Ocean Acidification Impacts on Larval Shell Formation by Commercial Shellfish
Arctic Research Initiative/OCCI Funded Project: 2008
Bivalved mollusks - including oysters, clams and scallops - represent some of the most economically valuable species in New England waters. These organisms build their protective shells from one of two forms of calcium carbonate (CaC03): calcite or aragonite, and some species use a combination of both forms. Today, the mid-latitude surface oceans are supersaturated with respect to both calcite and aragonite, and thus favorable for shell formation. However, the anthropogenic increase in atmospheric carbon dioxide, driven by fossil fuel combustion, is lowering the saturation state (Ω) of the surface oceans and recently-ventilated subsurface waters, a process known as ocean acidification (OA). This decrease in Ω is particularly strong at higher latitudes, due in large part to the strong temperature dependence of the solubility of carbon dioxide in seawater. Over the next 50-100 years, surface waters off the coast of New England will likely be under-saturated with respect to aragonite for several months of the year. This means that all shellfish, in protected or unprotected areas, will be exposed to corrosive seawater that will make shell formation more difficult and may even cause shells to dissolve. For this reason, there is growing concern over the potential for negative impacts of ocean acidification on recruitment and survival of commercially important shellfish species, should current trends in C02 emissions continue.
We propose a suite of experiments in our Marine Calcification Laboratory at WHO1 to assess the impact of elevated atmospheric pC02 on early shell formation by larvae of several commercially important New England shellfish species. The impact of OA on larval shell formation is little studied but of particular concern because rapid accretion of a normal "D-shell", usually within 24 hours following fertilization, is a prerequisite for the successful recruitment of spat. Larvae may be particularly vulnerable because the first shell or prodissoconch 1, of all New England shellfish species is built from aragonite, the more soluble form of CaC03, even in species with calcitic adult shells. Furthermore, there is some evidence that accretion of amorphous calcium carbonate (ACC), the most highly soluble phase of CaC03, precedes crystallization to aragonite within 24 hours of fertilization.
We will examine and quantify the impact of OA on development of the larval D-shell in several commercially important shellfish, including sea and bay scallops, oysters and quahogs. Fertilized eggs will be introduced to seawater treatments designed to mirror conditions expected for the New England region over the next century. Our laboratory contains a bank of 24 ten-gallon aquaria, the CaC03 saturation states of which are set and maintained by bubbling with air-C02 mixtures precisely controlled using mass flow controllers, and monitored by intercalibrated determinations of gas-phase pCO2 and seawater pH, alkalinity, and DIC. All of our experiments to date have been run at 25°C. In this study we will use chillers to enable us to carry out our experiments at lower temperatures, matching those experienced by larvae in their natural environment (for example, 14°C for the larval sea scallops). Fertilized eggs will be acquired from the Scientific Aquaculture Program at the Marine Biological Laboratory (http://www.mbl.edu/mrc/research/aquaculture.html) and the Aquaculture Resource Center in Dennis, Massachusetts. Larval culturing will be carried out in small (1L) plastic beakers fed from the main tanks. A dedicated set of mass flow controllers is needed to enable us to carry out these experiments while simultaneously carrying out our ongoing NSF-funded tropical coral experiments.
The larval shells are small (-50 μm across) so we use microscopy (polarizing and SEM) and imaging techniques to identify and quantify morphological pCO2 impacts on earliest calcification. We will also use Raman spectroscopy to characterize shell mineralogy (calcite, aragonite, ACC) of harvested specimens. Our expectation is that this initial survey will provide the basic data necessary with which to build a comprehensive research plan for a full, externally funded proposal.