Are Crabs Arachnids or Crustaceans or Insects? What are crabs classified as? Let’s Know In Detail

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Are Crabs arachnids or crustaceans or insects?” This one is a very often asked question amongst the learners and researchers who are going through the classification system of the Phylum Arthropoda.

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. 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.

All the invertebrate animals of the Animal Kingdom that have a hard exoskeleton made of chitin protein are included under the group (taxa) of the Phylum Arthropoda.

These invertebrates under the Phylum Arthropoda are all called Arthropods. They are the largest group of animals on earth and include insects, crustaceans, and arachnids overall.

Arthropods are animals with exoskeletons (external skeletons), segmented bodies, and jointed legs. Their body is divisible into head, thorax, and abdomen.

And, in fact, 75% of all animals on earth belong to Arthropods.

Here, we’ll know more about the concept. So, just keep reading…

What are arachnids?

Arachnids are those arthropods that are grouped under the Class Arachnida in the Subphylum Chelicerata of the Phylum Arthropoda.

Arachnids includes both terrestrial and aquatic species that have very simple eyes but no compound eyes.

All adult arachnids have 8 legs in total, but in some, you will see that the front pair of legs has converted to a sensory function.

While in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs.

They lack antennae or wings, and their body is divisible into cephalothorax (fused head and thorax) and abdomen.

Their cephalothorax is with 2 chelicerae (small mouthparts), 2 pedipalps, and with 4 pairs of walking legs.

They use their chelicerae for feeding and defense; pedipalps for feeding, locomotion, and reproductive functions; and walking legs for total locomotion.

Their abdomen portion lacks appendages. And, they respire by book-lungs which is similar to book-gills, by trachea, or by both.

Animals that includes scorpions, ticks, mites, harvestmen, solifuges, and even the horseshoe crabs are all included in arachnids.


What are crustaceans?

Crustaceans are those arthropods that are grouped under the Class Crustacea in the Subphylum Mandibulata of the Phylum Arthropoda.

As the subphylum suggests, Mandibulata is a group of arthropods characterized by the presence of mandibles (mouthparts) used for biting, cutting, and holding food.

They have cephalothorax that is their head and thorax is fused together. And, they do have a hard, chitinous, limy (calcareous) exoskeleton that protects their internal body parts.

The head is actually 5-segmented bearing 2 pairs of antennae, 1 pair of mandibles, and 2 pairs of maxillae.

The thorax has 5 pairs of thoracic limbs that are two-branched (biramous). The first 3 pairs of thoracic limbs form maxillipeds (appendages modified to function as mouthparts).

The carapace part which is the hard upper shell is well-developed, usually enclosing the gill chambers on the sides of the cephalothorax.

Respiration is usually done by gills or the body surface. Gills are usually in 3 series present on the thorax.

They have antennal glands for excretion that open at the base of maxillae.

Crustaceans include crabs, lobsters, crayfish, shrimp, krill, and barnacles. They are relatives of insects.


What are insects?

Insects are those arthropods that are grouped under the Class Insecta in the Subphylum Mandibulata of the Phylum Arthropoda.

They are also characterized by the presence of mandibles (mouthparts) used for biting, cutting, and for holding food.

The body is well divisible into the distinct head, thorax, and abdomen. They don’t have cephalothorax.

The body is well-segmented as its head has 6 fused segments, the thorax has 3 segments, and the abdomen has up to 11 segments.

It has a pair of compound eyes, a pair of antennae, a pair of mandibles, and two pairs of maxillae.

The thorax portion of the body has 3 pairs of jointed legs, and 1 or 2 pairs of winges which may or may not be present on all insects.

They respire by trachea and excrete by Malpighian tubules.

In aquatic insects, gills are usually outgrowths of the tracheal system. They are covered by a thin layer of cuticle that is permeable to both oxygen and carbon dioxide.

Nearly all insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of moulting.

Adult insects typically move about by walking, flying, or sometimes swimming. Many insects spend at least part of their lives under water, with larval adaptations that include gills, and some adult insects are aquatic and have adaptations for swimming.

Animals like ants, bees, wasps, silkworms, butterflies, moths, cockroaches, crickets, grasshoppers, etc. are all included in insects.


What is the difference between crustaceans and arachnids and insects?

1. Arachnids belong to Class Arachnida in the Subphylum Chelicerata, Crustaceans belong to Class Crustacea in the Subphylum Mandibulata, and Insects belong to Class Insecta in the Subphylum Mandibulata.

2. Arachnids have anterior cephalothorax and posterior abdomen. Crustaceans have anterior cephalothorax and its posterior abdomen is normally wrapped under the body thorax part. And, insects have a distinct head, thorax, and abdomen.

3. Antennae and true jaws absent in Arachnida. Crustaceans bear two pairs of antennae and mandibles that are often simply referred to as jaws. Insects have not only antennae but also mandibles and other mouth parts Labrum, Maxillae, Labium, etc.

4. Over 60,000 arachnid species are found on earth. Over 30,000 crustacean species are found on earth. And, over 900,000 different kinds of living insects are known.

