Declines over the last two decades of five intertidal invertebrate species in the western North Atlantic

Impact of ocean warming and food restriction on the fecundity of the sea star Asterias rubens

Sea stars are major predators in marine ecosystems that impact shellfish aquaculture because of their predation on cultured bivalves. As ectotherms, their reproductive success is expected to be impacted by rising ocean temperatures and change in prey availability induced by climate change. As reproduction is a key process in the reduction, maintenance, and proliferation of populations, the impacts of ocean warming and food ration on the fecundity of the sea star Asterias rubens were investigated. Males and females were exposed for 6 months either to monthly temperatures from the 1995-2014 periods or to temperatures projected at the end of the century by the "business as usual" scenario of greenhouse gases emission. Sea stars were also fed with a normal (3 mussels twice a week) or restricted (1 mussel twice a week) food ration. Biometric parameters (body weight, arm length, pyloric caeca weight, gonad weight, pyloric index, gonadosomatic index), gamete concentration (number of oocytes or sperm per gram of gonad) and fecundity (number of oocytes or sperm per individual) were assessed at the end of the experimental period. Sea stars exposed to elevated temperatures or to food restriction had reduced fecundity because of their reduced gonads weight. However, fecundity of food-restricted males was not reduced compared to well-fed ones despite their lower gonads weight, thanks to higher sperm concentration in their gonads. Overall, these results indicate that future environmental conditions induced by climate change could reduce the reproductive success of A. rubens.

Understanding change in benthic marine systems

BACKGROUND

The unprecedented influence of human activities on natural ecosystems in the 21st century has resulted in increasingly frequent large-scale changes in ecological communities. This has heightened interest in understanding such changes and effective means to manage them. Accurate interpretation of state changes is challenging because of difficulties translating theory to empirical study, and most theory emphasizes systems near equilibrium, which may not be relevant in rapidly changing environments.

SCOPE

We review concepts of long-transient stages and phase shifts between stable community states, both smooth, continuous and discontinuous shifts, and the relationships among them. Three principal challenges emerge when applying these concepts. The first is how to interpret observed change in communities - distinguishing multiple stable states from long transients, or reversible shifts in the phase portrait of single attractor systems. The second is how to quantify the magnitudes of three sources of variability that cause switches between community states: (1) 'noise' in species' abundances, (2) 'wiggle' in system parameters and (3) trends in parameters that affect the topography of the basin of attraction. The third challenge is how variability of the system shapes evidence used to interpret community changes. We outline a novel approach using critical length scales to potentially address these challenges. These concepts are highlighted by a review of recent examples involving macroalgae as key players in marine benthic ecosystems.

CONCLUSIONS

Real-world examples show three or more stable configurations of ecological communities may exist for a given set of parameters, and transient stages may persist for long periods necessitating their respective consideration. The characteristic length scale (CLS) is a useful metric that uniquely identifies a community 'basin of attraction', enabling phase shifts to be distinguished from long transients. Variabilities of CLSs and time series data may likewise provide proactive management measures to mitigate phase shifts and loss of ecosystem services. Continued challenges remain in distinguishing continuous from discontinuous phase shifts because their respective dynamics lack unique signatures.

Prevalence and effects of a parasitic trematode on the blue mussel, Mytilus edulis, in the Boston Harbor

The blue mussel, Mytilus edulis, is a keystone species in the North Atlantic that plays critical roles in nutrient cycling, water filtration, and habitat creation. Blue mussel populations have declined significantly throughout the North Atlantic due to various factors, including habitat loss, pollution, increasing water temperature, and parasites. One parasite is Proctoeces maculatus, a digenetic trematode, which uses M. edulis as an intermediate host. This parasite causes reduced growth, castration, and death in mussels. The range of P. maculatus has expanded northward from Cape Cod, MA to Maine which may be associated with rising temperatures in the Gulf of Maine. To evaluate the negative impacts of P. maculatus on mussels, we analyzed its infections in M. edulis throughout the Boston Harbor, MA. P. maculatus was present in every population and time point analyzed, with approximately 50% of mussels in the harbor infected. The parasite reduced gonadal development in infected mussels, which could lead to decreased fecundity. Severe P. maculatus infections induced a stress response, indicated by increased HSP70 expression. We developed a non-destructive hemolymph-based assay to determine if mussels are infected with P. maculatus, thus speeding up the evaluation process and eliminating the need to sacrifice individuals. With P. maculatus' continued expansion northward, more mussel populations will be under threat from this parasite.

Evidence of introgression, ecological divergence and adaptation in Asterias sea stars

Hybrid zones are important windows into the evolutionary dynamics of populations, revealing how processes like introgression and adaptation structure population genomic variation. Importantly, they are useful for understanding speciation and how species respond to their environments. Here, we investigate two closely related sea star species, Asterias rubens and A. forbesi, distributed along rocky European and North American coastlines of the North Atlantic, and use genome-wide molecular markers to infer the distribution of genomic variation within and between species in this group. Using genomic data and environmental niche modelling, we document hybridization occurring between northern New England and the southern Canadian Maritimes. We investigate the factors that maintain this hybrid zone, as well as the environmental variables that putatively drive selection within and between species. We find that the two species differ in their environmental niche breadth; Asterias forbesi displays a relatively narrow environmental niche while conversely, A. rubens has a wider niche breadth. Species distribution models accurately predict hybrids to occur within environmental niche overlap, thereby suggesting environmental selection plays an important role in the maintenance of the hybrid zone. Our results imply that the distribution of genomic variation in North Atlantic sea stars is influenced by the environment, which will be crucial to consider as the climate changes.

