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Cephalopoda |
| Cephalopods Fossil range: Late Cambrian - Recent |
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 A variety of cephalopod forms from Ernst Haeckel's 1904 Kunstformen der Natur.
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Subclass Nautiloidea
Subclass †Ammonoidea Subclass Coleoidea
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The cephalopods (Greek plural Κεφαλόποδα (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 octopuses, 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.
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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 have been described,2 although the soft bodied nature of cephalopods mean they are not easily fossilised.3
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).4
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,citation needed and their brain to body mass ratio falls between that of warm and cold blooded vertebrates.4 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.
Cephalopod vision is acute, and training experiments have shown that the Common Octopus can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects. 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.5 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.6 Evidence of color vision has been found in only one species, the Sparkling Enope Squid.5
Cephalopods have advanced vision, can detect gravity with statocysts, and have a variety of chemical sense organs.4 Octopuses use their tentacles to explore their environment and can use them for depth perception.4
Most cephalopods possess chromatophores - that is, coloured pigments - which they can use in a startling array of fashions.4 As well as providing camouflage with their background, some cephalopods bioluminesce, shining light downwards to disguide their shadows from any predators that may lurk below.4 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.4 Colouration can be changed in milliseconds as they adapt to their environment.4 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.4
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.7
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.8
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.9 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.3 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.3 Indeed, it makes cephalopods the fastest marine invertebrates.4 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.10
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.
Cuttlefish and squid have five pairs of muscular appendages surrounding their mouths. The longer two, termed tentacles, are actively involved in capturing prey;11 they can lengthen rapidly (in as little as 15 milliseconds11 ) to as long as 22m12 and may terminate by broadening into a sucker-coated club.11 The shorter four pairs are termed arms, and are involved in holding and manipulating the captured organism.11 They too have suckers, on the side closest to the mouth; these help to hold onto the prey.11
The tentacle consists of a thick central nerve cord (which must be thick to allow each sucker to be controlled independently)13 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%.11
All cephalopods have a two-part beak;4 most but not all have a radula.4
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.
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Please help improve this section by expanding it. Further information might be found on the talk page. (November 2008) |
The funnel of cephalopods develops on the top of their head, whereas the mouth develops on the opposite surface.14
The class developed during the Late Cambrian, and were diverse and dominant in the Paleozoic and Mesozoic seas. Small shelly fossils such as Tommotia were previously interpreted as early cephalopods, but today these tiny fossils are recognized as sclerites of larger animals,15 and the earliest accepted cephalopods date to the Late Cambrian Period.note 116 Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid monoplacophoran clade.17 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.18 The internal phylogeny of mollusca, however, is wide open to interpretation - see Mollusca#Phylogeny.
While most cephalopods float (i.e. are neutrally buoyant), they achieve this in different ways.9 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;9 others, like some fish, accumulate oils in the liver;9 and some octopuses have a gelatinous body with lighter chlorine ions replacing sulfate in the body chemistry.9
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,verification needed although the coeloids may be polyphyletic.11 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. 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 was a result of evolutionary pressure to increase manoeuvrability and resulted in a more fish-like habit.11 This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure.11 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;19 the ancestral state is thought to have had five pairs of tentacles which surround the mouth.19 Smell-detecting organs evolved very early in the cephalopod lineage.19
The earliest cephalopodsnote 2, like Nautilus and some coeloids, appeared to be able to propel themselves forwards by directing their jet backwards.11 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.11
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.20 Parentheses indicate extinct groups.
Class Cephalopoda
Other classifications differ, primarily in how the various decapod orders are related, and whether they should be orders or families.
Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms.21 22
Class Cephalopoda Cuvier 1795
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 24 25
The coeloids may represent a polyphyletic group.11
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