All cephalopods possess flexible limbs extending from their heads and surrounding their beaks. These appendages, which function as muscular hydrostats, have been variously termed arms, legs or tentacles. [upper-alpha 1]
In the scientific literature, a cephalopod arm is often treated as distinct from a tentacle , though the terms are sometimes used interchangeably, often with the latter acting as an umbrella term for cephalopod limbs. Generally, arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends. [4] Barring a few exceptions, octopuses have eight arms and no tentacles, while squid and cuttlefish have eight arms (or two "legs" and six "arms") and two tentacles. [5] The limbs of nautiluses, which number around 90 and lack suckers altogether, are called cirri. [5] [6] [7]
The tentacles of Decapodiformes are thought to be derived from the fourth arm pair of the ancestral coleoid, but the term arms IV is used to refer to the subsequent, ventral arm pair in modern animals (which is evolutionarily the fifth arm pair). [4]
The males of most cephalopods develop a specialised arm for sperm delivery, the hectocotylus.
Anatomically, cephalopod limbs function using a crosshatch of helical collagen fibres in opposition to internal muscular hydrostatic pressure. [8] [ better source needed ]
Cephalopod limbs bear numerous suckers along their ventral surface as in octopus, squid and cuttlefish arms and in clusters at the ends of the tentacles (if present), as in squid and cuttlefish. [9] Each sucker is usually circular and bowl-like and has two distinct parts: an outer shallow cavity called an infundibulum and a central hollow cavity called an acetabulum. Both of these structures are thick muscles, and are covered with a chitinous cuticle to make a protective surface. [10] Suckers are used for grasping substratum, catching prey and for locomotion. When a sucker attaches itself to an object, the infundibulum mainly provides adhesion while the central acetabulum is free. Sequential muscle contraction of the infundibulum and acetabulum causes attachment and detachment. [11] [12]
Many octopus arm anomalies have been recorded, [13] [14] including a 6-armed octopus (nicknamed Henry the Hexapus), a 7-armed octopus, [15] a 10-armed Octopus briareus , [16] one with a forked arm tip, [17] octopuses with double or bilateral hectocotylization, [18] [19] and specimens with up to 96 arm branches. [20] [21] [22]
Branched arms and other limb abnormalities have also been recorded in cuttlefish, [23] squid, [24] and bobtail squid. [25]
Cephalopod limbs and the suckers they bear are shaped in many distinctive ways, and vary considerably between species. Some examples are shown below.
For hectocotylized arms see hectocotylus variability.
| Shape of arm | Species | Family |
|---|---|---|
 | Todarodes pacificus | Ommastrephidae |
An octopus is a soft-bodied, eight-limbed mollusc of the order Octopoda. The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the centre point of the eight limbs. The soft body can radically alter its shape, enabling octopuses to squeeze through small gaps. They trail their eight appendages behind them as they swim. The siphon is used both for respiration and for locomotion, by expelling a jet of water. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of all invertebrates.
A squid is a mollusc with an elongated soft body, large eyes, eight arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida. Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.
A cephalopod is any member of the molluscan class Cephalopoda such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.
In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mainly like muscular hydrostats. Most forms of tentacles are used for grasping and feeding. Many are sensory organs, variously receptive to touch, vision, or to the smell or taste of particular foods or threats. Examples of such tentacles are the eyestalks of various kinds of snails. Some kinds of tentacles have both sensory and manipulatory functions.
Bobtail squid are a group of cephalopods closely related to cuttlefish. Bobtail squid tend to have a rounder mantle than cuttlefish and have no cuttlebone. They have eight suckered arms and two tentacles and are generally quite small.
Coleoidea or Dibranchiata is one of the two subclasses of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less". Unlike its extant sister group Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal shell called cuttlebone or gladius that is used for buoyancy or as muscle anchorage. Some species, notably incirrate octopuses, have lost their internal shell altogether, while in some it has been replaced by a chitinous support structure.
A hectocotylus is one of the arms of male cephalopods that is specialized to store and transfer spermatophores to the female. Structurally, hectocotyli are muscular hydrostats. Depending on the species, the male may use it merely as a conduit to the female, analogously to a penis in other animals, or he may wrench it off and present it to the female.
Decapodiformes is a superorder of Cephalopoda comprising all cephalopod species with ten limbs, specifically eight short arms and two long tentacles. It is hypothesized that the ancestral coleoid had five identical pairs of limbs, and that one branch of descendants evolved a modified arm pair IV to become the Decapodiformes, while another branch of descendants evolved and then eventually lost its arm pair II, becoming the Octopodiformes.
Euprymna tasmanica, also known as the southern dumpling squid or southern bobtail squid, is a bobtail squid that lives in the shallow temperate coastal waters of southern Australia's continental shelf. It lives for between 5 and 8 months and the adults can grow up to 6 or 7 cm long with a mantle length of 3 to 4 cm. They are found in seagrass beds or areas with soft silty or muddy bottoms from Brisbane on the east coast to Shark Bay on the west, as well as around Tasmania. Southern dumpling squid are nocturnal and during the day hide in sand or mud covered in a mucus-lined coat of sediment. If disturbed acid glans can quickly remove this coat as an additional decoy to ink squirting.
