Octopus tentacles are arguably the best-known feature of these intriguing sea animals. But did you know that octopuses don’t really have tentacles?
Unlike what many people think, octopuses have eight arms and no tentacles. These two terms are often mixed up and sometimes used interchangeably, but they are defined differently in the world of biology.
In this article, we’ll delve into the special features of an octopus’s arms. We’ll see why many people confuse them for having tentacles, and we’ll also look at the rare Seven-arm Octopus. Read along to learn about these and more!
Octopus Limbs, Arms, Tentacles, and Legs
The octopus is recognized for its eight long, flexible appendages. While some might refer to them as tentacles, limbs, or legs, the accurate term is ‘arms.’ In marine biology, these words mean different things.
Let’s look at how we define each one:
- Limbs: Limbs are parts of the body used to move, hold things, or touch. In humans, limbs are the arms and legs. Arms help us grab and carry stuff, and legs let us walk on two feet. In marine biology, however, the term has a broad meaning and can include various appendages used for movement or manipulation. Technically speaking, the arms of the octopuses are also limbs, but the term ‘limb’ is less specific.
- Tentacles: In marine biology, we usually use the term “tentacles” for appendages that are long and have thin bases and thicker ends. Meanwhile, shorter appendages are called “arms.”. By this definition, an octopus has eight appendages that cannot be tentacles because they don’t have the thin base and thick end found in tentacles.
- Legs: The term ‘legs’ is not typically used to describe an octopus’s appendages. Although octopuses can crawl using their arms in a way that resembles the function of legs, these arms are not primarily ‘weight-bearing structures’ like how legs are defined.
- Arms: For octopuses, ‘arms’ is the correct term. They have eight arms that bend easily and are covered in small, sticky circles called suckers. These arms help them move, eat, and feel around their environment. The term ‘arms’ is more specific than limbs and tentacles and has an entirely different definition than legs. In other words, arms is the best word to call an octopus’s appendages.
Simply put, the arms of an octopus qualify as ‘limbs’ and ‘tentacles.’ However, we still prefer using the word ‘arm’ because it is the most specific term we have.
Fun Fact: There is a species of octopus that can walk on land, the Algae Octopus (Abdopus aculeatus). They are often called “the only land octopus.” Despite this ability, however, their appendages still don’t qualify to be called legs.
Watch this video to see the Algae Octopus using its arms to walk:
The Seven-Arm Octopus Exception
The Seven-arm Octopus, known as Haliphron atlanticus, is an interesting type of octopus. While it’s called the Seven-arm Octopus, it actually has eight arms.
It is named this way because male Seven-arm Octopuses have one arm that is coiled in a sac beneath their right eye. This makes it look like they have only seven arms.
The hidden arm is called the hectocotylus and is found across many cephalopod species, including octopuses. This arm is used by males to place a sperm packet into the female’s body to start reproduction.
Aside from the unique placement of its hectocotylus, Seven-arm Octopuses are also known for their size. This species is one of the largest, second only to the giant Pacific octopus.
When fully grown, it can reach 12 feet long and weigh up to 170 pounds. Seven-arm octopuses are usually found in both the Atlantic and Pacific Oceans, often in deep waters as far down as 3,000 feet.
Fun Fact: Another fascinating thing about the Seven-arm Octopus is how it interacts with jellyfish. It not only consumes jellyfish but also potentially uses their stinging tentacles as a tool to catch other prey.
Watch this video to see how this unique relationship works:
Why Do Some Octopuses Have Less Than Eight Arms?
All types of octopus have eight arms, which is why they have “octo” in their name. However, they sometimes lose arms due to things like attacks, fights, or even on purpose.
Here are some reasons why some octopuses might have fewer than eight arms:
- Predation: Octopuses sometimes lose an arm when attacked by predators. This is a common risk in their natural habitat.
- Autonomy: Octopuses can deliberately detach an arm as a defense mechanism to escape threats. They are able to do this because their arms exhibit some form of autonomy, giving each arm some level of independence to ‘decide’ when it is wise to detach from the body.
- Injury: Some octopuses can get injured and lose an arm. This can happen from fights with other sea creatures or from accidents in their environment.
- Congenital Conditions: While there is not much scientific evidence to back this, in theory, it is possible for octopuses to be born with fewer than eight arms due to genetic or developmental factors.
- Disease: One potential reason for an octopus having fewer than eight arms could be disease. While our understanding of octopus diseases is still very limited, there’s a possibility that certain illnesses might affect their arms.
More often than not, environmental factors and predation are the main reasons why octopuses may have less than eight arms. There may be other reasons, too, but we don’t have conclusive evidence about them yet.
Watch this short clip to see an octopus handling one of its detached arms:
Roles and Functions of Octopus Arms
Octopuses use their arms for many things. Octopus arms help them move, feel their surroundings, and survive underwater life, which are all very important functions. Let’s learn more about what octopus arms do.
Movement and Locomotion
Octopus arms are essential for movement. They use their arms to crawl, swim, and navigate their underwater environment. Their arms are a key player in sensing their surroundings and moving around.
