Acid dulls the senses: impaired locomotion and foraging performance in a marine mollusc

From reproductive behaviour to responses to predators: Ocean acidification does not impact the behaviour of an herbivorous marine gastropod

Ocean acidification (OA), which reduces ocean pH and leads to substantial changes in seawater carbonate chemistry, may strongly impact organisms, especially those with carbonate skeletons. In marine molluscs, while the physiological effects of OA are well known, with a reduction of growth and shell calcification, there are few studies on behavioural effects. A large marine gastropod, Haliotis tuberculata, was exposed to ambient (pHT 8.0) or low pH (pHT 7.7) during a 5-month experiment. Because animal fitness can be affected through various behavioural changes, a broad spectrum of behavioural parameters was investigated, including situations involving no stress, responses to predators, righting to evaluate indirectly the level of energy reserves, and finally, reproductive behaviour. In addition, we measured the expression profile of the GABA A-like and serotonin receptor genes, often described as central neuromodulators of sensory performance and behaviour and known to be affected by OA in molluscs. No significant effect of low pH as compared to ambient pH was observed on abalone behaviour for any of these behavioural traits or gene expressions after either one week or several months of exposure to OA. The significance tests were corroborated by estimating the size of pH effects. The behaviour of this mollusc appears not to be affected by pH decrease expected by the end of the century, suggesting some resilience of the species to OA at the adult stage. This is probably related to the ecological niche of this abalone, where important pH variations can be observed at tidal, diurnal or seasonal scales.

Acid times in physiology: A systematic review of the effects of ocean acidification on calcifying invertebrates

The reduction in seawater pH from rising levels of carbon dioxide (CO₂) in the oceans has been recognized as an important force shaping the future of marine ecosystems. Therefore, numerous studies have reported the effects of ocean acidification (OA) in different compartments of important animal groups, based on field and/or laboratory observations. Calcifying invertebrates have received considerable attention in recent years. In the present systematic review, we have summarized the physiological responses to OA in coral, echinoderm, mollusk, and crustacean species exposed to predicted ocean acidification conditions in the near future. The Scopus, Web of Science, and PubMed databases were used for the literature search, and 75 articles were obtained based on the inclusion criteria. Six main physiological responses have been reported after exposure to low pH. Growth (21.6%), metabolism (20.8%), and acid-base balance (17.6%) were the most frequent among the phyla, while calcification and growth were the physiological responses most affected by OA (>40%). Studies show that the reduction of pH in the aquatic environment, in general, supports the maintenance of metabolic parameters in invertebrates, with redistribution of energy to biological functions, generating limitations to calcification, which can have severe consequences for the health and survival of these organisms. It should be noted that the OA results are variable, with inter and/or intraspecific differences. In summary, this systematic review offers important scientific evidence for establishing paradigms in the physiology of climate change in addition to gathering valuable information on the subject and future research perspectives.

Is Ocean Acidification Really a Threat to Marine Calcifiers? A Systematic Review and Meta-Analysis of 980+ Studies Spanning Two Decades

Ocean acidification is considered detrimental to marine calcifiers, but mounting contradictory evidence suggests a need to revisit this concept. This systematic review and meta-analysis aim to critically re-evaluate the prevailing paradigm of negative effects of ocean acidification on calcifiers. Based on 5153 observations from 985 studies, many calcifiers (e.g., echinoderms, crustaceans, and cephalopods) are found to be tolerant to near-future ocean acidification (pH ≈ 7.8 by the year 2100), but coccolithophores, calcifying algae, and corals appear to be sensitive. Calcifiers are generally more sensitive at the larval stage than adult stage. Over 70% of the observations in growth and calcification are non-negative, implying the acclimation capacity of many calcifiers to ocean acidification. This capacity can be mediated by phenotypic plasticity (e.g., physiological, mineralogical, structural, and molecular adjustments), transgenerational plasticity, increased food availability, or species interactions. The results suggest that the impacts of ocean acidification on calcifiers are less deleterious than initially thought as their adaptability has been underestimated. Therefore, in the forthcoming era of ocean acidification research, it is advocated that studying how marine organisms persist is as important as studying how they perish, and that future hypotheses and experimental designs are not constrained within the paradigm of negative effects.