Octopodiformes is a superorder of the subclass Coleoidea, comprising the octopuses and the vampire squid. All living members of Octopodiformes have eight arms, either lacking the two tentacles of squid or modifying the tentacles into thin filaments. Octopodiformes is often considered the crown group of octopuses and vampire squids, including all descendants of their common ancestor. Some authors use the term Vampyropoda for the same general category, though others use "Vampyropoda" to refer to the total group. Another term is Octobranchia, referring to cephalopods without prominent tentacles.
Metasepia pfefferi, also known as the flamboyant cuttlefish, is a species of cuttlefish occurring in tropical Indo-Pacific waters off northern Australia, southern New Guinea, as well as numerous islands of the Philippines, Indonesia and Malaysia.
Sepia mestus, also known as the reaper cuttlefish or red cuttlefish, is a species of cuttlefish native to the southwestern Pacific Ocean, specifically Escape Reef off Queensland to Murrays Beach off Jervis Bay. Reports of this species from China and Vietnam are now known to be misidentifications. S. mestus lives at a depth of between 0 and 22 m.
Cephalopod attacks on humans have been reported since ancient times. A significant portion of these attacks are questionable or unverifiable tabloid stories. Cephalopods are members of the class Cephalopoda, which includes all squid, octopuses, cuttlefish, and nautiluses. Some members of the group are capable of causing injury or death to humans.
A sucker in zoology is a specialised attachment organ of an animal. It acts as an adhesion device in parasitic worms, several flatworms, cephalopods, certain fishes, amphibians, and bats. It is a muscular structure for suction on a host or substrate. In parasitic annelids, flatworms and roundworms, suckers are the organs of attachment to the host tissues. In tapeworms and flukes, they are a parasitic adaptation for attachment on the internal tissues of the host, such as intestines and blood vessels. In roundworms and flatworms they serve as attachment between individuals particularly during mating. In annelids, a sucker can be both a functional mouth and a locomotory organ. The structure and number of suckers are often used as basic taxonomic diagnosis between different species, since they are unique in each species. In tapeworms there are two distinct classes of suckers, namely "bothridia" for true suckers, and "bothria" for false suckers. In digeneal flukes there are usually an oral sucker at the mouth and a ventral sucker posterior to the mouth. Roundworms have their sucker just in front of the anus; hence it is often called a pre-anal sucker.
Sepia trygonina, the trident cuttlefish, is a species of cuttlefish in the genus Sepia from the Red Sea and the western Indian Ocean. They are also a major source of food for larger marine life like dolphins, seals, and even birds.
Cephalopod fins, sometimes known as wings, are paired flap-like locomotory appendages. They are found in ten-limbed cephalopods as well as in the eight-limbed cirrate octopuses and vampire squid. Many extinct cephalopod groups also possessed fins. Nautiluses and the more familiar incirrate octopuses lack swimming fins. An extreme development of the cephalopod fin is seen in the bigfin squid of the family Magnapinnidae.
Acetabulum in invertebrate zoology is a saucer-shaped organ of attachment in some annelid worms and flatworms. It is a specialised sucker for parasitic adaptation in trematodes by which the worms are able to attach on the host. In annelids, it is basically a locomotory organ for attaching to a substratum. The name also applies to the suction appendage on the arms of cephalopod molluscs such as squid, octopus, cuttlefish, Nautilus, etc.
Cephalopods, usually specifically octopuses, squids, nautiluses and cuttlefishes, are most commonly represented in popular culture in the Western world as creatures that spray ink and use their tentacles to persistently grasp at and hold onto objects or living creatures.
Rossia pacifica, also known as the stubby squid, is a species of bobtail squid native to the northern Pacific Ocean. It usually occurs in winter on sandy slopes away from strong currents in moderately shallow water. In summer it moves to deeper water where it breeds. The female cements the egg capsules under a stone or in some other concealed location, and both male and female die soon after breeding.
Octopus minor, also known as the long arm octopus or the Korean common octopus, is a small-bodied octopus species distributed along the benthic coastal waters bordering China, Japan, and the Korean Peninsula. It lives at depths ranging from 0 to 200 metres. O. minor is commonly found in the mudflats of sub-tidal zones where it is exposed to significant environmental variation. It is grouped within the class Cephalopoda along with squids and cuttlefish.
To most of us it has always seemed obvious that an octopus has eight arms. Octopuses have two legs and six arms Claire Little, a marine expert from the Weymouth Sea Life Centre in Dorset, said: 'We've found that octopuses effectively have six arms and two legs.'
As result, marine biologists tend to refer to them as animals with two legs and six arms.
Cephalopod anatomy | ||||||||||||||
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| Shell |
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| Mantle & funnel |
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| Head & limbs |
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| General |
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Developmental stages: Spawn → Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult • Egg fossils • Protoconch (embryonic shell) | ||||||||||||||
