Is a Crab a fish?

In this post, we will answer the question “Is a Crab a fish?”. We will also explain more about their biology and ecology.

Is a Crab a fish? 

No, Crabs are not fish. Similarly to shrimp, Crabs are

representatives of a group of arthropods called Crustacea.


Crustaceans are members of the subphylum Crustacea. Many species that are part of this group have exoskeleton impregnated in calcium carbonate, forming a protective covering. This is what happens in lobsters, shrimp, and crabs, the best-known representatives of this group.

In addition to the exoskeleton, crustaceans have articulated appendages and a segmented body. In crustaceans, the head and thorax are fused into a single piece, the cephalothorax; on the cephalothorax portion, there are two pairs of antennae, and on the abdomen, there are the bireme swimming appendages (two segments attached to a base). On the head, in addition to the two pairs of antennae, two compound eyes usually pedunculated are present, and around the mouth, a pair of jaws and other appendages accessories to obtain food. 

Crustacean respiratory system

Crustaceans are animals adapted to live in aquatic environments, breathing through gills that usually are found at the base of the thoracic appendages.

Most crustaceans live in the marine environment, although there are many freshwater and estuarine representatives. Among them, there are microscopic forms that abound in the zooplankton to larger forms adapted to swimming and walking on the sea bottom. In addition, there are sessile species, with adults living fixed in rocks. Furthermore, there are also a few crustaceans that parasitize other animals.

Although most crustaceans are aquatic, some species have invaded the fully terrestrial environment, such as the Common woodlouse, roly-polies, and the terrestrial crabs or ghost crab (genus Ocypode), very common in the dryer parts of beaches and dunes.

Crustacean excretory system

The excretion of crustaceans occurs through a pair of green glands or antennae, located close to the antennae and opening outwards through an excretory pore at the ventral base of the second antennae. 

Reproduction of crustaceans

Most crustaceans have separate sexes, although there are hermaphrodite species. Their fertilization happens internally, involving copulation. In general, female crustaceans incubate their eggs in appendages of the body, as lobsters and crabs, or in egg-sacs formed when the eggs are expelled, as copepods. In most cases, their development is indirect, with free-swimming larvae, and in some cases with more than one type of larvae in the cycle. In general, from the egg comes a nauplius larva, which transforms into a zoea larva, but this pattern varies from group to group.

Crab biology and ecology

Crabs are decapod crustaceans of the Brachyura infraorder. They are defined taxonomically through the presence of ten legs and reduced tails found in other orders of crustaceans, such as lobster. Crabs are widely consumed around the world, with thousands of tons caught through fishing or produced in captivity.

We will detail the crab biology and ecology below:

–       Crab skeleton;

–       Ecology;

–       Anatomy;

–       Gill breathing;

–       Nervous system;

–       Reproduction;

–       Crab Fun Facts.

Crab skeleton

Some crabs have pointy legs adapted for locomotion on substrates such as rocky shores and the ocean floor. Others have paddle-shaped legs adapted for swimming and locomoting in the water column by paddling. 

In addition, their bodies are usually rounded, encompassing a reduced abdomen that folds under the cephalothorax. Both have a rigid chitin exoskeleton and the first pair of paws adapted in tweezers that help with food handling and protection. 


There are crab species that live in marine, freshwater and brackish environments, such as mangroves and estuaries. In general, they are very active animals. Crabs seek prey and sexual partners through complex communicative behaviours. These behaviours include rhythmic beats and pincer movements. 

Crabs live in a gregarious strategy, close and helping each other. Although crabs are sociable, males are usually aggressive and can fight each other for females or territory. Males normally have more prominent claws than females. 

Most crabs are omnivorous, that is they feed on both other animals (such as molluscs, worms and other crustaceans) and organic matter and algae debris, the main items in their diet.


The Crabs mouths are positioned at the bottom of the head, having several mouthparts that help with feeding. Their digestive tract is covered by a cuticle and subdivided into an anterior part (formed by the oesophagus and stomach) and a posterior part, where the intestine is located. 

Their stomachs have rigid chitin projections similar to teeth, with the function of crushing and sorting particles by size. What is absorbable passes to the digestive cecum, while the rest will be expelled back through the mouth or sent to the intestine, where they are metabolized for excretion. 

Nitrogen residues are expelled in the thinnest parts of the exoskeleton or through the gills, while other residues can be expelled by the bladder through a duct that communicates with the environment.

