This is part two of my posts on horseshoe crabs. If you missed the first one, it’s here.
The horseshoe crab shares only a superficial resemblance to other arthropods, such as spiders, crabs and beetles. For example, like other members of this huge phylum it is completely covered and protected by an exoskeleton, which needs to be shed periodically as the animal grows. But in many ways this ancient animal is unique anatomically, and it is the lone surviving member of the class of arthropods called Merostomata.
The photos I’m using below to illustrate the parts of this animal were taken of one of my live adult males.
We’ll get into the differences between male and female horseshoe crabs below, but generally you can tell the difference between adults based on size alone. Females are much larger than the males, as you can see in the photo above. Although the one on the right is a model, the size proportions are accurate. You’ll have to excuse my knee. The live one wouldn’t stay still for the photo.
You’ll notice that the body of this creature is divided into three segments. The shield-like main part of the body is called the prosoma, the middle section is the opisthosoma, and the long tail is called the telson. The telson really just has one function, which is to turn the animal over when it is upside-down. It is not a defensive weapon and the horseshoe crab does not stick it up so that waders will step on it. And it certainly doesn’t contain poison or have any type of stinger on the end.
The middle section of the body, the opisthosoma, contains the gills as well as the musculature used to move the telson. If you look closely at the joint that attaches these two segments you’ll see a flexible white tissue, which are the tendons that allow the tail to move up and down and side to side. This is delicate and can easily tear if the crab is held like this:
Please don’t hold one this way.
Most of the organs are protected by the prosoma, and we’ll get into this more as we look at the ventral part of the body. On the top, the most prominent features are the large lateral eyes. Horseshoe crabs are in a group of arthropods called Chelicerates, along with trilobites, scorpions and spiders. Unlike all other living members of this group, horseshoe crabs have compound eyes like insects and crustaceans do (spiders do not). However, these are just the two most obvious eyes on the body. Horseshoe crabs actually have nine of them. There are two rudimentary lateral eyes lying right next to the compound eyes, two median eyes, which are found in the middle of the leading edge of the shell, a pair of ventral eyes near the mouth beneath the body (used mainly to sense if the animal is rightside up or upside down) and an endoparietal eye smack in the middle of the "forehead". The endoparietal eye senses ultraviolet light that is reflected off the moon at night. This is how the species is able to coordinate spawning to occur at the highest neap tide each spring.
A: Median eyes
B: Lateral compound eye
C: Endoparietal eye
On the underside of the opisthosoma is a series of six page-like structures called book gills. These are versatile organs used not only to breathe, just as fish use their gills for respiration, but are also used for swimming. Horseshoe crabs swim upside down by flapping the gills like wings. Swimming is an alternative mode of transportation used in emergencies, mainly to escape from predators or if the animal finds itself in rough surf. When it settles back down onto the bottom it is upside down and must use the telson to right itself. The book gills also serve to absorb water into the body when it is time to molt, a process I described in this diary on ecdysis.
(Bracelet is from Tanzania.)
Now for the legs. Horseshoe crabs have six pairs, five of them ambulatory. We’ll start with the pair closest to the book gills. These two legs are called "pushers" and provide most of the power the animal needs to crawl across the bottom. As you can see below, these legs are tipped with five appendages, four oar-shaped ones and a longer, forked one. These work together to propel the animal forward by digging into the sand and pushing off the bottom. Interestingly, horseshoe crabs cannot move in reverse. If they become trapped in a dead end, for example cornering themselves between rocks, they must flip over and try to swim their way out. I mentioned that one of the appendages is long and forked. This is called the tarsus and is used like a windshield wiper to clean the gills, keeping them functioning efficiently by clearing the gill surface of sand and slime.
Next we have three sets of identical "walking legs". Each one of these is tipped with a pair of pincers which are used to pluck worms and other food from the sand as the animal moves. The next pair of legs are called pedipalps. In the females these legs look identical to the walking legs. However, in males the tip of these structures resemble mittens, or boxing gloves. Notice the hook-like extension at the end. These are used to latch onto the sides of the opisthosoma of the female during breeding season. On immature specimens, all of the legs have pincers. The males don’t develop the mittens until they mature at around twelve years of age.
The final pair of legs are tiny little claws set into the anterior part of the body. These are called chelicera and are used to position food into the mouth. The mouth of the horseshoe crab is a strange, alien looking structure called a gnathobase. It is located on the underside of the body right at the base of all the walking legs. It is surrounded by dozens of thin spines which point inward. The feeding behavior is unique in that it is the only animal I know of that cannot swallow its food unless its legs are moving. The base of the legs are basically needed for chewing. Here’s how it works:
As the horseshoe crab moves, the claws on the walking legs probe the sand for food. Horseshoe crabs are carnivores, but the food must be small enough to be swallowed whole since the claws are too weak to tear larger prey up into smaller bits. Any food that is found is transferred to the tiny chelicera, which position the prey into the mouth opening. As the legs move they pull the spines around the mouth outward and then inward. These spines grasp the food on the inward rotation, but slide past the food when moving outward again. This process pushes the prey into the esophagus ratchet-like, and the food is swallowed. Because the spines all point inward, the horseshoe crab cannot eject unwanted materials from the mouth once it has started down the throat. For this reason you should never try to feed a horseshoe crab shells or gravel or anything else inedible. It will have no choice but to swallow it. In the image below you can see the base of the legs, the small chelicera and the spiny mouth opening. If you remember the sand creature "Sarlacc" from Return of the Jedi, I'd be willing to bet Lucas modeled this after the horseshoe crab's gnathobase. If he didn't, he should have.
On the left side of the mouth, notice those little white blobs. These are commensal flatworms known as "Limulus leeches". They are found only on horseshoe crabs and will not survive if removed from the host. They feed on scraps of food left over from the crab's meals, and although they are not parasitic, if found in large numbers they can interfere with the horseshoe crab's ability to breathe.
In closing, here’s an image that gives you an idea of how powerful those pusher legs are, mentioned above. They are the driving force behind this animal’s ability to burrow under and travel beneath the sand.
Other diaries in this series can be found here.