Octopuses are invertebrates meaning they don’t have bones. Their only solid body part is their beak made out of chitin. They have hydrostatic skeletons (cavities filled with water). Octopuses use their muscles and jet propulsion to move.
Octopuses are known for their many arms and ability to move through incredibly small holes. But how is this possible? Are their bones bendable? Do they even have bones?
In this article, you will learn everything about octopuses’ skeletal structure (or lack thereof) and answer the anatomical question: Do octopuses have bones?
Do Octopuses Have Bones?
Octopuses do not have bones. Their bodies are bendable in all directions, allowing them to squeeze through extremely small holes.
Octopuses are invertebrates, which have no endoskeleton (internal skeletons). They are also mollusks, and some of them (like snails) have exoskeletons (outer skeletons), but octopuses lack that too.[1]
The boneless nature of octopuses benefits them greatly. They evolved this way to reduce the chances being killed.
Without bones, octopuses can do the following:
- Fit into small crevices to escape predators.
- Swim faster due to skeletons not weighing them down.
- Move their limbs in any direction.
- Maneuver with high flexibility.
- Blend in with the surface they rest on (helped by camouflage).
Do Octopuses Have Beaks?
Octopuses have beaks. An octopus’ beak is at the base of its arms. It is made out of chitin and it is the only solid part of the octopus.
Octopus beaks are similar to parrot beaks. They are two-part, sharp mouths with small openings. They are retractable as well.
The beak is the only solid part of an octopus. That also means, that if the beak can fit through a hole, so can the rest of the animal.[2]
The size of an octopus’ beak determines the octopus’ size too. Dead octopuses only leave their beaks behind, so scientists use these to determine how big was the individual.[3]
Octopuses also have venomous bites. The salivary glands are located behind the beak, producing and housing venom with tetrodotoxin.
Although the venom can paralyze, only four octopus species produce venom that is lethal to humans.
Do Octopuses Have Teeth?
No, octopuses do not have teeth. Their beaks look and function similarly to birds’ beaks. Octopuses use their sharp beaks to break through the shells of crabs and clams.
Octopus Anatomy
The three basic body parts of octopuses are the mantle, the head, and the arms. The mantle houses the organs, the head has the eyes, central brain, and mouth, and the arms have the suckers and mini-brains.
As octopuses lack bones, their internal structure is entirely made up of soft tissues, muscles, cartilage,[4] and a little fat.
All of these body parts are covered in skin, as opposed to exoskeletons.[5]
| Body Part | Description | Inside |
| Mantle | Rotund, oval, squishy | Organs, like hearts, gills, digestive system, reproductive glands |
| Head | Small, two prominent eyes, slightly elongated | Central brain, eyes, mouth |
| Arms | Eight long and narrowing appendages covered with suckers | mini-brains, ganglia |
Mantle
The octopus’ mantle is their primary body part which houses some of the most vital organs.
The mantle is oval but looks like a deflated ball if the octopus is ashore. It is fully covered with skin.
A notable part of the mantle is the funnel (or siphon) which is a tube-like structure. The octopus uses this to fill its cavities with water, which it then releases for propulsion.[6]
The inside of the mantle houses many organs:
- Kidney
- Liver
- Three hearts
- Stomach
- Intestines
- Gills
Head
The octopus’ head is its smallest body part. It is wedged between the mantle and the arms. The top of the head is marked by the octopus’ two big, prominent eyes.
The head houses the following:
- Beak (found at the base of the arms)
- Oral cavity
- Optic lobes
- Eyes
- Esophagus
- Doughnut-shaped central brain.
Arms
At the bottom of the head is the beginning of the eight arms. Each arm varies in length and function. Two of the bottom limbs are used for crawling, while the others are used for grabbing. Octopus arms also regrow when they are separated.
The arms are usually covered with two rows of suckers on the bottom. They are wide near the attachment point and gradually get narrower near the tip.
Inside the arms are groups of complex neuron networks. These are called ganglia and act as eight mini-brains.
Do Octopuses Have Skeletons?
Octopuses have hydrostatic skeletons. These are cavities that fill with water to stay semi-rigid. The cavities are filled and emptied by will, much like a water jet.
While a boneless structure has its benefits, some situations require a sturdy body. Octopuses have hydrostatic skeletons, which they fill with water to achieve this.
These skeletons are empty cavities throughout the body which octopuses can fill with water through their funnel. The incompressible water makes the octopus’ body parts (mantle and arms) rigid, so the octopus can change shape.[7]
It’s important to note that while hydrostatic skeletons act like skeletons, they are not real skeletal structures. Octopuses are invertebrates, meaning they don’t have bones.
The biological definition also states that skeletal structures are made out of rigid tissue.[8]
How Do Octopuses Move Without Bones?
Octopuses use their muscles to move. They can drastically change shape while moving.
Bones don’t make something move, but muscles do. As octopuses have muscles, they can move. You can compare octopuses to our tongues. They lack bones, yet they are able to move.
How we move our tongues and how octopuses move their arms aren’t dissimilar either. Both of them are muscular hydrostats (biological structures with dense muscle fibers and no skeletal structure). You move your tongue contacting some muscles while relaxing others.
1. Crawling
Octopuses usually crawl on the seabed as it is less tiring than swimming. They use their two back limbs to walk.
Octopus arms retract and extend as they move. Octopuses relax their muscles to extend their arms, then contract them to pull themselves forward.
2. Swimming
Octopuses are adept swimmers. They use all their limbs to propel themselves forward. They extend their arms outward and slam them together behind them, similar to how swimmers move their legs while breaststroke.[9]
Swimming is tiring for octopuses, which is why they often crawl. This is because their heart stops when they swim.
3. Jet Propulsion
Jet propulsion is a movement unique to octopuses. They use their funnel to siphon water into their mantle cavities and squirt it out rapidly, creating a jet stream.
This gives octopuses momentum while swimming and allows them to swim up to 25 mph.
Conclusion
Octopuses do not have bones. Their only solid body part is their beak at the base of their arms. They can squeeze through any hole their beaks can. They have three main body parts: mantle, head, and arms.
Octopuses have a skeleton-like structure called a hydrostatic skeleton. These are cavities filled with water. Octopuses move by retracting and tensing their muscles.
FAQs
What Color is Octopus Blood?
Octopus blood is blue. This is due to their poor oxygen carrier, hemocyanin, which contains high amounts of copper. When copper reacts with oxygen in the octopus’ blood, it turns blue.
Are Octopuses Intelligent?
Octopuses are the most intelligent invertebrates in the world. Their brain-to-body ratio is comparable to some mammals. Their problem-solving capabilities are impressive, as they can open bottles, and even communicate with humans for help.[10]
What Does Invertebrate Mean?
Invertebrates are animals without a backbone or internal skeletons. The word comes from the Latin “vertebra” (joint) and the “in-” is a privative prefix, meaning “not”. Invertebrates can have exoskeletons (such as crabs or lobsters) but octopuses do not have that either.
