Re-turning the favor

Since I can remember, summers for me meant the shore. When my siblings and I weren’t playing leap frog with the waves or making drizzle sand towers, we patrolled the beach. We buried the jellyfish that washed up on shore, lest anyone fall victim to an ill-placed foot. We picked up stray trash so that it wouldn’t hurt animals like those 6-pack rings did. My favorite duty, however, was gingerly righting the horseshoe crabs that had the misfortune of winding up on their backs.

Looking like a primordial holdover, the horseshoe crab easily evoked my sympathies. It reminded me of The Little House: sturdily built to last the ages, it was made to look lumbering by the civilization that sprung up around it. And to think 300 million years ago it was the dominant creature on the planet. Further proof they get no respect: they aren’t even crabs! The horseshoe crab is an arthropod that’s more closely related to spiders or scorpions than any of the crustaceans.

From The Little House by Virginia Lee Burton

My grandma’s house sits on the southern tip of New Jersey on the Delaware Bay, which happens to be prime breeding ground for the horseshoe crab. Every year as my family descended on the beach on the bay, the horseshoe crabs came to mate and lay their pearly green eggs.

When horseshoe crabs emerge from the surf and slowly traverse the sand, their legs remain hidden from view beneath their shells. It gives them the appearance of helmets commandeered by stowaways (like the old cartoon bucket or traffic cone shtick). I remember crouching down close to them, squinting hard to see if I could make out an eyeball where the bump on the shell suggested one would be (they actually have 10). Often one would sit so still, I’d be sure it was a once-but-no-longer “living fossil”, when suddenly it would lurch forward.

When they were on their backs, however, the difference between life and death was stark: they were either an empty plate where a seagull’s dinner used to be or a frenzy of legs. The horseshoe crab’s tail can prop itself up, helping the wake to flip it over. Farther from shore, a little human intervention goes a long way.

The more pressing issue for the horseshoe crabs, and the migratory birds who rely on their eggs to fuel their journey, is how many make it ashore to spawn. Starting in the 90s, the number of horseshoe crabs began to dwindle, triggered by their use as bait for eel and conch that was sold for sushi. This period of decline went largely unnoticed by my siblings and me (NJ declared a moratorium on harvesting them in 2006). We were growing up; we spent less of our summer at the shore, and we simply didn’t look out for them the way that we used to.

I miss the days when I saw myself as the horseshoe crab’s protector, when the only problem that needed to be solved was what lay – bottom’s up –  in front of me. Anyway, I’d had it wrong: I was under the horseshoe crab’s protection. Most of us are.

******

The horseshoe crab is something of a court taster in the biomedical industry. Which is rather funny considering it’s the real blue blood.

Up to a 1/3 of the horseshoe crab’s blood is collected in the lab before they’re released. Photo credit: Andrew Tingle

In the Middle Ages, the court taster’s job was to intercept poisoned food before it was served to his lord. We may not worry about hemlock or foxglove nowadays, but injectable drugs and implanted devices (e.g. pacemakers) threaten to serve up a different kind of poison, called endotoxin. Endotoxin is a molecule that decorates the outer surface of gram-negative bacteria, so named because they don’t stain purple in the Gram bacteria identification test.

Since the advent of the hypodermic needle, endotoxin has been a headache for the pharmaceutical industry. In the late 19th century, doctors noticed that injecting sterile drugs still caused some people to develop “injection fever”. What they didn’t know at the time is that even if the bacteria is killed, the remnants of its outer membrane – including endotoxin – can stick around. Injection fever, and in the most extreme case sepsis, isn’t actually caused by something that the bacteria does to our body. It’s our body’s immune response to the endotoxin that can prove deadly (read more about sepsis in Maryn McKenna’s SA article). Because they can’t filter endotoxin or heat-kill its fever causing effect, pharmaceutical companies need to ensure that it isn’t there in the first place. This is where they defer to the horseshoe crab.

The horseshoe crab “taste test” depends on a remarkable extract of its powder blue blood called Limulus amoebocyte lysate, or LAL. What makes LAL such a sensitive test – and worth its whopping $15,000 price tag per quart – is that horseshoe crabs know our potential poisoner very well. In fact, they’ve been locked in battle with gram-negative bacteria for millions of years. In its oceanic home, the horseshoe crab is constantly under threat of bacterial invasion. It also has a relatively open circulatory system that allows blood to directly contact large areas of tissue. Taken together, it would be easy to imagine how a minor crack in its shell could give bacteria easy access to tissue, spelling out disaster for the horseshoe crab. Luckily horseshoe crabs have evolved a type of immune cell that senses when bacteria is present in the blood. It knows this by gram-negative bacteria’s calling card – endotoxin.

When the horseshoe crab’s immune cells, called amoebocytes, detect endotoxin they swarm the bacteria, releasing clotting agents that stop up breaches in the shell and trap bacteria. Once the bacteria is sequestered, amoebocytes release killer proteins. The LAL used in the lab is the clotting agent collected by lysing, or rupturing, the amoebocytes. The LAL is freeze-dried in powder form, and gets reconstituted in endotoxin-free water when it’s time to test a sample. If endotoxin is present, the sample becomes a gel, and the drug batch or device gets flagged as contaminated.

Normal amoebocytes in a horseshoe crab gill

Amoebocytes respond to endotoxin in Bang’s 1956 paper.

This unique property of horseshoe crab blood was observed by Dr. Fred Bang in the 1950s. He infected several horseshoe crabs with a gram-negative bacteria from sea water. He noticed that one in particular became sluggish and looked ill. On inspecting the dead horseshoe crab, Bang was surprised to discover that almost its entire blood volume had become a gel. He went on to show that this clotting effect was triggered by endotoxin and carried out by amoebocytes. Today, producing LAL is a $50 million industry. Companies capture horseshoe crabs and bring them back to the laboratory where they collect up to 30% of their blood. The horseshoe crabs are released back into the wild within 72 hours, and figures suggest that as many as 10-15% of horseshoe crabs die following collection.

Flipping over horseshoe crabs won’t ensure their future, but that doesn’t mean that we can’t act as their protectors. After hundreds of million of years on this planet, their survival depends on the preservation of their spawning habitat and the regulation of their harvesting. The public’s awareness of the horseshoe crab’s life-saving contribution is an important piece of the puzzle.

Remember that the same horseshoe crab that looks so helpless on its back might have saved your life. Return the favor.

 *******

Hurricane Sandy posed the latest challenge to horseshoe crab spawning, as it stripped them of up to 70% of their mating habitat. After negotiating the man-made barriers of debris, the crabs would be met with thick clay and vegetation where there had once been sand. Fortunately, environmental agencies stepped in to replace sand on breeding grounds ahead of the arrival of the crabs and birds. However, the long term impact of habitat destruction remains to be seen.

Sources:

Bang, Fred. 1956 paper 

PBS “Crash: a Tale of Two Species”

FDA guidelines LAL testing

McKenna, M. Researchers Struggle to Develop New Treatments for Sepsis. Scientific American.

MBL, The Horseshoe Crab

Solon, O. Annual Blood Harvest of the Horseshoe Crab. Wired.

Other sources in links

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2 comments

  1. Anna

    Great post, Kristen! It’s interesting that horseshoe crabs evolved to have this system to identify and trap the bacteria rather than evolving to have a less vulnerable circulatory system. Any thoughts on that?
    Also, I’m glad that their blood can be harvested without having to kill them.

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