Posts Tagged 'echinoderms'



Marine mass mortality in a global change context: impacts on individuals, populations and communities

Human actions are pushing natural systems into states that have no historical precedent. In response, empirical and theoretical researchers are increasingly focused on developing ways to predict the responses of ecological systems to change. However, significant knowledge gaps remain, often leading to “ecological surprises” where observed impacts of global change do not align with existing theory or hypotheses. In this dissertation, I study the response to perturbations of a well-characterized system for ecological research, the Northeast Pacific rocky intertidal, to advance our understanding of and ability to predict the impacts of global change on individuals, populations and communities. In 2013 and 2014, sea star species along the west coast of North America were affected by an outbreak of Sea Star Wasting Syndrome (SSWS), resulting in an epidemic of mass mortality that spanned unprecedented geographic and temporal scales and resulted in the near extirpation of multiple sea star species from many locations along the coast. One of the species that was most strongly affected in the intertidal zone was Pisaster ochraceus, an iconic predatory sea star that has the ability to play a keystone role in its community through foraging on and ultimately controlling the lower boundary of mussel prey populations. The first two chapters of this dissertation take advantage of SSWS as a “natural” form of top predator removal to assess the consequences of this type of perturbation on ecosystem resilience. In Chapter 2, I tested the hypotheses that P. ochraceus loss would facilitate a population expansion of a smaller, mesopredator sea star, Leptasterias sp., and that this expansion would have negative effects on P. ochraceus population recovery. This result would follow expectations of competitive release and aligns with existing research on the competitive relationship between these species from the Northeast Pacific intertidal. I used field surveys to track Leptasterias populations just before the onset of and up to three years after SSWS. Contrary to expectation, I did not see an increase in the distribution or density of Leptasterias, and instead saw a decrease in individual size post-SSWS. Further, I found no evidence of competition between P. ochraceus recruits and Leptasterias for resources. Thus, although my hypotheses were grounded in theory and previous research, they were not supported by data. These results suggest that Leptasterias will not provide a bottleneck for P. ochraceus population recovery from SSWS, nor compensate for lowered P. ochraceus predation. The dynamics of P. ochraceus at the recruit (early benthic juvenile) life-history stage has long been considered a gap in our understanding of the species, as recruits have been historically rare in the intertidal and hard to study. Post-SSWS, however, many sites along the coast experienced unprecedented recruitment of P. ochraceus into intertidal ecosystems. In Chapter 3, I used a field experiment to test the hypothesis that this pulse of recruitment was facilitated by SSWS-related adult loss, the consequent decrease in predation by adult P. ochraceus, and increase in prey availability for recruits. Instead of finding evidence that adults dominate recruits in food competition and inhibit recruit success, I found that recruits have a negative effect on P. ochraceus adult densities. Further, treatments where recruits were excluded and only adults had access to prey communities showed the highest control of sessile invertebrate prey populations at the end of the year-long experiment. Thus, these results suggest that adult P. ochraceus will not hinder recruit recovery, but propose a mechanism whereby high recruit densities may increase vulnerability to SSWS-induced shifts in community structure. Outbreaks of mass mortality, particularly those as widespread as SSWS, are one of many ecological challenges driven by anthropogenic environmental changes such as warming and ocean acidification. However, predicting the vulnerability of species and populations to global change is an ongoing and significant challenge for researchers and managers. In Chapter 4 I assessed whether intraspecific physiological variability could help predict P. ochraceus recruit response to ocean acidification and warming. I found that individual metabolic rate interacted with ocean acidification and food availability to drive sea star growth, and that an interaction between metabolic rate and temperature also predicted sea star predation on Mytilus spp. mussels. Thus, these results have implications not only for P. ochraceus but also for its food web interactions. Incorporating these results into predictive frameworks may improve our ability to anticipate and scale up responses to global change across levels of ecological organization. In summary, my dissertation, although chock-full of surprises, presents several paths forward for improving predictive ability in the face of accelerating anthropogenic global changes. Further, we reinforce the notion that management strategies should be cautious and anticipate ecological surprises. Predicting the future is challenging even when predictions are well-informed, particularly in environmental contexts that have never been encountered before.

Continue reading ‘Marine mass mortality in a global change context: impacts on individuals, populations and communities’

Transcriptional profiles of early stage red sea urchins (Mesocentrotus franciscanus) reveal differential regulation of gene expression across development

The red sea urchin, Mesocentrotus franciscanus, is an ecologically important kelp forest species that also serves as a valuable fisheries resource. In this study, we have assembled and annotated a developmental transcriptome for M. franciscanus that represents eggs and six stages of early development (8- to 16-cell, morula, hatched blastula, early gastrula, prism and early pluteus). Characterization of the transcriptome revealed distinct patterns of gene expression that corresponded to major developmental and morphological processes. In addition, the period during which maternally-controlled transcription was terminated and the zygotic genome was activated, the maternal-to-zygotic transition (MZT), was found to begin during early cleavage and persist through the hatched blastula stage, an observation that is similar to the timing of the MZT in other sea urchin species. The presented developmental transcriptome will serve as a useful resource for investigating, in both an ecological and fisheries context, how the early developmental stages of this species respond to environmental stressors.

