Crabs and spiders are related, but not as close cousins as they might look.
You can trace them back to the same huge animal group, the phylum Arthropoda, inside the animal kingdom.
They share a basic body plan, yet they belong to very different branches of the arthropod family tree.
The Shared Arthropod Blueprint
Both animals fit the arthropod pattern, which also includes insects, centipedes, crustaceans, and arachnids.
That shared plan explains why they both have a hard outer shell and many jointed parts, even though they live very different lives.
What Spiders And Crabs Inherit From Arthropoda
As members of arthropoda in the phylum Arthropoda, both animals share a segmented body and paired limbs.
That same design helps arthropods succeed in land, water, and many other habitats.
Why Jointed Appendages And Exoskeletons Matter
Their jointed appendages give them flexibility and speed.
The exoskeleton protects soft tissue.
A recent overview of crabs and spiders as arthropods explains that this body plan is one reason the group became so successful.
How The Open Circulatory System And Hemolymph Fit In
Both use an open circulatory system, where body fluid called hemolymph moves through open spaces instead of sealed blood vessels.
That system works well for many arthropods, even though it is very different from yours.
Where Their Lineages Split
The big split comes after the shared arthropod plan.
Spiders sit on the chelicerata branch, while crabs belong to the crustacea side within mandibulata.
Their heads, legs, and mouthparts are built in different ways.
Chelicerata And Class Arachnida
Spiders belong to class arachnida.
Their group uses chelicerae and pedipalps, which are key traits that set them apart from crabs and other arthropods.
Crustacea, Mandibulata, And Order Decapoda
Crabs are crustacea and sit in order decapoda, which includes many ten-legged forms.
Their lineage is tied to mandibles, the jaw-like mouthparts that help them crush food.
Chelicerae Vs Mandibles
Spiders use chelicerae to handle prey.
Crabs use mandibles and other mouthparts for chewing and grinding.
Body Plan Differences Readers Notice First
You can spot the main differences almost right away.
Spiders and crabs both have a cephalothorax, yet the rest of the body, leg count, and breathing setup tell two very different stories.
Cephalothorax, Carapace, And Limb Count
A cephalothorax is the fused head and chest region in both animals.
A carapace helps protect that area.
Spiders usually have eight legs.
Crabs typically have ten limbs, including chelipeds that form their claws.
Gills Vs Book Lungs
Crabs breathe with gills, which fit life in water.
Spiders use book lungs to breathe air, which helps them live on land.
Why Land Crabs Still Are Not Arachnids
Even land crabs still stay in the crab branch because their core anatomy remains crustacean.
Their body shape may seem spider-like at a glance, yet they are still far from arachnids.
The Exceptions That Cause Most Confusion
Some animals get mislabeled so often that they blur the picture.
Names like horseshoe crab and sea spider sound close to true crabs or spiders, yet their places in the tree of life tell a more specific story.
Why Horseshoe Crabs Are Not True Crabs
A horseshoe crab is not a true crab at all.
Horseshoe crabs belong to a very old branch of chelicerata.
Research on their place in the tree of life shows that xiphosura sits outside true crabs, even though the name is misleading.
Xiphosura, Sea Spiders, And Misleading Names
Sea spiders are not spiders in the usual sense.
Their names can confuse you, since common names often sound more familiar than the actual classification.
What Arthropod Evolution Says About Their Deep History
Arthropod evolution and evolutionary history show long evolutionary divergence during the Cambrian. Many lineages branched early and survived through later mass extinction events.
Jesús Ballesteros and colleagues recently analyzed this history. Their work supports the deep split between chelicerata and mandibulate arthropods, which helps explain why spiders and crabs only seem close on the surface.