Northern shrimp from multiple origins show similar sensitivity to global change drivers, but different cellular energetic capacity

Species with a wide distribution can experience significant regional variation in environmental conditions, to which they can acclimatize or adapt. Consequently, the geographic origin of an organism can influence its responses to environmental changes, and therefore its sensitivity to combined global change drivers. This study aimed at determining the physiological responses of the northern shrimp, Pandalus borealis, at different levels of biological organization and from four different geographic origins, exposed to elevated temperature and low pH to define its sensitivity to future ocean warming and acidification. Shrimp sampled within the northwest Atlantic were exposed for 30 days to combinations of three temperature (2, 6 or 10°C) and two pH levels (7.75 or 7.40). Survival, metabolic rates, whole-organism aerobic performance and cellular energetic capacity were assessed at the end of the exposure. Our results show that shrimp survival was negatively affected by temperature above 6°C and low pH, regardless of their origin. Additionally, shrimp from different origins show overall similar whole-organism performances: aerobic scope increasing with increasing temperature and decreasing with decreasing pH. Finally, the stability of aerobic metabolism appears to be related to cellular adjustments specific to shrimp origin. Our results show that the level of intraspecific variation differs among levels of biological organization: different cellular capacities lead to similar individual performances. Thus, the sensitivity of the northern shrimp to ocean warming and acidification is overall comparable among origins. Nonetheless, shrimp vulnerability to predicted global change scenarios for 2100 could differ among origins owing to different regional environmental conditions.

A Review of Asteroid Biology in the Context of Sea Star Wasting: Possible Causes and Consequences

AbstractSea star wasting-marked in a variety of sea star species as varying degrees of skin lesions followed by disintegration-recently caused one of the largest marine die-offs ever recorded on the west coast of North America, killing billions of sea stars. Despite the important ramifications this mortality had for coastal benthic ecosystems, such as increased abundance of prey, little is known about the causes of the disease or the mechanisms of its progression. Although there have been studies indicating a range of causal mechanisms, including viruses and environmental effects, the broad spatial and depth range of affected populations leaves many questions remaining about either infectious or non-infectious mechanisms. Wasting appears to start with degradation of mutable connective tissue in the body wall, leading to disintegration of the epidermis. Here, we briefly review basic sea star biology in the context of sea star wasting and present our current knowledge and hypotheses related to the symptoms, the microbiome, the viruses, and the associated environmental stressors. We also highlight throughout the article knowledge gaps and the data needed to better understand sea star wasting mechanistically, its causes, and potential management.

Independent Effects of Species Removal and Asynchrony on Invariability of an Intertidal Rocky Shore Community

Ecological stability depends on interactions between different levels of biological organization. The insurance effects occur when increasing species diversity leads to more temporally invariable (i.e., more stable) community-level properties, due in part to asynchronous population-level fluctuations. While the study of insurance effects has received considerable attention, the role of dominant species that contribute with particular functional traits across different level of organizations is less understood. Using a field-based manipulative experiment, we investigated how species richness and different types of parameters at the population level, such as the invariability of dominants, population invariability, and population asynchrony, influence the community invariability. The experiment involved the repetitive removal of the canopy forming alga Mazzaella laminarioides (hereafter “Mazzaella”) during 32 months in two rocky intertidal sites of northern-central Chile. We predicted that the invariability of dominants enhances community invariability, that the effect of multispecies population-level parameters on community invariability are dependent on species richness, and that subdominant algae are unable to fully compensate the loss of canopies of the dominant species. Biomass of algae and mobile invertebrates was quantified over time. We observed independent effects of Mazzaella removal and community-wide asynchrony on community invariability. While canopy removal reduced community invariability, population asynchrony boosted community invariability regardless of the presence of canopies. In addition, filamentous and foliose algae were unable to compensate the loss of biomass triggered by the experimental removal of Mazzaella. Canopy removal led to a severe decrement in the biomass of macrograzers, while, at the same time, increased the biomass of mesograzers. Asynchrony stemmed from compensatory trophic responses of mesograzers to increased abundances of opportunistic algae. Thus, further work on consumer-resource interactions will improve our understanding of the links between population- and community-level aspects of stability.

Variability effects by consumers exceed their average effects across an environmental gradient of mussel recruitment

The implicit assumption that properties of natural systems deduced from the average statistics from random samples suffice for understanding them focuses the attention of ecologists on the average effects of processes and responses, and often, to view their variability as noise. Yet, both kinds of effects can drive dynamics of ecological systems and their covariation may confound interpretation. Predation by crabs and snails on competitively dominant mussels has long been recognized as an important process structuring communities on rocky shores of the Northwest Atlantic Ocean. We experimentally manipulated the average intensity of predation in plots across a gradient of mussel recruitment to separately estimate the average and variability of responses of mussel recruitment and community composition. Predation did not affect the average number of mussels recruited to plots, nor the average multivariate composition of the community. Plots from which predators were excluded showed a ~ 30% increase in spatial variability of mussel recruitment. After 1 year, the spatial variability in community composition was greater than that observed among plots that predators could access. An important, but less recognized, aspect of predation is its dampening effect on variability of community structure. As accelerating rates of environmental change disrupt species interactions, variability effects of ecological processes and corresponding responses are likely to be increasingly important determinants of community dynamics.