Horseshoe Crab: Monster or Miracle

This post was insired by a Twitter conversation I had with @oceanshaman and by the fact that it is one of many creatures that I was terrified of as a kid, but as I learn more, I realize my fears are unfounded.

I went to North Carolina with about 20 other family members every summer for a handful of years. I played in the water a lot, but was completely terrified of anything that moved and lived in the ocean–fish especially. But one afternoon while waist-deep in the water, I felt something crawl over my left foot and by the time I had looked down, it was going over my right one.  As soon as my foot was free of its prickly, slow gait, I fled the water screaming to seek refuge at my ‘safe’ beach towel. 

What Is A Horseshoe Crab?

Like many things, this creature is misnamed. It is not a true crab, but

more closely related to spiders, ticks, and scorpions.  It is a remnant of its ancient lineage and hasn’t physically changed much for 250 million years.  In fact, their shape is really the only thing that resembles a crabs, who have gills while these ‘crabs’ have book lungs. 

In fact, its taxonomy explains this creature very well:

Phylum: Arthropoda Subphylum: Cheilcerata Class: Merostomata Subclass: Xiphosura  Order: Xiphosurida  Family: Limulidae
Genus: Limulus Species: polyphemus

…Or a joint-legged animal with no jaws having a mouth surrounded by legs and a sword-like tail with one living member left in the family having large compound eyes on the sides of its head.

Sounds like I just described a monster, doesn’t it?  It fits perfectly with the creature I felt crawling over my foot. But this fierce, primative-looking creature from the deep is nothing more than a worm slurper.

Dangerous or A Passive Predator

Remember reading above that this creature had no jaws and a mouth surrounded by legs? With this in mind, I am sure you are thinking that it would be rather difficult to chew. Indeed! When a horseshoe crab crawls over the sandy bottoms, it senses worms and mollusks with its claw-tipped feet. Once a food item is found, it picks it up with those claws, passes it to where its legs meet its shell.  Once there, the legs rip up the food as the horseshoe crab walks and the food bits that make it to the bristles near its shell are eventually passed to the mouth to be consumed. The horseshoe crab also has a gizzard-like organ in which sand helps further grind up the food. 

Book gills are to the right

So all the armor and fierce decoration on this creature works more as a deterrent than as a weapon.  The little claws on the end of the legs are not strong enough to break a person’s skin and since it has no jaw, it can’t bite, but it is possible to get pinched by placing your fingers near where the legs meet the body. The tail looks sharp as if it were the same as a stingray, but if you watch a horseshoe crab for a while, you’ll see the tail is necessary for maneuvering in the sand and surf. In fact, this creature is docile enough to keep in aquarium touch tanks that you can find across the country.

Interesting Features

It’s often stated that horseshoe crabs have 10 eyes, but more specifically they have one set of compound eyes, which look most like eyes to us, and a pair of simple eyes towards the front of the shell and then a collection of light sensing cells scattered around the top. In total, areas that can ‘see’ and detect light equal ten, but not ten blinking eyes that most people picture.

It’s really easy to tell males and females apart. Females, for one, are much larger than their male counterparts, but to be sure flip the crab over and look at the first clawed foot.  If it looks like they rest of them, then it’s a female [or maybe an immature male].  But if the first claw looks like an oven mit or boxing glove, then it’s a male.  He uses this specialized claw to hang on to an egg laying female while she digs holes in which to deposit her eggs. As the male is along for the ride, he is pulled over the newly deposited eggs and fertilizes them. It takes almost a decade for horseshoe crabs to mature.

Important–Of Course!

Since there really isn’t anything to worry about when handling a horseshoe crab and it only eats worms and other invertebrates, is there really any reason to worry about this creature?

Indeed, there is! As an important spoke in the food web, the horseshoe crab serves as a food item for many creatures, as do the eggs of the horseshoe crabs and the larvae that briefly float around in the water column. Birds, fish, and crabs eat the eggs and larva of the crabs and adults are eaten by sea turtles.  Not just a food source, they also house many creatures, from barnacles to worms in their gills. Though not listed as endangered, they do warrant protection.

But if you’re not interested in who eats whom out in the deep, then maybe this will be of interest.  In some states you are not allowed to harvest horseshoe crabs except to allow for their blood to be drawn.  Their blood is used in standard testing for detecting impurities in our IVs and other medical fluids. They don’t go and bleed the horseshoe crabs dry, though. They catch wild individuals, draw some blood, and then release them back to the same location from which they were caught.  So if you’ve ever been to the hospital, you have a horseshoe crab to thank for not getting any microbial infections from your IVs!

Sources/Good Reads:

Horseshoe Crab Biology

Horseshoe Crabs, SCDNR

Horseshoe Crab.org

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