Are Crabs Arachnids? What Are Crabs Classified As?

Christian Mba
Oct 19, 2023 By Christian Mba
Originally Published on Oct 29, 2021
Yellow land crab. Cuba.
Age: 3-18
Read time: 6.5 Min

Hermit crabs are perhaps regarded as the most entertaining species of crabs.

Crabs belong to the subphylum Crustacean. Crustacea is a group of marine arthropods, with members like lobster and shrimp.

When you hear the word crab, you make a picture of a red animal with six legs and two sharp claws which can cut anything, right? But, crabs are of many types and colors.

Some interesting ones include coconut crabs, king crabs, hermit crabs, vampire crabs, ghost crabs, rainbow land crabs. Isn’t it amazing and interesting?

Well, in this article there are many more interesting facts about crabs, spiders, and insects, and the evolutionary relationships between all of these. Molecular scientists have found evidence that crabs and spiders are a bit closely related.

Available evidence from fossil records led scientists to believe that evolution happened rather quickly in these animals. Also, you will get to know some cool and amazing biological terms which you may have never heard earlier.

After reading about the biology of crabs, do read what do crabs eat and how many legs does a crab have.

Are crabs sea spiders?

Crabs and spiders may look alike, but they are not related to each other. Spiders belong to the Arachnida class. Do you know what an arachnid is?

Arachnida is a class of invertebrate animals in the subphylum Chelicerata. This class includes spiders, mites, scorpions, ticks, and many more, but not crabs. Crabs and lobsters belong to the crustacean group.

So, as you now know that spiders and crabs belong to different families, so a crab can not be a spider. Do you know what is the biggest difference between a crab and a spider? A spider has four pairs of limbs, while a crab has five pairs.

All the true crabs have a mandible. It is a kind of jaw that an arachnid doesn't have.

Arachnids have chelicerae, which are some small parts in front of the mouth of all the creatures of this family. Have you ever seen a spider in a movie, cartoon, or in real life?

If you have seen it, then you can understand this as the mouth of the spider, which is the most horrifying thing you can imagine about a spider.

Sea spiders come under the subphylum Pycnogonida. Sea spiders are neither spiders nor crabs.

They are just some small, marine animals that look like spiders. Sea spiders are exclusively marine, while crabs can go anywhere on land and in water. And also, crabs are found in any kind of water, but sea spiders are found only in the sea, as they are named.

Sea spiders do not have respiratory and excretory systems. They just have a thick exoskeleton to absorb oxygen, because they do not have lungs or gills. South ocean giant sea spider is one of the most common sea spiders which is found in Antarctica.

So, now you know that a crab is not a sea spider, and can never be. Though they look similar, their body structure is not the same when talking biologically.

Are crustaceans insects?

Crustaceans, insects, and arachnids belong to the same animal group, Arthropoda. Did you know that arthropods are the largest group of animals on earth?

As insects and crustaceans are two different classes under the same group, how can they be the same? There may be some similarities between them because they come under the same animal group, but there are many differences as well, both internal and external.

Crustaceans have two pairs of antennae and mandibles. These are often referred to as their jaws. Insects have only one pair of antennae and no mandibles. Insects also do not have other mouthparts such as Labrum, Labium, Maxillae.

Crustaceans are mainly marine and live in water. While insects live everywhere on this planet. Insects generally prefer to live on land, but they are also found in water. Because of this crustaceans have their body modified according to this environment.

They have limbs that help them swim, crawl, and eat food like algae, dead fish, and sea worms. While insects have bodies modified to crawl, fly, jump, run, and sometimes swim. Crustaceans have five pairs of limbs, and insects have only three pairs of limbs.

Steamed Crab on white background

Did crabs evolve from spiders?

As explained earlier that arachnids and crabs belong to different families, then how can crabs evolve from spiders? But for a fact, they both evolved from the same animal. Yes, crabs and spiders have the same ancestors which belong to an arthropod species.

Arthropods are those invertebrate animals that have an exoskeleton and pairs of jointed limbs. The most common members of this group are insects, crab, spiders, shrimp, centipedes, and crayfish. They both have very complicated evolutionary relationships on the family tree.

Just as we all humans have a common ancestor, these all arthropods have a common ancestor. They all evolved from one arthropod. So neither the crabs evolved from spiders, nor the spiders evolved from crabs.

They are like cousins to each other. They both have the same grandparents and belong to the same family, but do not belong to the same set of parents. They are related to each other in some way but not closely related.

Since both crabs and spiders have the same ancestor, they have some features in common. Both of them have exoskeletons and joined pairs of legs, although the number is different.

Another similarity is that the bodies of both of them are triploblastic, segmented, and bilaterally symmetrical. These are so heavy terms, let’s simplify them. Triploblastic means they have three layers in their cells, segmented means their body parts are divided such as head, legs, and bilaterally symmetric means their left half and right half are identical.

Both of these creatures have organs separated in their body system and have separate circulatory systems also. Both crabs and spiders have jointed legs which help them in moving.

Are horseshoe crabs related to spiders?

All true crabs are crustaceans. But, there are some exceptions in this case. Horseshoe crabs and extinct sea scorpions are not crustaceans and not even actually crabs.

They are actually closely related to spiders. Yes, though crabs are not related to spiders, these two types of crabs are like close buddies to the spiders.

There’s an interesting thing about these creatures. These have been on this planet for millions of years, and we have been researching animals and plants for thousands of years, but still do not know what they actually are. These are just some mysterious creatures having similarities with some of the known animals.

Horseshoe crab belongs to the arthropod group but almost all of the arthropods live on land instead of water. Horseshoe crabs are those arthropods that dwell in waters. Horseshoe crabs are also closely related to ticks, scorpions, and extinct trilobites other than spiders.

What are crabs actually related to?

Crabs are not related to spiders and nor to sea spiders. Then whom are they related to? Want to know, no? Crabs are actually closely related to insects. Yes, insects.

Can you imagine such a big insect? Insects are generally considered to be small. Well, crabs are not true insects, they are only related to them. All the true crabs are true crustaceans. And all crustaceans are very much similar to insects, although they are not the same.

Have you ever seen a crab moving? They move sideways. This movement is such a unique part of crabs that this movement is named crabwise. Have you seen any other animal moving like this? Next time when you see a crab just notice its movement. But be careful from its claws, they might hurt you.

Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestions for are crabs arachnids, then why not take a look at how big do hermit crabs get or horseshoe crab facts?

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Written by Christian Mba

Bachelor of Science specializing in Computer Science

Christian Mba picture

Christian MbaBachelor of Science specializing in Computer Science

Christian Mba is an experienced blogger and content writer with over a decade of experience. He holds a Bachelor of Science degree in Computer Science from Nigeria and has a keen interest in Python programming. Along with his writing and blogging expertise, he is also an SEO specialist with more than six years of experience. Chris, as he is commonly known, has a passion for music and enjoys playing the piano.

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