Crab blood isn't a single, uniform color. The color depends largely on the type of crab and whether the blood is exposed to air.
The Color of Crab Hemolymph
Most crabs, unlike humans, don't have blood containing hemoglobin. Instead, they have hemolymph, which contains a copper-based respiratory pigment called hemocyanin. This gives their hemolymph a slightly different color than human blood.
- Un-oxygenated: When not exposed to air, crab hemolymph is often colorless or pale, slightly blue. [Source: Quora]
- Oxygenated: Upon exposure to air, the hemocyanin can oxidize, resulting in a change of color that can range from a pale blue to a blue-green. [Source: Study.com, Horseshoe Crab Medical Uses]
It's important to note that the cooked, white substance often seen on crabs after cooking is not their blood. This is a different substance. [Source: Do Crabs have Blood?]
The Case of Horseshoe Crabs
Horseshoe crabs are often cited in discussions about blue blood because their hemolymph is a distinctive, bright blue even when oxygenated. This is due to the high concentration of hemocyanin. [Sources: National Zoo, Vox, NHM, Horseshoe Crab Blood] This blue blood is also crucial in medical applications, specifically as a bacterial endotoxin test in vaccines and medical devices. [Source: PBS]
While some sources may simplify the answer to "blue," it's more accurate to state that crab hemolymph can appear colorless, pale blue, or blue-green, depending on the species and oxygen exposure. The vibrant blue blood of the horseshoe crab is a notable exception.
