In crustaceans such as shrimp, crayfish, and lobster, the metallic base of the respiratory pigment is copper. The respiratory pigment in crustaceans is called hemocyanin, and it is responsible for carrying oxygen throughout their bodies. Unlike mammals, crustaceans do not have hemoglobin, which uses iron as its metallic base.

Hemocyanin contains copper ions instead of iron. Copper is a transition metal that can form different complexes, and it plays a crucial role in binding and transporting oxygen in crustaceans. As crustaceans have an open circulatory system, their hemolymph, which is equivalent to blood, comes into direct contact with their tissues. Oxygen attaches to the copper ions in the hemocyanin molecules and is transported to cells for respiration.

It is important to note that while iron is the main metallic base of the respiratory pigment in mammals, it is not the case for crustaceans. Therefore, option 2, iron, is not the correct answer.