Is an Octopus a fish?

In this post, we will learn if the octopus is a fish and its main biological and behavioural characteristics.

Is an Octopus a fish?

No, octopuses are not fish, they are molluscs. Octopuses belong to the mollusc group and are closely related to squids, slugs and snails.

The molluscs and their characteristics

Molluscs are soft-bodied animals that usually have a shell.

The shell is present in oysters, shellfish, and snails. In some molluscs, such as squid, the shell is internal and in others, the shell is absent, as in octopus. Shells are important to protect the molluscs’ soft bodies and prevent water loss.

Molluscs live in marine or freshwater aquatic environments and humid terrestrial environments.

The phylum Mollusca is the second largest in the number of species, approximately 50,000 (fifty thousand!), only behind the arthropods.


These animals have their bodies divided into head, foot and the visceral mass. Their sensory organs are found in the head. Their foot is responsible for the locomotion and, in some animals, such as the octopus, it can be replaced by tentacles. The visceral mass is where all the organs are found.

Digestive System and Food

Molluscs have a complete digestive system, with a mouth and an anus. Food is transported through the digestive tract, where it undergoes the action of enzymes. Nutrients are absorbed and distributed throughout the body through the blood.

Cephalopods and gastropods have the radula, a kind of tongue with sharp teeth, used to scrape the food out from the substrate.


Once molluscs are found in a wide variety of environments, they have different types of breathing strategies.

Branchial respiration is performed by molluscs that live in the water, such as octopuses, squids and oysters. Lung respiration is present in molluscs that live in a terrestrial environment, such as snails. Skin respiration occurs in slugs that also live in terrestrial environments, under the ground and in trees.

Circulatory system

The circulatory system distributes nutrients and oxygen thought the body from the digestive and respiratory systems. The excretory system is responsible for concentrating and eliminating metabolic waste.

The circulatory system is open, and the heart is located in the dorsal portion of the visceral mass. The heart contractions send blood through the vessels and then through gaps located between the tissues along the body.


Molluscs have sexual reproduction, with internal or external fertilization. Most molluscs have separate sexes, the exception is the bivalves (clams), which are typically hermaphrodites.

In the case of external fertilization, males release sperm and females release eggs directly into the water column, where the two gametes meet and become a fertilized egg. In the case of internal fertilization, the sperm are released into the female’s body.

Octopus and its general characteristics

Octopus – which means “many feet” – are invertebrate animals of the Cephalopoda class. It is estimated that there are around 700 species in the Octopoda order. They are strictly marine animals and are found in oceans worldwide, mainly in tropical waters. Thus, they are called cosmopolitan. Cephalopods have feet fused to the head, that is, tentacles attached to the animals head, circulating the mouth aperture. Unlike squid, these animals do not have the equivalent of an internal skeleton.

Octopuses use their arms to move around and capture their prey, as they are predatory animals and feed mainly on small fish, crustaceans and other invertebrates. Their arms are equipped with a series of suction cups that are very sensitive to touch. Each cup has tiny chemoreceptors that can pick up tiny molecules from the marine environment and deliver information to your brain ganglia. In this way, the animals can feel the taste of the objects and places they touch.

In addition, octopuses can autotomize their limbs, that is, they can “let go” their arms when they feel threatened, distracting the predator while fleeing. This behaviour is similar to what lizards and spiders do with their extremities. The area regenerates and this ability is called autotomy.

They are solitary animals that live among rocks and small caves. Some species weigh 3 kilograms and others can reach 40 kilograms; some are up to 3 metres, sometimes even called sea monsters.

Nervous system

Nerve structures called ganglia are large and form a structure similar to a brain. This structure is subdivided into parts responsible for memory and learning. They have neurons and are animals with well-studied captivity behaviour due to their intelligence. Their intelligence is considered unique among the invertebrates and even superior to some vertebrates. 

Their eyes are well-developed, with a binocular vision. This means they can form images and see colours. Few studies show that they can only distinguish the polarization of colours. 

