Green Eggs & Sand: A Horseshoe Crab Workshop

Two weekends ago, I drove over to Delaware to participate in a workshop called Green Eggs & Sand. The workshop revolved around one of my favourite marine animals—the horseshoe crab!

For those of you that haven’t had the pleasure of meeting a horseshoe crab, allow me to do a quick e-introduction. There are four species of horseshoe crabs, three in Asia, and one in North America—the Atlantic horseshoe crab (Limulus polyphemus). Despite their intimidating ‘armoured stingray’ appearance, they are not stingrays, nor are they true crabs. These harmless creatures are related to spiders, ticks and scorpions more than anything else. Horseshoe crabs have been around for a long, long time, appearing in the fossil record around 445 million years ago, hundreds of millions of years before dinosaurs! And while they have changed ‘under the hood’ over the years, they have kept their basic body plan – similar to the Volkswagen beetle.

A horseshoe crab pair (female in the front, male clinging on in the back--note the higher arch of the male's shell--this allows him to 'fit' tightly onto the female's back). Image Credit: Megan Chen
A horseshoe crab mating pair (female in the front, male clinging on in the back–note the higher arch of the male’s shell–this allows him to ‘fit’ tightly onto the female’s back). Image Credit: Megan Chen

Horseshoe crabs (Limulus polyphemus) are an important resource. They not only serve great ecological importance, but also are central to the health and livelihoods of people. They are intrinsically valuable to some people who travel to view their spawning events (wink wink), their eggs are food for fish and shorebirds, attracting bird enthusiasts, fishermen or watermen use them as bait to catch eel and whelks (large sea snails), and their blood is widely used in the biomedical industry. The workshop introduced us to the challenge of managing horseshoe crab harvesting, and featured a wealth of experts from many user groups. Here are a few highlights:


Horseshoe Crabs Live!

Seeing these guys in their natural habitat was squeal-like-a-little-girl-worthy. We showed up before high tide on the eve before the full moon when tides are high. The first thing that struck me was how strong the wave action was. The crashing waves were tossing several horseshoe crabs around like rubber ducks in a bathtub occupied by an exuberantly splashing baby. Horseshoe crabs are not normally jostling for space on the wavy shores, they follow high tides up for one purpose—bumping uglies. To be clear, horseshoe crab spawning is external, the female digs about 10-15 cm into the sand kept liquefied by rushing waves to deposit clumps of tiny eggs, and the males—some clinging desperately onto the back spines of the female–will release sperm to fertilize those eggs. Each egg is about the size of a small candy nerd and each clump can contain around 2,000-4,000 eggs (females can lay up to 80,000 eggs in a year).  These eggs in turn fuel the highest density of feeding, migratory shorebirds in the world!  Choosing how far up the shore to lay eggs is a delicate balance.  Too high, and the eggs risk drying out, and the horseshoe crabs may get stranded as the tide recedes.  Too low and the eggs develop too slowly in the low-oxygen environment and run the risk of getting poisoned by hydrogen sulfide.   As the sun set and the tide inched in, a thick band of scrambling horseshoe crabs hugged the shoreline for miles.  Here, we were witnessing an ancient, million year-old tradition…it was something special!

Horseshoe crab egg laying scramble, an ancient ritual. Photo Credit: Megan Chen
Horseshoe crab mating scramble, an ancient ritual. Photo Credit: Megan Chen


Meet Your Molt

meet your molt
Looking closely at a horseshoe crab molt. Photo credit: Megan Chen

In order to grow, horseshoe crabs molt, or shed their shell. They will molt 16-18 times until undergoing a final molt at around 10-12 years of age. This terminal molt represents a big stage in a horseshoe crab’s life… they are now reproductive! Horseshoe crabs molts look exactly like an empty shell of their walking counterparts. In this session, we sat down with some horseshoe crab molts to take a closer look at their anatomy. I highly encourage you to click this link for an excellent anatomical guide put together by one of the workshop organizers, Gary Kreamer.

#Protip: For educators with access to horseshoe crab molts, if you soak a horseshoe crab molt in a solution of 50% water and 50% glycerin, the molt will have some flexibility, allowing you to move the legs and bookgills without snapping them.

We also took a trip to DuPont Nature Center where I filmed these developing horseshoe crab eggs.  If you look closely, you might be able to see horseshoe crab molts inside the eggs!


The Horseshoe Crab Blood Test

Horseshoe crabs play a huge role in the biomedical industry. Anyone that has received a vaccine, injection-based drug, or inner body implant (e.g. heart valves) was spared the possibility of fever, shock and possibly death because of a sterility test made possible by horseshoe crab blood. I’ll spare the science, but the basic idea is that horseshoe crabs have blue, copper-based blood that clots when it encounters certain types of bacteria—an advantageous adaptation for an animal living in the ocean with constant bacterial exposure. The biomedical industry uses an extract of horseshoe crab blood to detect trace amounts of toxins in our medicine—and during one session, we tried out the test firsthand! We poured equal amounts of bottled water into one vial and water swished around in our bacteria-laden mouths in another. Both vials had white powder called LAL or Limulus Amebocyte Lysate, a product of processing horseshoe crab blood. After 15 minutes, the vial with bottled water remained runny, while the vial with the mouth-swished water formed a gel-like substance at the bottom of the test tube. Side note: this test is also done with an extract from the blood of an Asian (Tachypleus) horseshoe crab, and that test is appropriately called the TAL test for Tachypleus Amebocyte Lysate.

LAL test
Left: LAL vials filled with (1) bottled water, (2) water swished around in the mouth. Right: Water that had been swished around in the mouth formed a clot after 15 minutes. Photo credit: Megan Chen


Having been in love with horseshoe crabs for a long time, attending this workshop was an honoured experience. I met some incredibly smart and passionate people who are working towards keeping horseshoe crab populations healthy. The Atlantic Horseshoe crab is currently listed as Near Threatened on the IUCN Red List which may in part to harvesting pressure and habitat degradation. Hundreds of thousands of horseshoe crabs also die each year from being stuck upside down during spawning.

Stranded horseshoe crabs. Photo credit: Megan Chen
Stranded horseshoe crabs. Photo credit: Megan Chen

So next time you’re at the beach, if you see a stranded horseshoe crab, reach down and just flip ‘em. You’ll be doing your part to keep these ancient and important creatures around for another 445 million years.

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