Save the atmosphere: Cultivate shellfish

Allow me to share four interesting and readily available facts about shellfish:

1. There’s a lot of shell in shellfish.

2. Shellfish shell is mineralised carbon dioxide from the atmosphere.

3. Shellfish shell is not digested and is chemically stable for geological periods of time.

4. The shellfish industry (and everyone else for that matter) seems to be totally unaware that the shellfish cultivation industry is the ONLY industry on the planet that currently REMOVES serious quantities of carbon dioxide permanently from the atmosphere. The shellfish industry is the ONLY industry on the planet that could save the atmosphere by massively increasing its production.


ONE: There’s a lot of shell in shellfish.

Think of your average shellfish meal for two:

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Mussels: fresh weight 810 g, shell dry weight 296 g
 = 36.5% of the fresh mussel is shell
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Cockles: fresh weight 950 g. shell dry weight 557 g
= 58.6% of the fresh cockle is shell


And on a global scale? Data from FAO Fisheries and Aquaculture Information and Statistics Branch (as of 25 May 2019) show that over the years 2010 to 2017 aquaculture harvests across the globe totalled 53,512,850 metric tonnes of crustaceans and 122,527,372 metric tonnes of molluscs (a combined total of 176,040,222 metric tonnes in 8 years. It’s a reasonable assumption that the shell represents an average 50% of the animal’s mass, so the total shellfish-shell produced was 88 million tonnes over 8 years. An average of 11 million tonnes of shell per year.


TWO: Shellfish shell is mineralised carbon dioxide from the atmosphere.

Molluscan shell is a biomineral composed of CaCO3 with a small amount of matrix proteins included that direct the species-specific crystal growth; arthropod (crab, shrimp, lobster) exoskeletons are composed largely of chitin hardened with calcium-magnesium carbonate nanocrystals. Either way, shellfish shell is about 95% crystalline calcium carbonate.


The animals make this by absorbing calcium through specific transporters in their tissues and reacting it with bicarbonate (HCO3-), which is synthesised from CO2. Some of the bicarbonate is absorbed directly from the surrounding water (or gaseous atmosphere for terrestrial species), the rest derives from CO2 generated from the animal’s food. The fractions derived from these two sources differ widely. But since all food chains depend on fixation of photosynthetic carbon from the atmosphere at their root, the carbon in the food of predators, scavengers, filter-feeders and detritus feeders alike, aquatic and terrestrial, is derived from photosynthetic fixation of atmospheric carbon. There is no other source of metabolic carbon.


THREE: Shellfish shell is not digested and is chemically stable for geological periods of time.

·       And remember the fossils from deep time? Ammonites (65 to 240 million years ago), trilobites (520 million years ago), brachiopods (550 million years ago), shellfish all. Certainly, these fossil shells are changed considerably in chemistry by now (over extended time periods carbonates can recrystallise into calcite, or exchange with silica or iron sulphide in the surrounding rock), but the shell carbonates survive over geological time. The carbonates in shells are neither digested nor degraded. High temperatures are required to release the CO2 from carbonates (to produce quicklime) – ask the cement industry, which uses fossil limestone as a feedstock for cement production (in 2014, cement production accounted for 6% of the fossil CO2 emissions from industrial sources). In the natural world, the carbonates of shells are only likely to release their CO2 when/if they encounter volcanic conditions. How much more permanent, do we need permanent to be?


Of course, photosynthetic carbon capture by trees is widely considered to be possibly our most effective strategy to limit the rise of CO2 concentrations in the atmosphere. Trouble is, carbon capture by green plants (trees, kelp forests and peat mosses alike) is temporary because when they die the plants are subject to decay and digestion and the ultimate end-product of digestion is the release of CO2 back to the atmosphere. On a global scale, the world’s forests release billions of tons of CO2 to the atmosphere each year, a similar magnitude, in fact, to the annual CO2 emissions from fossil fuel combustion. We can all observe that the decomposition of seasonally shed leaves, petals, ripe fruit, and dead wood releases CO2 to the atmosphere in the same year it was fixed.

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And when the tree dies, there are hundreds of animals, bacteria and, especially, wood decay fungi in every forest waiting to help the decay and digestion along: living trees store carbon; dead trees give it back to the atmosphere. That’s life.


Of course, sustainably managed forests can be harvested to provide wood fuels as environmentally benign alternative to fossil fuels (but still returning their CO2 to the atmosphere), or timber for buildings and furniture. There are about 60 or so indoor wood decay fungi from which you need to protect your timber buildings and furniture, including dry rot, wet rot, cellar rot, and oak rot. The longevity of the carbon pools represented by wood products derived from harvested timber depends upon their use: lifetimes may range from less than one year for fuelwood, to several decades or centuries for furniture and timber in buildings; but compared with the permanence of shellfish shells, timber is only ever a temporary remedy for the atmosphere. And anyway, how much furniture do you want stacked up in your timber shed?


If you hope that terrestrial green plants can really effectively sequester carbon from the atmosphere, you are bound to be disappointed; you are expecting too much of them. So, if the forests can’t capture CO2, are we doomed? Well, no, actually; we just need to change our focus; turn away from trees (but still plant them; they’re good for us in so many ways) and concentrate on shellfish - the contrast of shellfish shell with trees is that shellfish shell is permanently solidified (mineralised) atmospheric CO2. And shellfish cultivation is the ONLY industry on the planet that currently REMOVES serious quantities of carbon dioxide permanently from the atmosphere.



