C, RCP 8.5, IPCC et al., 2019) and deciphering the subcellular mechanisms underlying their acclimation. A combination of transcriptomics, physiological (e.g. growth rate, gonad growth, ingestion rate and oxygen consumption), and behavioral-based approaches were used on individuals incubated at 1, 3 and, 5 °C for 22 weeks. Mortality was low at warmer temperatures (20%) and oxygen consumption and ingestion rate seemed to reach a stable state around 16 weeks suggesting that S. neumayeri might be able to acclimate to warmer temperatures (until 5 °C). Transcriptomic analyses highlighted adjustments of the cellular machinery with the activation of replication, recombination, and repair processes as well as cell cycle and division and repression of transcriptional and signal transduction mechanisms and defense processes. These results suggest that acclimation to warmer scenarios might require more than 22 weeks for the Antarctic Sea urchins S. neumayeri but that projections of climate change for the end of the century may not strongly affect the population of S. neumayeri of this part of the Antarctic.

Effects of Seawater Acidification on Echinoid Adult Stage: A Review

The continuous release of CO2 in the atmosphere is increasing the acidity of seawater worldwide, and the pH is predicted to be reduced by ~0.4 units by 2100. Ocean acidification (OA) is changing the carbonate chemistry, jeopardizing the life of marine organisms, and in particular calcifying organisms. Because of their calcareous skeleton and limited ability to regulate the acid–base balance, echinoids are among the organisms most threatened by OA. In this review, 50 articles assessing the effects of seawater acidification on the echinoid adult stage have been collected and summarized, in order to identify the most important aspects to consider for future experiments. Most of the endpoints considered (i.e., related to calcification, physiology, behaviour and reproduction) were altered, highlighting how various and subtle the effects of pH reduction can be. In general terms, more than 43% of the endpoints were modified by low pH compared with the control condition. However, animals exposed in long-term experiments or resident in CO2-vent systems showed acclimation capability. Moreover, the latitudinal range of animals’ distribution might explain some of the differences found among species. Therefore, future experiments should consider local variability, long-term exposure and multigenerational approaches to better assess OA effects on echinoids.

Interpopulational differences in the nutritional condition of Aequiyoldia eightsii (Protobranchia: Nuculanidae) from the Western Antarctic Peninsula during austral summer

The Western Antarctic Peninsula (WAP) is a hotspot for environmental change and has a strong environmental gradient from North to South. Here, for the first time we used adult individuals of the bivalve Aequiyoldia eightsii to evaluate large-scale spatial variation in the biochemical composition (measured as lipid, protein and fatty acids) and energy content, as a proxy for nutritional condition, of three populations along the WAP: O’Higgins Research Station in the north (63.3°S), Yelcho Research Station in mid-WAP (64.9°S) and Rothera Research Station further south (67.6°S). The results reveal significantly higher quantities of lipids (L), proteins (P), energy (E) and total fatty acids (FA) in the northern population (O’Higgins) (L: 8.33 ± 1.32%; P: 22.34 ± 3.16%; E: 171.53 ± 17.70 Joules; FA: 16.33 ± 0.98 mg g) than in the mid-WAP population (Yelcho) (L: 6.23 ± 0.84%; P: 18.63 ± 1.17%; E: 136.67 ± 7.08 Joules; FA: 10.93 ± 0.63 mg g) and southern population (Rothera) (L: 4.60 ± 0.51%; P: 13.11 ± 0.98%; E: 98.37 ± 5.67 Joules; FA: 7.58 ± 0.48 mg g). We hypothesize these differences in the nutritional condition could be related to a number of biological and environmental characteristics. Our results can be interpreted as a consequence of differences in phenology at each location; differences in somatic and gametogenic growth rhythms. Contrasting environmental conditions throughout the WAP such as seawater temperature, quantity and quality of food from both planktonic and sediment sources, likely have an effect on the metabolism and nutritional intake of this species.

