Which sea creature has blue blood?

Horseshoe crabs use haemocyanin, which carries oxygen on copper rather than iron and turns blue when it does. They are also not crabs. They are closer to spiders and scorpions, and the design has barely changed in something like 450 million years - they were already old when the dinosaurs turned up.

Their blood is worth more per litre than almost any other substance, because of what it does in the presence of bacteria. Their immune system has no antibodies; instead their blood cells clot violently around bacterial toxins. That reaction was turned into a test so sensitive it can detect contamination at parts per trillion, and it became the standard safety check for injectable drugs, vaccines and implanted devices. If you have ever had an injection, a horseshoe crab was probably involved.

The harvest is the uncomfortable part. Hundreds of thousands of animals are caught each year, bled by about a third of their volume, and returned to the sea; estimates of how many die afterwards vary from a few per cent to nearly a third, and the survivors appear to breed less. A synthetic replacement exists and has been approved in Europe, and the transition in the United States has been slow.