Microplastics can aggravate the impact of ocean acidification on the health of mussels: Insights from physiological performance, immunity and byssus properties

Ocean acidification may increase the risk of disease outbreaks that would challenge the future persistence of marine organisms if their immune system and capacity to produce vital structures for survival (e.g., byssus threads produced by bivalves) are compromised by acidified seawater. These potential adverse effects may be exacerbated by microplastic pollution, which is forecast to co-occur with ocean acidification in the future. Thus, we evaluated the impact of ocean acidification and microplastics on the health of a mussel species (Mytilus coruscus) by assessing its physiological performance, immunity and byssus properties. We found that ocean acidification and microplastics not only reduced hemocyte concentration and viability due to elevated oxidative stress, but also undermined phagocytic activity of hemocytes due to lowered energy budget of mussels, which was in turn caused by the reduced feeding performance and energy assimilation. Byssus quality (strength and extensibility) and production were also reduced by ocean acidification and microplastics. To increase the chance of survival with these stressors, the mussels prioritized the synthesis of some byssus proteins (Mfp-4 and Mfp-5) to help maintain adhesion to substrata. Nevertheless, our findings suggest that co-occurrence of ocean acidification and microplastic pollution would increase the susceptibility of bivalves to infectious diseases and dislodgement risk, thereby threatening their survival and undermining their ecological contributions to the community.

Mission impossible: Reach the carrion in a lithium pollution and marine warming scenario

In this study we investigated the independent and synergistic effects of lithium (Li, 0.08 mM) contamination and the rising seawater temperature (21 °C; control- 15 °C) on survival and trophic interactions (foraging behaviour, success, search time, carrion preference, feeding time, and tissue consumption-the dry weight basis) of the opportunistic intertidal scavenger Tritia neritea. Trophic interactions were assessed in a two-choice test using a Y-maze design using the same amount of two carrion species (Solen marginatus and Mytilus galloprovincialis) given to all snails simultaneously. Lithium pollution and synergestic warming have the effect of reducing the survival rate of T. neritea, triggering potential global change scenarios. The foraging behaviour of T. neritea under Li-contaminated conditions was characterised by a decrease in the snail's effectiveness in finding a carrion. Lithium changes the feeding behaviour as well as increasing the time it takes for snails to reach their food. T. neritea did not show preference for any of the carrion species offered in controls, but a shift in feeding behaviour towards more energetic carrion under Li contamination which may indicate a strategy to compensate for the greater energy expenditure necessary to survive. There were no differences in feeding time at the different treatments and regardless of the treatment tested T. neritea consumed more mussels tissue probably due to its greater palatability. Results showing foraging modifications in an intertidal scavenger mollusc in global change scenarios indicate potential changes in complex trophic interactions of marine food webs.

Long-term thermal acclimation drives adaptive physiological adjustments of a marine gastropod to reduce sensitivity to climate change

Ocean warming is predicted to challenge the persistence of a variety of marine organisms, especially when combined with ocean acidification. While temperature affects virtually all physiological processes, the extent to which thermal history mediates the adaptive capacity of marine organisms to climate change has been largely overlooked. Using populations of a marine gastropod (Turbo undulatus) with different thermal histories (cool vs. warm), we compared their physiological adjustments following exposure (8-week) to ocean acidification and warming. Compared to cool-acclimated counterparts, we found that warm-acclimated individuals had a higher thermal threshold (i.e. increased CT_max by 2 °C), which was unaffected by the exposure to ocean acidification and warming. Thermal history also strongly mediated physiological effects, where warm-acclimated individuals adjusted to warming by conserving energy, suggested by lower respiration and ingestion rates, energy budget (i.e. scope for growth) and O:N ratio. After exposure to warming, warm-acclimated individuals had higher metabolic rates and greater energy budget due to boosted ingestion rates, but such compensatory feeding disappeared when combined with ocean acidification. Overall, we suggest that thermal history can be a critical mediator of physiological performance under future climatic conditions. Given the relatively gradual rate of global warming, marine organisms may be better able to adaptively adjust their physiology to future climate than what short-term experiments currently convey.

