Cephalopods

The cephalopods (Greek plural ' (kephalópoda); "head-feet") are the mollusc class Cephalopoda''' characterized by bilateral body symmetry, a prominent head, and a modification of the mollusk foot, a muscular hydrostat, into the form of arms or tentacles. Teuthology, a branch of malacology, is the study of cephalopods.

The class contains two extant subclasses. In the Coleoidea, the mollusk shell has been internalized or is absent; this subclass includes the octopi, squid, and cuttlefish. In the Nautiloidea the shell remains; this subclass includes the nautilus. About 786 distinct living species of cephalopods have been identified. Two important extinct taxa are Ammonoidea, the ammonites, and Belemnoidea, the belemnites.

Cephalopods are found in all the oceans of Earth, at all depths. None of them can tolerate freshwater, but a few species tolerate more or less brackish water.

Distribution
There are around 800 species of cephalopod, although new species continue to be described. It is estimated that around 11,000 extinct taxa have been described, although the soft-bodied nature of cephalopods means that they are not easily fossilised.

Cephalopods occupy most of the depth of the ocean, from hydrothermal vents to the sea surface. Their diversity is greatest near the equator (~40sp retrieved in nets at 11°N by a diversity study) and decreases towards the poles (~5 species captured at 60°N).



Nervous system and behaviour
Cephalopods are widely regarded as the most intelligent of the invertebrates and have well developed senses and large brains; larger than the brains of gastropods or bivalves. The nervous system of cephalopods is the most complex of the invertebrates, and their brain to body mass ratio falls between that of warm and cold blooded vertebrates. The giant nerve fibers of the cephalopod mantle have been a favorite experimental material of neurophysiologists for many years; their large diameter (due to lack of myelination) makes them easier to study.

Cephalopods are social creatures; when isolated from their own kind, they will take to shoaling with fish.

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.

Senses
Cephalopods have advanced vision, can detect gravity with statocysts, and have a variety of chemical sense organs. Octopuses use their tentacles to explore their environment and can use them for depth perception.

Vision
Most cephalopods rely on vision to detect predators and prey, and to communicate with one another. 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. Cephalopods' eyes are also sensitive to the plane of polarization of light. Surprisingly in light of their ability to change color, most are probably color blind - certainly all octopus are. 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 probably comes from cells such as iridophores and leucophores that reflect light from the environment. Evidence of color vision has been found in only one species, the Sparkling Enope Squid.

Unlike many other cephalopods, nautiluses do not have good vision; their eye structure is highly developed but lacks a solid lens. They have a simple "pinhole" lens 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.

Use of light
Most cephalopods possess chromatophores - that is, coloured pigments - which they can use in a startling array of fashions. 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. 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. It is not certain that whether bioluminescence is actually of epithelial origin or if it is a bacterial production. Colouration can be changed in milliseconds as they adapt to their environment, and the pigment cells is expandable by muscular contraction. 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.

Ink
With the exception of the Nautilidae and the species of octopus belonging to the suborder Cirrina), all known cephalopods have an ink sac, which can be used to expel a cloud of dark ink to confuse predators. 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.  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. For more information, see Inking behaviors.



Circulatory system
Cephalopods are the only molluscs 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.

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.

Respiration
Cephalopods exchange gasses with the seawater by forcing water through gills, which are attached to the roof of the organism. Water enters the mantle cavity on the outside of the gills, and the entrance the 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 are much more efficient than other molluscs', and are attached to the ventral surface of the mantle cavity. 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, and complex systems to ensure that water is constantly washing through their gills even when the organism is stationary. The water flow is controlled by contractions of the radial and ciricular mantle cavity muscles.

Their gill lamellae are supported by a cartilage framework. The gills are also thought to be involved in excretion, with NH4+ being swapped with K+ from the seawater.

Locomotion and buoyancy
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. The relative efficiency of jet propulsion decreases further as animal size increases. Since the Paleozoic, 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. The stop-start motion provided by the jets, however, continues to be useful for providing bursts of high speed - not least when capturing prey or avoiding predators. Indeed, it makes cephalopods the fastest marine invertebrates, and they can outaccelerate most fish. 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. 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.

