Some animals have blue blood because their oxygen-carrying protein is hemocyanin, which uses copper rather than the iron-containing hemoglobin common in vertebrates. Oxygen binding changes hemocyanin’s light-absorbing properties, making oxygenated blood appear blue. The color and exact respiratory system vary among species.
Two oxygen carriers, different metals
Human blood looks red because hemoglobin contains iron and carries oxygen in red blood cells. Many mollusks and arthropods instead use hemocyanin, a copper-containing protein dissolved in their hemolymph. When oxygen binds to hemocyanin, the fluid can appear blue. [1]
The color depends on oxygenation
Horseshoe crab blood may look pale or nearly colorless when it carries little oxygen and blue when oxygenated. It is the oxygen-binding chemistry of hemocyanin that produces the visible color, not blue pigment added like paint. The blood’s appearance can therefore depend on the animal’s physiological state and the conditions under which it is observed. [2]
Not all invertebrates have blue blood
“Some animals” is the important qualifier. Respiratory pigments vary across the animal kingdom, and not every invertebrate uses hemocyanin. Other oxygen-binding proteins can involve different metals and produce other colors. The blue-blood examples most familiar to people include horseshoe crabs, octopuses and some other mollusks.
A useful evolutionary comparison
Hemocyanin and hemoglobin solve a similar problem—transporting oxygen—but are different proteins with different chemistry. Their comparison shows that evolution has produced multiple ways to carry oxygen through animal bodies. The color is a visible clue to that underlying molecular difference.
Sources and evidence
- University of Texas Marine Science Institute — The Real Blue Bloods of the Sea ↗
Explains copper-based hemocyanin and the blue color of oxygenated horseshoe crab blood.
- Open University — Oxygen, Blood and the Body ↗
Compares animal respiratory pigments, including iron-containing hemoglobin and copper-containing hemocyanin.
- PubMed — Functional Differences in the Multiple Hemocyanins of the Horseshoe Crab ↗
Research describing horseshoe crab hemocyanin as a copper protein that binds oxygen.
Sources consulted: 14 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; wording distinguishes blue oxygenated hemocyanin from blood color across all animals.