Hunting and Feeding
In their natural habitats, octopuses use their arms for “speculative bottom searching.” This involves extending their arms into holes and crevices to find food.
Their arms come with a built-in ability to search for food autonomously. This ability is enhanced by their suckers, which can taste and smell, allowing octopuses to hunt successfully in areas they cannot see.
Sensory Functions
The suckers on octopus arms are not just for gripping; they also contain sensory receptors. These receptors are capable of detecting textures, particles, and even color, helping with the octopus’s limited vision.
This ability is helpful for distinguishing different objects and creatures in their environment, such as differentiating a rock from a crab. This is also one of the reasons why octopus intelligence is so unique.
Fun Fact: The number of suckers on an octopus depends mainly on its species’ size. However, all octopuses share the characteristic of having a double row of suckers on each arm.
These rows are not always perfectly aligned and can sometimes come in a zigzagged arrangement.
Camouflage and Defense
Octopus arms assist in camouflage by changing color and texture to blend into their surroundings. Octopuses can also detach arms to escape predators effectively.
The detached arm can wiggle for hours, drawing the predator’s attention while the octopus escapes. This tactic, combined with their ink, is key in evading attackers or outsmarting prey.
Here’s a short clip demonstrating how effectively octopuses use their arms for camouflage and how they utilize their ink for evasion:
Reproduction
Male octopuses have a specialized arm known as the hectocotylus. This arm is primarily used for reproduction. It has grooves for storing sperm, which the male transfers to the female.
Fun Fact: Did you know that octopuses have a relatively short lifespan? Most species live for just 1 to 3 years, largely because of their reproductive cycle.
6 Fascinating Facts About Octopus Arms
Octopuses use their arms for basically everything they do, but did you know that they have other nifty tricks up their sleeves? Let’s take a look at more fascinating facts about octopus arms.
1. Octopus arms can ‘think’ for themselves
Octopus arms can operate independently due to their special neural structure. Each arm possesses a neural ring that allows it to bypass the brain, enabling the arms to communicate and coordinate autonomously.
This means the octopus’s brain might not always know where its arms are, but the arms can still ‘talk’ to each other during complex tasks.
What’s fascinating about octopuses is the distribution of their neurons. They have around 500 million neurons, with many concentrated in their arms.
On a related note, I’ve been following the works of Dominic Sivitilli, an octopus expert, for a few years now. In 2019, at an astrobiology conference (AbSciCon2019), Sivitilli shared intriguing insights into octopuses.
Here’s a glimpse into one aspect of Sivitilli’s research, illustrating how information is exchanged among the octopus’s suckers, arms, and brain:
His work left a pretty interesting perspective on how intelligence can come in forms that are unique to what we usually recognize.
2. Octopuses can regenerate lost arms
Octopuses have the remarkable ability to regrow lost arms, a process influenced by their age, size, and environmental conditions.
The regeneration involves specialized cells developing into muscles, nerves, and skin. Generally, this process can take from a few weeks to several months, with smaller arms regrowing faster.
Read more about octopus arm regeneration in this article.
Fun Fact: Unlike squids, where not all species can regenerate arms, all octopuses possess the ability to regenerate.
3. Octopus arms inspire robotic applications
The structure of octopus arms has led to significant innovations in the field of robotics. Scientists have developed octopus-inspired robots that can grip, move, and manipulate different objects.
Noteworthy examples include a soft robotic arm made from the collaboration between Harvard SEAS and Beihang University. This arm replicates the adaptive flexibility and suction of octopus arms.
Another example is Cecilia Laschi’s team’s underwater robot at the BioRobotics Institute.
This robot demonstrates the practical use of soft, flexible robotics inspired by octopus movements for performing complex underwater tasks.
4. The arms of octopuses allow them to use tools
Octopuses use their arms to manipulate objects like rocks, shells, and even human-made items for various needs.
One example is the Common Blanket Octopus, which uses Portuguese Man o’ War tentacles as weapons. Another example is the Veined Octopuses in Indonesia, which have been observed collecting coconut shells for shelter.
5. Octopuses eat their own arms
Octopuses sometimes exhibit a strange behavior: they eat their own arms. This self-cannibalism, known as autophagy, can occur due to stress, captivity, or illness.
While it might seem harmful, octopuses can regenerate these eaten arms over time. However, researchers are still trying to fully understand the reasons behind this unusual behavior.
6. Octopuses can have more than eight arms!
Did you know that it is possible for some octopuses to have more than eight arms? In fact, the world record for the octopus with the most number of arms is 96.
It is an extraordinary specimen found in Matoya Bay, Japan, in December 1998. Before passing away, it survived for five months in captivity, according to Guinness World Records.
The remains of the octopus are exhibited in Shima Marineland Aquarium in Shima, Japan.
Hopefully, this guide has taught you a lot about the fascinating arms of octopuses. Do you have other facts to share? Let us know what you think about Octopus’s arms by leaving a comment below!
