Fossil evidence indicates that horseshoe crabs were already present some 450 million years ago, which means they have been on Earth longer than most species alive today. Such a long history points to a remarkably hardy animal, able to cope with a wide range of physiological (internal) and ecological (environmental) challenges.
Their distribution is a case in point. Horseshoe crabs inhabit coastal waters and bays from Maine to Mexico, so they tolerate a very wide range of water temperatures and salinities (salt levels). They can even survive for several hours out of water, as sometimes happens when they become stranded on beaches during the breeding season. Although they are probably easiest to find in the US mid-Atlantic States, in places such as Chesapeake Bay and Delaware, they occur all along the East Coast and in the Gulf of Mexico, and related species (Tachypleus tridentatus) live in Asia. Anyone hoping to see them need only visit a beach at high tide in May or June; come across one of their favorite breeding sites, and you may witness hundreds of them laying eggs.
Their breathing system is equally remarkable. When a horseshoe crab flaps its gill plates to “breathe,” blood flows through the pages of its book gills, so called because, on close inspection, they resemble the pages of a book, while seawater containing oxygen flows over the outside and the oxygen passes into the blood. There, the oxygen binds to a pigment called hemocyanin. Unlike hemoglobin, the iron-containing pigment inside our own red blood cells, hemocyanin is a copper-containing molecule, so when it binds oxygen it turns blue rather than red.
Horseshoe crab blood also contains a substance of great importance in the fight against infection. Discovered in 1956, this substance, known as LAL, reacts very strongly when it encounters certain bacteria that can cause dangerous infections in humans. For this reason, vaccines, medical devices and any other material that will be injected or placed into the human body are first tested with LAL to make sure they are not contaminated with those bacteria.
Supplying hospitals across the country and around the world requires enormous quantities of LAL. In any given year, more than 500,000 horseshoe crabs are bled so that it can be extracted for medical use. The cost to the animals is considerable: once 30% of their blood has been removed, between 20% and 30% of them die, and many of the survivors are left with low energy and become less active. The author’s own research has found that bled females lay eggs only half as many times as those that have not been bled, a finding that raises concerns about the crabs’ ability to breed and about the stability of the population as a whole.
