You steam a crab, put it on the table — and the first thing everyone notices is not the golden roe but the clumps of white, egg-white-like solid wedged in the shell seams and leg joints. Is it dirt? A disease? Or just protein that happens to look underwhelming?
It is blood. Real, cooked blood — and here is the biology that makes it look nothing like the blood you are thinking of.
What Is That White Solid on Cooked Crab?
Crab "blood" is not like ours. Humans move oxygen through hemoglobin, an iron-based protein inside red blood cells — that is why our blood is red. Crabs (and insects, spiders, and other arthropods) use hemocyanin, a copper-based protein dissolved directly in the plasma, not packaged in cells.
While the crab is alive, hemocyanin is nearly colorless or carries a faint blue-gray tint. It only turns blue when oxygenated — think of the blue-tinged fluid in a live crab's joints. Then the heat of steaming denatures the protein, coagulating it into the solid white lumps you find in the shell seams. It is perfectly edible and harmless; it just tastes bland — the texture is the only thing "egg-white-like" about it.
Why Crab Blood Isn't Red: Hemocyanin vs. Hemoglobin
The contrast is one of the neatest lessons in animal physiology. Both proteins transport oxygen; the difference is the metal and the packaging:
- Hemoglobin (humans): iron-based, packaged inside red blood cells, turns blood red; an open circulatory system is not needed — ours is a closed, pressurized system.
- Hemocyanin (crabs): copper-based, dissolved freely in plasma, colorless to pale blue; two copper atoms bind each oxygen molecule, which is why oxygenated hemolymph looks bluish.
Arthropods run an open circulatory system: hemolymph bathes organs directly rather than staying inside vessels. That is also why a crab's "blood" can pool loosely in body cavities and, once cooked, set into those white lumps instead of staying liquid.
The Crab's "User Manual": What Each Part Actually Is
The white lumps are just the start. Most of us follow handed-down crab-eating steps without knowing what we are actually eating.
The intestine (蟹肠). "The gutless gentleman," as the crab is nicknamed, does have an intestine — a short midgut and a longer hindgut hidden at the belly flap. When you snap off the crab's abdomen, the black thread-like line you see is the intestine, and it carries waste: do not eat it. The gills and the stomach are also places where debris collects, best avoided.
