bio

Haemocyanin

The copper-based respiratory pigment of most arthropods and some molluscs - transparent when deoxygenated, deep blue when it binds oxygen. It is why horseshoe crab and octopus blood is blue, and also the source of a common false inference: blue skin does not imply copper-based blood. More importantly, haemocyanin cannot be packed into cells; it circulates free in the fluid, so it must be very large to avoid being filtered out by the kidney. At the concentration needed to match a vertebrate's oxygen-carrying capacity, that fluid becomes both viscous and osmotically ferocious - too thick to push through a capillary.