So, 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. Crabs like Horseshoe crabs and Sea Scorpions (now extinct) are not actually true crabs at all. They are much more closely related to spiders and other arachnids.
Arachnids that include spiders belong to Class Arachnida in the Subphylum Chelicerata. Whereas, Crustaceans that include crabs belong to Class Crustacea in the Subphylum Mandibulata.
One similarity between spiders and crabs is that both are arthropods under Phylum Arthropoda of the Animal Kingdom.
Crustaceans include crabs, lobsters, crayfish, shrimp, krill, and barnacles. They are relatives of insects but not Arachnids like spiders.
True crabs are not arachnids because true crabs have a mandible, a type of jaw that arachnids don’t have. Arachnids like the spiders have chelicerae, which are the small parts that appear before the mouth.
Moreover, crabs have 10 legs (5 pairs of legs), but Arachnids have 8 legs (4 pairs of legs).
Crabs better use their limbs to swim, crawl, manipulate food, etc. But, Arachnids like spiders use their appendages to walk.
Did crabs evolve from spiders?
No, crabs did not evolve from spiders, but in fact, both crabs and spiders have together evolved into two separate types from a common Arthropod species.
Meaning that spiders are not the ancestors of crabs, but both crabs and spiders had a common ancestor from which they have evolved into two separate types according to the phylogenetic tree of evolution.
Modern arthropods include insects, spiders, centipedes, shrimp, crabs, and crayfish. All arthropods are the descendents of a single common arthropod ancestor.
Just like we and our biological cousins can trace our ancestry back to a common set of grandparents, in the same way, all arthropods can trace their ancestry back to a common arthropod ancestor.
Just as you and your biological cousins can trace your ancestry back to a common set of grandparents, all arthropods can trace their ancestry back to a common arthropod ancestor.
Yes, it’s true that neither crabs had evolved from spiders, nor spiders had evolved from crabs. The evolution of spiders can actually be traced around 380 million years ago from a common ancestor that was an Arthropod.
Any species descended from that ancestor is an arthropod. And any species not descended from that ancestor is not an arthropod.
So, it is believed that Spiders evolved from ancestors that are similar in nature to the design of crabs but were not crabs. In fact, the first true spiders (thin-waisted arachnids) evolved from crab-like chelicerate ancestors.
How are crabs related to spiders?
Although both crabs and spiders are a lot different but there are some far common similarities and relations between the two as they both belong to the Phylum Arthropoda of the Animal Kingdom and share a common ancestor.
In simple words, both crabs and spiders are grouped in the same phylum, but this does not mean that they are necessarily closely related.
But, yes the relation is there but it’s a very far relation thus, making them look and work differently from each other.
For instance, all animals with a spine are in the Phylum Chordata. But, it is clear that although birds and humans both have spines, they are very different. That’s what the same case is with the spiders and crabs.
They both are far related and so they’re classed together as Arthropods because they all have chitinous exoskeletons and jointed legs.
Another relation is that their body is triploblastic (that is having ectoderm, mesoderm, and endoderm), segmented (divided into head, thorax, and abdomen), and bilaterally symmetrical (that is the left and right halves of the body are approximate mirror images).
Another relation is that they both have an organ-system level of body organization. And, their body has jointed appendages or limbs which help them in locomotion and movement.
Another far relation can be traced as they both have an open circulatory system present and their coelomic cavity is filled with blood. Both are unisexual and both perform internal fertilization.
Are crabs sea spiders?
No, crabs are not sea spiders and are not related to them. Sea spiders are grouped under the Subphylum Pycnogonida (also called Pantopoda) of the Phylum Arthropoda.
Sea spiders are neither true spiders nor even arachnids, in fact, they are small, marine, and spider-like animals of the Animal Kingdom.
Previously, Sea spiders were being grouped under Chelicerates (Subphylum Chelicerata) but now, they are grouped under Pycnogonids (Subphylum Pycnogonida) because there are better genetic, molecular, anatomical, and embryological evidence to prove that Chelicerates and Sea spiders are not closely related.
That’s also primarily why crabs are not related to sea spiders.
Sea spiders are exclusively marine. For example, the Southern Ocean giant sea spider is one of the most common sea spiders in the waters around Antarctica.
Their abdomen is a completely vestigial organ meaning a useless body-part. They have a mouth on a long proboscis.
They have a head region, the cephalosoma, which always carries a proboscis, and usually has 4 eyes and four pairs of appendages.
The 4 pairs of appendages are chelifores, palps, the first pair of legs, and structures called ovigers used in reproduction. Some species lack eyes and the number of appendages varies.
Moreover, they don’t have any respiratory and excretory organs.
The sea spider has no gills or lungs, and its exoskeleton is a bit thick for absorbing oxygen and help it to respire.
What are crabs actually related to?
True crabs like the hermit crabs, ghost crabs, coconut crabs, rainbow land crabs, vampire crabs, etc. are all crustaceans and are closely related to insects.
Whereas, those like the Horseshoe crabs are “living fossils” are arachnids and are more closely related to spiders and scorpions than they are to crabs.
True crabs are not insects but are closely related to insects. Crabs fall under the Order Decapoda of Class Crustacea whereas, Insects fall under Class Insecta.
The present day genetic, molecular, anatomical, and embryological evidence shows that both insects and crabs are closely related. That’s also why they both are included under the Subphylum Mandibulata of the Phylum Arthropoda.
Both have mandibles as they are included under the Subphylum Mandibulata.
Crabs have a pair of big mandibles larger than it’s other legs, used either for biting, cutting, and holding the food that looks like flat pincers or jaw of a mouth.
Insects also have a pair of mandibles situated near the insect’s mouth and look like small pincers much smaller than it’s other legs.
Both have a pair of compounds eyes. Each of the compound eyes consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which distinguish brightness and color.
All insects are close relatives to crustaceans like crabs and often considered a land-dwelling version of them. Genetic researches have proved and accepted this thing.
Final Words: All true crabs are Crustaceans
All true crabs other than the Horseshoe crab and Sea scorpions (now extinct) are crustaceans.
All crabs are crustacean because their body is generally dorso-ventrally compressed and consist of a large and broad cephalothorax and a stumpy abdomen.
Crab’s cephalothorax is broader than its length. They have carapace which is fused with the epistome at the sides and remains nearly always in the middle. Moreover, they don’t have rostrum.
Crabs belong to the subphylum Crustacea, the largest group of marine arthropods, which also includes lobster, shrimp, and krill, a shrimp-like crustacean.
There are small antennules, antennae, and eyespots in the sockets of the carapace. They have well-developed five pairs of thoracic legs.
Crabs move sideways, walking on four pairs of legs, and holding their two legs with claws away from their body.
The abdomen is reduced and fixed under the cephalothorax by fitting into a groove in the thoracic sterna, thus remaining invisible in the dorsal view of the animal.
Are Crabs Arachnids or Crustaceans or Insects? What are crabs classified as? Let’s Know In Detail
This one is a very often asked question amongst the learners and researchers who are going through the classification system of the Phylum Arthropoda.