Coral larvae under ocean acidification: survival, metabolism, and metamorphosis

Ocean acidification may negatively impact the early life stages of some marine invertebrates including corals. Although reduced growth of juvenile corals in acidified seawater has been reported, coral larvae have been reported to demonstrate some level of tolerance to reduced pH. We hypothesize that the observed tolerance of coral larvae to low pH may be partly explained by reduced metabolic rates in acidified seawater because both calcifying and non-calcifying marine invertebrates could show metabolic depression under reduced pH in order to enhance their survival. In this study, after 3-d and 7-d exposure to three different pH levels (8.0, 7.6, and 7.3), we found that the oxygen consumption of Acropora digitifera larvae tended to be suppressed with reduced pH, although a statistically significant difference was not observed between pH conditions. Larval metamorphosis was also observed, confirming that successful recruitment is impaired when metamorphosis is disrupted, despite larval survival. Results also showed that the metamorphosis rate significantly decreased under acidified seawater conditions after both short (2 h) and long (7 d) term exposure. These results imply that acidified seawater impacts larval physiology, suggesting that suppressed metabolism and metamorphosis may alter the dispersal potential of larvae and subsequently reduce the resilience of coral communities in the near future as the ocean pH decreases.     

Effects of Reduced pH on Macoma balthica Larvae from a System with Naturally Fluctuating pH-Dynamics

Ocean acidification is causing severe changes in the inorganic carbon balance of the oceans. The pH conditions predicted for the future oceans are, however, already regularly occurring in the Baltic Sea, and the system might thus work as an analogue for future ocean acidification scenarios. The characteristics of the Baltic Sea with low buffering capacity and large natural pH fluctuations, in combination with multiple other stressors, suggest that OA effects may be severe, but remain largely unexplored. A calcifying species potentially affected by low pH conditions is the bivalve Macoma balthica (L.). We investigated larval survival and development of M. balthica by exposing the larvae to a range of pH levels: 7.2, 7.4, 7.7 and 8.1 during 20 days in order to learn what the effects of reduced pH are on the larval biology and thus also potentially for the population dynamics of this key species. We found that even a slight pH decrease causes significant negative changes during the larval phase, both by slowing growth and by decreasing survival. The growth was slower in all reduced pH treatments compared to the control treatment. The size of 250 µm that is considered indicative to imminent settling in our system was reached by 22% of the larvae grown in control conditions after 20 days, whereas in all reduced pH treatments the size of 250 µm was reached by only 7–14%. The strong impact of ocean acidification on larvae is alarming as slowly growing individuals are exposed to higher predation risk in response to the longer time they are required to spend in the plankton, further decreasing the ecological competence of the species.     

Experimental influence of pH on the early life-stages of sea urchins I: different rates of introduction give rise to different responses

Many early life-stage response studies to ocean acidification utilize gametes/offspring obtained from ambient-sourced parents, which are then directly introduced to experimentally altered seawater pH. This approach may produce a stress response potentially impacting development and survival. Hence, this study determined whether this approach is suitable by subjecting embryos/larvae to different rates of introduction to lowered seawater pH to assess larval success under acute and staggered experimental pH scenarios. Embryos and 4-armed larvae of the sea urchin Psammechinus miliaris were introduced to pH conditions, widely used in ocean acidification studies, from ambient conditions utilizing 380, 470, 560, 700 and 840?ppm CO₂ changed at incremental steps at two rates: fast (every 3rd hour) or slow (every 48th hour). Direct transfers from ambient to low seawater pH gave rise to dramatic negative impacts (smaller size and low survival), but slower rates of introductions gave rise to lesser negative responses (low survival). There was no treatment effect on settled juveniles. Fast introductions utilized in many studies are likely not ideal approaches when assessing pre-settlement larval developmental responses. Therefore, careful consideration of the pattern of response is needed when studies report the responses of offspring, derived from ambient conditions, introduced directly to forecasted ocean acidification conditions.     

