A cephalopod (Greek Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical ancient Greek literature and the New Testament of plural Κεφαλόποδα (kephalópoda); "head-feet") is any member of the mollusc The Mollusca, common name molluscs or mollusks,[note 1] is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. This is the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. Molluscs are highly diverse, class In biological classification, rank is the level in a taxonomic branched ordering of living things. The most specific level is species, the next most specific is genus, and then family, class, etc. Sometimes (but only rarely) the term "taxonomic category" is used and more often the term "rank" is used -- the ranking, or ordering, Cephalopoda, characterized by bilateral body symmetry Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry, a prominent head, and a modification of the mollusc foot, a muscular hydrostat A muscular hydrostat is a biological structure found in animals. It is used to manipulate items or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in a separate compartment, as in a hydrostatic skeleton. The principle behind the hydrostatic skeleton is that water is, into the form of arms By definition, cephalopod arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends.[citation needed] or tentacles Tentacles refers to the elongated flexible organs present in animals, especially invertebrates, and sometimes to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like other muscular hydrostats. Teuthology, a branch of malacology Malacology is the branch of invertebrate zoology which deals with the study of mollusca , the second-largest phylum of animals in terms of described species after the arthropods. Mollusks include snails and slugs, clams, octopus and squid and numerous other kinds, many (but by no means all) of which have shells. One division of malacology,, is the study of cephalopods. These creatures became dominant around the Ordovician The Ordovician [/ɔɹdəˈvɪʃən/] is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago (ICS, 2004,. It follows the Cambrian Period and is followed by the Silurian Period. The Ordovician, named after the Welsh tribe of the Ordovices, was defined by Charles period. The fishing industry It is defined by the FAO as including recreational, subsistence and commercial fishing, and the harvesting, processing, and marketing sectors. The commercial activity is aimed at the delivery of fish and other seafood products for human consumption or as input factors in other industrial processes. Directly or indirectly, the livelihood of over 500 name for this class is inkfish Categories: Fisheries science | Edible molluscs | Cephalopods | Fish common names | , referring to many cephalopods' ability to squirt ink.

The class contains two extant Extant is a term commonly used in biology to refer to taxa that are still in existence (living). The term extant contrasts with extinct. For example, Brandt's Cormorant is an extant species, while the Spectacled Cormorant is an extinct species. Likewise, of the group of molluscs known as the cephalopods, there are approximately 600 extant species subclasses In biological classification, rank is the level in a taxonomic branched ordering of living things. The most specific level is species, the next most specific is genus, and then family, class, etc. Sometimes (but only rarely) the term "taxonomic category" is used and more often the term "rank" is used -- the ranking, or ordering,. In the Coleoidea Subclass Coleoidea is the grouping of cephalopods containing all the primarily soft-bodied creatures. Unlike its sister group Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal bone or shell that is used for buoyancy or support. Some species have lost their bone altogether, while in some it, the mollusk shell has been internalized or is absent; this subclass includes the octopus The octopus is a cephalopod mollusk in the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms. Most octopuses have no internal or external skeleton, allowing them to squeeze through tight places, squid Squid are marine cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two longer tentacles, and cuttlefish Cuttlefish are marine animals of the order Sepiida belonging to the class Cephalopoda . Despite their common name, cuttlefish are not fish but molluscs. Recent studies indicate that cuttlefish are among the most intelligent invertebrates. Additionally, it is noted that cuttlefish have one of the largest brain-to-body size ratios of all. In the Nautiloidea Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and developed an extraordinary diversity of shell shapes and, the shell remains; this subclass includes the nautilus Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of suborder Nautilina. It comprises six species in two genera, the type of which is genus Nautilus. Though it more specifically refers to species Nautilus pompilius, the name chambered nautilus is also used for any species of the Nautilidae. About 800 distinct living species In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as based on similarity of DNA or of cephalopods have been identified. Two important extinct taxa A taxon is a group of (one or more) organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a taxonomic group is done by a taxonomist. It is not uncommon for one taxonomist to disagree with another on what exactly belongs to are Ammonoidea, the ammonites Ammonites, as they pertain specifically to the order Ammonitida, are an extinct group of marine animals belonging to the cephalopod subclass Ammonoidea. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods, and Belemnoidea Belemnites are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related[citation needed] to the modern cuttlefish. Like them, the belemnites possessed an ink sac, but, unlike the squid, they possessed ten arms of roughly equal length, and no tentacles. The name "Belemnoid" comes from the, the belemnites.

