Salute to the Gladiators of the Sea

Have you been vaccinated recently? Took medicine without any mishaps?

The merit goes to our clanky fellow—the horseshoe crab (Limulus polyphemus)—for its precious blue blood.

Nope, they are not royalty, their blood is literally blue—it contains hemocyanin, a copper-based molecule carrying copper [1,2] which, when oxidized, turns bluish-green [1,3]. Meanwhile, our blood uses hemoglobin which carries oxygen (has iron in it), thus the reddish hue [1,3].

These ‘crabs’ are not true crabs, not even crustaceans [2,3]. In fact, they are under the subphylum Chelicerata [3,7], more akin to scorpions and spiders than they are to crabs [2,7]. They boast 10 eyes: large compound eyes, in particular, aid in locating a mate [3,4]. Their tails may look like a scathing weapon against predators; in fact, they use it to propel in different directions [3,4], or to flip them right up when capsized [2,3].

Thousands of these ‘living fossils’ form throngs in Delaware Bay every May and June, ready to mate. A female can release as much as 90,000 eggs per clutch but only around 10 are deemed to reach adulthood [3].

Horseshoe crabs are fine, robust, armor-clad creatures, as the paleontologist Richard Fortney remarked [1]. Time has been their ally, predating the dinosaurs for more than 200 million years [2,7]. A big hole on the head, a lump on the thorax, or a cracked tail spike did not obliterate these 450-million-year old ‘gundams.’ [1,4].

What helps them become almost invincible?

When a horseshoe crab gets wounded, its blood instantly releases an army of blood-clotting granules which seal the invading bacteria, preventing further infection [1]— the same, humbling reason, why we get to be safely injected with vaccines for four decades now [3].

Today, their blood is extensively used to test products, intravenous drugs and medical devices that come into contact with blood. Essentially, its active ingredient is a sentinel against “negative” bacteria, which is confirmed present if the cells clot in contact with a product [1,2,4,5]. Suffice it to say, horseshoe crabs have been saving millions of lives from unsanitary injections [3].

Photo credit: Popular Mechanics

One quart (almost 1 liter) of horseshoe blood is sold by Atlantic fishermen to pharmaceutical companies for an astounding $15,000, a lucrative business with more than 600,000 'donors' being bled [3,6].

To obtain the blue blood they are hosed up, sucking 30 percent of their blood [2,3,6]. They are released back into the sea after 48 hours, dizzy after a clueless donation [3]. It is estimated that 3 to 15 percent of these crabs die after being bled [1], while those that survive become sluggish [3]. Also, scientists saw a decline in the population of horseshoe crab in Delaware and so prompted the creation of a sanctuary [1]. They have been assessed as Vulnerable since last year [8]. Scientists, hence, are on their way to creating synthetic amebocytes [3].

We may not live for as long as they have, but next time we receive a safe vaccination or feel well after a medication, we ought to thank a horseshoe crab.

To know more about horseshoe crabs, visit SeaLifeBase.

If you have more information on horseshoe crabs and other non-fish organisms, we'll be happy to have you as one of our collaborators. Let us know by sending us an email or visiting our FaceBook page.

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[1] Krulwich, R. (2012, June 1).What the vampire said to the horseshoe crab: ‘your blood is blue?’ Retrieved from https://goo.gl/66sdMC

[2] National Ocean Service (2015). Are horseshoe crabs really crabs? Retrieved from https://goo.gl/J9zEw6

[3] Mancini, M. (2015, September 21). 10 hard-shelled facts about horseshoe crabs. Mental Floss. Retrieved from https://goo.gl/JNBSHT

[4] Walker, K. (2014, July 15). 10 facts about horseshoe crabs. Retrieved from https://goo.gl/WTmg7M

[5] Jones, L. (2015, April 13). Are there some animals that have stopped evolving? BBC Earth. Retrieved from https://goo.gl/y8AQ22

[6] Moss, L. (2014, March 11). Why is horseshoe crab so vital to pharmaceuticals? Mother Nature Network. Retrieved from https://goo.gl/23czRv

[7] Edgecomb, M. (2002, June 21). Horseshoe crabs remain mysteries to biologists. National Geographic. Retrieved from https://goo.gl/Tz9Hys

[8] Smith, D.R., Beekey, M.A., Brockmann, H.J., King, T.L., Millard, M.J. & Zaldívar-Rae, J.A. 2016. Limulus polyphemus. The IUCN Red List of Threatened Species 2016: e.T11987A80159830. https://goo.gl/L8nZvT