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Does an octopus have tentacles?
The etymology of the word Octopus means Eight Legs. This alone suggests the answer to the question of whether an octopus has tentacles or legs. “Octopus” is a new word that was derived from the Greek word “oktopous”. This greek word was literally translated as eight-footed. Therefore, it can be said that octopuses have eight legs. However, the octopus species are characterized and described to have eight arms (appendages). Although there are confusions on whether the appendages of an octopus should be called tentacles or arms.
Octopuses are cephalopods. They belong to the same class as squids and cuttlefish. The Latin term cephalopods are translated to mean “headfeet”. This term is used to group these animals because their limbs are attached to their head. However, although a lot of people commonly describe the eight limbs of an octopus to be tentacles. Some specialist, however, has emphasized a distinction between tentacles and arms.
Tentacles are said to retract and have suckers only around their tips. However, they usually come in pairs. Whereas, an arm is shorter and stronger compared to a tentacle. Arms usually have suckers all over. Hence in this regard, it is correct to say that octopuses don’t have tentacles but arms.
Further observations on octopuses suggest that the rear two of the octopus arms are mainly used by the octopus as legs whereas, the other six are used as arms. Octopuses seem to prefer using the first 3 pairs of their appendages for feeding, grabbing, and handling objects while the rear two appendages function as legs. Hence, it is assumed that they have six arms and two legs.
What exactly are tentacles?
As earlier said octopuses do not have tentacles. However, let us understand what a tentacle is and compare it with the appendages an octopus has. It is important to know that all animals that belong to the class cephalopod, including the octopus have flexible appendages. Their appendages extend from their head and surround their beaks.
These appendages in cephalopods are muscular hydrostats and are variously called tentacles, arms, or legs. The fact is, calling these appendages arms or tentacles depends on the organism and the structure of the appendages. However, in the context of an octopus, calling its appendages arms or legs is way acceptable than calling them tentacles. Even though arms and tentacles are used interchangeably when referring to cephalopods, there is a huge difference between these two. In scientific literature, the arms of these animals are distinct from tentacles.
Tentacles can be defined in zoology as the flexible, mobile, and elongated organs that some animal species possess. The anatomy of an animal’s tentacle is mainly like muscular hydrostats. Several forms of tentacles are used for feeding and grasping. The majority of tentacles are sensory. They are variously receptive to vision, touch, or to the taste or smell of certain food or threats. An example of such sensory tentacles is the eyestalks of several kinds of snails. Moreso, some tentacle types possess both sensory and manipulatory functions.
Tentacles are similar to cirri (singular: cirrus), though cirri lack the strength, flexibility, size, and sensitivity of a tentacle. A nautilus, for instance, has cirri whereas, squids possess tentacles. Several mollusks possess tentacles of different forms. The pulmonate land snails are the most familiar. They usually have two sets of tentacles on the head. As these snails extend, the upper pair of tentacles have eyes at their tips. Whereas, the lower pair are chemoreceptors. Both pairs, however, are fully retractable muscular hydrostats. Though they are not used to capture prey or for manipulation. Also, there are many small tentacles around the edge of the mantle of some marine snails like abalone and Trochidae (top snails). Such tentacles are called pallial tentacles.
Squids happen to have spectacular tentacles amongst the cephalopods. These tentacles are like highly mobile muscular hydrostats with several appendages such as suction disks and thorny hooks, sometimes. Tentacles were interchangeably called arms even up to the early twentieth century. However, the modern convention now is to describe an appendage as a tentacle if it has a relatively thin “peduncle” or “stalk” with “clubs” at its tips. Thus, the relatively shorter appendages are referred to as “arms”.
How many tentacles does an octopus have?
Hence, in reference to the above explanation, octopuses do not have tentacles. The 8 appendages of an octopus count as arms even though they are quite long. However, it is important to note that even though tentacles are distinct from arms, they could be used as an umbrella term for limbs. Giant squid and colossal squid’s tentacles possess powerful suckers and pointed teeth at the ends. The teeth at the end of the tentacles of a giant squid resemble bottle caps and serve like tiny hole saws. Whereas, the colossal squid’s tentacles wield 2 long rows of swiveling tri-pointed hooks.