Contents

Distribution

There are around 800 extant species of cephalopod,[1] although new species continue to be described. It is estimated that around 11,000 extinct taxa A taxon is a group of (one or more) organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a taxonomic group is done by a taxonomist. It is not uncommon for one taxonomist to disagree with another on what exactly belongs to have been described,[2] although the soft-bodied nature of cephalopods means that they are not easily fossilised.[3]

Cephalopods are found in all the oceans An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (~3.61 X 1014 m2) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas of Earth Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet,[note 6] or by its Latin name, Terra.[note 7]. None of them can tolerate freshwater Freshwater or fresh water is naturally occurring water on the Earth's surface in bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Freshwater is characterized by having low concentrations of dissolved salts and other total dissolved solids. The term specifically excludes seawater and, but the brief squid, Lolliguncula brevis, found in Chesapeake Bay The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers 64,299 square miles (166,534 km2) in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West Virginia. More than 150 may be a notable exception in that it tolerates brackish water Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salten" or "salty". Certain human activities can which has a low salinity.[4]

Cephalopods occupy most of the depth of the ocean, from hydrothermal vents A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots to the sea surface. Their diversity is greatest near the equator (~40 species retrieved in nets at 11°N by a diversity study) and decreases towards the poles (~5 species captured at 60°N).[5]:11

Nervous system and behaviour

See also: Cephalopod intelligence Cephalopod intelligence has an important comparative aspect in our understanding of intelligence, because it relies on a nervous system fundamentally different from that of vertebrates. The cephalopod class of mollusks, particularly the Coleoidea subclass , are considered the most intelligent invertebrates and an important example of advanced and squid giant axon The squid giant axon is the very large axon that controls part of the water jet propulsion system in squid. It was discovered by English zoologist and neurophysiologist John Zachary Young in 1936. Squid use this system primarily for making brief but very fast movements through the water An octopus opening a container with a screw cap

Cephalopods are widely regarded as the most intelligent of the invertebrates An invertebrate is an animal without a backbone. The group includes 95% of all animal species — all animals except those in the chordate subphylum Vertebrata and have well developed senses and large brains The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary; larger than the brains of gastropods The Gastropoda or gastropods are a large taxonomic class of mollusks that are more commonly known as snails and slugs. This class of animals includes snails and slugs of all kinds and all sizes: huge numbers of marine snails and sea slugs, as well as freshwater snails and freshwater limpets, and the terrestrial snails and slugs. The class. The nervous system The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous system contains the brain, spinal cord, and retina of cephalopods is the most complex of the invertebrates,[6] and their brain to body mass ratio falls between that of warm and cold blooded vertebrates.[5]:14 The giant nerve A peripheral nerve, or simply nerve is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system. In the central nervous system, the analogous structures are known as tracts fibers of the cephalopod mantle The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself have frequently been used as an experimental material of neurophysiologists Neurophysiology is a part of physiology. Neurophysiology is the study of nervous system function. Primarily, it is connected with neurobiology, psychology, neurology, clinical neurophysiology, electrophysiology, biophysical neurophysiology, ethology, neuroanatomy, cognitive science and other brain sciences for many years; their large diameter (due to lack of myelination) makes them easier to study.[7]

Cephalopods are social creatures; when isolated from their own kind, they will take to shoaling In biology, any group of fish that stay together for social reasons are said to be shoaling , and if, in addition, the group is swimming in the same direction in a coordinated manner, they are said to be schooling (pronounced /ˈskuːlɪŋ/). In common usage, the terms are sometimes used rather loosely. About one quarter of fishes shoal all their with fish.[8]

Some cephalopods are able to fly distances up to 50 m. While the organisms are not particularly aerodynamic, they achieve these rather impressive ranges by use of jet-propulsion; water continues to be expelled from the funnel while the organism is in flight.[9]

Senses

Cephalopods have advanced vision, can detect gravity with statocysts The statocyst is a balance organ present in some aquatic invertebrates . It consists of a sac-like structure containing a mineralised mass (statolith) and numerous innervated sensory hairs (setae). The statolith's inertia causes it to push against the setae when the animal accelerates. Deflection of setae by the statolith in response to gravity, and have a variety of chemical sense organs.[5]:34 Octopuses use their tentacles to explore their environment and can use them for depth perception.[5]

The primitive nautilus Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of suborder Nautilina. It comprises six species in two genera, the type of which is genus Nautilus. Though it more specifically refers to species Nautilus pompilius, the name chambered nautilus is also used for any species of the Nautilidae eye functions similarly to a pinhole camera A pinhole camera is a simple camera without a lens and with a single small aperture — effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box. The human eye in bright light acts similarly, as do cameras using small apertures.