Vulnerability of the calcifying larval stage of the Antarctic sea urchin Sterechinus neumayeri to near‐future ocean acidification and warming

Stenothermal polar benthic marine invertebrates are highly sensitive to environmental perturbations but little is known about potential synergistic effects of concurrent ocean warming and acidification on development of their embryos and larvae. We examined the effects of these stressors on development to the calcifying larval stage in the Antarctic sea urchin Sterechinus neumayeri in embryos reared in present and future (2100+) ocean conditions from fertilization. Embryos were reared in 2 temperature (ambient: ?1.0 °C, + 2 °C : 1.0 °C) and 3 pH (ambient: pH 8.0, ?0.2–0.4 pH units: 7.8,7.6) levels. Principle coordinates analysis on five larval metrics showed a significant effect of temperature and pH on the pattern of growth. Within each temperature, larvae were separated by pH treatment, a pattern primarily influenced by larval arm and body length. Growth was accelerated by temperature with a 20–28% increase in postoral (PO) length at +2 °C across all pH levels. Growth was strongly depressed by reduced pH with a 8–19% decrease in PO length at pH 7.6–7.8 at both temperatures. The boost in growth caused by warming resulted in larvae that were larger than would be observed if acidification was examined in the absence of warming. However, there was no significant interaction between these stressors. The increase in left‐right asymmetry and altered body allometry indicated that decreased pH disrupted developmental patterning and acted as a teratogen (agent causing developmental malformation). Decreased developmental success with just a 2 °C warming indicates that development in S. neumayeri is particularly sensitive to increased temperature. Increased temperature also altered larval allometry. Altered body shape impairs swimming and feeding in echinoplutei. In the absence of adaptation, it appears that the larval phase may be a bottleneck for survivorship of S. neumayeri in a changing ocean in a location where poleward migration to escape inhospitable conditions is not possible.     

CO2-induced fertilization impairment in Strongylocentrotus droebachiensis collected in the Arctic

Fertilization depends on distribution and aggregation patterns of sea urchins which influence gamete contact time and may potentially enhance their vulnerability to ocean acidification. In this study, we conducted fertilization experiments to assess the effects of selected pH scenarios on fertilization success of Strongylocentrotus droebachiensis, from Spitsbergen, Arctic. Acidification was achieved by aerating seawater with different CO₂ partial pressures to represent pre-industrial and present conditions (measured ~180–425 µatm) and future acidification scenarios (~550–800, ~1,300, ~2,000 µatm). Fertilization success was defined as the proportion of successful/unsuccessful fertilizations per treatment; eggs were classified according to features of their fertilization envelope (FE), hyaline layer (HL) and achievement of cellular division. The diagnostic findings of specific pathological aberrations were described in detail. We additionally measured intracellular pH changes in unfertilized eggs exposed for 1 h to selected acidification treatments using BCECF/AM. We conclude that (a) acidified conditions increase the proportion of eggs that failed fertilization, (b) acidification may increase the risk of polyspermy due to failures in the FE formation supported by the occasional observation of multiple sperms in the perivitelline space and (c) irregular formation of the embryo may arise due to impaired formation of the HL. The decrease in fertilization success could be also related to the observed changes in intracellular pH at pCO₂ ~ 1,000 μatm or higher.     

Calcification, growth and mortality of juvenile clams Ruditapes decussatus under increased pCO2 and reduced pH: Variable responses to ocean acidification at local scales?

We investigated the effects of ocean acidification on juvenile clams Ruditapes decussatus (average shell length 10.24 mm) in a controlled CO₂ perturbation experiment. The carbonate chemistry of seawater was manipulated by diffusing pure CO₂, to attain two reduced pH levels (by −0.4 and −0.7 pH units), which were compared to unmanipulated seawater. After 75 days we found no differences among pH treatments in terms of net calcification, size or weight of the clams. The naturally elevated total alkalinity of local seawater probably contributed to buffer the effects of increased pCO₂ and reduced pH. Marine organisms may, therefore, show diverse responses to ocean acidification at local scales, particularly in coastal, estuarine and transitional waters, where the physical-chemical characteristics of seawater are most variable. Mortality was significantly reduced in the acidified treatments. This trend was probably related to the occurrence of spontaneous spawning events in the control and intermediate acidification treatments. Spawning, which was unexpected due to the small size of the clams, was not observed for the pH −0.7 treatment, suggesting that the increased survival under acidified conditions may have been associated with a delay in the reproductive cycle of the clams. Future research about the impacts of ocean acidification on marine biodiversity should be extended to other types of biological and ecological processes, apart from biological calcification.     

