Because horseshoe crabs (Limulus polyphemusz) have lived for millions of years in relatively unchanged form, some biologists refer to them as living fossils. Relatives of the living species have inhabited the world’s oceans for at least 400 million years. Before their 400-million-year reign began, horseshoe crabs developed a number of adaptations that allow them to survive, including numerous eyes, hard shells, a specialized assortment of appendages and a primitive immune-like response to bacteria.
Marine biologists identify 10 different eyes and other light-sensing organs on horseshoe crabs. Seven eyes are on the top of the animal’s carapace; the lateral eyes are the two most obvious, and are compound in design. Additionally, horseshoe crabs have a pair of rudimentary eyes behind each lateral eye, and a cluster of three eyes at the front of their carapace. Two very simple eyes are located near the mouth, on the underside of their carapace, but their function is not clear. The final light sensing organ is located along the length of their tail, scientists think that they help the arthropods to synchronize their activity pattern with the lunar cycle.
One of the most important adaptations of horseshoe crabs is their hard shell, termed a carapace. Though their carapace does not provide absolute protection from all predators, it discourages the majority of small- and medium-sized predators. Many organisms cling to the carapace of horseshoe crabs, including algae, barnacles and mollusks. Before they develop the hard carapace, young horseshoe crabs avoid the bottom of the ocean, and instead swim higher in the water column.
Horseshoe crabs have five pairs of walking legs, and one additional pair that has been modified into chelicerae -- pincers -- that help bring food to their mouth. Horseshoe crabs primarily eat creatures buried in the sediment. Using their legs, they dig -- primarily at night -- for flatworms, mollusks and other prey.
Primitive Immune Response
Horseshoe crabs do not produce antibodies to fight infection. However, they do demonstrate a novel approach to dealing with pathogens. Presumably, this allows these long-lived creatures to survive in their bacteria-laden habitats. When a horseshoe crab’s body detects the presence of endotoxin -- a compound associated with a variety of gram-negative bacteria -- its blood cells begin to exhibit massive clotting. This effectively seals off the invading pathogens before they can harm the horseshoe crab.
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