Cephalopod

? Cephalopods
Juvenile cephalopod from plankton Antarctica
Scientific classification
Kingdom: Animalia Phylum: Mollusca Class: Cephalopoda Cuvier, 1797
Orders
Subclass Nautiloidea †Plectronocerida †Ellesmerocerida †Actinocerida †Pseudorthocerida †Endocerida †Tarphycerida †Oncocerida †Discosorida Nautilida †Orthocerida †Ascocerida †Bactritida Subclass †Ammonoidea †Goniatitida †Ceratitida †Ammonitida Subclass Coleoidea †Belemnoidea †Aulacocerida †Belemnitida †Hematitida †Phragmoteuthida Neocoleoidea (most living cephalopods) Sepiida Sepiolida Spirulida Teuthida Octopoda Vampyromorphida

Cephalopods (Greek plural Κεφαλόποδα (kephalópoda); "head-foot") are marine mollusks (phylum Mollusca) of the class Cephalopoda, a group that includes the familiar octopus and squid. Cephalopods are characterized by bilateral body symmetry, a prominent head, and a modification of the mollusk foot into the form of arms or tentacles surrounding the mouth, which has beak-like jaws. Cephalopods have the most complex nervous system among invertebrates, and large and image-focusing, vertebrate-like eye]s.

The class contains two extant (living) subclasses. In the Coleoidea, the mollusk shell has been internalized or is absent; this subclass includes the octopuses, squids, and cuttlefish. In the Nautiloidea, the shell remains; this subclass includes the nautilus. There are around 800 known extant species of Cephalopods, although an estimate 10,000 or more are known from the fossil record. Two important extinct taxa are Ammonoidea, the ammonites, and Belemnoidea, the belemnites.

Adult cephalopods range from about 2 centimeters (less than one inch) to over 20 meters (about 60 feet) in total length, with the largest organism weighing over one ton (Vecchione 2006). This class includes the largest living invertebrates.

Cephalopods are found in all the oceans of the world and at all depths, from 0 to over 5,000 meters (Vecchione 2006). Some species undergo daily migrations, such as from depths of 400 to 1,000 meters during the day and 200 meters or so during the night (Vecchinone 2006). None of them can tolerate freshwater, but a few species do tolerate more or less brackish water; none are found at salinities less than 17.5 PSU (practical salinity units) (Vecchione 2006). (Ocean water is about 35 PSU, or 35% salinity, or 3.5% salt.)

Cephalopods are important ecologically and commerically, and are increasingly being target for the fishing industry. This is the case for thousands of years; Plato and Aristotle both describe fishing methods (Lane 1960; Wood 2001). Cephalopods protein is considered to be low calorie and including unsaturated fatty acids, which are considered to have health benefits, including lowering risk of heart disease (Okuzumi & Fujii,2000).

The name cephalopod ("head-foot") refers to the large, well-developed head and the prominent foot of tentacles. Octopuses have 8 tentacles, squids have 10 tentacles, and nautiluses have up to 90 tentacles. These typically have suckers or hooks, with the exception of Nautilus.

Teuthology is the study of cephalopods.

Anatomy

Cephalopods are soft-bodied and generally without a hard covering. Only Nautilus has an external shell, although female argonauts have a shell-like egg case (Vecchione 2006).

Cephalopods have two gills 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 (Wells 1980). 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 (Ghiretti-Magaldi and Ghiretti 1992).

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. With the exception of Nautilus, they have special skin cells called chromatophores that change color and are used for communication and camouflage.

The nervous system of cephalopods is generally highly developed. The giant nerve fibers of the cephalopod mantle have been a favorite experimental material of neurophysiologists for many years.

Like vertebrates, cephalopods can focus their eyes. 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 (Messenger and Hanlon 1998). 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 (Hanlon and Messenger 1998). Evidence of color vision has been found in only one species, the Sparkling Enope Squid (Hanlon and Messenger 1998).

Locomotion

Cephalopods' primary method of movement is by jet propulsion, a very energy-consuming way to travel compared to the tail propulsion used by fish. The relative efficiency of jet propulsion degrades with larger animals. This is probably the reason why many species prefer to use their fins or arms for locomotion if possible.

In cephalopod locomotion, oxygenated water is taken into the mantle cavity. 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.

Reproduction and life cycle

Cephalopod sexes are separate, being male and female, with many exhibiting external sexual differentiation, whether size differences or structural differences.

With a few exceptions, Coleoidea (octopuse, squid) live short lives with rapid growth and high metabolism. Most of the energy extracted from their food is used for growing. They tend towards a semelparous reproduction strategy; they lay many small eggs in one batch and die afterwards. For most species, life expectancy appears to be about one or two years, but the giant squid (Architeuthis spp.) and the giant octopus (Enteroctopus spp.) seemingly have at least somewhat longer lives (Vecchione 2006). Some species such as pyroteuthids complete their life cycles in less than six months, and some species die after spawning (Vecchione 2006).

The Nautiloidea, on the other hand, stick to iteroparity. They produce a few large eggs in each batch and live for a long time.

In Coleoidea, the penis in the males 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.

