An ancient saying that is still very famous, widespread in various parts of Europe, claims that people of noble origin have the “blue blood“, different from that of normal people. If the origin of this adage has typically human and imaginative roots, In nature there really are animals with blue bloodlike the horseshoe crab, a characteristic common to species that are even very distant from each other. The unusual color, and other variations such as the green or purple blooddepend on the different proteins that organisms use for transport oxygen or for get rid of the “waste”: over the course of millions or billions of years, in fact, evolution has seen the appearance of different tools for carrying out the same functions, as in the case of proteins hemocyanin and hemoglobin. Some animals, such as icefish, have actually “dismissed” these proteins during their evolutionary path. The result: blood altogether colorless.
Blue blood in marine animals
Different aquatic animals like crustaceans, cuttlefish and octopus instead of hemoglobin, they usehemocyanin, another protein capable of binding to oxygen to transport it throughout the body. The protein takes on a blue color in contact with oxygen because it contains copper: this is the main distinction compared to hemoglobin (widespread in most vertebrates, especially terrestrial) based on iron and with the usual red colour. Among the various blue-blooded species, the most famous is probably that of Limuliwhose blood also contains further substances from valuable features for the healthcare industry. This was “exploited” by scientists to create the test LAL (Limulus Amebocyte Lysate), used since the 1970s to ensure that vaccines and medicines are not contaminated with dangerous bacteria. While this test is critical to human health, it poses a problem for this species, which is caught every year to produce it.
Hemocyanin is a very ancient protein, which appeared for the first time 2.5 billion years ago and used as “defense” from oxygena secondary product of the photosynthesis cycle and initially not widespread in the Earth’s atmosphere. Organisms that had evolved for millennia in oxygen-free environments, they had to adapt to an increasingly hostile world, and hemocyanin was one of the first responses to counteract the oxidizing power of O2. Afterwards, with him development of aerobic lifethis protein became essential for the transport of oxygen to organs and fabrics.
THE’hemoglobinborn only 400 million years ago, has a totally different evolutionary path: it appears in the Devonian, a period in which vertebrates began to conquer the emerged landsand has favored the development of more complex and efficient organisms and respiratory systems.
The color of green, purple and transparent blood: emeritin and bilirubin
Proteins similar to hemocyanin have evolved in different species, sometimes independently, and with specific variations: different molluscs possess a more purple blood thanks to emerythrina protein similar to our hemoglobin but from more intense color.
Sometimes, the use of these oxygen transport proteins has even been abandoned in the course of evolution: this is the case of ice fishof the family Channichthyidae, animals that live in the very cold waters of Antarctica, so rich in oxygen to allow him to live without “help” to transport him in the blood. The lack of hemoglobin actually helps him lower blood viscositythus counteracting the natural increase in viscosity due to low temperatures of its habitat.
The green blood of some species of lizards belonging to the genus Prasinohaema, widespread on the island of New Guinea, is instead due to the high concentrations of biliverdin. This protein is normally one bile “waste”. due to degradation of heme groups (part of hemoglobin) and is toxic to most animalsto the point that high concentrations in the blood lead to pathologies such asjaundice. In the case of lizards, however, evolution seems to have led to one greater tolerance of the body to this toxic waste: the evolutionary advantage in this case is given by the properties antioxidants and from one increased resistance to infections.