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 ), they achieve this in different ways. 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; others, like some fish, accumulate oils in the liver; and some octopuses have a gelatinous body with lighter chlorine ions replacing sulfate in the body chemistry.

Shell
Nautiluses are the only extant cephalopods with a true external shell. Cuttlefish, squid, spirula, and cirrate octopuses have small internal shells. The majority of octopuses – those in the suborder most commonly known, Incirrina – have almost entirely soft bodies with no vestige of an internal shell.

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 in certain areas as fossils.



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

The tentacle consists of a thick central nerve cord (which must be thick to allow each sucker to be controlled independently) 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%.

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.

Feeding
All cephalopods have a two-part beak; most but not all have a radula. They feed by capturing prey with their tentacles, drawing it in to their mouth and taking bite-size bites. They have a nasty mix of toxic digestive juices, some of which are manufactured by symbiotic algae, which they spit on to their prey in the mouth, from their salivary glands; this separates the flesh of their prey from the bone or shell. The salivary gland has a small tooth at its end which can be poked into an organism to digest it from within.

The digestive gland itself is rather short. 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. 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.

Reproduction and life cycle
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. They 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.

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.

The eggs may be brooded, the female either making a shelter for them as in the paper nautilus, or else laying them under rocks and aerating them until they hatch. Often though, the eggs are left to their own devices, as for example in squids, which 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.

Cephalopods are occasionally long-lived, specially 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. Most live for one to two years, reproducing and then dying shortly thereafter.

Embryology
The funnel of cephalopods develops on the top of their head, whereas the mouth develops on the opposite surface. In Nautilus, and by extension other cephalopods, the tentacles and arms develop from the foot, not the head. The early embryological stages are reminiscent of ancestral gastropods and extant monoplacophora.

Evolution
The class developed during the Late Cambrian, and underwent pulses of diversification during the Ordovician period to become diverse and dominant in the Paleozoic and Mesozoic seas. Small shelly fossils such as Tommotia were once interpreted as early cephalopods, but today these tiny fossils are recognized as sclerites of larger animals, and the earliest accepted cephalopods date to the Late Cambrian Period. During the Cambrian, cephalopods are most common in shallow near-shore environments, but they have been found in deeper waters too. Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid monoplacophoran clade. However genetic studies suggest that they are more basal, forming a sister group to the scaphopoda but otherwise basal to all other major mollusc classes. The internal phylogeny of mollusca, however, is wide open to interpretation - see Mollusca.



The cephalopods are thought to have evolved from a monoplacophoran-like ancestor with a curved, tapering shell, and to be closely related to the gastropods (snails). The development of a siphuncle allowed their shells to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separates the true cephalopods from putative ancestors such as Knightoconus, which lacked a siphuncle. Negative buoyancy (i.e. the ability to float) came later, followed by swimming in the Plectroneocerida and eventually jet propulsion in more derived cephalopods. However, because chambered shells are found in a range of molluscs - monoplacophora and gastropods as well as cephalopods - a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda. The earliest such shells do not have the muscle scars which would be expected if they truly had a monoplacophoran affinity.

The earliest cephalopod order to emerge was the Ellesmerocerida, which were quite small organisms; their shells were slightly curved, and the internal chambers were closely spaced. The siphuncle penetrated the septa with meniscus-like holes. Early cephalopods haad ine shells which could not cope with the pressures of deep water. In the mid Tremadoc, these were supplemented by larger shells around 20 cm in length; these larger forms included straight and coiled shells, and fall into the orders Endocerida (with wide siphuncles) and Tarphycerida (with narrow siphuncles). By the mid Ordovician these orders are joined by the Orthocerids, whose chambers are small and spherical, and Lituitids, whose siphuncles are thin. The Oncocerids also appear during this time; they are restricted to shallow water and have short conchs which surround the stomach. 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. The wide-siphuncled Actinocerida and the Discocerida both emerged during the Darriwilian. The direction of 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. (Endogastric means that the tip of the coiling shell points in the same direction as the funnel; exogastric shells coil the other way, allowing the funnel to be pointed backwards beneath the shell.)

