A horseshoe crab: it is only when you see the shell wet from the water, close up, that you know they are real

They have milky blue blood that can detect toxins – and people in lab coats want it

Every day in bright clinical rooms in countries all over the world, horseshoe crabs are strapped into specially designed harnesses and drained of a third of their blood by people in lab coats. Then they are put back into rivers and oceans to swim-scuttle out their days.

Horseshoe crabs are prehistoric and they look it: a fossilised Roomba most of the way through eating a stingray. The horseshoe crab looks mainly like it should not be alive right now.

They are about 20 cm across and twice that length including their tails, and belong in museum dioramas, not the sea – or the bright clinical rooms. They belong on dusty papier-mache painted to look like rocks or in solid resin water, not strapped into perfect rows in rooms full of steel.

It is only when you see the shell wet from the water, close up, that you know they are real and alive. They are beautiful, finely cracked, a patch of dark olive on an old painting.

The blood. It is milky blue. And like no other substance it quickly, accurately detects toxins that can contaminate medicine. This is how we got here: half a million horseshoe crabs are caught and bled in laboratories every year. Below them in the harnesses are large glass jugs. Even the jugs seem strange, homely almost.

How do the crabs arrive? They are not living in tanks. No, men in peak caps and cargo shorts, in ordinary human clothes, go to the sea at night and yank the crabs from the water by their tails, chucking them into a pile on a boat.

Then they are chucked into trucks and driven to the bleeding facilities. To get the blood, a needle is injected through a hinge in the crab’s shell and into a membrane running along its heart (its heart is shaped like a caterpillar).

“Anyone who gets a flu or Covid shot, childhood immunisation, heart stent or hip replacement … is protected by [a test made using] the blue blood of the horseshoe crab,” according to Deborah Cramer, who has written a book about horseshoe crabs and the birds that eat their tiny green eggs.

The eggs: they are laid in a spawning event of millions of crabs over thousands of kilometres, females climbing the beach while dragging the males attached to their backs. They have been doing this at full moon and high tide since before dinosaurs. They look like hundreds of army helmets abandoned on the sand – until a wave flips one over, and you see legs flailing in the moonlight.

Magnified, wedged between biscuit-coloured grains of sand, the green eggs reveal themselves as transparent, but holding small green creatures, which, with their gently convex shells seem to delight in somersaulting against a concave wall. They have no idea that of all of the hundreds of millions of years they could have been born, with all of the things happening around them, they were born in this age; that they will meet vampires and live (most of them do, anyway) to tell the tale.

Helen Sullivan is a Guardian journalist. Her first book, a memoir called Freak of Nature, will be published in 2024

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