Question

In seas, mainly how do marine organism build their shells?

Answer 1

To build shells, organisms extract calcium ions $\left(C{a}^{2+}\right)$ and carbonate $\left(C{O}_3^{2-}\right)$ from sea water, which combine into the solid crystals of calcium carbonate $\left(CaC{O}_3\right)$ that shells are made of.