Invertebrates
Horseshoe Crab
Xiphosura · also called horseshoe crabs, king crab
A horseshoe crab is not a crab, and it is older than almost everything. The same helmet-shaped, spike-tailed body has been crawling along the sea floor for more than 400 million years, through five mass extinctions. It is a cousin of spiders and scorpions, and it has pale blue blood so good at detecting bacterial contamination that the medical industry catches half a million of them a year to bleed.
Overview
Horseshoe crabs are the Xiphosura, four living species — one on the Atlantic and Gulf coasts of North America, three in Asia. They are chelicerates, kin to spiders, scorpions, ticks, and mites, not crustaceans. They plough soft sediment for worms, clams, and carrion, crushing food between their leg bases because they have no jaws.
A living fossil
Fossil horseshoe crabs almost identical to today’s date back around 445 million years — before dinosaurs, before trees. The animal has about ten eyes, some sensitive to ultraviolet light, and its long spiky tail (telson) is a rudder and a lever for flipping upright, not a weapon.
Blue blood
Horseshoe crab blood is blue (copper-based hemocyanin) and clots instantly around bacterial toxins. That reaction became the LAL test, used worldwide to check that vaccines, injectable drugs, and implants are free of contamination. Around 500,000 crabs a year are bled and released; an estimated 5–30% die, which has driven a now-approved synthetic replacement.
Status
The American horseshoe crab is Vulnerable and declining, reduced by decades of harvest for bait and biomedical use. The Asian tri-spine horseshoe crab is Endangered after collapsing across China and Southeast Asia from coastal development and overharvest. On Delaware Bay, the crabs’ spawning fuels the migration of red knots and other shorebirds.
Horseshoe Crab: common questions
Is a horseshoe crab actually a crab?
No. A horseshoe crab is not a crustacean and is not closely related to crabs, lobsters, or shrimp. It belongs to the chelicerates, the same major group as spiders, scorpions, ticks, mites, and the extinct sea scorpions, and within that group it is the sole survivor of an ancient lineage. Its closest living relatives are the arachnids. The resemblance to a crab is superficial, coming from the hard shell and the many legs. Anatomically it is very different, with a mouth surrounded by the leg bases, no antennae, no jaws, book gills rather than crustacean gills, and a pair of small pincer-like appendages called chelicerae, the feature the whole group is named for.
Why is horseshoe crab blood so valuable?
Its blood contains cells called amebocytes that react to tiny amounts of bacterial toxin, called endotoxin, by clotting almost instantly. In the 1970s this reaction was turned into the Limulus amebocyte lysate, or LAL, test, which became the global standard for checking that anything injected into the human body, including every vaccine, intravenous drug, and implanted device, is free of endotoxin contamination. There was no equally reliable alternative for decades, so a small, specialised industry grew up around collecting live horseshoe crabs, drawing off part of their blood, and releasing them. Raw horseshoe crab blood has been quoted at figures like 15,000 US dollars per litre, reflecting how much processing and how many animals lie behind each batch.
Does bleeding kill the horseshoe crabs?
Many survive, but a significant fraction do not. Crabs are collected from spawning beaches or by trawl, held out of water, drained of up to about 30 percent of their blood, and then returned, usually to a different area. Estimates of the death rate from this process range from around 5 percent to as high as 30 percent depending on handling, temperature, and time out of water, which adds up to tens of thousands of animals a year. Survivors may also have reduced activity and, in females, reduced spawning for some time afterwards. Concern about this, combined with supply worries, has driven the development of a synthetic substitute called recombinant factor C, which is now approved in Europe and is slowly being adopted elsewhere.
What does the horseshoe crab's tail do?
The long, pointed tail, properly called the telson, is a tool, not a weapon. It is not venomous, cannot sting, and is not used in defence. The animal uses it mainly as a rudder to steer while swimming and crawling, and as a lever to right itself. A horseshoe crab flipped onto its back by waves arches its body and pushes with the telson against the ground to flip back over, and one that cannot do this will die stranded in the sun, which is why beachgoers are asked to pick up an upside-down horseshoe crab by the edges of the shell, never by the tail, and turn it the right way up.