Combined effect of microplastics and global warming factors on early growth and development of the sea urchin (Paracentrotus lividus)

The aim of this work was to estimate the potential risk of the combined effect of global change factors (acidification, temperature increase) and microplastic (MP) pollution on the growth and development of the sea urchin P. lividus. Embryo-larval bioassays were conducted to determine growth and morphology after 48 h of incubation with MP (1000 and 3000 particles/mL); with filtered sea water at pH = 7.6; and with their combinations. A second experiment was conducted to study the effect of pH and MP in combination with a temperature increase of 4 °C compared to control (20 °C). We found that the inhibition of growth in embryos reared at pH = 7.6 was around 75%. Larvae incubated at 3000 MP particles/mL showed a 20% decrease in growth compared to controls. The exposure to MP also induced an increase in the postoral arm separation or rounded vertices. The combined exposure to a pH 7.6 and MP caused a significant decrease of larval growth compared to control, to MP and to pH 7.6 treatments. Morphological alterations were observed in these treatments, including the development of only two arms. Increasing the temperature resulted in an increased growth in control, in pH 7.6 and pH 7.6 + MP3000 treatments, but the relative stomach volume decreased. However, when growth parameters were expressed per Degree-Days the lower growth provoked by the thermal stress was evidenced in all treatments. In this work we demonstrated that MP could aggravate the effect of a decreased pH and that an increase in water temperature generated an additional stress on P. lividus larvae, manifested in a lower growth and an altered development. Therefore, the combined stress caused by ocean warming, ocean acidification, and microplastic pollution, could threaten sea urchin populations leading to a potential impact on coastal ecosystems.

When site matters: Metabolic and behavioural responses of adult sea urchins from different environments during long-term exposure to seawater acidification

CO₂-driven ocean acidification (OA) affects many aspects of sea urchin biology. However, even in the same species, OA effects are often not univocal due to non-uniform exposure setups or different ecological history of the experimental specimens. In the present work, two groups of adult sea urchins Paracentrotus lividus from different environments (the Lagoon of Venice and a coastal area in the Northern Adriatic Sea) were exposed to OA in a long-term exposure. Animals were maintained for six months in both natural seawater (pH_T 8.04) and end-of-the-century predicted condition (-0.4 units pH). Monthly, physiological (respiration rate, ammonia excretion, O:N ratio) and behavioural (righting, sheltering) endpoints were investigated. Both pH and time of exposure significantly influenced sea urchin responses, but differences between sites were highlighted, particularly in the first months. Under reduced pH, ammonia excretion increased and O:N decreased in coastal specimens. Righting and sheltering were impaired in coastal animals, whereas only righting decreased in lagoon ones. These findings suggested a higher adaptation ability in sea urchins from a more variable environment. Interestingly, as the exposure continued, animals from both sites were able to acclimate. Results revealed plasticity in the physiological and behavioural responses of sea urchins under future predicted OA conditions.

Effects of ocean acidification on acid-base physiology, skeleton properties, and metal contamination in two echinoderms from vent sites in Deception Island, Antarctica

Antarctic surface waters are expected to be the first to experience severe ocean acidification (OA) with carbonate undersaturation and large decreases in pH forecasted before the end of this century. Due to the long stability in environmental conditions and the relatively low daily and seasonal variations to which they are exposed, Antarctic marine organisms, especially those with a supposedly poor machinery to eliminate CO₂ and protons and with a heavily calcified skeleton like echinoderms, are hypothesized as highly vulnerable to these environmental shifts. The opportunities offered by the natural pH gradient generated by vent activities in Deception Island caldera, Western Antarctic Peninsula, were used to investigate for the first time the acid-base physiologies, the impact of OA on the skeleton and the impact of pH on metal accumulation in the Antarctic sea star Odontaster validus and sea urchin Sterechinus neumayeri. The two species were sampled in four stations within the caldera, two at pH (total scale) 8.0-8.1 and two at reduced pH 7.8. Measured variables were pH, alkalinity, and dissolved inorganic carbon of the coelomic fluid; characteristic fracture force, stress and Young's modulus using Weibull statistics and Cd, Cu, Fe, Pb and Zn concentrations in the integument, gonads and digestive system. Recorded acid-base characteristics of both studied species fit in the general picture deduced from temperate and tropical sea stars and sea urchins but conditions and possibly confounding factors, principally food availability and quality, in the studied stations prevented definitive conclusions. Reduced seawater pH 7.8 and metals had almost no impact on the skeleton mechanical properties of the two investigated species despite very high Cd concentrations in O. validus integument. Reduced pH was correlated to increased contamination by most metals but this relation was weak. Translocation and caging experiments taking into account food parameters are proposed to better understand future processes linked to ocean acidification and metal contamination in Antarctic echinoderms.