History

Cephalopods appeared during the late Cambrian and were dominant and diverse during the Paleozoic and Mesozoic eras. Tommotia, a basal cephalopod, had squid-like tentacles but also a snail-like foot it used to move across the seabed. Early cephalopods were at the top of the food chain.

The ancient (cohort Belemnoidea) and modern (cohort Neocoleoidea) coleoids, as well as the ammonoids, all seemed to have diverged from the external shelled nautiloid during the middle Paleozoic Era, between 450 and 300 million years ago. While Nautiloid was dominant in the Paleozoic, ammonites were dominant during the Mesozoic.

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. 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.

Phanerozoic eon (542 mya - present)
Paleozoic era	Mesozoic era	Cenozoic era

Classification

Wood (2001) reports that 786 distinct living species of cephalopods have been identified. However, there are often new species of cephalopods discovered or delineated, and taxonomic changes made. Brune (2004) estimates there may be as many as 1,000 to 1,200 living species.

There are many more fossil species identified. It is estimated there are around 11,000 extinct taxa of cephalopods (Ivanov et al. 2001).

The classification as listed here is one possible arrangement. The three subclasses are traditional, corresponding to the three orders of cephalopods recognized by Bather (1888b). 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
    • (Order Aulacocerida)
    • (Order Phragmoteuthida)
    • (Order Hematitida)
    • (Order Belemnitida)
  • Cohort Neocoleoidea
    • Superorder Decapodiformes (also known as Decabrachia or Decembranchiata)
      • 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
• Subclass Endoceratoidea Teichert, 1933
• Subclass Actinoceratoidea Teichert, 1933
• Subclass Nautiloidea Agassiz, 1847
• Subclass Orthoceratoidea Kuhn, 1940
• Subclass Bactritoidea Shimansky, 1951
• Subclass Ammonoidea Zittel, 1884
• Subclass Coleoidea Bather, 1888

The first mention of Coleoidea appears in (Bather, 1888a) among this article's references.

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 (Berthold & Engeser, 1987; Engeser 1997).

References

ISBN links support NWE through referral fees

• Wells, M. J. 1980. Nervous control of the heartbeat in octopus. Journal of Experimental Biology 85(1): 112.

• Ghiretti-Magaldi, A., and F. Ghiretti. 1992. The pre-history of hemocyanin. The discovery of copper in the blood of molluscs. Cellular and Molecular Life Sciences 48(10): 971-972.

• Messenger, J. B., and R. T. Hanlon. 1998. Cephalopod Behaviour. Cambridge: Cambridge University Press. ISBN 0-521-64583-2}}</ref>

• Turgeon D., J. F. Quinn, A. E. Bogan, E. V. Coan, F. G. Hochberg, W. G. Lyons, P. M. Mikkelsen, R. J. Neves, C. F. E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F. G. Thompson, M. Vecchione, and J. D. Williams. 1998. Common and Scientific Names of Aquatic Invertebrates from the United States and Canada: Mollusks, Second Edition. American Fisheries Society Special Publication 26.

.

• 1998 - 703 recent species
• 2001]] - 786 recent species
• 2004 - approximate guess, from 1000 to 1200 species

• Bather, F.A. 1888a. Shell-growth in Cephalopoda (Siphonopoda). Annals and Magazine of Natural History, Series 6, Vol. 1: 298-310
• Bather, F.A. 1888b. Professor Blake and Shell-Growth in Cephalopoda. Annals and Magazine of Natural History. Series 6, Vol. 1: 421-426.
• Berthold, Thomas, & Engeser, Theo. 1987. Phylogenetic analysis and systematization of the Cephalopoda (Mollusca). Verhandlungen Naturwissenschaftlichen Vereins in Hamburg. (NF) 29: 187-220.
• Engeser, Theo. 1997. Fossil Nautiloidea Page. <http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautcontent.htm>
• Felley, J., Vecchione, M., Roper, C. F. E., Sweeney, M. & Christensen, T., 2001-2003: Current Classification of Recent Cephalopoda. internet: National Museum of Natural History: Department of Systematic Biology: Invertebrate Zoology: http://www.mnh.si.edu/cephs/
• Shevyrev, A.A. 2005. The Cephalopod Macrosystem: A Historical Review, the Present State of Knowledge, and Unsolved Problems: 1. Major Features and Overall Classification of Cephalopod Mollusks. Paleontological Journal. 39(6):606-614. Translated from Paleontologicheskii Zhurnal No. 6, 2005, 33-42.

External links

Wikispecies has information related to:

Cephalopoda

Wikibooks Dichotomous Key has more about this subject:

Cephalopoda

• CephBase - cephalopod database
• TONMO.COM - The Octopus News Magazine Online - cephalopod articles and discussion
• Tree of Life Web Project - Cephalopoda
• Mikko's Phylogeny Tree
• Articles on various fossil cephalopod topics for non-specialists
• Fish vs. Cephalopods
• Will Fast Growing Squid Replace Slow Growing Fish?
• Bipedal Octopuses with links to video and original paper
• Palcephalopoda/Neocephalopoda Hypothesis
• Biomineralisation in modern and fossil cephalopods
• Cephalopods

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