What Is Ocean Acidification and How Does It Affect Mollusks and Their Shells?
5 min read / 2026-08-28
Ocean acidification is the gradual drop in seawater pH caused by absorbed carbon dioxide, and it makes it harder for mollusks, corals, and other sea creatures to build the calcium carbonate shells they depend on for survival.
What it means
Ocean acidification happens when seawater absorbs extra carbon dioxide (CO2) from the air and becomes slightly more acidic. Oceans naturally soak up about a quarter of the CO2 humans release each year, mostly from burning fossil fuels and from land-use changes. As CO2 dissolves in water, it forms carbonic acid, which lowers the water's pH, a scale that measures how acidic or basic a liquid is. Lower pH means fewer of the carbonate ions that mollusks need to build their shells.
How it works
A mollusk's mantle, the thin layer of tissue that lines its shell, pulls calcium and carbonate ions from seawater to form calcium carbonate, the mineral that makes up roughly 95% of a shell's dry weight. When seawater has fewer free carbonate ions because of acidification, this building process slows down and the shell that forms can end up thinner or more brittle. Some young mollusks and coral larvae struggle to form a shell at all in water that is too acidic, which affects survival before they even reach adulthood.
A simple example
Think of building a brick wall when the mortar supply keeps shrinking. A mollusk secreting shell layers is like a mason adding bricks; calcium carbonate is the mortar. In more acidic water, the mortar becomes scarce, so walls, or shells, come out thinner and weaker, similar to how a poorly cured concrete pillar holds less weight than a well-cured one, even though both look finished from the outside.
Why people talk about it
Scientists, fisheries, and coastal communities pay attention because shell-forming species anchor entire food webs and economies, from oyster farms to reef tourism. Researchers at institutions like NOAA track ocean pH the way meteorologists track temperature, because even small shifts change how well shellfish grow. This matters for India's coastal fisheries too, where mussel and clam harvests support local livelihoods, similar to how a poor monsoon can affect a farmer's crop yield.
What to remember
Ocean acidification does not instantly dissolve shells; it slows and weakens the biological process of building them over time. The effect varies by species, water temperature, and local ocean chemistry, so not every mollusk population is affected equally right now. Long-term monitoring, like the shell studies described by the Smithsonian Institution, helps scientists understand which coastal habitats need the most protection as ocean chemistry keeps shifting.
Key words
Calcium carbonate
A mineral compound that forms most of a mollusk shell's structure and also makes up coral skeletons and limestone.
pH
A scale used to measure how acidic or basic a liquid is; lower numbers mean more acidic.
Mantle
The thin layer of tissue in a mollusk that secretes the minerals forming its shell.
Ocean acidification
The gradual decrease in seawater pH caused by the ocean absorbing extra carbon dioxide from the atmosphere.
Key facts
- 1Oceans absorb roughly a quarter of the carbon dioxide humans emit annually, according to NOAA research.
- 2Calcium carbonate typically makes up about 95% of a mollusk shell's dry weight.
- 3Carbonic acid forms when CO2 dissolves in seawater, lowering the water's pH over time.
- 4Young mollusks and coral larvae are often more sensitive to low pH than fully grown adults.
- 5Ocean pH has dropped by about 0.1 units since the pre-industrial era, which represents a measurable rise in acidity.
Why it matters
Shell-building creatures support fisheries, coastal food chains, and habitats for countless other species, so changes in ocean chemistry ripple far beyond a single mollusk.
Sources
- Smithsonian National Museum of Natural History
- NOAA
- Smithsonian Magazine