At present we cultivate shellfish for the meat (the shell is food-waste) and the industry is scaled according to that market. We must change the paradigm: cultivate the bivalves, and those other shellfish, to sequester permanently CO2 from the atmosphere and accept the food as the by-product.


All it needs is the same sort of positive attitude as that which prompted the Intergovernmental Panel on Climate Change Special Report of 2018 to suggest that we plant an extra 1 billion hectares of continuous forest in order to limit global warming to 1.5°C by 2050. Another positive is that cultivating shellfish does not threaten agricultural land. About 70 percent of the Earth’s surface is covered by water, we might as well use that.


Let’s put some numbers on the present situation to get an idea of the scale of the operation required. On a molar mass basis, CO2 represents 44% of the mass of calcium carbonate. So, that global production of 11 million tonnes of shell per year, referred to above, is equivalent to 4.84 million tonnes of CO2 per year being captured, and mineralised, from the atmosphere by current aquaculture activities. In carbon-offset terms, that’s equivalent to one million business class return flights between London Heathrow and JFK New York (6 billion miles of flying per year, every year).


The following is an alternative calculation for a single offshore farm designed to produce 10,000 tonnes of mussels per year:

10,000 tonnes mussels = 3,650 tonnes of shell = 1,606 tonnes CO2 permanently removed from the atmosphere annually.


According to [] one business class return flight LHR (London Heathrow) to JFK (New York) = 2.17 tonnes of CO2 and would cost between £6 and £13 per tonne to offset, depending on the offsetting programme you wish to support (planting trees in the UK, promoting renewable energy, overseas community projects, etc).

The same website calculates that the carbon footprint of my Ford Focus (EU 2015 FORD All New Focus , Model Year Post 2015 1/2 1.6 Duratorq TDCi (115PS) With Stop/Start - 5 Door) is 0.22 tonnes CO2 per 1000 miles (costing between £2 and £5 to offset, depending on the programme).

So, the shells of 10,000 tonnes of mussels:

  1. offset 740 return business class tickets (equivalent to about £14,430 in offsetting fees); OR

  2. offset my driving 7,300,000 miles (equivalent to about £25,550 in offsetting fees).


If we had a million mussel farms like this around the world, they’d be removing 1.606 billion tonnes CO2 permanently from the atmosphere each year. Global carbon emissions in 2014 from fossil fuel use were 35.9 billion tonnes of carbon dioxide []. So, a million mussel farms would permanently remove about 4.5% of the global CO2 emissions in each year.


Don’t be put off by the call for a million mussel farms; we already have 570,000,000 farms of various sorts on our scarce agricultural land. yet 70% of the surface of the Earth is covered in ocean. Imagine a mussel farm on every offshore wind turbine, every oil and gas rig, every pier, wharf and jetty, every breakwater or harbour wall; imagine cultivating cockles (and other clams) in every shallow sandy/muddy bay. Imagine restocking and extending every fished-out Oyster or Scallop fishery; imagine cultivating Giant Clams to restore all those bleached-out coral reefs. We could start tomorrow.


The shellfish industry is the ONLY industry on the planet that could save the atmosphere by a massive increase in production.


FOUR: The shellfish cultivation industry (and everyone else for that matter) seems to be totally unaware of the OPPORTUNITIES that exist for this UNIQUE industry.

The shellfish industry could tell its customers that, for example, every bowl of moules marinière removes about 20 g of CO2 from the atmosphere, OR every tonne of fresh mussels removes about 220 kg of CO2 from the atmosphere. Why not use some of these facts in promotional and advertising materials [‘Eat more shellfish. SAVE the atmosphere’]?


Remind the customer that this is a permanent removal (compare with this sentence ‘While trees are growing, they absorb carbon dioxide from the atmosphere; dead trees release it again.’ []).


We have to start pointing out that the international scientific guidelines do not include ‘blue carbon habitats such as oyster reefs, seagrass beds and saltmarsh’. Shellfish-industry associations and representatives would have to get involved here. Surely, they can get over the point that shellfish cultivation is the only industry in which a massive increase in productivity will benefit the atmosphere. If central governments could be persuaded to fund shellfish cultivation to sequester (permanently) atmospheric carbon it would contribute to their carbon neutrality goals.


A high priority should be given to having ‘cultivation of shellfish shells’ included as a project on carbon offset websites. Many of the projects currently offered for carbon offsetting (of things like air travel and car journeys) are based on planting trees. But their projects also include overseas conservation projects. Philip Dor’s Giant Clam restocking and coral reef rehabilitation in the Philippines looks like an ideal candidate for this []. The primary CO2 emitter industries might also welcome a different kind of help to balance their carbon footprints.

FIVE: Further Information.

READ THIS published paper in the Mexican Journal of Biotechnology for the science

CHECK OUT THIS article on The Fish Site website

VIEW THIS social media thread

WATCH THIS (spoof documentary) YouTube video

VIEW THIS mussel farm website

VIEW THIS giant clam coral reef restoration website

VIEW THIS Matthias Heilweck's website

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Written and produced by David Moore, BSc, PhD, DSc, FLS, who retired from the Faculty of Life Sciences of the University of Manchester in 2009 after 43 years service to the university. He was born in Liverpool (1942) but has lived in south Manchester since 1966. Today, he considers himself to be a freelance writer. He lives in Stockport with his wife, Elizabeth.

February 2020