Vision

Main articles: Cephalopod eye and mollusk eye

Most cephalopods rely on vision to detect predators and prey, and to communicate with one another.[10] Consequently, cephalopod vision is acute: training experiments have shown that the Common Octopus can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects. The morphological construction gives cephalopod eyes the same performance as sharks'; however, their construction differs as cephalopods lack a cornea, and have an everted retina.[10] Cephalopods' eyes are also sensitive to the plane of polarization Polarization is a property of certain types of waves that describes the orientation of their oscillations. Electromagnetic waves such as light exhibit polarization; acoustic waves (sound waves) in a gas or liquid do not have polarization because the direction of vibration and direction of propagation are the same of light. Surprisingly—given their ability to change color—all octopuses[11] and most cephalopods[12] are color blind Color blindness or color vision deficiency is the inability to perceive differences between some of the colors that others can distinguish. It is most often of genetic nature, but may also occur because of eye, nerve, or brain damage, or exposure to certain chemicals. The English chemist John Dalton published the first scientific paper on the. When camouflaging themselves, they use their chromatophores to change brightness and pattern according to the background they see, but their ability to match the specific color of a background may come from cells such as iridophores and leucophores Chromatophores are pigment-containing and light-reflecting cells found in amphibians, fish, reptiles, crustaceans, and cephalopods. They are largely responsible for generating skin and eye colour in cold-blooded animals and are generated in the neural crest during embryonic development. Mature chromatophores are grouped into subclasses based on that reflect light from the environment.[13] They also produce visual pigments throughout their body, and may sense light levels directly from their body.[14] Evidence of color vision Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit. The nervous system derives color by comparing the responses to light from the several types of cone photoreceptors in the eye. These cone photoreceptors are sensitive to different portions of the has been found in the Sparkling Enope Squid The Sparkling Enope Squid , also known as the Firefly Squid, is a species of squid in the family Enoploteuthidae. It is the sole species in the genus Watasenia (Watasenia scintillans),[12][15] which achieves color vision by the use of three distinct retinal Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision. Bound to proteins called type 1 rhodopsins, retinal allows certain microorganisms to convert light into metabolic energy molecules (A1, sensitive to red; A2, to purple, and A4, to yellow?) which bind to its opsin.[16]

Unlike many other cephalopods, nautiluses Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of suborder Nautilina. It comprises six species in two genera, the type of which is genus Nautilus. Though it more specifically refers to species Nautilus pompilius, the name chambered nautilus is also used for any species of the Nautilidae do not have good vision; their eye structure is highly developed but lacks a solid lens. They have a simple "pinhole" eye through which water can pass. Instead of vision, the animal is thought to use olfaction as the primary sense for foraging, as well as locating or identifying potential mates.

Hearing

Cephalopods can use their statocyst to detect sound.[17]

Use of light

This Broadclub Cuttlefish (Sepia latimanus) can go from camouflage tans and browns (top) to yellow with dark highlights (bottom) in less than a second.

Most cephalopods possess chromatophores - that is, coloured pigments - which they can use in a startling array of fashions.[5] As well as providing camouflage with their background, some cephalopods bioluminesce, shining light downwards to disguise their shadows from any predators that may lurk below.[5] The bioluminescence is produced by bacterial symbionts; the host cephalopod is able to detect the light produced by these organisms.[18] Bioluminescence may also be used to entice prey, and some species use colourful displays to impress mates, startle predators, or even communicate with one another.[5] It is not certain whether bioluminescence is actually of epithelial origin or if it is a bacterial production.[19]

Colouration

Colouration can be changed in milliseconds as they adapt to their environment,[5] and the pigment cells are expandable by muscular contraction.[19] Colouration is typically more pronounced in near-shore species than those living in the open ocean, whose functions tend to be restricted to camouflage by breaking their outline.[5]:2

Evidence of original colouration has been detected in cephalopod fossils dating as far back as the Silurian; these orthoconic individuals bore concentric stripes, which are thought to have served as camouflage.[20] Devonian cephalopods bear more complex colour patterns, whose function may be more complex.[21]

Ink

Main articles: Cephalopod ink and ink sac

With the exception of the Nautilidae and the species of octopus belonging to the suborder Cirrina[22], all known cephalopods have an ink sac, which can be used to expel a cloud of dark ink to confuse predators.[11] This sac is a muscular bag which originated as an extension of the hind gut. It lies beneath the gut and opens into the anus, into which its contents – almost pure melanin – can be squirted; its proximity to the base of the funnel means that the ink can be distributed by ejected water as the cephalopod uses its jet propulsion.[11] The ejected cloud of melanin is usually mixed, upon expulsion, with mucus, produced elsewhere in the mantle, and therefore forms a thick cloud, resulting in visual (and possibly chemosensory) impairment of the predator, like a smokescreen. However, a more sophisticated behaviour has been observed, in which the cephalopod releases a cloud, with a greater mucus content, that approximately resembles the cephalopod that released it (this decoy is referred to as a pseudomorph). This strategy often results in the predator attacking the pseudomorph, rather than its rapidly departing prey.[11] For more information, see Inking behaviors.

