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10 Hard-Shelled Facts About Horseshoe Crabs

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The plodding sea creatures have weird blood, weirder swimming habits, and a secret weapon that’s probably saved your life.  

1. HORSESHOE CRABS ARE INCREDIBLY OLD.

Discovered in 2008, the 25 millimeter-wide Lunataspis aurora crawled over Manitoba 445 million years ago. This makes it the world’s oldest-known horseshoe crab. Four species are with us today, all of which closely resemble their long-extinct ancestors.

Supposedly frozen in time, horseshoe crabs are often hailed as “living fossils” by the media. Yet, appearances can be misleading. Evolution didn’t really leave these invertebrates behind. They’ve transformed quite a bit over the past half-billion years. For instance, some prehistoric species had limbs that split out into two branches, but today's specimens have only one.

2. THEY'RE NOT CRABS.

In fact, they aren’t even crustaceans. Unlike real crabs and their kin, horseshoe “crabs” lack antennae. So, where do the strange ocean-dwellers belong on the arthropod family tree? Biologists classify them as chelicerates, a subphylum that also includes arachnids. Members possess two main body segments and a pair of unique, pincer-like feeding appendages called chelicerae (hence the name).

3. EACH ONE HAS A HUGE ARRAY OF SIGHT ORGANS.

Rachel Oh via Wikimedia Commons // CC BY-SA 3.0

Large compound eyes rest on the sides of their shells. Come mating season, these bean-shaped units help amorous crabs locate a partner. Behind each one, there’s a small, primitive photoreceptor called a lateral eye. Towards the front of the shell are two tiny median eyes and a single endoparietal eye. On its underside, a horseshoe has two “ventral eyes,” which presumably help it navigate while swimming.

Most interesting to scientists are the compound pair. By virtue of their relatively simple wiring, they’re easy to study and have taught us a great deal about how our own eyes function.

4. BABIES CAN SWIM UPSIDE DOWN.

Walking around on the ocean floor is generally how horseshoe crabs get from point A to point B. Nevertheless, young ones will often flip over and start propelling themselves through the water, using their gills as extra paddles. With age, they do this less frequently.

5. THE SPIKED TAIL HAS SEVERAL USES.


Stinging isn't one of them, despite what many falsely believe. Among its uses are assuming rudder duties and helping the arthropod right itself after getting stuck on its back. 

6. ADULTS MAINLY EAT BIVALVES.

Both larvae and fully grown horseshoes eat aquatic worms. Though adults will also devour algae and carrion, they predominantly consume clams and mussels. When feeding time comes, these low-profile predators mash food between the spiky upper regions of their legs before pushing it into the mouth. 

7. HUNDREDS OF THOUSANDS GATHER IN DELAWARE BAY FOR A MASSIVE ANNUAL ORGY.

Every year in May and June, the bay becomes the largest Atlantic horseshoe crab spawning zone on Earth. During the night, a female will climb ashore with a male (or several) in hot pursuit. After she digs a hole and deposits her eggs, the males fertilize them. Migratory shore birds descend upon the bay in huge numbers, fattening themselves on the nutrient-rich eggs. Among these avians are scores of red knots, which use the crab fest as a final pit stop during their yearly migration from the Arctic to South America.

8. VERY FEW SURVIVE INTO ADULTHOOD.

A mother can lay as many as 90,000 eggs per clutch. Even so, it’s estimated that only about 10 of those individual embryos will ever become adults. Before they get a chance to hatch, fish, sea turtles, and birds gorge themselves on the eggs. With so many animals utterly dependent on this fodder, nesting horseshoe crabs are vital to the ecology of Delaware Bay and countless other regions around the world.   

9. ATLANTIC HORSESHOE CRAB FEMALES ARE 25 TO 30 PERCENT BIGGER THAN MALES.

When it comes to reproduction, females also mature more slowly: While males are ready to mate by age 8 or 9, their counterparts don’t start breeding until age 10 or 11. 

10. IF YOU'RE UNDER 40 AND HAVE BEEN VACCINATED, THANK A HORSESHOE CRAB.

While human beings don’t have blue blood, horseshoe crabs do. Whereas our blood uses iron-based hemoglobin to transport oxygen throughout the body, horseshoe crabs rely on hemocyanin, which contains copper. Copper turns bluish-green when it oxidizes.

Strange coloration isn’t the only remarkable thing about horseshoe crab blood. Shorelines are downright squalid: a single gram of undersea sediment contains roughly 1 billion bacteria. Unlike us, the arthropods lack infection-fighting white blood cells. Instead, special cells called amebocytes attack pathogens in the horseshoe crab's body by sealing them inside a gooey physical barrier, thus halting the malady’s spread.

Ever since Johns Hopkins University physician Frederick Bang discovered this characteristic in 1956, medical scientists have been capitalizing on it. To ensure that a vaccine or injectable drug is safe, they introduce horseshoe crab amebocytes into a sample. If the cells start releasing their goo, it’s because they’ve encountered bacteria and, therefore, the product isn’t ready yet. In the 1970s, the FDA made this test mandatory for experimental drugs and surgical implants. Without these magnificent animals, thousands—perhaps even millions—of people might have died during the past four decades from unsanitary injections.

As you might expect, horseshoe crab blood is worth a pretty penny: sellers now command $15,000 per quart. The magic elixir is extracted from over 600,000 “donors” every year, each of whom parts with 30 percent of its blood before being released within 48 hours. Sadly, though, many don’t last that long. About 10–15 percent of captured crabs die somewhere in the process, and survivors can exhibit lethargic behavior down the road. Scientists aren’t insensitive to this problem. Researchers have been trying to develop synthetic amebocytes.

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iStock // Ekaterina Minaeva
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Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
May 21, 2017
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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May 23, 2017
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