The Horseshoe Crab: World’s Most Successful Animal

June 17, 2013

Hundreds of thousands of horseshoe crabs come ashore to lay eggs in the sands of Delaware Bay in the eastern United States. Photo: Asturnut under a Creative Commons license.

By Craig Leisher, senior social scientist

Horseshoe crabs are, arguably, the most successful animals on earth, having survived for 445 million years. That’s 440 million years longer than humans and 130 million years longer than the über-survivor cockroach.

It’s the ‘crab moon’ right now when hundreds of thousands of horseshoe crabs come ashore to lay eggs in the sands of Delaware Bay in the eastern United States. The crabs have been coming to beaches like these for a long, long time.

If there were actuarial tables for species survival, horseshoe crabs would have the world’s lowest species survival insurance premiums. They have already made it through asteroids hitting the earth, at least three ice ages, changes in sea level, and large fluctuations in atmospheric CO2.

But their biggest challenge in this aeon may be surviving people harvesting them by the pickup-truck load to be ground into fertilizer or used as bait for eel and conch fishing. A large female can sell for $5 to an eel fisherman, and horseshoe crabs are ridiculously easy to catch during spawning season. You just pick them up with your hands. They don’t bite, and they can’t run.

In addition to being the first input on the way to catching eels for sushi or conch for a sandwich, horseshoe crabs are big in medicine.

There’s a $50-million (US) industry that does nothing but collect blood from horseshoe crabs. The animals are true blue bloods with a copper-based blood that’s the color of lapis lazuli.

Horseshoe crab blood has a unique property: in the presence of bacterial, it turns to jelly and creates a barrier against the bacteria. The space shuttle team used horseshoe crab blood to wipe down shuttle surfaces and ensure they were bacterial free.

The medical industry uses the blood to ensure injectable drugs are free of endotoxins. After the blood letting, the crabs are released back into the wild, and of course, all but a small percentage survive.

What does it take to make it 445 million years? In addition to anti-pathogen blue blood, the horseshoe crab has gills that are like the pages of a book allowing it to breath out of water so long as its gills stay wet, ten eyes for detecting potential mates, food and predators, and the ability to produce 90,000 fertile eggs a year.

In the Delaware Bay, migratory shore birds time their travels to arrive just as the horseshoe crabs start to emerge from the water to lay their eggs. The Bay is a prime refueling station for the Red Knot.

The rufa Red Knot is a legendary frequent flyer that migrates from the high artic to the tip of South American and back every year. One Red Knot, ‘B95’, has done the round trip so many times, he’s flown the distance to the moon and half way back. In May 2013, he was photographed on the beach in the Delaware Bay eating horseshoe crab eggs 18 years after he was first banded with the identification number B95.

The horseshoe crab may be weird and ugly, and more closely related to spiders than crabs, but it’s the world’s most successful animal and will probably outlast even the cockroach.

445 million years...and counting. Photo: Flickr user David Goehring under a Creative Commons license.
445 million years…and counting. Photo: Flickr user David Goehring under a Creative Commons license.

Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.

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  1. This is so interesting, Craig! I’m heading to Delaware this week and may go pay my tributes to this true endurance king!

  2. 445 million years, how accurate is that, plus or minus 450 million years. Dont believe it, these are the same people who think Carbon dating and isotope decay are actually accurate, heck the earth could only be 100 million years old, THEY DONT KNOW!!!

  3. This is so fascinating! I’m supposed to be reading a book! I just stopped to look up one word 15 mins ago!! I really must stop and read my book! I really don’t want too!! Thank you so much!!

  4. Neat intro to one of the most amazing and fabulous creatures on the planet!

    Do take the family to beaches on the Delaware Bay for the spawning (check moon phases and tide tables: full or new moon, high tide, best at night).

    Do pick up wandering crabs gently and show to any random kids. They look scary but are harmless to handle, and total survivors of FIVE mass extinctions. Also if you’ve ever had a medical procedure, thank a crab.

  5. I found this to be very interesting and well written. I thought the answer was cockroach.