Dominance Hierarchies in Marine Invertebrates

AbstractDominance hierarchies have been well studied in myriad terrestrial animals, but surprisingly little is known about hierarchies in marine invertebrates; examples are limited to a few species of decapod crustaceans and cephalopods. Is the marine environment less conducive to the establishment of dominance hierarchy structures, or does this just underline the lack of detailed behavioral information about most marine invertebrates? In this review, we highlight the published information about marine invertebrate dominance hierarchies, which involve ranks established through fights or displays. We focus on the method of hierarchy formation, examine the ecological implications of this population structure, and compare the habitat and behavioral characteristics of species that exhibit this behavior. Because dominance hierarchies can influence habitat use, population distributions, energetics, mating, resource exploitation, and population genetic structure, it is crucial to understand how this trait evolves and which species are likely to exhibit it. A better understanding of marine invertebrate hierarchies could change the way we think about population dynamics of some species and could have important implications for fisheries, conservation, or even modeling of social and economic inequality.

Linking energy budget to physiological adaptation: How a calcifying gastropod adjusts or succumbs to ocean acidification and warming

Accelerating CO₂ emissions have driven physico-chemical changes in the world's oceans, such as ocean acidification and warming. How marine organisms adjust or succumb to such environmental changes may be determined by their ability to balance energy intake against expenditure (i.e. energy budget) as energy supports physiological functions, including those with adaptive value. Here, we examined whether energy budget is a driver of physiological adaptability of marine calcifiers to the near-future ocean acidification and warming; i.e. how physiological energetics (respiration rate, feeding rate, energy assimilation and energy budget) relates to adjustments in shell growth and shell properties of a calcifying gastropod (Austrocochlea concamerata). We found that ocean warming boosted the energy budget of gastropods due to increased feeding rate, resulting in faster shell growth and greater shell strength (i.e. more mechanically resilient). When combined with ocean acidification, however, the gastropods had a substantial decrease in energy budget due to reduced feeding rate and energy assimilation, leading to the reduction in shell growth and shell strength. By linking energy budget to the adjustability of shell building, we revealed that energy availability can be critical to determine the physiological adaptability of marine calcifiers to the changing oceanic climate.

Hypoxia aggravates the effects of ocean acidification on the physiological energetics of the blue mussel Mytilus edulis

Apart from ocean acidification, hypoxia is another stressor to marine organisms, especially those in coastal waters. Their interactive effects of elevated CO₂ and hypoxia on the physiological energetics in mussel Mytilus edulis were evaluated. Mussels were exposed to three pH levels (8.1, 7.7, 7.3) at two dissolved oxygen levels (6 and 2 mg L^-1) and clearance rate, absorption efficiency, respiration rate, excretion rate, scope for growth and O: N ratio were measured during a14-day exposure. After exposure, all parameters (except excretion rate) were significantly reduced under low pH and hypoxic conditions, whereas excretion rate was significantly increased. Additive effects of low pH and hypoxia were evident for all parameters and low pH appeared to elicit a stronger effect than hypoxia (2.0 mg L^-1). Overall, hypoxia can aggravate the effects of acidification on the physiological energetics of mussels, and their populations may be diminished by these stressors.

Boosted nutritional quality of food by CO2 enrichment fails to offset energy demand of herbivores under ocean warming, causing energy depletion and mortality

