By Pierre Ratte
Old tack on the side of a barn, with bits and bobs of this and that: a buckle, a bell and a horseshoe. Seems like the weathered horse bit is made of copper, judging by its patina, while the rest of the metal menagerie is iron, judging by its reddish-brown rust.
Fun Facts: Rust is primarily oxidation-producing iron oxides, most commonly Fe3O4 and Fe2O3. Other metals undergo oxidation, but the term rust is exclusively used for iron.
Copper’s oxidation (or corrosion) creates a blue-green color, copper carbonate (Cu2 CO3), commonly described as patina. The Statue of Liberty, clad with hand-hammered sheets of copper, has copper’s distinctive blue-green patina.
Human blood is dark maroon, almost rust color, due to the presence of iron which is used to transport oxygen. The more oxygen in blood, the redder the color. Hemoglobin contains iron which binds oxygen molecules in the lungs. Oxygen-rich blood then flows through the body releasing oxygen molecules to tissues where it is needed, simultaneously picking up carbon dioxide to expel via exhalation.
While human and other animal blood is red, horseshoe crabs have blue-green blood. Their oxygen exchange is based on copper, not iron. The copper oxygen transportation and exchange takes place via hemocyanin.
Horseshoe crabs’ blue blood plays a vital role in medical safety testing because it clots in the presence of toxins. This characteristic allows it to be used to test vaccines and other injectables for endotoxins or other contaminants. Horseshoe crab blood is harvested through biomedical transfusions, after which the animals are mostly returned to the water. Other animals with blue blood include octopuses, squid and some marine snails.
Horseshoe crabs (Limulus polyphemus) are not “true” crabs. They are more closely related to spiders, scorpions, ticks and mites under the phylum Arthropoda, subphylum Chelicerata.
Crabs, lobsters and shrimp are crustaceans. Horseshoe crabs are living fossils, little changed over 400 million years. Their long tail is called a “telson.” The telson is not a stinger; it is exclusively used for flipping itself over. The telson has an eye, one of nine eyes, or more accurately light sensitive spots enabling a wide, almost circular, field of vision.
Horseshoe crabs walk right-side-up, but swim upside-down. They are found along the Atlantic Coast and in Southeast Asia.