Temperature affects the reproductive outputs of coral-eating starfish Acanthaster spp. after adult exposure to near-future ocean warming and acidification

Outbreaks of the coral-eating crown-of-thorns starfish Acanthaster spp. (COTS) have become to be amongst the most severe threats to coral reefs worldwide. Although most research has focused on COTS early development, it remains unclear how COTS populations will keep pace with changing ocean conditions. Since reproduction is a key process contributing to outbreaks, we investigated the reproductive success of adult COTS acclimated for 3-4 months to different treatment combinations of ambient conditions, ocean warming (+2 °C) and acidification (-0.35 pH). Our results suggest that the optimal breeding season in New Caledonia is concentrated around the end of the calendar year, when water temperature reaches >26 °C. We found negative effects of temperature on egg metrics, fertilisation success, and GSI, conflicting with previously documented effects of temperature on echinoderm reproductive outputs. Fertilisation success dropped drastically (more than threefold) with elevated temperature during the late breeding season. In contrast, we detected no effects of near-future acidification conditions on fertilisation success nor GSI. This is the first time that COTS reproduction is compared among individuals acclimated to different conditions of warming and acidification. Our results highlight the importance of accounting for adult exposure to better understand how COTS reproduction may be impacted in the face of global change.

Short-Term Exposure to Storm-Like Scenario Microplastic and Salinity Conditions Does not Impact Adult Sea Urchin (Arbacia punctulata) Physiology

The effects of microplastic pollution on sea urchins has received little attention despite their ecological and economical importance. This is the first study to focus on adult sea urchins (Arbacia punctulata). These organisms were exposed to storm-like sediment resuspension of microplastic concentrations (9-μm polystyrene 25,000 spheres L^-1) combined with salinity reductions (salinity 25 vs. 33) associated with high precipitation. Urchins were exposed to these parameters for 24 h before assessing righting times and for 48 h before assessing oxygen consumption rates. No significant impacts on urchin physiology were observed showing resilience to short-term exposures of storm-like induced microplastics and salinity. No microplastic particles blocked the madreporite pores indicating the active removal of particles by cilia and pedicellariae. Gut tissue samples indicated consumption of microplastics. Studies on more species are urgently required to determine their responses to plastic pollution to inform management decision-making processes.

Combined effects of warming and freshening on the physiological energetics of the edible whelk Trophon geversianus

The interacting effects of climate change pressures and human use of natural resources are increasingly affecting marine biodiversity. Variations in key abiotic factors such as temperature and salinity may therefore negatively influence marine organisms that are already threatened by intensive fisheries. Herein, we tested the hypothesis that future ocean warming and freshening will affect the fitness and survival of the overexploited snail Trophon geversianus in Southern Patagonia. To test this hypothesis, we investigated the effect of a 50 day incubation period of five temperatures (1, 5, 9, 12 and 15 °C) and two salinities (25 and 30 psu), (which correspond to current and projected conditions for Antarctic and Sub-Antarctic regions), on the physiological energetics (ingestion rate, absorption efficiency, oxygen uptake and scope for growth (SFG)) of the edible whelk T. geversianus. Our results showed no significant effects for salinity or the combination of temperature and salinity on T. geversianus bioenergetics. On the contrary, incubation at low temperatures (1 and 5 °C) was shown to affect the ingestion rate, absorption efficiency, oxygen uptake and SFG for T. geversianus, whereas for specimens incubated at 12 and 15 °C, physiological rates remained similar to control. Our data suggests that T. geversianus might be robust to warming and future variations of salinity, but longer term experiments are needed to ensure that no reduction of performance will occur after an extended incubation time from an increase in temperature.

Ocean warming and acidification alter the behavioral response to flow of the sea urchin Paracentrotus lividus

Ocean warming (OW) and acidification (OA) are intensively investigated as they pose major threats to marine organism. However, little effort is dedicated to another collateral climate change stressor, the increased frequency, and intensity of storm events, here referred to as intensified hydrodynamics. A 2-month experiment was performed to identify how OW and OA (temperature: 21°C; pHT: 7.7, 7.4; control: 17°C-pHT7.9) affect the resistance to hydrodynamics in the sea urchin Paracentrotus lividus using an integrative approach that includes physiology, biomechanics, and behavior. Biomechanics was studied under both no-flow condition at the tube foot (TF) scale and flow condition at the individual scale. For the former, TF disk adhesive properties (attachment strength, tenacity) and TF stem mechanical properties (breaking force, extensibility, tensile strength, stiffness, toughness) were evaluated. For the latter, resistance to flow was addressed as the flow velocity at which individuals detached. Under near- and far-future OW and OA, individuals fully balanced their acid-base status, but skeletal growth was halved. TF adhesive properties were not affected by treatments. Compared to the control, mechanical properties were in general improved under pHT7.7 while in the extreme treatment (21°C-pHT7.4) breaking force was diminished. Three behavioral strategies were implemented by sea urchins and acted together to cope with flow: improving TF attachment, streamlining, and escaping. Behavioral responses varied according to treatment and flow velocity. For instance, individuals at 21°C-pHT7.4 increased the density of attached TF at slow flows or controlled TF detachment at fast flows to compensate for weakened TF mechanical properties. They also showed an absence of streamlining favoring an escaping behavior as they ventured in a riskier faster movement at slow flows. At faster flows, the effects of OW and OA were detrimental causing earlier dislodgment. These plastic behaviors reflect a potential scope for acclimation in the field, where this species already experiences diel temperature and pH fluctuations.

