The primordial ocean dwellers are squarely situated in the arachnid family tree, scientists claim in a new study.
Despite their name, horseshoe crabs—bizarre, ancient aquatic critters—don’t look like horseshoes and they’re definitely not crabs.
They’re arachnids. Just like spiders, scorpions, amblypygids, mites and a bunch of other leggy, land-dwelling animals.
Scientists recently came to this conclusion after sequencing a huge amount of genetic material from spiders and horseshoe crabs and then using that data to make the most likely family tree. (Read more about horseshoe crab evolution. ).
According to study leader Jesús Ballesteros of the University of Wisconsin–Madison, “this part of the tree of life has always been quite challenging to solve.” The results were recently published in the journal Systematic Biology.
“One surprising thing about this study was that the horseshoe crabs are always nested inside the arachnids on the family tree, no matter how we treated the data.” ”.
Crabs are iconic marine animals that hold a special place in our cuisine and culture Their unique sideways scuttle across the seafloor and massive front claws make them instantly recognizable But where exactly do crabs fall in the taxonomy of the animal kingdom? Specifically, many wonder – is a crab an arachnid? Let’s dive into the details and settle the debate on crab classification once and for all.
Decoding the Arachnid Designation
First, what are arachnids? Arachnids are a class of joint-legged invertebrates within the arthropod phylum. They are characterized by four pairs of walking legs no antennae or wings, simple eyes, and a body divided into two segments – a fused head/thorax (cephalothorax) and abdomen.
Well-known arachnid examples include spiders, scorpions, ticks, harvestmen, and mites. Arachnids are predominantly terrestrial rather than marine. They breathe via book lungs or trachea, not gills, and lack maxillipeds or mandibles.
Crabs Fall Under the Crustacean Class
In contrast to arachnids, crabs belong to the subphylum Crustacea within the arthropod phylum. Specifically, they are members of the order Decapoda, along with other crustaceans like lobsters, shrimp, and crayfish.
As crustaceans, crabs possess key anatomical differences from arachnids:
- 5 pairs of legs (arachnids have 4 pairs)
- Mandibles as mouthparts (arachnids have chelicerae)
- Antennae (no antennae in arachnids)
- Compound eyes (arachnids have simple eyes)
- Gill respiration (arachnids use book lungs/trachea)
- 2 body segments – cephalothorax and abdomen (arachnids have distinct head+thorax+abdomen)
So by taxonomy, crabs fall definitively under the crustacean designation based on shared crustacean anatomy like mandibles, antennae, maxillipeds, and gills. Their body construction is also quintessentially crustacean with a cephalothorax and reduced abdomen.
Exceptions: Horseshoe Crabs and Sea Spiders
There are two marine creatures often mistaken for crabs that are actually classified as arachnids – horseshoe crabs and sea spiders.
Horseshoe Crabs: Despite the name, horseshoe crabs are more closely related to scorpions and spiders than true crabs. They have chelicerae, 4 pairs of legs, book gills, and a body divided into 3 distinct segments like arachnids.
Sea Spiders: Sea spiders resemble land spiders but inhabit the ocean floor. They share key arachnid traits like chelicerae, 4 pairs of legs, book lungs, and segmented bodies. Their spindly legs give them a slight crab-like silhouette.
What Defines a Crab?
Crabs differentiate from other crustaceans by a few key anatomical features:
- Flattened body optimized for life on the seafloor
- Reduced abdomen curled underneath thorax
- Large front claws (chelipeds) for defense/feeding
- Sideways walking motion using other 4 pairs of legs
- Carapace (thick shell) covering cephalothorax
So in essence, a true crab is any crustacean that possesses these signature crab adaptations like sideways walking, front claws, and a pancaked body and shell ideal for bottom-dwelling.
Marine species that only mimic crabs in passing are not classified in the same infraorder (Brachyura) as true crabs.
