During the warmer months, especially
at night during the full moon horseshoe crabs emerge
from the sea to spawn. Waiting for them are teams of lab workers who capture the horseshoe crabs by the hundreds of
thousands, take them to labs, harvest their cerulean blood, then return them to the sea.
Oddly enough, we capture
horseshoe crabs on the beach because that's the only place we know we can find
them. A female horseshoe crab lays as many as 20 batches of up to 4,000 eggs on
her annual visit to the beach. When the eggs hatch, the juvenile horseshoe
crabs often stay near shore, periodically shedding their shells as they grow. Once
they leave these shallow waters, they don't return until they reach sexual
maturity ten years later.
Despite our best
efforts, we don't know where they spend those years. Though we've spotted the occasional
horseshoe crab as deep as 200 meters below the ocean's surface, we only see
large groups of adults when they come ashore to spawn. Horseshoe crab blood
contains cells called amebocytes that protect them from infection by viruses, fungi, and bacteria. Amebocytes
form gels around these invaders to prevent them from spreading infections. This
isn't unusual. All animals have protective immune systems.
But horseshoe crab
amebocytes are exceptionally sensitive to bacterial endotoxins. Endotoxins are
molecules from the cell walls of certain bacteria, including E. coli. Large
amounts of them are released when bacterial cells die, and they can make us
sick if they enter the blood stream. Many of the medicines and medical devices
we rely on can become contaminated, so we have to test them before they touch
our blood.
We do have tests called
Gram stains that detect bacteria, but they can't recognize endotoxins which can
be there even when bacteria aren't present. So scientists use an extract called
LAL produced from harvested horseshoe crab blood to test for endotoxins. They
add LAL to a medicine sample, and if gels form, bacterial endotoxins are
present. Today, the LAL test is used so widely that millions of people who've
never seen a horseshoe crab have been protected by their blood. If you've ever
had an injection, that probably includes you.
How did horseshoe crabs
end up with such special blood? Like other invertebrates, the horseshoe crab
has an open circulatory system. This means their blood isn't contained in blood
vessels, like ours. Instead, horseshoe crab blood flows freely through the body
cavity and comes in direct contact with tissues. If bacteria enters their
blood, it can quickly spread over a large area. Pair this vulnerability with
the horseshoe crab's bacteria-filled ocean and shoreline habitats, and it's
easy to see why they need such a sensitive immune response. Horseshoe crabs
survived mass extinction events that wiped out over 90% of life on Earth and
killed off the dinosaurs, but they're not invincible.
And the biggest
disruptions they've faced in millions of years come from us. Studies have shown
that up to 15% of horseshoe crabs die in the process of having their blood
harvested. And recent research suggests this number may be even higher. Researchers
have also observed fewer females returning to spawn at some of the most
harvested areas. Our impact on horseshoe crabs extends beyond the biomedical
industry, too. Coastal development destroys spawning sites, and horseshoe crabs
are also killed for fishing bait. There's ample evidence that their populations
are shrinking. Some researchers have started working to synthesize horseshoe
crab blood in the lab. For now, we're unlikely to stop our beach trips, but
hopefully, a synthetic alternative will someday eliminate our reliance on the
blood of these ancient creatures.
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