Continue reading ‘Transcriptional profiles of early stage red sea urchins (Mesocentrotus franciscanus) reveal differential regulation of gene expression across development’

Effects of coralline algal diffusion boundary layers on growth of newly settled sea urchins: implications for ocean acidification conditions

Macroalgae are able to modify their local environment via biological processes, thereby creating a diffusive boundary layer (DBL) where the chemical and physical environment differs from the overlying bulk seawater. In slow flow environments, the DBL has the potential to modulate effects of reduced seawater pH associated with ocean acidification (OA). OA poses a major threat to marine ecosystems and particularly to calcifying organisms. While implications for macroalgae and corals in the DBL have been studied, the effects on invertebrates settling and inhabiting the DBL are not well understood. This study examines
the oxygen and pH conditions within coralline algal DBLs that change as a result of irradiance, flow and bulk seawater pH, in order to understand the effects of these variable conditions on growth of juvenile sea urchins in the DBL. Oxygen concentrations, used as a proxy for pH based on previous research, were measured above crustose coralline algal surfaces to determine DBL thickness and pH levels within the DBL. Newly settled juvenile sea urchins Pseudechinus huttoni were subsequently grown in these conditions for up to 11 days. Morphological measurements (test diameter and spine length) and scanning electron microscopy were used to examine growth and calcification of sea urchins.

Seawater pH levels above CCA varied as a result of irradiance, flow and bulk seawater pH. In static flow, CCA increased pH at its surface up to approximately 0.8 units above the overlying bulk seawater in the light, but only decreased pH up to nearly 0.09 units below bulk seawater in the dark. DBLs were thickest at zero or slow flow (1 cm s-1 ) in the light. pH levels in the DBL varied from approximately pHT 7.4 to 8.6, but there was no strong effect of these varying pH levels within the DBL on post-settlement growth of P. huttoni juveniles. Life in
the diffusion boundary has allowed juveniles to adapt to grow and calcify in naturally fluctuating pH environments. This finding supports observations seen in other juvenile sea urchins, and is significant because it indicates that the early post-settlement stage may not be as sensitive to OA as the larval stage, where negative effects have been previously documented. Life in thick diffusion boundary layers above CCA in slow-flow fjord environments may have increased tolerance of juvenile P. huttoni to reduced bulk seawater pH, thereby conferring greater resilience to future ocean acidification conditions.

Continue reading ‘Effects of coralline algal diffusion boundary layers on growth of newly settled sea urchins: implications for ocean acidification conditions’

The importance of inter‐individual variation in predicting species’ responses to global change drivers

Inter‐individual variation in phenotypic traits has long been considered as “noise” rather than meaningful phenotypic variation, with biological studies almost exclusively generating and reporting average responses for populations and species’ average responses. Here, we compare the use of an individual approach in the investigation of extracellular acid–base regulation by the purple sea urchin Paracentrotus lividus challenged with elevated pCO2 and temperature conditions, with a more traditional approach which generates and formally compares mean values. We detected a high level of inter‐individual variation in acid–base regulation parameters both within and between treatments. Comparing individual and mean values for the first (apparent) dissociation constant of the coelomic fluid for individual sea urchins resulted in substantially different (calculated) acid–base parameters, and models with stronger statistical support. While the approach using means showed that coelomic pCO2 was influenced by seawater pCO2 and temperature combined, the individual approach indicated that it was in fact seawater temperature in isolation that had a significant effect on coelomic pCO2. On the other hand, coelomic [HCO3−] appeared to be primarily affected by seawater pCO2, and less by seawater temperature, irrespective of the approach adopted. As a consequence, we suggest that individual variation in physiological traits needs to be considered, and where appropriate taken into account, in global change biology studies. It could be argued that an approach reliant on mean values is a “procedural error.” It produces an artefact, that is, a population’s mean phenotype. While this may allow us to conduct relatively simple statistical analyses, it will not in all cases reflect, or take into account, the degree of (physiological) diversity present in natural populations.

Continue reading ‘The importance of inter‐individual variation in predicting species’ responses to global change drivers’

Rare genetic variation and balanced polymorphisms are important for survival in global change conditions

Standing genetic variation is important for population persistence in extreme environmental conditions. While some species may have the capacity to adapt to predicted average future global change conditions, the ability to survive extreme events is largely unknown. We used single-generation selection experiments on hundreds of thousands of Strongylocentrotus purpuratus sea urchin larvae generated from wild-caught adults to identify adaptive genetic variation responsive to moderate (pH 8.0) and extreme (pH 7.5) low-pH conditions. Sequencing genomic DNA from pools of larvae, we identified consistent changes in allele frequencies across replicate cultures for each pH condition and observed increased linkage disequilibrium around selected loci, revealing selection on recombined standing genetic variation. We found that loci responding uniquely to either selection regime were at low starting allele frequencies while variants that responded to both pH conditions (11.6% of selected variants) started at high frequencies. Loci under selection performed functions related to energetics, pH tolerance, cell growth and actin/cytoskeleton dynamics. These results highlight that persistence in future conditions will require two classes of genetic variation: common, pH-responsive variants maintained by balancing selection in a heterogeneous environment, and rare variants, particularly for extreme conditions, that must be maintained by large population sizes.