Examples of True Crab Species
With over 6,700 species, crabs display incredible diversity across our oceans. Some well-known examples of true crabs include:
- Dungeness Crab – Commercially fished; edible claws
- King Crab – Massive size; legs and claws eaten
- Snow Crab – Found in cold waters; targeted for meaty legs
- Coconut Crab – Tropical land crab; world’s largest terrestrial arthropod
- Hermit Crab – Lives in discarded shells; soft abdomen
- Fiddler Crab – One claw much larger than the other
- Mole Crab – Adapted for digging and burrowing underground
A tale of two sisters
Chelicerata is the group that horseshoe crabs and arachnids are in. Scientists have known for a long time that these two groups are closely related, but it’s been hard to figure out how closely.
Horseshoe crabs are blue-blooded burrowers that belong to the genus Xiphosura. The first fossils of these animals were found about 450 million years ago. Around that same time, arachnids crawled into existence.
In the old version of the story, spiders and horseshoe crabs both came from the same ancestor, which was some kind of aquatic chelicerate. They then split into sister branches. One lineage quickly moved onto land and diversified into as many as 100,000 species, becoming today’s arachnids. (See pictures of fossil spiders found with “glowing” eyes. ).
Another group, the horseshoe crabs, stayed put and survived major mass extinctions pretty much the same way they were. There are now only four species of horseshoe crabs left in the world’s oceans, and some of them can be more than a foot long.
See Photos of Amazing Underwater Wildlife
“The problem with Xiphosurans is that there are only four extant species,” says Matthias Obst of Sweden’s University of Gothenburg, who also studies the evolutionary relationships within chelicerates.
So there’s “very little internal diversity, which usually helps to stabilize certain groups in the tree of life.”
But new genomic sequencing work by Ballesteros and others has called this story into question. It suggests that horseshoe crabs are actually arachnids, not a sister group to them.
This new study came to that conclusion by looking at the family trees that best show how genetic sequences are spread among 53 species of arachnids, including horseshoe crabs, sea spiders (which are not really spiders, despite their name), and different types of crustaceans and insects.
The majority of those gene trees—roughly two-thirds—place Xiphosura well within the arachnid lineage, appearing as a sister group to hooded tick spiders.
But Rosa Fernández, who also studies arachnid evolution at the Center for Genomic Regulation in Barcelona, notes that there are lingering uncertainties. For instance, one-third of the sequenced genes keep horseshoe crabs outside arachnids.
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“The authors have done a magnificent job,” says Fernández. But “I would not be surprised if alternative methodologies yield a different result in the future. ”.
Are spiders and crabs closely related?
FAQ
Are crabs a type of arachnid?
Why are crabs not spiders?
What are crabs classified as?
Is a lobster an arachnid?
Are crabs Arachnids or insects?
True Crabs are all crustaceans and especially decapods and they are neither arachnids nor insects. Crabs are considered crustaceans because they have mandibles and their head remains joined with the thorax to form a cephalothorax under the carapace. They also typically have a very short projecting tail-like abdomen.
Are arachnoid cysts cancerous?
Arachnoid cysts are fluid-filled swelling that grows on the brain or spinal cord. They are non-cancerous sacs. The symptoms of this are headaches, dizziness, seizure, vomiting, and limited range of movement in case of a large sac. Usually, arachnoid cysts do not cause any symptoms. The treatment consists of drainage of fluid or surgical removal.
Are crabs related to spiders?
No, true crabs are not related to spiders and it is because crabs are not in the same family of classification as those of the spiders. True Crabs are all crustaceans and especially decapods (having 5 pairs of limbs) and they are neither arachnids nor insects. Whereas, spiders are all arachnids. Although there’s an exception.
Are horseshoe crabs arachnids?
A crab’s tail and reduced abdomen remain entirely hidden under the thorax. Crabs like Horseshoe crabs are not actually true crabs at all. They are much more closely related to spiders and other arachnids. So, they are not crustaceans. In 2019, a molecular phylogenetic study also placed Horseshoe crabs in Class Arachnida as arachnids.