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Carbon Sequestration with Giant Clams

Everybody knows about carbon dioxide, or CO2, the main greenhouse gas responsible for climate change. Not everybody knows, however, that the oceans are the biggest repositories of atmospheric CO2, which dissolves in seawater. And even fewer people know that this dissolved atmospheric carbon is incorporated by shell-producing marine organisms into shells made of calcium carbonate (CaCO3). Because of this, clam shells provide a long-term sink for atmospheric CO2; in contrast, the carbon contained in most plant and animal tissues returns to CO2 within a few years. 

Highly Effective Carbon Sequestration

Shellfish such as clams use carbon that has been dissolved in seawater in the form of carbonate (CO3), combined with calcium (Ca), to generate calcium carbonate (CaCO3) shells. This carbon is incorporated into the clamshell, storing it in the strong CaCO3 chemical bonds forever, at least on a human timescale.


How we calculate total carbon removal from clam mariculture

Several independent studies have been carried out to determine the carbon content of both clam tissues and shells by drying, weighing, and grinding them into fine powders. In each case, stable isotope mass spectrometry was then used to determine the proportion of carbon in the samples. From these data, clam shells are about 12% carbon by weight. This indicates that 12% of the total mass of CaCO3 (calcium carbonate, which is the substance that makes up clam shells) consists of carbon. 
When it comes to removing carbon from the atmosphere, only carbon counts, which is why the "O3" in CaCO3 is not considered in determining CO2 sequestration. Therefore, the formula below can be written as the following for a 250kg giant clam:
C(tridacnae) = W(shell) x P, where C = carbon content, W = weight, and P = proportion of CO2

C(t) = 250 kg x 0.12
= 30 kg of carbon in 250 kg giant clam shell

Additionally, bivalves such as clams not only sequester carbon in their shells, but also in their tissues while they are growing. This is especially true of giant clams as they have formed a symbiotic relationship with a type of algae found in the similar calcium carbonate structures of coral reefs. Zooxanthellae algae work together with giant clams (as well as corals found in reefs) to provide more food through photosynthesis. In exchange for providing this extra source of nourishment, the algae are given a safe and sturdy place to call home inside a tough calcium carbonate shell. It is only through this extraordinary partnership between organisms that giant clams are able to grow as big as they do - and for coral reefs to form as large as they are. There simply is not enough food energy provided by filter feeding to support such large masses. 

So, we have these humble algae to thank for our reefs, giant clams, and all the carbon they sequester in their shells and coral structures that house the polyps. With the help of these incredible algae, not only is more carbon stored in these structures, but there is also more carbon stored in the living tissue of giant clams as they grow thanks to the photosynthesis of their symbiotic roommates. This is factored in when we measure the carbon sequestration rates of the giant clams being grown in the IMARCS mariculture project: the living tissue contains even more carbon than the shell by proportion! Therefore, when we calculate the weight of a living giant clam, we are actually underestimating the total carbon stored in the organism by weight. 

Other benefits of giant clam mariculture

There are many other positive outcomes associated with giant clam mariculture besides storing and removing carbon. Some of these additional benefits include the following: 

Restoring Reefs

The giant clam, scientifically known as Tridacnae, is rarely found among coral reefs these days due to years of overharvesting. However, giant clam aquaculture has become a successful alternative to collection in the wild. Ocean acidification has become a problem that is also limiting the growth of giant clams and other mollusks, corals, and crustaceans, as it reduces the ability of these organisms to build their shells (due to fewer available carbonate ions — see diagram along)
By growing them in onshore mariculture and then introducing them into reefs after maturity, they can be a great benefit to the entire ecosystem — and, since they are already at their peak size, will not be affected by increased acidification. More CO2 in oceans does not dissolve calcium carbonate shells, it just prevents them from growing larger at a regular rate. 
Disclaimer: Ocean acidification chemistry is fairly well understood, but the long-term effects of this are not known with any certainty; all predictions and projections of future states of the ocean are hypothesized and estimated as a "best guess".

Food Security

Giant clams provide a lot of meat — organism for organism they provide more than any other mollusk or invertebrate on the planet. Though the soft body only accounts for about 10% of the body weight of giant clams, it is nearly pure, healthy protein.

Water Filtration

They power all that bulk by filter-feeding microbes and particles from the water, siphoning hundreds of gallons of water per day. Like corals, they also have a symbiotic relationship with photosynthetic algae called zooxanthellae that live in the fleshy part of the clam and provide it with food.

Community benefits from clam sales

Helping locals through the sale of these beautiful species for meat, decorative shells, and the aquarium trade, as well as the monetization of carbon removal that is collected by IMARCS and then distributed back to the community. 

Strengthening Reefs

Clams help contribute to the formation of reefs by aggregating together, providing shelter and associated food sources for fish and other marine organisms. This indirectly contributes to sequestering carbon as reefs store carbon in a manner similar to forests and mangrove wetlands.

Support our giant clam mariculture efforts

None of the work that we are doing to remove CO2, restore reefs, protect biodiversity, and help local communities would be possible without your support.

Please consider contributing to the flagship IMARCS project by donating, offsetting your carbon, or subscribing to "adopt-a-clam." If you are not ready to give, you can still support us by following our socials and spreading the word about our mission to restore, replenish, and rethink our reefs - all while helping mitigate climate change and bringing benefits to local communities.