Gill breathing

Crabs perform gas exchange through the gills. They maintain a constant flow of water in these structures. In addition, many species also have bristles in the portions where the water enters the body, preventing them from being blocked by environmental residues. 

Gases are efficiently transported inside the body through the hemolymph, which contains hemocyanin, a respiratory pigment with a function similar to the haemoglobin. 

Nervous system

Crabs have a developed nervous system with a brain composed of a cluster of neural cells in the anterior portion of the head. This brain ganglion connects with the animal’s eyes and antennae, receiving visual and motion signals from the environment and water currents. 

Dorsal ventricular nerves depart from the head, responsible for the nervous stimulation of the entire body and connecting the cerebral ganglion to the thoracic ganglion. The lateral peripheral nerves, which are related to the movement of the paws, depart from the thoracic ganglion. 

Crabs also have mechanoreceptors at the base of their antennae called statocysts, which give them the sense of gravitational equilibrium. 


Crabs are dioecious animals with marked sexual dimorphism. This sexual dimorphism is visible when comparing the size of the pincers and the shape of the abdominal plates of the crabs. In females, the abdominal plate is rounded and wide, and in males, it is narrow and triangular. This difference is due to the fertilized eggs carried in this region of the body, which is only done by females. 

Their fertilization occurs internally, with spermatophores transference between male and female. The fertilized eggs remain attached to the female body for a while. Then the larvae (indirect development) are released into the environment. After at least 6 stages of development, the larvae become adult-like juvenile forms. The juvenile after dozens of growth cycles (or moults, which involve the formation of larger carapaces) become reproductively active adults. 

Crab Fun Facts

–       An average of 1.5 million tons of crabs is consumed per year by humans. Being a food appreciated around the world;

–       They are omnivorous animals, just like humans. In addition, crabs feed on other dead species and plants, sea algae;

–       Crabs have a very resistant exoskeleton, composed of chitin, which protects their fragile body;

–       A female crab is capable of laying up to 700,000 eggs in its fertile period;

–       Their reproduction strategy is sexual and the female sends chemical signals through the water to notify her partner of the moment;

–       When crabs get hurt, they can easily tear off their limbs and build new ones;

–       There are more than 4,500 species of crabs around the world;

–       There is evidence that the first crab species have risen around more than 100 million years ago.


In this post, we answered the question “Is a Crab a fish?”. We also detailed their taxonomy, biology and ecology.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): Is a Crab a fish?

What is a crustacean?

Crustaceans constitute a group of arthropods whose most striking feature is the presence of two pairs of antennae in the head region.

Why is it important to know the biology and life cycle of the crab?

Understanding crab biology and ecology is important to develop and create management and fishing strategies to not reduce their stocks by overfishing or destroying their habitats

Where do crabs live?

Crabs are crustaceans that live in salt, brackish or freshwater. They also live in estuarine and terrestrial areas. They inhabit coastal regions around the world, with some species preferring mangrove areas.

How many babies are born from a crab?

A female can lay up to 700,000 eggs during one fertile period. Crabs are animals that reproduce very quickly, although a lot of care is needed during the egg phase. During this period, the females carry their eggs until they hatch into larvae.

Where does the Spider Crab live?

Giant Spider Crabs live in the deep sea at depths from 100 to 200 metres. This crab is found off the coast of Japan and Taiwan. Thus they are also known as Japanese Spider Crabs

What is the biggest land crustacean?

Birgus latro, popularly called Coconut Crab, Thief Crab or Coconut Thief, is a large Anomura crustacean, terrestrial, found in several tropical islands of the Indian and Pacific oceans. They are also the largest terrestrial arthropod in the world.

How much does the Japanese Spider Crab weigh?

The Japanese Spider Crab can weigh up to 18 kilograms.


Burggren, W. W., & McMahon, B. R. (Eds.). (1988). Biology of the land crabs. Cambridge University Press.

Sakai, T. (1976). Crabs of Japan and the adjacent seas. Crabs of Japan and the Adjacent Seas.

Bertini, G., Fransozo, A., & De Melo, G. A. (2004). Biodiversity of brachyuran crabs (Crustacea: Decapoda) from non-consolidated sublittoral bottom on the northern coast of São Paulo State, Brazil. Biodiversity & Conservation, 13(12), 2185-2207.

Berrill, M. (1982). The life cycle of the green crab Carcinus maenas at the northern end of its range. Journal of Crustacean Biology, 2(1), 31-39.