Effects of ocean acidification on the early life history processes of the breadcrumb sponge Halichondria panicea

Ocean acidification (OA) is predicted to result in reduced survival, growth, reproduction, and overall biodiversity of marine invertebrates, and yet we lack information about the response to OA of some major groups of marine organisms. In particular, we know relatively little about how OA will impact temperate sponges, which will experience more extreme low pH conditions than tropical species. In this study, we quantified OA-induced changes in early life history patterns (larval mortality and condition, settlement rate, recruit survival, and size) in the non-calcifying breadcrumb sponge Halichondria panicea collected from a temperate intertidal site in the California Current Large Marine Ecosystem. Sponge larvae were exposed to OA conditions for 15 days, and early life history patterns were observed. Compared with baseline (“present”) conditions, larval mortality and settlement rates increased in the acidified treatment (“future”). This effect was restricted to larval stages; treatment had no effect on the growth and survival of recruits. This study is significant in that it shows that H. panicea may be particularly vulnerable to changes in ocean pH during the larval stage, which could ultimately reduce total sponge abundance by diminishing the number of larvae that survive to settlement.     

TESTING THE EFFECTS OF OCEAN ACIDIFICATION ON ALGAL METABOLISM: CONSIDERATIONS FOR EXPERIMENTAL DESIGNS1

Ocean acidification describes changes in the carbonate chemistry of the ocean due to the increased absorption of anthropogenically released CO₂. Experiments to elucidate the biological effects of ocean acidification on algae are not straightforward because when pH is altered, the carbon speciation in seawater is altered, which has implications for photosynthesis and, for calcifying algae, calcification. Furthermore, photosynthesis, respiration, and calcification will themselves alter the pH of the seawater medium. In this review, algal physiologists and seawater carbonate chemists combine their knowledge to provide the fundamental information on carbon physiology and seawater carbonate chemistry required to comprehend the complexities of how ocean acidification might affect algae metabolism. A wide range in responses of algae to ocean acidification has been observed, which may be explained by differences in algal physiology, timescales of the responses measured, study duration, and the method employed to alter pH. Two methods have been widely used in a range of experimental systems: CO₂ bubbling and HCl/NaOH additions. These methods affect the speciation of carbonate ions in the culture medium differently; we discuss how this could influence the biological responses of algae and suggest a third method based on HCl/NaHCO₃ additions. We then discuss eight key points that should be considered prior to setting up experiments, including which method of manipulating pH to choose, monitoring during experiments, techniques for adding acidified seawater, biological side effects, and other environmental factors. Finally, we consider incubation timescales and prior conditioning of algae in terms of regulation, acclimation, and adaptation to ocean acidification.     

Climate change and coral reef connectivity

This review assesses and predicts the impacts that rapid climate change will have on population connectivity in coral reef ecosystems, using fishes as a model group. Increased ocean temperatures are expected to accelerate larval development, potentially leading to reduced pelagic durations and earlier reef-seeking behaviour. Depending on the spatial arrangement of reefs, the expectation would be a reduction in dispersal distances and the spatial scale of connectivity. Small increase in temperature might enhance the number of larvae surviving the pelagic phase, but larger increases are likely to reduce reproductive output and increase larval mortality. Changes to ocean currents could alter the dynamics of larval supply and changes to planktonic productivity could affect how many larvae survive the pelagic stage and their condition at settlement; however, these patterns are likely to vary greatly from place-to-place and projections of how oceanographic features will change in the future lack sufficient certainty and resolution to make robust predictions. Connectivity could also be compromised by the increased fragmentation of reef habitat due to the effects of coral bleaching and ocean acidification. Changes to the spatial and temporal scales of connectivity have implications for the management of coral reef ecosystems, especially the design and placement of marine-protected areas. The size and spacing of protected areas may need to be strategically adjusted if reserve networks are to retain their efficacy in the future.     