Viscera of Chtenopteryx sicula
Viscera of Ocythoe tuberculata

Circulatory system

Cephalopods are the only mollusks with a closed circulatory system. They have two gill hearts (also known as branchial hearts) that move blood through the capillaries of the gills. A single systemic heart then pumps the oxygenated blood through the rest of the body.[23]

Like most molluscs, cephalopods use hemocyanin, a copper-containing protein, rather than hemoglobin to transport oxygen. As a result, their blood is colorless when deoxygenated and turns blue when exposed to air.[24]

Respiration

Cephalopods exchange gasses with the seawater by forcing water through their gills, which are attached to the roof of the organism.[25]:488[26] Water enters the mantle cavity on the outside of the gills, and the entrance of the mantle cavity closes. When the mantle contracts, water is forced through the gills, which lie between the mantle cavity and the funnel. The water's expulsion through the funnel can be used to power jet propulsion. The gills, which are much more efficient than those of other molluscs, are attached to the ventral surface of the mantle cavity.[26] There is a trade-off with gill size regarding lifestyle. To achieve fast speeds, gills need to be small - water will be passed through them quickly when energy is needed, compensating for their small size. However, organisms which spend most of their time moving slowly along the bottom do not naturally pass much water through their cavity for locomotion; thus they have larger gills, along with complex systems to ensure that water is constantly washing through their gills, even when the organism is stationary.[25] The water flow is controlled by contractions of the radial and circular mantle cavity muscles.[27]

The gills of cephalopods are supported by a skeleton of robust fibrous proteins; the lack of mucopolysaccharides distinguishes this matrix from cartilage.[28][29] The gills are also thought to be involved in excretion, with NH4+ being swapped with K+ from the seawater.[26]

Locomotion and buoyancy

Octopuses swim headfirst, with arms trailing behind

While all cephalopods can move by jet propulsion, this is a very energy-consuming way to travel compared to the tail propulsion used by fish.[30] The relative efficiency of jet propulsion decreases further as animal size increases. Since the Paleozoic era, as competition with fish produced an environment where efficient motion was crucial to survival, jet propulsion has taken a back role, with fins and tentacles used to maintain a steady velocity.[3] Whilst jet propulsion is never the sole mode of locomotion,[3]:208 the stop-start motion provided by the jets continues to be useful for providing bursts of high speed - not least when capturing prey or avoiding predators.[3] Indeed, it makes cephalopods the fastest marine invertebrates,[5]:Preface and they can out-accelerate most fish.[25] Oxygenated water is taken into the mantle cavity to the gills and through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. The size difference between the posterior and anterior ends of this organ control the speed of the jet the organism can produce.[31] The velocity of the organism can be accurately predicted for a given mass and morhpology of animal.[32] Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions.[33] Some cephalopods accompany this expulsion of water with a gunshot-like popping noise, thought to function to frighten away potential predators.[34]

Nautilus belauensis seen from the front, showing the opening of the hyponome

Early cephalopods are thought to have produced jets by drawing their body into their shells, as Nautilus does today.[35] Nautilus is also capable of creating a jet by undulations of its funnel; this slower flow of water is more suited to the extraction of oxygen from the water.[35] The jet velocity in Nautilus is much slower than in coleoids, but less musculature and energy is involved in its production.[36] Jet thrust in cephalopods is controlled primarily by the maximum diameter of the funnel orifice (or, perhaps, the average diameter of the funnel)[37]:440 and the diameter of the mantle cavity.[38] Changes in the size of the orifice are used most at intermediate velocities.[37] The absolute velocity achieved is limited by the cephalopod's requirement to inhale water for expulsion; this intake limits the maximum velocity to eight body-lengths per second, a speed which most cephalopods can attain after two funnel-blows.[37] Water refills the cavity by entering not only through the orifices, but also though the funnel.[37] To accommodate the rapid changes in water intake and expulsion, the orifices are highly flexible and can change their size by a factor of twenty; the funnel radius, conversely, changes only by a factor of around 1.5.[37]

Some octopus species are also able to walk along the sea bed. Squids and cuttlefish can move short distances in any direction by rippling of a flap of muscle around the mantle.

While most cephalopods float (i.e. are neutrally buoyant; in fact most cephalopods are about 2-3% denser than seawater[8]), they achieve this in different ways.[30] Some, such as Nautilus, allow gas to diffuse into the gap between the mantle and the shell; others allow purer water to ooze from their kidneys, forcing out denser salt water from the body cavity;[30] others, like some fish, accumulate oils in the liver;[30] and some octopuses have a gelatinous body with lighter chlorine ions replacing sulfate in the body chemistry.[30]

Shell

See also: Cuttlebone and Mollusc shell Cuttlebone of Sepia officinalis
Gladius of Sepioteuthis lessoniana
Gladius of Lepidoteuthis grimaldii

Nautiluses are the only extant cephalopods with an external shell. However, all molluscan shells are formed from the ectoderm (outer layer of the embryo); in cuttlefish (Sepia spp.), for example, an invagination of the ectoderm forms during the embryonic period, resulting in a shell that is internal in the adult.[39] The same is true of the chitinous gladius of squid[39] and octopus.[40] Cirrate octopuses have cartilaginous fin supports,[41] which are sometimes referred to as a "shell vestige" or "gladius".[42]: The Incirrina have no vestige of an internal shell,[citation needed] and some squid also lack a gladius.[43] Interestingly, the shelled coleoids do not form a clade or even a paraphyletic group.[44] The Spirula shell begins as an organic structure, and is then very rapidly mineralized.[45] Shells that are "lost" may be lost be resorption of the calcium carbonate component.[46]

Females of the octopus genus Argonauta secrete a specialised paper-thin eggcase in which they reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal.

