Horseshoe Crab

Image: la-blue-eyez via Flickr

I have always found Horseshoe Crabs to be exceptionally strange and peculiar animals. Looking at them, it's really difficult to understand where all their body parts are and how it all fits together. It looks like a mystery wrapped in an enigma covered in a shell. Indeed, that all engulfing shell doesn't help matters. Neither does their lifestyle of ploughing through the mud and sand at the bottom of shallow seas. But things actually get weirder when you find out more. Good. Let's do that then.


Firstly, horseshoe crabs aren't crabs at all. They aren't even crustaceans. They instead belong to the subphylum Chelicerata, alongside the arachnids, making them more closely related to spiders. There are four species, one of which can be found all along the eastern coast of the US, the others coming from Asia and ranging between Japan and India. The Atlantic horseshoe crab can reach 60 centimetres (24 inches) in length including tail, the others are smaller. They are also extremely ancient; the first horseshoe crabs arose some 450 million years ago. Eventually, they hit the jackpot and have barely changed at all for some 250 million years.

Image via Wikipedia

Horseshoe crabs are made up of three parts which can be seen quite clearly in the shell. The first two are much like in spiders; the head-thorax region is covered with the big, round shell, the abdomen is covered mostly by that smaller, thorny, vicious looking shell. A tough, stiff tail helps the horseshoe crab flip itself over if it gets upside down. Did you ever see something like Robot Wars? The ones that could right themselves always had a bit of an advantage.

Horseshoe crabs have numerous eyes. The largest pair are compound eyes that are on top of their shell. This is actually quite strange when you think about it, there aren't many creatures that have eyes on top of a protective carapace. When you see a horseshoe crab in the mud and focus on those eyes, it starts to look like the angry glare of a gigantic and suitably terrifying burrowing beast. Probably for the best that it's just a harmless horseshoe crab. I guess. There are several more simpler eyes on the shell and a couple more on the underside, where they are joined by a small chemoreceptor organ to taste the ground they're walking on. Add to this a host of light sensors all along their tail, and the horseshoe crab has all the information it needs to live and feed.

Image via Wikipedia

Horseshoe crabs have 8 legs, 2 pedipalps that have become legs and 2 chelicerae. The legs are for walking around and mostly end in little pincers. The last pair are a bit more complicated and are used to push through the mud and sand. Pedipalps are like the pincers of scorpions or those little leg-like appendages on spiders. In the horseshoe crab they look and work just like the legs. The chelicerae are like spider fangs or those horrible crunching, munching armaments on our very own camel spider. Here, they look quite a lot like smaller versions of the legs, but these pincers are used to put food into their mouth.

Strangely, that mouth can be found at the centre of all those legs. Seems like a pretty weird place to put your mouth. Not a very pretty place either. It works well though. The horseshoe crab seeks out worms and molluscs, they have no jaws so they use their pincered legs to cut them to pieces, and yes, they can also crack mussels open. Since they effectively use their feet to chew, it's natural to put your mouth in the middle of them, right? They also have a gizzard, a sort of pouch inside that they fill with sand and pebbles to help them crush food after they've swallowed it. Be thankful for your molars.

Image via Wikipedia

Lastly there are the book gills. These are used to breathe, there are five pairs of them and each one has about 100 layers that make it look like pages of a book. They also let the horseshoe crab swim upside down at the water's surface, which is pretty cool. It's good to have options.

At breeding time, horseshoe crabs clamber up to the shore. Males, the smaller of the species, grab onto the females and are dragged along like the lazy so-and-so's they are. Females dig holes in the sand and lay several thousand eggs before moving on, allowing the males to fertilise them. They repeat this until some 100,000 eggs are laid before they descend back beneath the waves. Youngsters moult 6 times in their first year, getting about 30% bigger each time. Horseshoe crabs kind of remind me of our similarly ancient tadpole shrimp, but those guys moulted once or twice a day. Horseshoes are thought to live for about 20 or 40 years, so I guess they have a lot more time to play with.

They also have some incredible blood, and it isn't even because it goes blue when in contact with oxygen. Rather, it's because they have amoebocytes which act, funnily enough, like our very own amoeba. Scientists very carefully extract their blood to use in tests for bacterial endotoxins in pharmaceuticals. Bacterial endotoxins are bad things to have in pharmaceuticals. No better method of testing has been found and the horseshoe crabs are soon returned to their watery home, so it doesn't seem to be too bad of a deal.

Who would think that such a beastly, thorny, misnamed monster and its horde of mercenary amoebae could prove such a boon? That's why it's good not to kill stuff until there's none of them left. Just saying.

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Video taken from Life in the Undergrowth.

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