The correct answer is crab shells.
The carbon cycle is based on carbon dioxide that can be found in the air in the gaseous form and in water in dissolved form. Carbon dioxide that is accumulated in water will be present as either bicarbonate of carbonate ions (crab shells are made up of carbonates).
These ions are an essential component of natural buffers, which prevent the water from turning into too basic or too acidic. When the warming of the water surface is done by the Sun, the carbonate and bicarbonate ions will get reimbursed to the atmosphere as carbon dioxide.