Horseshoe Crab: Anatomy & Taxonomy

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

The horseshoe crab is a tried and true animal on Earth. It's older than the dinosaurs and has changed very little during its tenure here. In this lesson, we'll take a closer look at this un-crab, identifying its taxonomy and physical anatomy.

A Living Fossil

On Earth for over 300 million years, horseshoe crabs are one of the oldest living animals on Earth. These critters aren't 'true' crabs like the ones you might eat for dinner. In fact, they are not crustaceans at all. So what are they? You're about to find out!

Horseshoe crabs are one of the oldest living animals on Earth
horseshoe crab

Horseshoe Crab Taxonomy

There are four species of horseshoe crab. One, Limulus polyphemus occurs along the eastern coasts of North and Central America. The other three, Tachypleus gigas, Tachypleus tridentatus, and Carcinoscorpius rotundicauda, occur in the Indo-Pacific along the coasts of Japan, the Philippines, China, India, Java, and Sumatra.

All four species belong to the family Limulidae and the order Xiphosura. Limulidae is only made up of the four horseshoe crab species. They belong to the Xiphosura order because of their sword-like tail, and the prefix 'Xiphos' comes from the Greek for 'sword.'

Moving up the hierarchy we find horseshoe crabs in the class Merostomata because their legs are at their mouths. Again, breaking down the word we see that the name makes sense. 'Meros' is from the Greek for 'thigh' and 'stoma' for 'mouth' or 'opening.' The other member of this group, the sea scorpions, are extinct, so we again find horseshoe crabs holding their own.

Let's travel up to the next level, subphylum Chelicerata. Here, horseshoe crabs have rejoined the living as they share this classification with others such as the arachnids (spiders, ticks, mites, and scorpions), and sea spiders. They are classified together because they lack antennae and jaws. However, horseshoe crabs do have pincer-like mouth parts called chelicerae.

We've got one more level to go, and that's up to phylum Arthropoda. This is a large phylum that includes not only the chelicerates, but also insects, millipedes and centipedes, crustaceans, and even the now extinct trilobites. Arthropods are invertebrates that have jointed legs, segmented bodies, and a hard exoskeleton protecting their bodies.

External Anatomy

Externally, horseshoe crabs can be divided into three sections: the prosoma, the front most section, the opisthosoma, the mid region, and the telson, that sword-like tail. You can also remember them as the cephalothorax, which is a fused head and thorax region, the abdomen, and the tail, respectively.

Horseshoe crabs have a large number of eyes, ten in total. There are two compound eyes (like the eyes of a fly) that are the most obvious, one on each side of the prosoma. Five more eyes are spread around the prosoma, and they allow the horseshoe crab to detect ultraviolet light and stay in sync with the lunar cycle, which is particularly important for spawning purposes. Two eyes are located underneath near the mouth. It's not clear what purpose these eyes serve. And the last eye is in a place you might never guess - at the tail! This eye is believed to help the crab's brain synchronize with light cycles.

The underside of a horseshoe crab
underside of a horseshoe crab

On the underside of the crab we find the mouth with its chelicerae pincers and jointed appendages typical of an arthropod, gills that help the crab breathe underwater, and the anus, which is where the crab excretes waste.

Remember how I said that the crab's legs are at its mouth? These legs serve multiple purposes, despite what may seem like an awkward location. There are six pairs of appendages. The first is the chelicerae, the feeding appendages. The next four pairs are the pedipalps or walking legs. They each have a small claw on the end and help the crab move around. The final pair is the pusher legs that are designed for helping the crab burrow into sediment by pushing and clearing the ground away.

The gills of a horseshoe crab are special because they are book gills, so called because they have structures called lamellae that look like pages of a book stacked up. There are about 150 lamellae on each gill that allow the crab to extract oxygen from the water without taking any water in.

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