Early cephalopods were likely predators near the top of the food chain.



The ancient (cohort Belemnoidea) and modern (cohort Neocoleoidea) coleoids, as well as the ammonoids, all diverged from the external shelled nautiloid during the middle Paleozoic Era, between 450 and 300 million years ago, although the coeloids may be polyphyletic. 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 coeloids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters. However, some of the straight-shelled nautiloids evolved into belemnites, out of which some evolved into squid and cuttlefish. The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit. This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure. Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous.

The tentacles of the ancestral cephalopod developed from the mollusc's foot; the ancestral state is thought to have had five pairs of tentacles which surround the mouth. Smell-detecting organs evolved very early in the cephalopod lineage.

The earliest cephalopods, like Nautilus and some coeloids, appeared to be able to propel themselves forwards by directing their jet backwards. Because they had an external shell, they would not have been able to generate their jets by contracting their mantle, so must have used alternate methods: such as by contracting their funnels or moving the head in and out of the chamber.

Classification
The classification as listed here (and on other cephalopod articles) follows largely from Current Classification of Recent Cephalopoda (May 2001), plus fossil groups from several sources. The three subclasses are traditional, corresponding to the three orders of cephalopods recognized by Bather. Parentheses indicate extinct groups.

Class Cephalopoda
 * Subclass Nautiloidea: all cephalopods except ammonoids and coleoids
 * (Order Plectronocerida): the ancestral cephalopods from the Cambrian Period
 * (Order Ellesmerocerida): include the ancestors of all later cephalopods
 * (Order Endocerida)
 * (Order Actinocerida)
 * (Order Discosorida)
 * (Order Pseudorthocerida)
 * (Order Tarphycerida)
 * (Order Oncocerida)
 * Order Nautilida: nautilus and its fossil relatives
 * (Order Orthocerida)
 * (Order Ascocerida)
 * (Order Bactritida): include the ancestors of ammonoids and coleoids
 * (Subclass Ammonoidea): extinct ammonites and kin
 * (Order Goniatitida)
 * (Order Ceratitida)
 * (Order Ammonitida): the true ammonites
 * Subclass Coleoidea
 * (Cohort Belemnoidea): extinct belemnites and kin
 * (Genus Jeletzkya)
 * (Order Aulacocerida)
 * (Order Phragmoteuthida)
 * (Order Hematitida)
 * (Order Belemnitida)
 * Cohort Neocoleoidea
 * Superorder Decapodiformes (also known as Decabrachia or Decembranchiata)
 * (?Order Boletzkyida)
 * Order Spirulida: Ram's Horn Squid
 * Order Sepiida: cuttlefish
 * Order Sepiolida: pygmy, bobtail and bottletail squid
 * Order Teuthida: squid
 * Superorder Octopodiformes (also known as Vampyropoda)
 * Order Vampyromorphida: Vampire Squid
 * Order Octopoda: octopus

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

Shevyrev classification
Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms.

Class Cephalopoda Cuvier 1795
 * Subclass Ellesmeroceratoidea Flower 1950
 * Order Plectronocerida
 * Order Protactinocerida
 * Order Yanhecerida
 * Order Ellesmerocerida
 * Subclass Endoceratoidea Teichert, 1933
 * Order Endocerida
 * Order Intejocerida
 * Subclass Actinoceratoidea Teichert, 1933
 * Order Actinoceratoidea
 * Subclass Nautiloidea Agassiz, 1847
 * Order Basslerocerida
 * Order Tarphycerida
 * Order Lituitida
 * Order Discosorida
 * Order Oncocerida
 * Order Nautilida
 * Subclass Orthoceratoidea Kuhn, 1940
 * Order Orthocerida
 * Order Ascocerida
 * Order Dissidocerida
 * Order Bajkalocerida
 * Subclass Bactritoidea Shimansky, 1951
 * Subclass Ammonoidea Zittel, 1884
 * Subclass Coleoidea Bather, 1888

Cladistic classification
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

Monophyly of coeloids
The coeloids may represent a polyphyletic group.