Horseshoe Crabs' True Relationships

Madison C.

2022

I like horseshoe crabs... They aren't crabs.

Crabs

Because the word crab is in their name, it is easy to believe horseshoe crabs are closely related to crabs. Despite this, there is evidence that proves otherwise. Rather than being closely related to crabs, the horseshoe crab is more closely related to scorpions and spiders. There are many differences between crabs and horseshoe crabs and despite the differences between horseshoe crabs, spiders, and scorpions, the similarities between the three species outweighs the differences.

Crabs and horseshoe crabs are both arthropods but there is one main dividing factor between them. Crabs are crustaceans while horseshoe crabs are arachnids. Arthropods are a phylum, a large group of many different species. A Phylum then breaks off into different classes, like crustaceans and arachnids; these classes are more specific in the species they contain and their traits. Crustaceans are animals like crabs, lobster and shrimp. On the other hand, arachnids are animals like scorpions and spiders which typically have eight legs and pincer-like mouthparts called chelicerae, (Tarsitano). Being part of the crustacean class puts crabs in a different branch of arthropods from horseshoe crabs. When comparing crabs and horseshoe crabs visually, viewers can see that both have shells but crabs do not have a tail like horseshoe crabs do. The shell feature, which is present in both species, along with the number of appendages crabs and horseshoe crabs have, can lead to falsely classifying horseshoe crabs and crabs as closely related. The SCDNR (South Carolina Department of Natural Resources) states that, “true crabs have two pairs of antennae, a pair of jaws, and five pairs of legs with one set of claws,” (Whitaker). From this we can gather that crabs have antennae and jaws while horseshoe crabs do not have any antennae and do not have jaws. The SCDNR also states that the horseshoe crab has five pairs of claws instead of the one that makes a “true crab,” (Whitaker). According to the Salem Press Encyclopedia of Science, “…the horseshoe crab is more closely related to scorpions and spiders because of its pincher-like mouthparts,” (“Horseshoe Crab”). These mouthparts are a significant differentiating factor between crabs and horseshoe crabs because the jaws of crabs and the pincher mouths of horseshoe crabs have more differences between them than the mouths of scorpions and spiders do when compared to that of horseshoe crabs’. According to the academic journal Science Scope, “a recent study helps firmly plant ancient horseshoe crabs within the arachnid family tree,” (McGinnis 10). Since horseshoe crabs are placed among spiders and scorpions as arachnids, they are genetically closer to them than they are to crabs. While both crabs and horseshoe crabs are arthropods, they are in different classes; arachnids and crustaceans. Like the different branches of a tree, the crustacean branch is separate from the arachnid branch but still part of the same arthropod family tree.

Although scorpions, spiders and horseshoe crabs are on the same branch, it does not make them the same leaf, or in this case the same species. Some of the main differences between the species are more noticeable than others. For example, horseshoe crabs are marine animals while scorpions and spiders are land dwelling. Because they live in completely different habitats, the way they breathe and what they eat is different. A less noticeable difference is between how venomous each species can be and how many eyes they each have. Finally, the number of limbs each species has differs. According to the Salem Press Encyclopedia of Science, “many modern spiders … have book lungs, which are hollow, leaflike structures through which blood flows,” (Weng). These structures enable a spider to breathe on land. While they are not all venomous, most spiders do have venom. Spiders also have eight simple eyes which they use to see and locate their prey, (Weng). In order to move around, the spider uses its eight legs. Similar to spiders, Samuel Tarsitano states, “scorpions breathe by means of book lungs …,” (Tarsitano). The scorpion also has venom which it delivers through the stinger, or telson, on the end of its tail, (Tarsitano). According to the Salem Press Encyclopedia of Science, scorpions “…have a pair of simple eyes located in the center of the carapace. Additionally, there are from two to five pairs of eyes located along the anterior and lateral margins as the carapace,” (Tarsitano). Like the spider, a scorpion has eight legs. In addition to this it also has two claws. In contrast to these both the scorpion and the spider, the horseshoe crab breathes with gills, (McGinnis). The SCDNR states that atop the body of the horseshoe crab there are “two compound eyes, similar to those of many insects,” and that they have “an additional eight eyes [which] run along the sides of the body,” (Whitaker). Unlike the other arachnids, the horseshoe crab lacks any venom. While it does have a telson, according to the University of Florida, the horseshoe crab uses its telson to flip itself upright when it gets flipped onto its back. It is not advised to pick up a horseshoe crab by its tail though as this can damage the muscles in the telson and prevent the horseshoe crab from putting themselves upright in the future; instead, grab the shell and gently flip it over, (Barry). As for limbs, the horseshoe crab has ten legs. In brief, horseshoe crabs use gills to breathe under the water while spiders and scorpions use lungs. Spiders have eight limbs, scorpions have ten (eight of which are for walking) and horseshoe crabs have ten. Spiders, scorpions and horseshoe crabs all have a different possible number of eyes and the strength and presence of their venom varies.

