During the 24 million years before the pre-industrial era of the earth, carbon dioxide levels in the world’s oceans were relatively unchanging. Although it is possible that higher levels of carbon dioxide arouse, it does not suggest lower pH and carbonate ion levels accompanied the rise of CO2. This could occur due to the oceans cushioning the rising CO2 levels. This now suggests an increase in carbon dioxide level in the world’s oceans, occurring since the 19th century, has led to the decline of carbonate ions availability and lowered the oceans pH levels. Thus putting selective pressure on shellfish and how they evolve.
Human carbon dioxide emissions not only affect the atmosphere causing global warming, but also lower the ocean’s pH level and the carbonate ion abundance. This is known as ocean acidification. This severely affects organisms that produce calcium carbonate body parts (i.e. shellfish, mussels, clams). From a multitude of other aspects, growth, metamorphosis and survival are all affected by increased carbon dioxide emissions entering the ocean.
The problem starts with the larvae of these organisms before they have shells and have matured. Due to the ocean acidification that has occurred since the 19th century, the survival and maturity rates of shellfish larvae’s are being hindered. Thus dwindling some of these shellfish populations.
Larvae of the Northern quahog (Mercenaria mercenaria) and the bay scallop (Argopecten irradians) were used to find the differences of future CO2 levels and present-day CO2 levels compared to pre-industrial CO2 levels. Both of these shellfish are valuable resources, considering annual harvesting brings in a combined $750 million.
Stephanie C. Talmage and Christopher J. Gobler published Effects of past, present, and future ocean carbon dioxide concentrations on the growth and survival of larval shellfish, in PNAS. In this article, it explains the experiment in which the two organisms were grown under 3 controlled CO2 level environments. The first CO2 level was pre-industrial (~250 parts per million). The second CO2 level was present-day (~390 ppm). The third was the estimated future CO2 levels >400, being set around 1,500 ppm and 750 ppm.
The experiment done by Stephanie C. Talmage and Christopher J. Gobler resulted in the larvae growing in the pre-industrial CO2 level having the highest survival, metamorphosis and growth rates. As the CO2 levels increased to 390 ppm, 750 ppm and 1,500 ppm, these rates decreased in both species. Not only were the metamorphosis rates higher, they were faster with the larvae in lower CO2 levels.
Overall the experiment proved that shellfish larvae would survive and grow better in environments with lower CO2 levels. Shells were thicker, metamorphosis was faster and survival was higher. There was much more shell integrity and hinge joints were more intact compared to shellfish in higher CO2 levels. What this presents is a problem with the current shellfish populations and the future populations. Shell and hard parts that are produced with calcium carbonate by shellfish will have a greater chance of being malformed. This weakens the integrity of the shell which in turn leaves the shellfish more susceptible to predators, which could lead some species to declining populations, even extinction.
The above figure shows how shell size, hinge joint and integrity in larval shellfish are distinguished depending on the CO2 levels present.
Evolution could allow different shellfish to adapt to the increased levels of CO2 in the oceans. It is uncertain how resistant shellfish could be to the increase of CO2, as some shellfish may prosper in the enriched CO2 levels found in the future. In the two shellfish used for the determining experiment each could have different outcomes of the increased levels of CO2 in the future.
Moreover, each species could have a different result from futuristic CO2 levels. The Northern quahog has a more diverse and larger geographical habitat unlike the bay scallop. This provides the Northern quahog to adapt and to evolve without the pressure of increased CO2 levels. The bay scallop will have much more of a struggle and pressure to adapt to the increased CO2 levels then the Northern quahog.
If shellfish such as the Northern quahog and bay scallop go extinct then this could affect the fishing economy drastically. The U.S harvesting economy uses both species as a large source of income and as a valuable resource. Ecosystems would also be affected by the extinction of such vital shellfish species.
Blake Turner
Reference: S. Talmage, C. Gobler. “ Effects of past, present, and future ocean carbon dioxide concentrations on the growth and survival of larval shellfish.” Proceedings of the National Academy of Scientists (2010): vol. 107 no. 40 17246-17251, 10/05/10.
Link to site: http://www.pnas.org/content/107/40/17246.full
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