The CO₂-boosted trophic transfer from primary producers to herbivores has been increasingly discovered at natural CO₂ vents and in laboratory experiments. Despite the emerging knowledge of this boosting effect, we do not know the extent to which it may be enhanced or dampened by ocean warming. We investigated whether ocean acidification and warming enhance the nutritional quality (C:N ratio) and energy content of turf algae, which is speculated to drive higher feeding rate, greater energy budget and eventually faster growth of herbivores. This proposal was tested by observing the physiological (feeding rate, respiration rate and energy budget) and demographic responses (growth and survival) of a common grazing gastropod (Phasianella australis) to ocean acidification and warming in a 6-month mesocosm experiment. Whilst we observed the boosting effect of ocean acidification and warming in isolation on the energy budget of herbivores by either increasing feeding rate on the more nutritious algae or increasing energy gain per feeding effort, their growth and survival were reduced by the sublethal thermal stress under ocean warming, especially when both climate change stressors were combined. This reduced growth and survival occurred as a consequence of depleted energy reserves, suggesting that the boosting effect via trophic transfer might not sufficiently compensate for the increased energy demand imposed by ocean warming. In circumstances where ocean acidification and warming create an energy demand on herbivores that outweighs the energy enhancement of their food (i.e. primary producers), the performance of herbivores to control their blooming resources likely deteriorates and thus runaway primary production ensues.

Trans-life cycle acclimation to experimental ocean acidification affects gastric pH homeostasis and larval recruitment in the sea star Asterias rubens

AIM

Experimental simulation of near-future ocean acidification (OA) has been demonstrated to affect growth and development of echinoderm larval stages through energy allocation towards ion and pH compensatory processes. To date, it remains largely unknown how major pH regulatory systems and their energetics are affected by trans-generational exposure to near-future acidification levels.

METHODS

Here, we used the common sea star Asterias rubens in a reciprocal transplant experiment comprising different combinations of OA scenarios, to study trans-generational plasticity using morphological and physiological endpoints.

RESULTS

Acclimation of adults to pH_T 7.2 (pCO₂ 3500 μatm) led to reductions in feeding rates, gonad weight and fecundity. No effects were evident at moderate acidification levels (pH_T 7.4; pCO₂ 2000 μatm). Parental pre-acclimation to pH_T 7.2 for 85 days reduced developmental rates even when larvae were raised under moderate and high pH conditions, whereas pre-acclimation to pH_T 7.4 did not alter offspring performance. Microelectrode measurements and pharmacological inhibitor studies carried out on larval stages demonstrated that maintenance of alkaline gastric pH represents a substantial energy sink under acidified conditions that may contribute up to 30% to the total energy budget.

CONCLUSION

Parental pre-acclimation to acidification levels that are beyond the pH that is encountered by this population in its natural habitat (eg, pH_T 7.2) negatively affected larval size and development, potentially through reduced energy transfer. Maintenance of alkaline gastric pH and reductions in maternal energy reserves probably constitute the main factors for a reduced juvenile recruitment of this marine keystone species under simulated OA.

Resilience of the larval slipper limpet Crepidula onyx to direct and indirect-diet effects of ocean acidification

Ocean acidification (OA) is known to directly impact larval physiology and development of many marine organisms. OA also affects the nutritional quality and palatability of algae, which are principal food sources for many types of planktonic larvae. This potential indirect effect of OA via trophic interactions, however, has not been fully explored. In this study, veligers of Crepidula onyx were exposed to different pH levels representing the ambient (as control) and low pH values (pH 7.7 and pH 7.3) for 14 days, and were fed with Isochrysis galbana cultured at these three respective pHs. pH, diet, nor their interactions had no effect on larval mortality. Decrease in pH alone had a significant effect on growth rate and shell size. Structural changes (increased porosity) in larval shells were also observed in the low pH treatments. Interactions between acidification and reduced diet quality promoted earlier settlement. Unlike other calcifying molluscs, this population of slipper limpets introduced to Hong Kong in 1960s appeared to be resilient to OA and decreased algal nutritional value. If this robustness observed in the laboratory applies to the field, competition with native invertebrates may intensify and this non-native snail could flourish in acidified coastal ecosystems.

Chemical mediation as a structuring element in marine gastropod predator-prey interactions

Some diterpenoid compounds protect the sacoglossansElysiasp. andCyerce nigricansfrom their carnivorous predator the dorid nudibranch,Gymnodorissp., unlike chemically unprotected gastropods that are consumed by this voracious nudibranch (photo Philippe Bourseiller).