Horseshoe Crab: Characteristics and Curiosities

Written and verified by the biologist Cesar Paul Gonzalez Gonzalez

Last update: 27 December, 2022

The horseshoe crab is a marine arthropod that is characterized by having a flattened and convex shape. In addition, it has an extremely hard shell, which it uses as a defense. Despite its name, the closest relatives of this crab are from the Arachnida group – scorpions and spiders – so it isn’t a crab as such.

This organism is important for two reasons: for being a living fossil and for the properties of its blood. Because of this, it has become a species with great potential in the fields of medicine and research. Read on to find out more about this blue-blooded arthropod.

Characteristics and shape of the horseshoe crab

The horseshoe crab, (Limulus polyphemus), is an aquatic species that has an impressive carapace (exoskeleton) as a defense. If you look at it from above, you’ll see a brown horseshoe-shaped organism with a fairly long tail, which can reach up to 60 centimeters (24 inches) in length.

From the outset, three sections are easily distinguished on its body: prosoma – horseshoe-shaped head -, opisthosoma – intermediate zone with some spines – and telson – tail. This is the common body plan of all merostomate species ( Xiphosura ), the taxa that encompasses the horseshoe crab.

Its shell greatly limits its movements. The only regions with mobility are the areas where the three regions of the body are divided. Like turtles, their greatest enemy is being turned upside down, as it’s quite difficult for them to turn themselves over again. Because of this, they have to use their tail as a lever and manage to return to the starting position.

Viewed from above, a horseshoe or inverted U-shape can be seen on its head. In this area, two bumps protrude, one in each corner. In addition, in the second half of the body some mobile spines stand out, ending with a very striking tail, which looks more like an elongated spine.

Depending on the species, it can have up to four eyes, one on each bump, and just in the center, one or two ocelli. Although horseshoe crabs may seem like they look perfect, remember that their eyes are quite simple, so they can barely detect changes in brightness.

If a specimen is observed from below, six pairs of legs will be detected, which are quite similar to those of a crab. Most of the limbs end in a pincer shape, which allows the animal to move and hold its prey when feeding.

What does it feed on?

This organism feeds on crustaceans, worms, mussels, clams, and some fish. Its victims are eaten through its mouth, but it has no teeth to chew them. Because of this, the digestion process must begin before entering the mouth.

This mechanism is started by means of its tweezers, with which it manages to chop up its food. Then, it brings the ground mixture close enough to its belly, where he can break it up even more, by means of some thorns that protrude from his legs. These spines function as external teeth.

Maybe you think that eating like this is a bit complicated, especially not being able to control what comes out of its mouth, but, for this arthropod, it’s no problem at all. To avoid problems, it has a kind of stop, just near the mouth, which serves as a collector. So, when something comes out of the mouth, it gets stuck in this area and it can easily retrieve it.

How does the horseshoe crab breathe?

Because this species is marine, it has gills to breathe. These gills are present in lamellar forms in the region of the opisthosoma. In fact, they’re covered by what is called the ‘genital operculum’, which is the region where the eggs and sperm come out.

Once the gills take direct oxygen from the water, they pass it to their circulatory system. Although it’s an arthropod, the circulatory system is quite complex, with cells that look like lymphocytes (amoebocytes) and others that look like erythrocytes (cyanocytes).

Usually, the red color of our blood is thanks to the heme group of erythrocytes, which has iron as its central atom. However, the horseshoe crab has cyanocytes that contain hemocyanin, which has copper as its central atom. This difference is what causes its blood to be blue.

Reproduction

In this species, you can differentiate between males and females, by the shape of the claws on their first pair of legs. The male caliper isn’t fully developed, so it looks more like a glove. In females, the claw is the same as that of the other legs.

It may seem that it doesn’t make sense, but the reality is that this simple characteristic helps the male a lot. Thanks to this ‘glove’, the male can grab the female and anchor to her in times of reproduction.

Fertilization occurs externally, which means that the female lays the eggs and then the male arrives to fertilize them. If the male clings to the female at all times, he will be the first to fertilize the eggs, which is why males need that glove shape on their legs, instead of clamps.

The laying of the eggs takes place in the sand on the beach. Usually, it only has one mating season in the spring, but some southern species can have up to two. This occurs only at high tide, since the female needs to make a hole to deposit the eggs and that the nest is hidden at low tide.

Almost a trilobite

Perhaps, at some point, you’ve heard of fossil creatures called trilobites. These arthropods first appeared in the Paleozoic, but they dwindled and disappeared about 250 million years ago.

Interestingly, the horseshoe crab has a larval stage shaped like a trilobite (trilobitiform), indicating its relationship to these arthropod ancestors.

This doesn’t stop there, since this species is part of a group of organisms (Xiphosura), which appeared more than 200 million years ago, so there are fossil species almost identical to the horseshoe crab, such as Mesolimulus walchii.

This means that the morphology of the species has not changed much in millions of years, hence it is known as a living fossil.

Royal blood

As mentioned, this crab has a fairly complex circulatory system, through which blue blood circulates. This blood is important, because it clots in the presence of bacteria, making it possible to use it in chemical tests.

Additionally, it also darkens when encountered with bacteria in culture media, such as salmonella. Because of this, it’s used as a simple, but effective, bacterial detection method.

This blue blood also appears to have positive effects in the fight against HIV, as it could have the ability to reduce the activity of the human immunodeficiency virus, as indicated by studies. This would be achieved through proteins that are present in the blood, such as polyphemusin and tachyplesin.

Multipurpose horseshoe crab

If that weren’t enough, this arthropod is also used as bait to fish for eels and mollusks, as well as fertilizer in agriculture and as food for livestock. In addition, within nature, it’s usually part of the diet of some sea turtles. Even their eggs are the preferred food of birds and crabs.

Dangers and conservation of the horseshoe crab

Unfortunately, this species is listed as vulnerable, with clear population declines, but, currently, it isn’t too worrying. The main reason their numbers have dropped is because of bycatch.

It’s a fairly long-lived species (it can live for about 20 years) but it takes a long time for it to reproduce. For example, a female needs to wait until she’s 10 or 11 years of age to become fertile. This makes it difficult to maintain their population, in addition to the inherent risk of late sexual maturation.

Although the benefits that can be obtained from it are quite good, if it’s overexploited, this species could disappear. Fortunately, several ways around it have been found, but only for now. If the blood of this organism shows the capacity to produce useful drugs, its use will have to be further controlled.