Octopuses, cuttlefish, and squids are all cephalopods. They all evolved from a mollusk ancestor over 400 million years ago. Later, they evolved appendages in two forms. The octopus has 8 appendages. Each of these appendages has rows of suckers running its length. Thus, in strict anatomical terms, these appendages are arms and not tentacles. Unlike arms, tentacles only have suckers on their spade-shaped ending. Squids and cuttlefishes have tentacles and also arms. Then, nautiluses have about 90 appendages that lack suckers altogether. Hence, these appendages of nautiluses are all tentacles. The tentacles and arms of cephalopods lack bones. Rather, they are made from an intricate tapestry of coiling muscles fibers. Cuttlefishes shoot their tentacles at their prey by contracting fibers that are along the entire length of the tentacles. The octopus can work its arm into a tiny recess by contracting only some fibers.
What are arms?
Arms are different from tentacles. In comparison with tentacles, arms have suckers along most of their length. Whereas, tentacles only have suckers near their ends. Hence, an octopus has eight arms with no tentacles, while squid and cuttlefish have 8 arms and 2 tentacles. The mouth of an octopus is surrounded by its arms. These arms are attached near their base to each other by a webbed structure of tissues. They can be described based on the side and sequence position they take in the octopus. A description example includes L1, R1, L2, R2.
Every arm of an octopus has two rows of fleshy suckers that make them capable of gripping. Since the octopus lack skeletal support, the arms serve as muscular hydrostats. There are transverse, longitudinal, and circular muscles around a central axial nerve in the arms. Thus, they can extend and contract. They can also twist and bend in any direction. Furthermore, the circular adhesive suckers at the interior surfaces of these arms allow an octopus to manipulate objects or anchor itself. The suckers on an arm are usually circular. These suckers are made up of two distinct parts: the outer shallow cavity called an infundibulum and a central hollow cavity called an acetabulum.
The infundibulum and acetabulum are thick muscles. However, there is a protective chitinous cuticle that covers the infundibulum and acetabulum. Moreso, when an octopus’s sucker attaches to a surface, the orifice between the surface and the sucker is sealed. The sucker’s infundibulum gives adhesion as the acetabulum stays free. However, for attachment and detachment to occur, there is muscle contraction of the sucker. Also, these arms can sense and respond to light. Thus, even if the octopus head is concealed, it can control its arms.
Octopuses, however, are said to have 6 arms and 2 legs. They even have a preference for which arm they use for eating. In a 2008 study, scientists revealed that they maneuver and crawl around the seafloor using two rear arms while the remaining six arms are predominantly used for feeding and propulsion. Scientists observed them pushing off with their back arms to get over rocks and the seabed. Thus, using the remaining arms for swimming or propulsion along the ocean floor.
Following about 2,000 observations of octopuses, scientists concluded that cephalopods are able to use their left and right arms equally well. Though many of them prefer to use their third arm from the front to eat. However, it was discovered by researchers that only octopuses with a weak eye seemed to favor one side of arms. The average octopus has 240 suckers. Octopuses that are large can hold up to 35 pounds. An octopus can operate their arms individually using them for movement and to catch its prey.
Disturbingly, in rare situations, captive octopuses have been known to eat their own arms when they get bored. They tend to do this when restricted to an environment that doesn’t provide them enough stimulation. This awkward behavior is known as autophagy. Furthermore, there are some arm abnormalities found in octopuses. Here are some arm anomalies that have been recorded:
- An octopus with a forked arm tip
- 7-armed octopus
- Octopuses with double or bilateral hectocotylization
- 6-armed octopus nicknamed Henry the Hexapus
- Octopus with up to 96 arm branches
- 10-armed Octopus briareus
It is important to note that octopuses do not have tentacles. Yea, a lot of people may think of cephalopods when they hear the word tentacles. However, the fact is all members of this group possess arms but only the squid and cuttlefish have tentacles.
The major difference between tentacles and arms is that tentacles are retractable and longer than arms. In addition, they have a spade-shaped or flattened tip. Also, arms possess suckers along most of their length, while there are only suckers near the ends of tentacles. In reference to this, it is conclusively acceptable to say that octopuses have eight arms and no tentacles.
Jamar holds an M.D. from Yale University as well as a B.S. in Biology from Brandeis University. He currently conducts research in the field of Microbiology with a specialized focus on bacteria. Outside of work Jamar enjoys spending time with his family and writing about his field of study to help students and other industry professionals better understand its effects on the world.