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Octopus, as the name suggests, is a marine animal that has 8 limbs. It belongs to the order Octopoda which means it is a mollusk with eight limbs. More than 300 such species are found in this particular order within Cephalopoda that includes squids, nautiloids, and cuttlefish. Let us find out more information about this animal in the following sections.

What is an Octopus?

Octopus is a creature that seems to be out of this world. Its appearance is different from any conventional animal we have seen in the zoo, on the television, or in pictures. Its exclusive appearance makes it an alien species that is often mentioned in fairy tales. Zoologists have properly analyzed the physical traits of this animal and concluded that it is a Cephalopod that represents an animal with long limbs.

As per the octopus classification, it falls in the order Octopoda, Class Cephalopoda, and Phylum Mollusca. This animal has no internal or external skeleton. Hence, it can squeeze inside small holes, ducts, rifts, cracks, etc. This is the most fascinating part of its physiology that the organs of an octopus do not get damaged due to extreme contorting of the entire body.

They are bilaterally symmetrical which means that an octopus can be divided equally into two parts. It has two eyes and a small brain. The mouth is positioned underneath the head and has a beak. The mouth is where all the limbs meet. The body is so soft that it can take any shape very fast and can escape from the smallest gaps. They swim majestically by pushing the eight limbs backward to move forward. One of the prime octopus characteristics is that it has two siphons behind the eyes. These siphons are used for respiration, as well as, locomotion. A jet of oceanic water is expelled from both the siphons causing the body to move forward in the opposite direction. The rate of water expulsion is controlled as per the requirement of speed and changes in direction.

Now that you have got the answer to the question ‘Is an octopus a mollusk?’ you should know that an octopus is the most intelligent among the invertebrates. It has a complex nervous system that helps it to evade predatory attacks, sneak and catch prey, get camouflaged to the surroundings, and think to add more safety.

Where Can We Find Octopuses?

As mentioned earlier, octopuses are marine mollusks that are found in the oceans and seas. They are generally found in and around coral reefs where the fauna is diverse and food is plenty. You will also find octopuses in the sea beds, pelagic waters, intertidal zones, and even at extreme depths.

They like to stay and rest inside small holes, crevasses, cracks, and in between rocks. This animal generally lives alone and is extremely protective about its territory. They are also found in the grass beds of seas closer to the shore. Some of the species like to remain on the warmer layer of oceanic water and some have adopted the cold high-pressure level of the atmosphere deep in the ocean. The deepest recorded presence of an octopus (Grimpoteuthis or Dumbo Octopus) is 6957 m or 22,825 ft. There is no freshwater octopus recorded by any zoologist. The most common name of octopus is Devil Fish.

Some of the species such as Vulcanoctopus live close to the hydrothermal vents where hot water oozes out relentlessly. The temperature of these vents can go up to 700°F. You can see there is excellent variance in habitat in the octopus family.

Physiology of Octopus

Among all the breeds, the largest known is the Giant Pacific Octopus. It weighs 15 kg on average and has an arm span ranging up to 4.3 meters. The largest of this breed documented is 71 kg. The smallest species, on the other hand, is Octopus wolfi. It is around 2.5 cm and weighs not more than 1 g.

As mentioned earlier, the octopus is a bilaterally symmetrical animal if you consider the dorso-ventral axis. The feet and head belong to the two ends of a body. The head consists of a small brain, a mouth, and two eyes. The feet have developed to form 8 extremely flexible appendages with excellent capabilities of folding and extending to a certain distance. All these flexible limbs are attached at the base. The base is where the mouth lies in the center.

You will be fascinated to know that only two of the appendages at the back are used for walking or locomotion on the ocean floor. The rest of the appendages are used for scavenging food or catching prey. The mantle is a bulb-like structure that protrudes from the head to form a visceral hump. This hump contains all the vital organs covered and protected by muscular walls. It also contains a pair of gills that ends in siphons. If you concentrate on the two structures protruding from the head below the eyes, you will find two pipe-like structures. These are siphons used for breathing and locomotion. The octopus passes a jet stream of oceanic water from the siphons to add more thrust to its movement. This is a prime feature of the octopus species. The mouth has a beak-like hard structure. It is used to crush food and ingest it with the help of the appendages.

Most of the body is soft and can be squeezed. It is made of soft tissues. The outer epidermis is very thin. It contains mucous cells to make the octopus animal slippery. It also contains sensory cells to sense any change in the external environment. Below the epidermis, it contains a large amount of collagen, a flexible connective tissue that helps the octopus to twist, contort, extend, contract, or lengthen its body parts. As there is no skeletal support (internal or external), the limbs act as support. They contain transverse, circular, and longitudinal muscles attached to a central axial neural connection.

All these arms have adhesive suckers that help the octopus to keep itself attached to a surface or grab tight a prey. A sucker is nothing but a small bowl-like circular and muscular organ made of thick muscular tissues.

