Physiology of Finfish and Shellfish (2+1)

2.1.28. Shellfishes

A metalloprotein containing copper is the respiratory protein of crustaceans (shrimps, crabs, etc.) and of some gastropods (snails). The protein, called hemocyanin, is pale yellow when not combined with oxygen (deoxygenated) and blue when combined with oxygen (oxygenated). The molecular weights of hemocyanins vary from 300,000 to 9,000,000. Hemocyanin, contains copper rather than iron . Haemocyanin is the respiratory pigment akin to haemoglobin but hemocyanin has copper as the cofactor instead of iron which you find in haemoglobin.
Animals belonging to class crustacea and phylum Mollusca have hemocyanin, and hence they have blue blood. Some notable crustaceans are crabs, lobsters, and shrimps. Some notable mollusks include Snails, Mytilus, Oysters, Chitons, Squids, Sepia, and Octopus.