The largest group of shelled cephalopods, the ammonites, are extinct, but their shells are very common as fossils.

The deposition of carbonate, leading to a mineralized shell, appears to be related to the acidity of the organic shell matrix (see Mollusc shell); shell-forming cephalopods have an acidic matrix, whereas the gladius of squid has a basic matrix.[47]

Left: A giant squid found in Logy Bay, Newfoundland, in 1873. The two long feeding tentacles are visible on the extreme left and right. Right: Detail of the tentacular club of Abraliopsis morisi

Head appendages

Main articles: Cephalopod arm and tentacle

Cuttlefish and squid have five pairs of muscular appendages surrounding their mouths. The longer two, termed tentacles, are actively involved in capturing prey;[48]:225 they can lengthen rapidly (in as little as 15 milliseconds[48]:225). In giant squid they may reach a length of 8 metres. They may terminate by broadening into a sucker-coated club.[48]:225 The shorter four pairs are termed arms, and are involved in holding and manipulating the captured organism.[48]:225 They too have suckers, on the side closest to the mouth; these help to hold onto the prey.[48]:226

The tentacle consists of a thick central nerve cord (which must be thick to allow each sucker to be controlled independently)[49] surrounded by circular and radial muscles. Because the volume of the tentacle remains constant, contracting the circular muscles decreases the radius and permits the rapid increase in length. Typically a 70% lengthening is achieved by decreasing the width by 23%.[48]:227

The size of the tentacle is related to the size of the buccal cavity; larger, stronger tentacles can hold prey as small bites are taken from it; with more numerous, smaller tentacles, prey is swallowed whole, so the mouth cavity must be larger.[50]

Feeding

All cephalopods have a two-part beak;[5]:7 most have a radula, although it is reduced in most octopus and absent altogether in Spirula.[5]:7[51]:110 They feed by capturing prey with their tentacles, drawing it in to their mouth and taking bites from it.[11] They have a mixture of toxic digestive juices, some of which are manufactured by symbiotic algae, which they eject from their salivary glands onto their captured prey held in their mouth. These juices separate the flesh of their prey from the bone or shell.[11] The salivary gland has a small tooth at its end which can be poked into an organism to digest it from within.[11]

The digestive gland itself is rather short.[11] It has four elements, with food passing through the crop, stomach and caecum before entering the intestine. Most digestion, as well as the absorption of nutrients, occurs in the digestive gland, sometimes called the liver. Nutrients and waste materials are exchanged between the gut and the digestive gland through a pair of connections linking the gland to the junction of the stomach and caecum.[11] Cells in the digestive gland directly release pigmented excretory chemicals into the lumen of the gut, which are then bound with mucus passed through the anus as long dark strings, ejected with the aid of exhaled water from the funnel.[11]

Radula

The two-part beak of the giant squid, Architeuthis sp.

The cephalopod radula consists of multiple symmetrical rows of up to nine teeth[52] – thirteen in fossil classes.[53] The organ is reduced or even vestigial in certain octopus species and is absent in Spirula.[53] The teeth may be homodont (i.e. similar in form across a row), heterodont (otherwise), or ctenodont (comb-like).[53] Their height, width and number of cusps is variable between species.[53] The pattern of teeth repeats, but each row may not be identical to the last; in the octopus, for instance, the sequence repeats every five rows.[53]:79

Cephalopod radulae are known from fossil deposits dating back to the Silurian.[53] They are usually preserved within the cephalopod's body chamber, commonly in conjunction with the mandibles; but this need not always be the case;[54] many radulae are preserved in a range of settings in the Mason Creek.[55]

Excretory system

Most cephalopods possess a single pair of large nephridia. Filtered nitrogenous waste is produced in the pericardial cavity of the branchial hearts, each of which is connected to a nephridium by a narrow canal. The canal delivers the excreta to a bladder-like renal sac, and also resorbs excess water from the filtrate. Several outgrowths of the lateral vena cava project into the renal sac, continuously inflating and deflating as the branchial hearts beat. This action helps to pump the secreted waste into the sacs, to be released into the mantle cavity through a pore.[56]

Nautilus, unusually, possesses four nephridia, none of which are connected to the pericardial cavities.