Despite these differences, there are many similarities between the horseshoe crab and spiders and scorpions. Spiders, scorpions and horseshoe crabs all go through the same process while growing. They molt. In order to get larger with a hard shell or exoskeleton, all three species will need to shed their shell or exoskeleton. The Encyclopædia Britannica states, “As in all arthropods, growth is accompanied by molting (ecdysis),” (Polis). While crabs are included in this group, this shows that molting is a common trait across the three. Another trait scorpions, spiders and horseshoe crabs share is their location in the food chain. All three fill the role as both predator and prey. A spider can make a web to catch its prey but other animals eat spiders. A scorpion eats different bugs, snails, and rodents but mammals, lizards, and snakes will eat scorpions, (Polis). Finally, the horseshoe crab will scavenge as well as eat shellfish and algae, (“Horseshoe Crab”). Meanwhile, according to the University of Florida, “shorebirds such as the endangered red knot depend on the eggs for energy during migration. At least 11 species of migratory bird use horseshoe crab eggs as a food supply during northern migration,” (Barry). The article then follows this up by stating that both turtles and fish will also consume horseshoe crabs and their eggs. Although one lives in the water while the other two are terrestrial, they all have a similar location in the food chain in addition to the similar breathing mechanism which is found across all three species. Horseshoe crabs use book gills and spiders and scorpions use book lungs. According to Science Scope, “they are hard-shelled like crabs but are the only marine animals known to breathe with book gills, which resemble the book lungs spiders and scorpions use to survive on land,” (McGinnis 10). The horseshoe crab being the only known marine creature which uses book gills to breathe is an important finding about the horseshoe crab’s relationship to scorpions and spiders. It shows that the horseshoe crab is unique in its habitat as its respiratory functions share no attributes with other marine life but do appear in a similar trait in both spiders and scorpions. It also helps to disprove the relationship between crabs and horseshoe crabs by establishing a difference in their respiratory systems.

Even though the three have their dissimilarities, spiders, scorpions and horseshoe crabs are closely related. The differences between crabs and horseshoe crabs outweigh those between horseshoe crabs, spiders and scorpions. In summary, crabs have jaws and antennae which horseshoe crabs do not have; instead, horseshoe crabs have book gills which are similar to the book lungs of spiders and scorpions. While crabs, horseshoe crabs, spiders and scorpions are all within the same phylum as arthropods, crabs are crustaceans while the other three are arachnids. Although the name is still deceiving, crabs are horseshoe crabs are not as similar as it may seem.

Works Cited

Barry, Savanna, et al. “The American Horseshoe Crab (Limulus Polyphemus).” University of Florida, Oct. 2020.

“Horseshoe Crab.” Salem Press Encyclopedia of Science, 2020.

McGinnis, Patty, et al. “Horseshoe crabs are relatives of spiders, scorpions.” Science Scope, Apr. 2019, pp. 10-11.

Polis, Gary, et al. “scorpion.” Encyclopædia Britannica, 29 Oct. 2020.

Tarsitano, Samuel. “Scorpions.” Salem Press Encyclopedia of Science, 2021.

Weng, Yujia. “Spiders.” Salem Press Encyclopedia of Science, 2021.

Whitaker, David, et al. “Horseshoe Crabs” Sea Science, SCDNR, 2019.