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Artifacts of Calcification
Most seashells are formed almost entirely through calcification. The chalky exterior and smooth, reflective interior of an abalone are both formed through the calcification process. Coral reefs are built up over many years as tiny organisms create calcified shells and die, allowing their descendants to build their own shells on top. Microscopic life forms such as algae create calcified exoskeletons and sink to the bottom of the sea as they die, creating deposits of calcified material.
Calcium Content of Seawater
Seawater contains significant levels of dissolved calcium carbonate. Calcium carbonate is commonly found on land as limestone and related minerals. When these minerals are eroded by wind and water, the particles of calcium carbonate are dissolved into rivers and carried to the ocean. When in this dissolved state, calcium carbonate is ionized in such a way as to allow its attraction by certain proteins.
Organic Calcification
In order to create a shell or exoskeleton through calcification, an organism creates a layer of protein that attracts and attaches to ionic calcium carbonate in seawater. As layers of calcium carbonate are attached to the protein, new layers of protein are created on top of the new calcium carbonate deposits to attract more material. Depending on the specific proteins created, the calcium carbonate may take the chalky form of an outer shell or combine with organic fibers to create smooth nacre to line the interior of the shell.
Shaping a Shell Through Calcification
Different organisms have varying requirements for shell design. For example, a clam will create a layer of protein along the open edge of the shell, growing the shell slowly one horizontal layer at a time. This produces the familiar striations of a clam shell. More complex shells, such as the knobbed shell of the conch or the spiral shell of the chambered nautilus, require complex and specific patterns of protein production.