Ammonium

The handling of ammonia is thought to be important in shell formation in terrestrial molluscs, and in other non-molluscan lineages.[57]

Because protein (i.e. flesh) is a major constituent of the cephalopod diet, large amounts of ammonium are produced as waste. The main organs involved with the release of this excess ammonium are the gills.[58]

The rate of this release is the lowest in the shelled cephalopods Nautilus and Sepia, probably as a result of their use of nitrogen to fill their shells with gas, in order to produce buoyancy.[58] Other cephalopods use ammonium in a similar way, storing the ions (as ammonium chloride) themselves in order to reduce their overall density and thus become more buoyant.[58]

Reproduction and life cycle

Female Argonauta argo with eggcase and eggs
Detail of the hectocotylus of Ocythoe tuberculata A dissected male specimen of Onykia ingens, showing a non-erect penis (the white tubular structure located below most of the other organs)
A specimen of the same species exhibiting elongation of the penis to 67 cm in length

With a few exceptions, Coleoidea live short lives with rapid growth. Most of the energy extracted from their food is used for growing. The penis in most male Coleoidea is a long and muscular end of the gonoduct used to transfer spermatophores to a modified arm called a hectocotylus. That in turn is used to transfer the spermatophores to the female. In species where the hectocotylus is missing, the penis is long and able to extend beyond the mantle cavity and transfers the spermatophores directly to the female. Deep water squid have the greatest known penis length relative to body size of all mobile animals, second in the entire animal kingdom only to certain sessile barnacles.[59] Penis elongation in Onykia ingens may result in a penis that is as long as the mantle, head and arms combined.[59][60]

Most cephalopods tend towards a semelparous reproduction strategy; they lay many small eggs in one batch and die afterwards. The Nautiloidea, on the other hand, stick to iteroparity; they produce a few large eggs in each batch and live for a long time.

External sexual characteristics are lacking in cephalopods, so cephalopods use colour communication. A courting male will approach a likely looking opposite number flashing his brightest colours, often in rippling displays. If the other cephalopod is female and receptive, her skin will change colour to become pale, and mating will occur. If the other cephalopod remains brightly coloured, it is taken as a warning.[61]

The male has a sperm-carrying arm, known as the hectocotylous arm, with which to impregnate the female. In many cephalopods, mating occurs head to head and the male may simply transfer sperm to the female. Others may detach the sperm-carrying arm and leave it attached to the female. In the paper nautilus, this arm remains active and wriggling for some time, prompting the zoologists who discovered it to conclude it was some sort of worm-like parasite. It was duly given a genus name Hectocotylus, which held for some time until the mistake was discovered[61]:227.

The eggs may be brooded: female paper nautilus construct a shelter for the young, while Gonatiid squid carry a larva-laden membrane from the hooks on their arms.[62] Other cephalopods deposit their young under rocks and aerate them with their tentacles hatching. Often, though, the eggs are left to their own devices; many squid lay sausage-like bunches of eggs in crevices or occasionally on the sea floor. Cuttlefish lay their eggs separately in cases and attach them to coral or algal fronds.[63] Fossilised egg clutches show that ammonites also laid clutches of eggs.[64]

Cephalopods are occasionally long-lived, especially in the deep water or polar forms, but most of the group live fast and die young, maturing rapidly to their adult size. Some may gain as much as 12% of their body mass each day.[11] Most live for one to two years,[11] reproducing and then dying shortly thereafter.[65]

In order to free up resources for reproduction, many squid are known to resorb the muscle tissue of their mantle and tentacles, breaking down the tissue and using the energy contained therein to produce more gametes.[66]

Egg cases laid by a female squid

Embryology

Unlike most other molluscs, cephalopods do not have a distinct larval stage. The fertilised ovum initially divides to produce a disc of germinal cells at one pole, with the yolk remaining at the opposite pole. The germinal disc grows to envelop and eventually absorb the yolk, forming the embryo. The tentacles and arms first appear at the hind part of the body, where the foot would be in other molluscs, and only later migrate towards the head.[56][67]

The funnel of cephalopods develops on the top of their head, whereas the mouth develops on the opposite surface.[68]:86 The early embryological stages are reminiscent of ancestral gastropods and extant Monoplacophora.[67]

The shells develop from the ectoderm as an organic framework which is subsequently mineralised.[39] In Sepia, which has an internal shell, the ectoderm forms an invagination whose pore is sealed off before this organic framework is deposited.[39]

The gene engrailed is expressed first in the arms, funnel and optic vesicles, and is only later present in the tentacles and eyelids.[39] It is expressed in embryonic stages 17–19 in all arm buds, and subsequently in the future-tentacles in stages 24–5, suggesting that it may serve a role in the differential development of tentacles. Sequential expression of Hox genes is also observed in cephalopod arms.[39]

Development

Chtenopteryx sicula paralarvae. Left: Two very young paralarvae. The circular tentacular clubs bear approximately 20 irregularly arranged suckers. Two chromatophores are present on each side of the mantle. Centre: Ventral, dorsal and side views of a more advanced paralarva. An equatorial circulet of seven large yellow-brown chromatophores is present on the mantle. Posteriorly the expanded vanes of the gladius are visible in the dorsal view. Right: Ventral and dorsal views of a very advanced paralarva. Left: Immature specimens of Chiroteuthis veranyi. In this paralarval form, known as the doratopsis stage, the pen is longer than the mantle and 'neck' combined

Right: A mature Chiroteuthis veranyi. This species has some of the longest tentacles in proportion to its size of any known cephalopod.

