Role of the Stress Gradient Hypothesis in Geoduck Survival 

 

Introduction: The increase in ambient global CO2 levels has disastrous consequences for bivalves, such as the Pacific Geoduck (Panopea generosa), because their carbonate based shells become degraded in the presence of acid. With increases in atmospheric carbon dioxide, the carbonic acid equation moves to the left. Heightened levels of free carbon dioxide react with water, forming free hydrogen ions that result in bicarbonate instead of carbonate, reducing the amount of free carbonate that bivalves form their shells with. The fitness impact is extreme, with shelled organisms subjected to increased levels of CO2 having reduced size and heightened vulnerability to their environment. Due to the connection between a global increase in levels of atmospheric carbon, the ease in which CO2 dissolves in water, the resulting increase in ocean acidification and the negative fitness impact on bivalves, it can be assumed that using Panopea generosa as a study system for changing marine ecosystems will serve as a proxy for additional species that use carbonate. Furthermore, a decrease in fitness, such as the size impact mentioned above, is often enough of a catalyst to increase selection pressure on a species, and yet is not the only one. In fact, interspecies competition usually drives selection to further favor the largest, most competitive individuals. To that end I propose a multi-faceted experiment to build on the research methods of previous scholars and combine the still vague Stress Gradient Hypothesis (SGH) with the study of Geoduck survival in simulated climate change environments. The SGH describes a phenomenon whereby individuals experiencing strong environmental selection react by decreasing competition within the species, for reasons yet undefined. I propose to correlate acquired fitness data in terms of Panopea generosa size with expected changes in behavioral competition in the presence of a stressful environment, defined here as one with CO2 increased beyond the ancestral levels. The importance of this experiment is further highlighted due to the substantial knowledge gap surrounding Panopea generosa in spite of their monetary value for the Pacific Northwest. Beyond the intrinsic desire to preserve threatened species, aquatic agriculture communities across the coast need to understand the impact acidification will have on their currently profitable crops, and the realistic chance of Panopea generosa evolving fast enough in a positive direction to offset their demise. A potential decrease in selection pressures, if the SGH is to believed, may push Panopea generosa to be smaller and weaker to protect against the need for limited carbonate. I hypothesize that interspecies competition in Pacific geoducks will increase selection pressure at a lesser rate in populations experiencing climate induced stress than populations raised in ancestral, ambient conditions. 

 

Aim 1: It is important to generate two comparative study groups to best randomize the level of genetic relatedness while inducing stress. In order to efficiently expose geoducks to enough environmental change, I propose a scheduled increase in CO2 levels on a monthly basis to not stress the cohort into extinction while modeling the unstable reality of atmospheric CO2 levels. I will raise one cohort of geoducks and split them in two environments, one with CO2 levels increasing monthly, and one maintaining the ancestral environment. 

 

Aim 2: For the purposes of this experiment, the competition levels of the two groups of geoducks will need to be recorded. I will assess the differences in competition between the two groups throughout the experiment to assess both the final differences in competition and also the minimum threshold of stress that marks a decrease in competition for the variable CO2 group. 

 

 

Experimental Approach: Generally, the experiment is intended to mimic two real-world environments. The ancestral environment with regular levels of CO2 and a predictive environment with CO2 levels that increase periodically. I predict that Panopea generosa will experience the fitness deficit of decreased size and therefore be under increased selection pressure, however the individuals in the ancestral environment will experience heightened levels of competition. Therefore both groups will experience selection but at different levels and in different directions. 

 

Methods: I will conduct this experiment in a laboratory setting over 3 years to best control the levels of atmospheric CO2 after harvesting local geoduck larvae from beaches up the Washington coast.  I will raise the Geoducks for 1 year in a common garden with the water having the same chemical composition as the average coastal Pacific Ocean using acidification data from NOAA, and the techniques that Gurr et. al. has demonstrated to be effective. The increase in acidification of their water environment will occur after the cohort has been split as juveniles, and randomly separated into experimental classes. Group 1 will experience a change in water acidification through monthly increases over 2 years in dissolved CO2 in their environment. These increases in CO2 will be based on predicted yearly CO2 levels scaled in the lab to monthly changes to best promote stress. Group 2 will remain in the ancestral CO2 environment for the duration of the experiment. Competition between individuals will be tested with the experimental design Kamermans et. al. pioneered, focusing mostly on the mortality rate and health of the species within their own group before a cross-comparison is conducted. Selection rate will be determined from statistically corrected mortality rates and fitness predictors. Comparison of selection rates across both groups will indicate whether or not the SGH was at work in this study system or not. 

 

Intellectual Merit: The impact of global climate change on ocean dwelling populations has long been a cause of concern for scientists, as these communities seem to be exceptionally sensitive to changes in ambient CO26. That being said, the role of multiple selection pressures on the extremely vulnerable bivalve populations remains extremely under researched. Further, Panopea generosa in particular are generally missing from most academic conversations that include bivalve vulnerability despite residing in an area of the world where CO2 adaptation is unlikely from an ancestral perspective. In general the ability to expand our collective knowledge base when it comes to impacts on bivalves across threatened coastal marine environments will allow scientists to make predictions about the areas most requiring protection, and the time frame in which effective changes can be implemented with positive outcomes. The Stress Gradient Hypothesis (SGH) has also not been tested in Panopea generosa even though the population is going to experience environmental stressors naturally that will force the expression of either the SGH or its null hypothesis. Getting ahead of the physical expression of the SGH and making predictions as to its viability in marine study systems allows for the predictive powers of this hypothesis to be tested more that it has been in the past.