Their highly flexible arms have tension receptors and suction cups that also act as chemoreceptors, giving them the ability to feel the texture of an object and even smell or taste it. These suction cups also help with locomotion and feeding.


Octopus reproduction is sexual. The female releases a sex pheromone in the water, which attracts males. The females can be fertilized by more than one partner. Octopuses have a modified arm, the hectocotylus, which will introduce spermatophores into the female. They can keep them inside the female until the eggs become mature. Thousands of eggs can be deposited in the most diverse places, depending on the species, for example, in nests. The mother takes care of the eggs until they hatch and dies shortly thereafter, once they do not feed during this process. Males die sometime after copulation. 

Once they are born, the hatchlings stay on the water column feeding on zooplankton, and when adults most species inhabit deep waters.

They are carnivores and feed mostly on fish and other invertebrates, such as crustaceans. Cannibalism is common in some species. They are economically important as in some cultures it is served as food.


The skin of octopuses is covered by special cells with pigments of various types. These cells are called chromatophores and iridocytes. They can camouflage similar to the environment when the animals need to and they are extremely quick. The curious thing about chromatophore cells is that they do not change colours, as each one already has a specific colour. What happens is an expansion of the chromatophores of the desired colour, while the others contract, resulting in the ideal camouflage.

These animals use camouflage to flee from predators, hunt their prey, communicate with each other (shock other octopuses or attract females) and even signal danger. The danger signalisation is the case of the blue-ringed octopus, which has well-rounded circles marked in bluish colour throughout its body indicating they are harmful. This species has a powerful venom.

The intelligence of octopuses

The octopus can learn and memorize. They can solve complex problems, discriminate through classical conditioning, and learn from observation.

They can transport materials to build their refuges, although they have difficulty moving and, thus, temporarily jeopardize their survival. In this way, they have the opportunity to survive longer.

Octopuses apply significantly different pressure when they are willing to handle different tools. Studies have shown that they retain prey, trapping them in their den to devour more calmly.

They can recognize their autotomized limbs from other individuals’ limbs. According to studies, 94% of the octopuses did not eat their own tentacles, they only transport them to their refuge with its beak.

Octopuses can also mimic poisonous species in their environment in an attempt to survive. They have extraordinary long-term memory, learning capability and defensive reflex memory.


In this article, we learnt if the octopus is a fish and its main biological and behavioural characteristics.

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

Frequently Asked Questions (FAQs): Is an Octopus a fish?

What is the predator of the octopus?

Octopuses are preyed on by moray eels, sharks, rays, some large fish, marine mammals, birds, and humans. To escape predators, these animals have well-developed techniques.

What are octopus suction cups?

In cephalopods, suckers on their tentacle have three main functions. They help the animal locomotion, that is the case of octopuses and other benthic molluscs, and they also serve to hold their prey and, finally, they can be used as chemoreceptors.

How long does an octopus live?

On average, an octopus lives between 3 and 4 years, but there are reports that the Giant Pacific octopus reaches 5 years old.

Why do female octopuses die after mating?

After mating, the female lays the eggs, about 150,000, inside a cave or a nest, and takes care of them to avoid predators. During this period, the female does not feed and, for this reason, she dies soon after the chicks hatch. 

Are octopuses smart?

Studies of intelligence in cephalopods have shown that such animals are endowed with a cognitive apparatus capable of providing them with several actions that can be understood as signs of high intelligence.

Does octopus eat crabs?

Yes, octopuses eat crabs. Octopus fees on a variety of food items, from crustaceans to fish.


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Smale, M. J., & Buchan, P. R. (1981). Biology of Octopus vulgaris off the east coast of South Africa. Marine Biology, 65(1), 1-12.

Hernández-Urcera, J., Garci, M. E., Roura, Á., González, Á. F., Cabanellas-Reboredo, M., Morales-Nin, B., & Guerra, Á. (2014). Cannibalistic behavior of octopus (Octopus vulgaris) in the wild. Journal of comparative psychology, 128(4), 427.

Boycott, B. B. (1965). Learning in the octopus. Scientific American, 212(3), 42-51.