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Is horseshoe crab blood really blue?

BBC Wildlife

|

July 2025

BEYOND THEIR ODD APPEARANCE AND ancient history, there is another thing that makes horseshoe crabs stand out from the crowd: their bright blue blood.

- Beki Hooper

Is horseshoe crab blood really blue?

The blue colour is caused by the protein haemocyanin, which is their equivalent of haemoglobin (the molecule that carries oxygen in human blood). Rather than being iron-based, as haemoglobin is, haemocyanin is copper-based.

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