Cephalopod eggs span a large range of sizes, from 1 to 30 mm in diameter.[69] The length of time before hatching is highly variable; smaller eggs in warmer waters are the fastest to hatch, and newborns can emerge after as little as a few days. Larger eggs in colder waters can develop for over a year before hatching.[69]

The process from spawning to hatching follows a similar trajectory in all species, the main variable being the amount of yolk available to the young and when it is absorbed by the embryo.[69]

Young do not pass through a larval stage sensu stricto. They quickly learn how to hunt, using encounters with prey to refine their strategies.[69]

Growth in juveniles is usually allometric, whilst adult growth is isometric.[70]

Evolution

Main article: Evolutionary history of cephalopods

The traditional view of cephalopod evolution holds that they evolved in the Late Cambrian from a monoplacophoran-like ancestor[71] with a curved, tapering shell,[72] which was closely related to the gastropods (snails).[73] The similarity of the early shelled cephalopod Plectronoceras to some gastropods was used in support of this view. The development of a siphuncle would have allowed the shells of these early forms to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separated the true cephalopods from putative ancestors such as Knightoconus, which lacked a siphuncle.[73] Negative buoyancy (i.e. the ability to float) would have come later, followed by swimming in the Plectronocerida and eventually jet propulsion in more derived cephalopods.[74]

However, some morphological evidence is difficult to reconcile with this view, and the re-description of Nectocaris pteryx, which did not have a shell and appeared to possess jet propulsion in the manner of "derived" cephalopods, complicated the question of the order in which cephalopod features developed.[75] Their position within the Mollusca is currently wide open to interpretation - see Mollusca#Phylogeny.

Early cephalopods were likely predators near the top of the food chain.[11] They underwent pulses of diversification during the Ordovician period[76] to become diverse and dominant in the Paleozoic and Mesozoic seas.[77] In the Early Palaeozoic, their range was far more restricted than today; they were mainly constrained to sub-littoral regions of shallow shelves of the low latitudes, and usually occur in association with thrombolites.[78] A more pelagic habit was gradually adopted as the Ordovician progressed.[78] Deep-water cephalopods, whilst rare, have been found in the Lower Ordovician - but only in high-latitude waters.[78] The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200 m.[76] The direction of shell coiling would prove to be crucial to the future success of the lineages; endogastric coiling would only permit large size to be attained with a straight shell, whereas exogastric coiling - initially rather rare - permitted the spirals familiar from the fossil record to develop, with their corresponding large size and diversity.[79] (Endogastric mean the shell is curved so as the ventral or lower side is longitudinally concave (belly in); exogastric means the shell is curve so as the ventral side is longitudinally convex (belly out) allowing the funnel to be pointed backwards beneath the shell.)[79]

An ammonitic ammonoid with the body chamber missing, showing the septal surface (especially at right) with its undulating lobes and saddles.

The ancestors of coleoids (including most modern cephalopods) and the ancestors of the modern nautilus, had diverged by the Floian Age of the Early Ordovician Period, over 470 million years ago.[80][78] It is widely held that the Bactritida, an Silurian–Triassic group of orthocones, are paraphyletic to the coleoids and ammonoids – that is, the latter groups arose from within the Bactritida.[81]:393 An increase in the diveristy of the coleoids and ammonoids is observed around the start of the Devonian period, and corresponds with a profound increase in fish diversity. This could represent the origin of the two derived groups.[81]

Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters.[73]:36 However, some of the straight-shelled nautiloids evolved into belemnites, out of which some evolved into squid and cuttlefish.[verification needed] The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit.[48]:289

Phylogeny

Cephalopod phylogeny
Nautiloids

Nautilus
Coleoids

Basal Octopods (e.g. Argonauta)

Vampyroteuthis

Heteroteuthis (bobtail squid)
*

Sepia (cuttlefish)

Idiosepius

Sepioteuthis

Spirula
*

Certain squid (e.g. Bathyteuthis)
Approximate consensus of extant cephalopod phylogeny, after Strugnell et al. 2007[82] Mineralized taxa are in bold. The attachment of the clade including Sepia and Spirula is unclear; either of the points marked with an asterisk may represent the root of this clade.