A broad transition zone between an inner Baltic hybrid swarm and a pure North Sea subspecies of Macoma balthica (Mollusca, Bivalvia)

The populations of the bivalve clam Macoma balthica in the low-salinity Northern Baltic Sea represent an admixture of two strongly diverged genomic origins, the Pacific Macoma balthica balthica (approx. 60% genomic contribution) and Atlantic Macoma balthica rubra (40%). Using allozyme and mtDNA characters, we describe the broad transition from this hybrid swarm to the pure M. b. rubra in the saline North Sea waters, spanning hundreds of kilometre distance. The zone is centred in the strong salinity gradient of the narrow Öresund strait and in the adjacent Western Baltic. Yet the multilocus clines show no simple and smoothly monotonic gradation: they involve local reversals and strong differences between neighbouring populations. The transitions in different characters are not strictly coincident, and the extent of introgression varies among loci. The Atlantic influence extends further into the Baltic in samples from the southern and eastern Baltic coasts than on the western coast, and further in deeper bottoms than at shallow (< 1 m) sites. This fits with the counterclockwise net circulation pattern and with a presumably weaker salinity barrier for invading Atlantic type larvae in saline deeper water, and corresponding facilitation of outwards drift of Baltic larvae in diluted surface waters. Genotypic disequilibria were strong particularly in the shallow-water samples of the steepest transition zone. This suggests larval mixing from different sources and limited interbreeding in that area, which makes a stark contrast to the evidence of thorough amalgamation of the distinct genomic origins in the inner Baltic hybrid swarm of equilibrium structure.     

Ocean acidification has little effect on developmental thermal windows of echinoderms from Antarctica to the tropics

As the ocean warms, thermal tolerance of developmental stages may be a key driver of changes in the geographical distributions and abundance of marine invertebrates. Additional stressors such as ocean acidification may influence developmental thermal windows and are therefore important considerations for predicting distributions of species under climate change scenarios. The effects of reduced seawater pH on the thermal windows of fertilization, embryology and larval morphology were examined using five echinoderm species: two polar (Sterechinus neumayeri and Odontaster validus), two temperate (Fellaster zelandiae and Patiriella regularis) and one tropical (Arachnoides placenta). Responses were examined across 12–13 temperatures ranging from ?1.1 °C to 5.7 °C (S. neumayeri), ?0.5 °C to 10.7 °C (O. validus), 5.8 °C to 27 °C (F. zelandiae), 6.0 °C to 27.1 °C (P. regularis) and 13.9 °C to 34.8 °C (A. placenta) under present‐day and near‐future (2100+) ocean acidification conditions (‐0.3 pH units) and for three important early developmental stages 1) fertilization, 2) embryo (prehatching) and 3) larval development. Thermal windows for fertilization were broad and were not influenced by a pH decrease. Embryological development was less thermotolerant. For O. validus, P. regularis and A. placenta, low pH reduced normal development, albeit with no effect on thermal windows. Larval development in all five species was affected by both temperature and pH; however, thermal tolerance was not reduced by pH. Results of this study suggest that in terms of fertilization and development, temperature will remain as the most important factor influencing species' latitudinal distributions as the ocean continues to warm and decrease in pH, and that there is little evidence of a synergistic effect of temperature and ocean acidification on the thermal control of species ranges.     

Dynamic response in the larval geoduck (Panopea generosa) proteome to elevated pCO2

Pacific geoducks (Panopea generosa) are clams found along the northeast Pacific coast where they are important components of coastal and estuarine ecosystems and a major aquaculture product. The Pacific coastline, however, is also experiencing rapidly changing ocean habitat, including significant reductions in pH. To better understand the physiological impact of ocean acidification on geoduck clams, we characterized for the first time the proteomic profile of this bivalve during larval development and compared it to that of larvae exposed to low pH conditions. Geoduck larvae were reared at pH 7.5 (ambient) or pH 7.1 in a commercial shellfish hatchery from day 6 to day 19 postfertilization and sampled at six time points for an in‐depth proteomics analysis using high‐resolution data‐dependent analysis. Larvae reared at low pH were smaller than those reared at ambient pH, especially in the prodissoconch II phase of development, and displayed a delay in their competency for settlement. Proteomic profiles revealed that metabolic, cell cycle, and protein turnover pathways differed between the two pH and suggested that differing phenotypic outcomes between pH 7.5 and 7.1 are likely due to environmental disruptions to the timing of physiological events. In summary, ocean acidification results in elevated energetic demand on geoduck larvae, resulting in delayed development and disruptions to normal molecular developmental pathways, such as carbohydrate metabolism, cell growth, and protein synthesis.     

