Conch produce their shells during the three day period in which they are in their egg stage. When they are born, the shell is already present. The mantle, which is inside of the shell, is what produces the shell. The conch incorporate calcium carbonate from the water or their food and then deposit the calcium to form the shell. They grow by adding whorls (the tip) to their shell, but once sexual maturity is reached, they begin to add shell on to their lip.