The eyes of an octopus are big and are similar to that of the fish. A pair of eyes resides above the siphons on the head on both sides. The pupils of the eyes form a slit-shaped retina that adjusts with the intensity of the light.

The Circulatory System of the Octopus

You will be enthralled to know that the octopus species has three hearts and the blood circulatory tissues remain inside the vessels. One of them is the main systemic heart which is responsible to circulate blood throughout the body. The rest two brachial hearts are present close to the gills. When an octopus is swimming, the systemic heart stops beating whereas the branchial hearts work. This makes the animal get tired quickly and it prefers crawling.

As per the octopus information, the blood in the circulatory system contains hemocyanin. It is a copper-rich pigment of the blood commonly found in mollusks that carries oxygen and carbon dioxide. This pigment makes the blood very thick. This is the reason why the heart of an octopus is very strong as it requires immense power to pump viscous blood. You must know that in cold aquatic conditions and high-pressure zones, hemocyanin works better than hemoglobin in transporting oxygen to the entire body. This pigment gives the blood of the octopus a bluish color. The presence of this pigment also answers the question ‘is an octopus a mollusk?’


The process of respiration in octopuses is a complex one. An aperture lets the water into the mantle cavity and then to the gills. In the end, the water is then passed through the gills and then through the siphons. This process also helps the octopus to propel its body and shoot to a distance very fast.

Apart from the respiratory gills, octopuses also have a mechanism of breathing through their skins. When they are stagnant, nearly 41% of the oxygen requirement comes from the thin skin. This percentage reduces to 33% when an octopus starts swimming. Once it has preyed or consumed and rested, the respiration process through the skin reduces as much as 3%.

Digestive System

As mentioned earlier, the mouth is present right at the center of the origin of the limbs. The buccal cavity is guarded by strong beak-like structures that act like teeth. This beak is made of chitin. The mouth has salivary glands and has a muscular tongue-like structure with embedded teeth on it.

The tiny teeth break down food and the tongue helps to ingest it through the esophagus, also called the food pipe. It then conveys the food to the gastrointestinal tract. This tract suspends from the mantle roof supported by numerous membranes. This tract has three different parts, crop, stomach, and caecum. The crop stores the ground food, the stomach digests it, and the caecum expels it. There are digestive glands present in this system that helps in breaking down the food and absorbing the ingredients properly.

Excretory System

Due to osmoregulation, fluid accumulates in the pericardial portion of the branchial hearts of the octopus. The prime octopus characteristics of the excretory system are its two nephridia. These organs resemble the kidneys of vertebrates. These organs help in eliminating the wastes such as ammonia produced from digesting and assimilating protein.

Inc sac

We all know or have watched the evasive techniques an octopus uses to distract predators. It has an ink sac located right below the digestive gland. This gland is connected to a sac that secretes an ink-like chemical. This sac is also located very close to the funnel of an octopus. Hence, the accumulated ink can be shot and sprayed outside in the water when an octopus feels threatened. Before shooting this ink, it mixes with the mucus to produce a thick curtain of darkness. The predator loses its sight of the prey and the octopus gets a few seconds to escape or seek to resort to any safe place. When cultured, the ink is found to be melanin, the common pigment found in our hair and skin. Only the Cirrate octopus does not have an ink sac.

Reproduction of Octopuses

The reproduction process in octopuses is a little different from that of the conventional methods. The male octopus introduces the gamete arm (generally the 3rd arm from the right) into the mantle cavity of a female octopus. The sperms are then deposited into the mantle where the oviduct lies. The union between the ova and sperms results in the formation of embryos. The female octopus stops movement and even eats to take care of the matured eggs. It takes almost 40 days to prepare the eggs inside the mantle cavity of a female octopus.

Nearly 400,000 eggs laid by a female octopus. She shows immense care and obsession to guard these eggs until they hatch. An interesting fact is that once the eggs hatch, the female octopus dies. After a few months also, the male octopus goes through the same fate and dies a natural death.

The babies of an octopus are called larvae. When they hatch, they drift apart and eat the larvae of other animals under the plankton clouds. Not all larvae survive only a small percentage of all the eggs survive and mature to become an adult. As per the octopus classification, the number of eggs varies.

FAQ (Frequently Asked Questions)

1. How Does an Octopus Behave?

We have learned what is an octopus. It is a marine animal that likes to lead a solitary life when not mating. They occur in very low densities. They have their dens for hiding and resting.

2. Does an Octopus Use Tools?

Some octopuses have been witnessed carrying empty coconut shells for protection. This is considered to be the first recorded use of tools in an invertebrate.

3. What are the Common Names of an Octopus?

A very common name of octopus is devil fish due to its unconventional appearance. It is also called scuttle, octopod, etc in different countries.