Experimental influence of pH on the early life-stages of sea urchins II: increasing parental exposure times gives rise to different responses

Many studies into the responses of early life-stages to ocean acidification utilise offspring obtained from parents reared under present-day conditions. Their offspring are directly introduced to altered-pH conditions. This study determined whether this approach is suitable by pre-exposing parent sea urchins (Psammechinus miliaris) to altered seawater pH (~1000?μatm) for several durations, spawning them and rearing their offspring to settlement. Parents acclimated when exposed to low seawater pH for extended periods (>42?d). Longer adult pre-exposures reduced larval survival and less competent offspring were removed from populations earlier than in controls. Control offspring were larger during earlier development stages (2–7?d), but smaller during later development stages (14?+?d) than offspring reared under low pH conditions. Juvenile settlement levels were similar across all treatments. After 17?d, offspring sourced from parents pre-exposed to low pH for 42 and 70?d were larger than those pre-exposed for 28?d and ambient sourced offspring directly transferred to low pH. These different responses show that the use of ambient derived offspring utilised in many studies is likely not an ideal approach when assessing larval development responses via morphometric measurements and survivorship prior to settlement. This study also suggests that calcifying organisms have capacities to acclimate and possibly adapt towards conditions beyond natural rates of ocean acidification.     

Empirical Evidence Reveals Seasonally Dependent Reduction in Nitrification in Coastal Sediments Subjected to Near Future Ocean Acidification

Research so far has provided little evidence that benthic biogeochemical cycling is affected by ocean acidification under realistic climate change scenarios. We measured nutrient exchange and sediment community oxygen consumption (SCOC) rates to estimate nitrification in natural coastal permeable and fine sandy sediments under pre-phytoplankton bloom and bloom conditions. Ocean acidification, as mimicked in the laboratory by a realistic pH decrease of 0.3, significantly reduced SCOC on average by 60% and benthic nitrification rates on average by 94% in both sediment types in February (pre-bloom period), but not in April (bloom period). No changes in macrofauna functional community (density, structural and functional diversity) were observed between ambient and acidified conditions, suggesting that changes in benthic biogeochemical cycling were predominantly mediated by changes in the activity of the microbial community during the short-term incubations (14 days), rather than by changes in engineering effects of bioturbating and bio-irrigating macrofauna. As benthic nitrification makes up the gross of ocean nitrification, a slowdown of this nitrogen cycling pathway in both permeable and fine sediments in winter, could therefore have global impacts on coupled nitrification-denitrification and hence eventually on pelagic nutrient availability.     

Noncalcifying larvae in a changing ocean: warming,not acidification/hypercapnia,is the dominant stressor on development of the sea star Meridiastra calcar

Climate change driven ocean warming and acidification is potentially detrimental to the sensitive planktonic life stages of benthic marine invertebrates. Research has focused on the effects of acidification on calcifying larvae with a paucity of data on species with alternate developmental strategies and on the interactive effects of warming and acidification. To determine the impact of climate change on a conspicuous component of the intertidal fauna of southeast Australia, the development of the noncalcifying lecithotrophic larvae of the sea star Meridiastra calcar was investigated in the setting of predicted ocean warming (+2 to 4 ^°C) and acidification (?0.4 to 0.6 pH units) for 2100 and beyond in all combinations of stressors. Temperature and pH were monitored in the habitat of M. calcar to place experiments in context with current environmental conditions. There was no effect of temperature or pH on cleavage stage embryos but later development (gastrula‐larvae) was negatively effected by a +2 to 4 ^°C warming and there was a negative effect of ?0.6 pH units on embryos reaching the hatched gastrula stage. Mortality and abnormal development in larvae increased significantly even with +2 ^°C warming and larval growth was impaired at +4 ^°C. For the range of temperature and pH conditions tested, there were no interactive effects of stressors across all stages monitored. For M. calcar, warming not acidification was the dominant stressor. A regression model incorporating data from this study and projected increasing SST for the region suggests an increase in larval mortality to 70% for M. calcar by 2100 in the absence of acclimation and adaptation. The broad distribution of this species in eastern Australia encompassing subtropical to cold temperate thermal regimes provides the possibility that local M. calcar populations may be sustained in a warming world through poleward migration of thermotolerant propagules, facilitated by the strong southward flow of the East Australian Current.     

