Which blood pigment gives a horseshoe crab’s blood its blue colour?

Hemocyanin gives horseshoe crab blood its blue colour.

Hemocyanin is a copper-containing protein that transports oxygen through the animal’s circulatory system. When oxygenated, copper makes the blood appear blue. Humans and most other mammals use iron-containing hemoglobin instead, which makes oxygenated blood look red.

Horseshoe crabs are marine arthropods, not true crabs. They are more closely related to arachnids such as spiders and scorpions. Their lineage has existed for hundreds of millions of years, and their hard shell, long tail spine and compound eyes are distinctive features.

Cells in horseshoe crab blood are used in tests for bacterial endotoxins in medical products. This has made the animals valuable to medicine, although conservation programmes and synthetic alternatives aim to reduce harm and protect wild populations.