What substance makes lobster blood appear blue?

Hemocyanin makes lobster blood appear blue. This copper-containing protein carries oxygen through the circulatory system of lobsters and many other mollusks and arthropods.

When hemocyanin binds oxygen, copper ions give it a blue color. Human and many other vertebrate blood cells use iron-containing hemoglobin instead, which is bright red when oxygenated. Lobsters have an open circulatory system, so their blue hemolymph moves through body spaces rather than remaining inside a network of closed blood vessels.

Hemocyanin is especially useful in cold marine environments, where oxygen availability and temperature can vary. Lobster blood is not normally blue because of food or seawater pigments; the color comes from the oxygen-carrying molecule itself. Other animals using hemocyanin include horseshoe crabs, spiders, scorpions, and many cephalopods.