5. Arachnids are mainly terrestrial. Crustaceans are mainly marine. Insects live all over the planet and are almost terrestrial and only a few insect species live in the oceans or in very cold places.

6. Arachnids use their appendages to walk. Crustaceans use their appendages to swim, crawl, manipulate food, etc. Insects use their appendages to crawl, run, fly, jump, and even swim

7. Arachnids excrete by using their coxal glands and malpighian tubules. Crustaceans excrete using their antennal glands. Insects excrete using their Malpighian tubules.

8. Arachnids respire using their trachea, book-lungs, or by both. Crustaceans respire by using their gills or body surface. Insects respire using their trachea or gills that are usual outgrowths of the tracheal system.

9. Arachnids have 4 pairs of limbs. Crustaceans have at least 5 pairs of legs. Insects have 3 pairs of legs.

10. Arachnids don’t have compound eyes but have simple eyes meaning that each eye has a single lens to receive and process visual information. Most crustaceans have compound eyes with apposition optics, while many with simple eyes, or no eyes at all. All insects have compound eyes that consist of thousands of ommatidia.


Are Crabs arachnids? If NO, then why?

No, true crabs are not Arachnids. True crabs are not arachnids because true crabs have a mandible, a type of jaw that arachnids don’t have.

Arachnids belong to the group of joint-legged invertebrate animals (arthropods) in the subphylum Chelicerata whereas, Crabs belong to the group of decapods (having eight walking legs and two grasping claws), in the subphylum Mandibulata.

Arachnids like the spiders have chelicerae, which are mouth parts that appear before the mouth. The chelicerae look-like the fangs of spiders.

Crabs have mandibles, which are a pair of mouthparts used for chewing or cutting. Whereas, Arachnids have a pair of chelicerae and lack the mouthparts for chewing, unlike mandibulates.

Crabs like the Horseshoe crabs and Sea scorpions (now extinct) are not true crabs but Arachnids. It’s because they do not have a mandible and instead have chelicerae in front of their mouthparts.

Crabs use their 3 series of gills present on the thorax to respire and breathe. But, Arachnids use trachea or their book-lungs which are similar to book-gills (having five pairs of them) but not true gills.

Crabs have antennal glands for excretion that open at the base of maxillae. But, Arachnids have only coxal glands or malpighian tubules for excretion purposes.

Crabs have a pair of compound eyes, two pairs of antennae, and five pairs of legs. Arachnids don’t have compound eyes but simple eyes, they lack antennae, and have 4 pairs of limbs.


Are Crabs crustaceans? If YES, then why?

Yes, all true crabs other than the Horseshoe crab and Sea scorpions (now extinct) are crustaceans.

Horseshoe crabs and Sea scorpions are not true crabs and so included in the Class Arachnida in the Subphylum Chelicerata.

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 remain 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.

The epistome of crabs is actually the transverse plate forming the anterior border of the buccal frame, that remains laterally fused with the carapace and anteriorly fused with the front.

There are small antennules, antennae, and eye spots 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 insects? If NO, then why?

All crabs are crustaceans and no crabs on earth are insects. Crabs fall under the Order Decapoda of Class Crustacea whereas, Insects fall under Class Insecta. Both have mandibules.

The main difference that shows that crabs are not insects is that all insects have distinct tri-segmented bodies consisting of the head, thorax, and abdomen parts. But, the crabs being crustaceans have fused head and thorax as cephalothorax under the carapace and their abdomen remain normally wrapped under the body and pressed against the thorax.

Insects have three pairs of legs attached to their thoracic region. Crustaceans like crabs have five pairs of legs attached to their thoracic region.

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.

Crabs respire by using their gills or body surface. The gills of crabs are located under the carapace near the first pair of walking legs. The oxygen that crabs need is taken into the gills either through water or moisture in the air.

Whereas, insects respire using their trachea or gills that are usual outgrowths of the tracheal system.

Crustaceans like crabs excrete using their antennal glands. But, insects excrete using their Malpighian tubules.

Nevertheless, 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: So, What are crabs classified as?

Crabs are classified as:

  1. PHYLUM: Arthropoda
    • Body is triploblastic, segmented, and bilaterally symmetrical.
    • Organ system level of organization.
    • Body has jointed appendages which help in locomotion.
    • Coelomic cavity is filled with blood.
    • Open circulatory system present.
    • Unisexual and fertilization is either external or internal.
  2. SUB-PHYLUM: Mandibulata
    • Body into head, thorax, and abdomen.
    • Compound eyes common.
  3. CLASS: Crustacea
    • Cephalothorax present.
    • Exoskeleton chitinous, hard, and limy.
    • Respiration by gills.
    • Excretion by antennal glands.
    • 2 pairs of antennae, 1 pair of mandibles, and 2 pairs of maxillae present.
  4. ORDER: Decapoda
    • Carapace well-developed. Usually enclosing gill chambers on sides of cephalothrorax.
    • First 3 pairs of thoracis limbs form maxillipedes.
    • Gills usually in 3 series present on thorax.
  5. SUB-ORDER: Reptantia
    • Body dorso-ventrally flattened. Rostrum absent.
    • Pleopods reduced. Not modified for swimming.

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