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What Animals Have Copper-Based Blood?

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Unlike most animals on earth, whose blood is iron-based, some mollusks (Mollusca) and arthropods (Arthropoda) have copper-based blood. The protein molecule hemocyanin carries oxygen through the body, in much the same way hemoglobin carries oxygen to cells in mammals and other animals. When the blood circulates through the mollusk or arthopod, it is clear, but when exposed to the air it turns bluish. While the best-known example of an arthropod with copper-based blood is the horseshoe crab, a number of other arthropods have blue blood.

Horseshoe Crabs

North America's horseshoe crab (Limulus polyphemus) lives along the East Coast shoreline, from Maine to the Yucatan Peninsula. Present-day horseshoe crabs closely resemble fossils that date back 230 million years, to the Triassic period. Similar fossil species go back even further, 400 million years, to the Devonian period. The horseshoe crab's copper-based blood is essential to the pharmaceutical companies, which harvest the creatures, remove one-third of their blood, then return them to the sea. The horseshoe crab's blood coagulates in the presence of certain bacterial toxins, allowing the companies to ensure that drugs and other products are safe for human use. The IUCN Red List classifies the horseshoe crab as near threatened due to human activities, including harvesting, pollution and habitat loss.

The Mollusks

Most mollusks have copper-based blood, including the Burgundy snail (Helix pomatia, also known as the vineyard snail), octopuses (order Octopoda) and squids (order Teuthida). Burgundy snails are land-dwelling mollusks, native to the Mediterranean, Central and Southeast Europe but spread by humans (Homo sapiens) across Asia, Europe and North America. Meanwhile, all the octopus and squid species are ocean-dwelling creatures, living from the tropics to the temperate zones. The protein in copper-based blood, called hemocyanin, functions better than iron-based hemoglobin would in carrying oxygen through the mollusks' bodies in the cold, oxygen-poor depths of the ocean.

The Crustaceans

Members of the crustacean family, the decapods (order Decapoda) such as crabs, crayfish, lobsters and shrimps, also have blue, copper-based blood. Commonly known as insects of the sea, the decapods are omnivores. They eat both plants and animals, including other crustaceans, fish and worms. Some crabs are also scavengers, cleaning up the beaches and ocean floor. While most decapods are ocean-dwellers, the crayfish, also known as crawdads and mudbugs, are freshwater crustaceans that live in ponds, rivers, creeks and ditches. Decapods are also arthropods.

The Arachnids

Among the other blue-blooded arthropods are members of the Arachnida class, including spiders and scorpions such as the wandering spider (Cupiennius saki), the tarantula (Eurypelma californicum) and the emperor scorpion (Pandinus imperator, also known as the imperial scorpion). While most spiders' venom is not dangerous to humans, scorpions' stings are painful and may (like bee stings) cause an allergic reaction, which might be fatal in sensitive individuals. Over 50,000 spider species and about 1,200 scorpion species have been identified.