The horseshoe crab is no crab, has nine eyes and bleeds blue
Its species name honours Polyphemus, the one-eyed giant, because early naturalists thought it had a single eye. The Atlantic horseshoe crab actually has nine, plus light sensors along its tail. It is not a crab either: its closest relatives are spiders and scorpions, and its blood turns dark blue in air.
Those eyes earned a place in medical history. Each large compound eye holds about 1,000 receptor units, loosely arranged, each wired to its own big, easily reached nerve fibre. That let researchers record how single cells respond to light and watch effects like lateral inhibition happen at the cellular level, work that contributed to the 1967 Nobel Prize in Medicine. Males also use brightness and shape to recognise potential mates. Even unhatched embryos seem able to sense light from inside their buried eggs.
The blood is equally unusual. Instead of iron-based haemoglobin it carries oxygen with a copper protein, haemocyanin, dissolved in the fluid rather than packed into red cells. It looks greyish-white to pale yellow inside the animal and darkens to blue when exposed to air. Its only blood cells, amebocytes, release a clotting factor when they meet bacterial toxins, walling off infections, which is why the crabs are harvested for a biomedical extract used in science.
The body has three parts: a head section containing brain and heart, an abdomen, and a spike-like tail used for steering and for flipping upright, since a crab turned over by currents can suffocate. It has no jaws, feeding with small pincers instead. So many algae, worms, barnacles and molluscs hitch rides on its shell that it has been called a walking museum. Females are 25 to 30 per cent larger than males and can reach 60 centimetres and 4.8 kilograms.
This is the only living horseshoe crab native to the Americas, ranging from Maine to Florida, around the Gulf and on Mexico's Yucatan Peninsula, with Delaware Bay its main migration hub. Shorebirds depend on its eggs, and sea turtles eat the adults. In 2022 scientists recovered roughly two-million-year-old DNA from northern Greenland assigned to this species, among the oldest DNA ever sequenced, suggesting it once spawned that far north when seas were 8 degrees warmer.
Source: Atlantic horseshoe crab
