Horseshoe crabs are not crabs, and their blood helps detect bacterial contamination
Despite the name, horseshoe crabs are closer kin to spiders, ticks and scorpions than to any crab or lobster. They have barely changed in some 250 million years, and people now harvest them for a substance in their blood, Limulus amebocyte lysate, that reveals bacterial toxins.
Their broad, shield-like front section, a fused head and thorax, is shaped like a horse's hoof and holds most of the eyes, limbs and organs. Behind it sit a three-lobed abdomen, rather like a trilobite's, and a long tail spine. Underneath are six pairs of appendages, starting with tiny pincers before the mouth, and five pairs of book gills, flaps used for breathing that double as paddles when the animal swims up from the seabed.
Relatives resembling them appear in rocks from the Early Ordovician, around 480 million years ago, and modern forms arrived in the Early Triassic. Their nearest cousins may be the extinct sea scorpions, among the largest arthropods that ever lived. Of 22 known species only four survive: the Atlantic horseshoe crab of North America's east coast and Gulf of Mexico, plus three Asian species, the mangrove, tri-spine and Indo-Pacific. Their last shared ancestor lived about 135 million years ago in the Cretaceous.
Their DNA is unusually bulky for an invertebrate. An ancestor shared with arachnids duplicated its whole genome, and the lineage leading to living horseshoe crabs did so at least twice more, leaving the mangrove and tri-spine species with genomes of roughly 1.72 gigabases. Females outgrow males, partly because they mature a year later after one extra moult and partly because bigger bodies carry more eggs.
Besides the blood harvest, Americans use them as fishing bait and some Asian cuisines treat them as a delicacy. Overharvesting and coastal habitat loss have driven populations down, prompting harvest rules and captive breeding in many areas. Delaware Bay remains a key migration site for the Atlantic species.
Source: Horseshoe crab
