Understanding the Effects of Ocean Acidification on Predator-Prey Interactions
by Sasha Seroy
Sasha Seroy is a graduate student in the Oceanography Department at the University of Washington, advised by Dr. Daniel Grünbaum.
Marine organisms are experiencing dramatic environmental changes due to global climate change. As atmospheric carbon dioxide concentrations rise, the oceans absorb increasing amounts of carbon dioxide, which results in acidification. While ocean acidification affects several different types of organisms, calcifiers — those that make their shells or skeletons from calcium carbonate like shellfish or corals — have been identified as particularly vulnerable. Acidification not only increases the likelihood of shell or skeleton dissolution, it can also make it more difficult for organisms to create calcium carbonate in the first place. Several studies have investigated the effects of ocean acidification on calcifiers in isolation; however, in nature, organisms interact with a wide variety of other organisms, from predators to prey to competitors. These interactions have the potential to amplify or reduce the effects of acidification with consequences that could propagate up to population and community levels. I am particularly interested in how interactions between predators and prey are influenced by changing ocean chemistry.
The encrusting bryozoan Membranipora membranacea is commonly found in the waters around San Juan Island and presents a good model system to investigate the effects of acidification on predator-prey interactions. Membranipora forms large circular colonies composed of zooids — the individual units within a colony (Figure 1) — on kelp blades. As they grow, colonies add subsequent rings of zooids to their outer edge. This structure makes it simple to divide colonies like cutting a pizza, and then expose genetically identical slices of the same colony to different environmental conditions via laboratory manipulations. Membranipora exhibits an inducible defense — a defense that is only formed in the presence of predators — which helps protect them from being eaten. Upon receiving chemical cues that the predatory sea slug Corambe steinbergae is close by, Membranipora produces spines on skeletons of newly-formed zooids along the outer growing edge of the colony (Figure 2). While these inducible spines are beneficial, they present a trade-off because they require energy to make, and leave less energy to put toward colony growth. Therefore, the cost associated with increased protection is a reduction in overall colony growth. Thus, similar to tree rings, we can see which rings of zooids were formed at a time of high predation by looking for defensive spines. Since these interactions are easy to quantify and Membranipora forms its skeleton from calcium carbonate, this system is a good model to understand how ocean acidification might affect predator-prey dynamics.
I was introduced to the Ocean Acidification Environmental Laboratory (OAEL) at FHL through the summer 2015 Ocean Acidification course, during which I was supported by the Adopt-a-Student Program. Through pilot studies as a part of the course, I learned how to manipulate water chemistry and familiarized myself with Membranipora. I have continued to return to conduct my own graduate research, and last summer I worked in the OAEL to investigate whether Membranipora could still form protective spines in acidified water. From my experiments, I found that colonies exposed to predator cues were still able to form spines under ocean acidification conditions, with a similar cost of defense. This potentially means that Membranipora will still be able to protect itself even under predicted future ocean conditions. I was also surprised to observe that colonies not exposed to predators grew more in moderately acidified conditions than colonies that weren't acidified. While many calcifiers do exhibit reductions in growth in acidified conditions, responses are variable and species-specific. Some calcifiers — including some other species of bryozoans — have displayed increased growth. However, there is likely a cost to living in sub-optimal pH conditions. For instance, colonies may be growing more but laying down brittle, less-calcified skeletons.
As part of my research this summer I have collected Membranipora colonies off of racks of plastic plates under the FHL dock, taking advantage of the bryozoan's planktonic larvae that settled en masse in the spring. I will continue to look at growth rates of both defended and undefended colonies in a wider range of pH conditions. In addition to understanding prey responses to ocean acidification (do Membranipora spines formed in acidified conditions confer the same protection as those formed in ambient conditions?), I am also interested testing whether predation rates change. These data will be integrated into an evolving mathematical model of bryozoan population dynamics that can be used to see the population-level implications of changes and tradeoffs observed at the organism level. Overall, understanding how these responses to ocean change scale up will help predict how ecological communities will fair in future conditions.
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