The internal phylogeny of the cephalopods is difficult to constrain; many molecular techniques have been adopted, but the results produced are conflicting.[82][83] Nautilus tends to be considered an outgroup, with Vampyroteuthis forming an outgroup to other squid; however in one analysis the nautiloids, octopus and teuthids plot as a polytomy.[82] Some molecular phylogenies do not recover the mineralized coleoids (Spirula, Sepia, and Metasepia) as a clade; however, others do recover this more parsimonious-seeming clade, with Spirula as a sister group to Sepia and Metasepia in a clade that had probably diverged before the end of the Triassic.[84][85]

Molecular estimates for clade divergence vary. One 'statistically robust' estimate has Nautilus diverging from Octopus at 415 ± 24 million years ago.[86]

Taxonomy

Chambered Nautilus (Nautilus pompilius) Common Cuttlefish (Sepia officinalis) Atlantic Bobtail (Sepiola atlantica) European Squid (Loligo vulgaris) Common Octopus (Octopus vulgaris)

The classification presented here, for recent cephalopods, follows largely from Current Classification of Recent Cephalopoda (May 2001), for fossil cephalopods takes from Arkell et al. 1957, Teichert and Moore 1964, Teichert 1988, and others. The three subclasses are traditional, corresponding to the three orders of cephalopods recognized by Bather.[87]

Class Cephalopoda († indicates extinct groups)

Other classifications differ, primarily in how the various decapod orders are related, and whether they should be orders or families.

Suprafamilial Classifiction of the Treatise

This is the older classification that combines those found in parts K and L of the Treatise on Invertebrate Paleontology, which forms the basis for and is retained in large part by classifications that have come later.

Nautiloids in general, (Teichert and Moore 1964) Sequence as given.

Subclass † Endoceratoidea. Not used by Flower, e.g. Flower and Kummel 1950, interjocerids included in the Endocerida.
Order † Endocerida
Order † Intejocerida
Subclass † Actinoceratoidea Not used by Flower, ibid
Order † Actinocerida
Subclass † Nautiloidea Nautiloidea in the restricted sense.
Order † Ellesmerocerida Plectronocerida subsequently split off as separate order.
Order † Orthocerida Includes orthocerids and pseudorthocerids
Order † Ascocerida
Order † Oncocerida
Order † Discosorida
Order † Tarphycerida
Order † Barrandeocerida A polyphyletic group now included in the Tarphycerida
Order Nautilida
Subclass † Bactritoidea
Order † Bactritida

Paleozoic Ammonoidea ( Miller, Furnish, and Schindewolf, 1957)

Suborder † Anarcestina
Suborder † Clymeniina
Suborder † Goniatitina
Suborder † Prolecanitina

Mesozoic Ammonoidea (Arkel et al., 1957)

Suborder † Ceratitina
Suborder † Phylloceratina
Suborder † Lytoceratina
Suborder † Ammonitina

Subsequent revisions include the establishment of three Upper Cambrian orders, the Plectronocerida, Protactinocerida and Yanhecerida; separation of the pseudorthocerids as the Pseudorthocerida, and elevating orthoceritoids as the Subclass Orthoceratoidea.

Shevyrev classification

Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms,[88][89] although this classification has been criticized as arbritary.[90]

Various species of ammonites Holotype of Ostenoteuthis siroi from family Ostenoteuthidae. A fossilised belemnite

Class Cephalopoda

Cladistic classification

Pyritized fossil of Vampyronassa rhodanica, a vampyromorphid from the Lower Callovian (164.7 million years ago)

Another recent system divides all cephalopods into two clades. One includes nautilus and most fossil nautiloids. The other clade (Neocephalopoda or Angusteradulata) is closer to modern coleoids, and includes belemnoids, ammonoids, and many orthocerid families. There are also stem group cephalopods of the traditional Ellesmerocerida that belong to neither clade.[92][93]

Monophyly of coeloids

The coeloids have been thought to possibly represent a polyphyletic group,[48]:289 although this has not been supported by the rising body of molecular data.[94]

Post-mortem decay

After death, if undisturbed, cephalopods[note 1] decay relatively quickly.[66] Their muscle softens within a couple of days, and may swell; egg sacs can swell so much that they rip through the mantle.[66] Subsequently, the organs shrink again; at this point the organism may start to break up into fragments. The eyes retain their size while the head shrinks around them. The gills may remain swollen at this point.[66] After around a week, the carcass collapses in on itself and begins to disintegrate. The ink sac solidifies around this point.[66] After a fortnight little is left but a blob with eyes, arms and ink sac visible.[66] After a couple of months, these are only recognisable as flattened dark stains - although in some cases the eye lenses can remain intact for up to a year.[66]

See also

Book:Cephalopoda
Books are collections of articles that can be downloaded or ordered in print.

Notes

  1. ^ Experiments were performed on a variety of squid

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Cephalopod Collective Noun

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ue, 03 Jul 2007 11:54:20 GM

Are there collective nouns for . cephalopods. in general? Just for octopuses? Squids? Etc? What do you call a group of octopuses? A tangle?

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What is your favorite Cephalopod?
Q. Just wondering. Mine would have to be Octopus vulgaris. And please attach pictures/videos if possible. Thank you! To thresher: Thresher sharks are actually my favorite shark! So that's really funny you chose that. However, what I want to know is, if you were FORCED to choose, what Cephalopod would you prefer?
Asked by Avocado - Tue Jan 26 22:40:14 2010 - - 6 Answers - 0 Comments

A. Definitely cuttlefish ... [Though octopuses are excellent]
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