Living fossils: Long live the Horseshoe Crab

To survive minor injuries, the animals rely on their blue blood. A horseshoe crab’s blood changes from straw-coloured to blue when oxygenated by the gills or exposed to the air, as from an open wound. The blue colour (as with the blood of squid and other molluscs) results from the presence of the protein haemocyanin, in which two copper atoms bind to an oxygen molecule. In mammals, oxygen binds to the iron atoms in haemoglobin, found in red blood cells, to form the red pigment. In horseshoe crabs, there are no blood cells to transport oxygen inside the body, because the haemocyanin itself circulates in the blood.

Horseshoe crabs do not have the intricate mammalian circulatory system, with its dense network of capillaries. Instead they have one in which large, blood-filled cavities make direct contact with the living parts of the body. This design creates two potential problems: Even a small injury can mean losing a lot of blood, and bacteria can easily gain access to the organism. Meanwhile, horseshoe crabs have no immune system to help them resist harmful bacteria. But their blue blood does have special cells, amaebocytes, that solve both problems. If an amaebocyte detects endotoxins, a constituent of bacterial cell walls, it secretes a protein that effectively stops blood loss and prevents new bacteria from entering the horseshoe crab’s body. Researchers believe that studying the animal’s blood could lead to new methods of detecting and perhaps treating human afflictions such as cancer.

All-seeing eyes
The horseshoe crab’s eyes are also unique. Haldan Keffer Hartline, a physiologist at the Rockefeller University in New York, shared the Nobel Prize in 1967 for his work with horseshoe crabs. His discoveries laid the foundations for our present knowledge of the function of the human eye. Hartline studied the two largest of the animal’s 10 eyes, the ones located high on its carapace. Using these eyes, the horseshoe crab can see in all directions. The two large seeing organs are, like those of insects, compound eyes consisting of 1,000 ommatidia, or receptors. Photoreceptors at the base of each ommatidium register visible light and infrared and ultraviolet radiation and send signals to the brain.

Surprisingly, each ommatidium functions well both at night and in direct sunlight, when it is bombarded by 100 trillion times as many photons as during the weakest illumination that the compound eye can detect.
To carry out the same tasks, the human eye is equipped with cones for seeing in daylight and rods for seeing at night. The ommatidia in horseshoe crabs are about 100 times as large as the rods and cones in the human eye’s retina, making them the largest known photoreceptors in the animal kingdom.

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