Impact of Ocean Warming and Ocean Acidification on Larval Development and Calcification in the Sea Urchin Tripneustes gratilla

Background

As the oceans simultaneously warm, acidify and increase in P _CO2, prospects for marine biota are of concern. Calcifying species may find it difficult to produce their skeleton because ocean acidification decreases calcium carbonate saturation and accompanying hypercapnia suppresses metabolism. However, this may be buffered by enhanced growth and metabolism due to warming.

Methodology/Principal Findings

We examined the interactive effects of near-future ocean warming and increased acidification/P _CO2 on larval development in the tropical sea urchin Tripneustes gratilla. Larvae were reared in multifactorial experiments in flow-through conditions in all combinations of three temperature and three pH/P _CO2 treatments. Experiments were placed in the setting of projected near future conditions for SE Australia, a global change hot spot. Increased acidity/P _CO2 and decreased carbonate mineral saturation significantly reduced larval growth resulting in decreased skeletal length. Increased temperature (+3°C) stimulated growth, producing significantly bigger larvae across all pH/P _CO2 treatments up to a thermal threshold (+6°C). Increased acidity (-0.3-0.5 pH units) and hypercapnia significantly reduced larval calcification. A +3°C warming diminished the negative effects of acidification and hypercapnia on larval growth.

Conclusions and Significance

This study of the effects of ocean warming and CO₂ driven acidification on development and calcification of marine invertebrate larvae reared in experimental conditions from the outset of development (fertilization) shows the positive and negative effects of these stressors. In simultaneous exposure to stressors the dwarfing effects of acidification were dominant. Reduction in size of sea urchin larvae in a high P _CO2 ocean would likely impair their performance with negative consequent effects for benthic adult populations.     

Ontogenetic changes in responses to settlement cues by Anemonefish

Population connectivity for most marine species is dictated by dispersal during the pelagic larval stage. Although reef fish larvae are known to display behavioral adaptations that influence settlement site selection, little is known about the development of behavioral preferences throughout the larval phase. Whether larvae are attracted to the same sensory cues throughout their larval phase, or exhibit distinct ontogenetic shifts in sensory preference is unknown. Here, we demonstrate an ontogenetic shift in olfactory cue preferences for two species of anemonefish, a process that could aid in understanding both patterns of dispersal and settlement. Aquarium-bred na?ve Amphiprion percula and A. melanopus larvae were tested for olfactory preference of relevant reef-associated chemical cues throughout the 11-day pelagic larval stage. Age posthatching had a significant effect on the preference for olfactory cues from host anemones and live corals for both species. Preferences of olfactory cues from tropical plants of A. percula, increased by approximately ninefold between hatching and settlement, with A. percula larvae showing a fivefold increase in preference for the olfactory cue produced by the grass species. Larval age had no effect on the olfactory preference for untreated seawater over the swamp-based tree Melaleuca nervosa, which was always avoided compared with blank seawater. These results indicate that reef fish larvae are capable of utilizing olfactory cues early in the larval stage and may be predisposed to disperse away from reefs, with innate olfactory preferences drawing newly hatched larvae into the pelagic environment. Toward the end of the larval phase, larvae become attracted to the olfactory cues of appropriate habitats, which may assist them in identification of and navigation toward suitable settlement sites.     

Description and quantification of pteropod shell dissolution: a sensitive bioindicator of ocean acidification

Anthropogenic ocean acidification is likely to have negative effects on marine calcifying organisms, such as shelled pteropods, by promoting dissolution of aragonite shells. Study of shell dissolution requires an accurate and sensitive method for assessing shell damage. Shell dissolution was induced through incubations in CO₂‐enriched seawater for 4 and 14 days. We describe a procedure that allows the level of dissolution to be assessed and classified into three main types: Type I with partial dissolution of the prismatic layer; Type II with exposure of underlying crossed‐lamellar layer, and Type III, where crossed‐lamellar layer shows signs of dissolution. Levels of dissolution showed a good correspondence to the incubation conditions, with the most severe damage found in specimens held for 14 days in undersaturated condition (Ω ~ 0.8). This methodology enables the response of small pelagic calcifiers to acidified conditions to be detected at an early stage, thus making pteropods a valuable bioindicator of future ocean acidification.