Antarctic environmental change and biological responses

Antarctica and the surrounding Southern Ocean are facing complex environmental change. Their native biota has adapted to the region's extreme conditions over many millions of years. This unique biota is now challenged by environmental change and the direct impacts of human activity. The terrestrial biota is characterized by considerable physiological and ecological flexibility and is expected to show increases in productivity, population sizes and ranges of individual species, and community complexity. However, the establishment of non-native organisms in both terrestrial and marine ecosystems may present an even greater threat than climate change itself. In the marine environment, much more limited response flexibility means that even small levels of warming are threatening. Changing sea ice has large impacts on ecosystem processes, while ocean acidification and coastal freshening are expected to have major impacts.

Robustness of Adamussium colbecki shell to ocean acidification in a short-term exposure

Atmospheric pCO₂ has increased since the industrial revolution leading to a lowering of the ocean surface water pH, a phenomenon called ocean acidification (OA). OA is claimed to be a major threat for marine organisms and ecosystems and, particularly, for Polar regions. We explored the impact of OA on the shell mechanical properties of the Antarctic scallop Adamussium colbecki exposed for one month to acidified (pH 7.6) and natural conditions (unmanipulated littoral water), by performing Scanning Electron Microscopy, nanoindentation and Vickers indentation on the scallop shell. No effect of pH could be detected either in crystal deposition or in the mechanical properties. A. colbecki shell was found to be resistant to OA, which suggests this species to be able to face a climate change scenario that may threat the persistence of the endemic Antarctic species. Further investigation should be carried out in order to elucidate the destiny of this key species in light of global change.

The impact of ocean acidification on the gonads of three key Antarctic benthic macroinvertebrates

CO₂ atmospheric pressure is increasing since industrial revolution, leading to a lowering of the ocean surface water pH, a phenomenon known as ocean acidification, with several reported effects on individual species and cascading effects on marine ecosystems. Despite the great amount of literature on ocean acidification effects on calcifying organisms, the response of their reproductive system still remains poorly known. In the present study, we investigated the histopathological effects of low pH on the gonads of three key macroinvertebrates of the Terra Nova Bay (Ross Sea) littoral area: the sea urchin Sterechinus neumayeri, the sea star Odontaster validus and the scallop Adamussium colbecki. After 1 month of exposure at control (8.12) and reduced (7.8 and 7.6) pH levels, we dissected the gonads and performed histological analyses to detect potential differences among treatments. Results showed significant effects on reproductive conditions of A. colbecki and S. neumayeri, while O. validus did not show any kind of alteration. Present results reinforce the need to focus on ocean acidification effects on soft tissues, particularly the gonads, whose damage may exert large effects on the individual fitness, with cascading effects on the population dynamic of the species.

Long-term exposure to acidification disrupts reproduction in a marine invertebrate

Climate change research is advancing to more complex and more comprehensive studies that include long-term experiments, multiple life-history stages, multi-population, and multi-trait approaches. We used a population of the barnacle Balanus improvisus known to be sensitive to short-term acidification to determine its potential for long-term acclimation to acidification. We reared laboratory-bred individuals (as singles or pairs), and field-collected assemblages of barnacles, at pH 8.1 and 7.5 (≈ 400 and 1600 μatm pCO₂ respectively) for up to 16 months. Acidification caused strong mortality and reduced growth rates. Acidification suppressed respiration rates and induced a higher feeding activity of barnacles after 6 months, but this suppression of respiration rate was absent after 15 months. Laboratory-bred barnacles developed mature gonads only when they were held in pairs, but nonetheless failed to produce fertilized embryos. Field-collected barnacles reared in the laboratory for 8 months at the same pH’s developed mature gonads, but only those in pH 8.1 produced viable embryos and larvae. Because survivors of long-term acidification were not capable of reproducing, this demonstrates that B. improvisus can only partially acclimate to long-term acidification. This represents a clear and significant bottleneck in the ontogeny of this barnacle population that may limit its potential to persist in a future ocean.