Continue reading ‘Rare genetic variation and balanced polymorphisms are important for survival in global change conditions’

Parental environments alter DNA methylation in offspring of the purple sea urchin, Strongylocentrotus purpuratus

Highlights
• Maternal environment impacts DNA methylation of early stage offspring.

• Developmental environment has a minimal impact on DNA methylation.

• DNA methylation differences across developmental stages are minimal.

• Differences in DNA methylation is associated with body size of embryos.

Abstract
Phenotypic plasticity, within and across generations, is a strategy by which organisms can respond rapidly to environmental change. Epigenetic modifications, such as DNA methylation, have been proposed to be involved in phenotypic plasticity. We examined the potential for the environment to mediate both transgenerational and intragenerational plasticity in DNA methylation and phenotypes in early stages of the purple sea urchin, Strongylocentrotus purpuratus, an ecologically important herbivore in kelp forest ecosystems. This approach involved a controlled laboratory experiment where adult urchins were acclimated during gametogenesis to upwelling (~1300 μatm pCO2 & 13 °C) or non-upwelling (~650 μatm pCO2 & 17 °C) conditions that are representative of their kelp forest habitat. Progeny from these adults were raised in either high (~1050 μatm) or low (~450 μatm) pCO2 treatments and sampled at three developmental stages. Differences in condition experienced by mothers were associated with differentially methylated genes in the offspring. However, differences in developmental conditions corresponded to little observable effects on gene methylation in the progeny. Variation in gene body methylation across treatments was correlated with body size of the embryos and larvae, illustrating a potential link between transgenerational phenotypic plasticity and DNA methylation. Overall, our results suggest that epigenetic factors such as DNA methylation have the potential to contribute to phenotypic plasticity in a transgenerational framework, and further, that epigenetic processes may act as a mechanism of rapid response to environmental change.

Continue reading ‘Parental environments alter DNA methylation in offspring of the purple sea urchin, Strongylocentrotus purpuratus’

Transgenerational effects in an ecological context: conditioning of adult sea urchins to upwelling conditions alters maternal provisioning and progeny phenotype

Highlights

• Differential maternal conditioning did not affect egg size or protein content.

• Simulated upwelling conditions increased maternal provisioning of lipids to eggs.

• Maternal conditioning to simulated upwelling increased embryo body size.

Abstract

Transgenerational plasticity occurs when the conditions experienced by the parental generation influence the phenotype of their progeny. This may in turn affect progeny performance and physiological tolerance, providing a means by which organisms cope with rapid environmental change. We conditioned adult purple sea urchins, Strongylocentrotus purpuratus, to combined pCO2 and temperature conditions reflective of in situ conditions of their natural habitat, the benthos in kelp forests of nearshore California, and then assessed the performance of their progeny raised under different pCO2 levels. Adults were conditioned during gametogenesis to treatments that reflected static non-upwelling (~650 μatm pCO2, ~17 °C) and upwelling (~1300 μatm pCO2, ~13 °C) conditions. Following approximately 4 months of conditioning, the adults were spawned and embryos were raised under low pCO2 (~450 μatm pCO2) or high pCO2 (~1050 μatm pCO2) treatments to determine if differential maternal conditioning impacted the progeny response to a single abiotic stressor: pCO2. We examined the size, protein content, and lipid content of eggs from both sets of conditioned female urchins. Offspring were sampled at four stages of early development: hatched blastula, gastrula, prism, and echinopluteus. This resulted in four sets of offspring: (1) progeny from non-upwelling-conditioned mothers raised under low pCO2, (2) progeny from non-upwelling-conditioned mothers raised under high pCO2, (3) progeny from upwelling-conditioned mothers raised under low pCO2, and (4) progeny from upwelling-conditioned mothers raised under high pCO2. We then assessed the effects of maternal conditioning along with the effects of developmental pCO2 levels on body size of the progeny. Our results showed that differential maternal conditioning had no impact on average egg size, although non-upwelling females produced eggs that were more variable in size. Maternal conditioning did not affect protein content but did have a modest impact on egg lipid content. Developing embryos whose mothers were conditioned to simulated upwelling conditions (~1300 μatm pCO2, ~13 °C) were greater in body size, although this effect was no longer evident at the echinopluteus larval stage. Although maternal conditioning affected offspring body size, the pCO2 levels under which the embryos were raised did not. Overall, this laboratory study provides insight into how transgenerational effects may function in nature. The impacts of parental environmental history on progeny phenotype during early development have important implications regarding recruitment success and population-level effects.

Continue reading ‘Transgenerational effects in an ecological context: conditioning of adult sea urchins to upwelling conditions alters maternal provisioning and progeny phenotype’


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