The limulus crab (which is not a real crab but is a helicoprotein, the only representative of the Limulus genus) exists for 450 million years (it is a living fossil), and it is reasonable to think that at this time he learned some trick to survive.
The crab's blood is, in fact, sensitive to bacteria. It can reveal the presence of bacteria even at concentrations of 1 part per trillion.
Special blood cells in the blood, called amebocytes, coagulate around bacterial threats , preventing the diffusion: the process lasts only 45 minutes.
It is for this reason that blood is used in the reliability tests of sanitary instruments, and allows you to save thousands of lives each year; it has even been used on the International Space Station to see if certain bacterial species exist.
600,000 specimens of limulus crab (which is not a real crab) are caught to pick up 30% of the blood, and then released to the sea.
The amount of blood taken is minimal, and the crab is supposed to survive: however, it seems that 10-30% of the crabs do not resist the operation.
Other curiosity: Blood is blue for the presence of hemocyanin, a molecule similar to hemoglobin but based on copper instead of iron. This molecule is transparent to the normal state, and becomes intense blue when it binds to oxygen.
