cephalopod

 

(sĕf'ələpŏd') , member of the class Cephalopoda, the most highly organized group of mollusks (phylum Mollusca), and including the squids, octopuses, cuttlefish, and nautiluses. The class as a whole has become adapted for a free-swimming existence. Cephalopods are able to move about rapidly, and most are aggressive carnivores. The part of the body that forms the foot in other mollusks is located anteriorly in cephalopods instead of ventrally. Part of the foot area surrounds the mouth and is modified into sucker-bearing tentacles, used to capture prey. The tentacles number 8 in octopuses, 10 in squids, and as many as 90 in nautiluses. The rest of the foot forms a muscular funnel, or siphon, which expels water from the mantle cavity, permitting cephalopods to move about by a kind of jet propulsion. Only one existing genus, the nautiluses, the sole survivors of an extinct group known as the nautiloids, possesses an external shell. In the squid and cuttlefish the shell has become internalized and reduced, and in the octopus it is completely absent. The cephalopod head is large and is equipped with prominent eyes that resemble those of vertebrate animals. The class Cephalopoda has a fossil record of 10,000 species, although only 600 exist today. The nautiloid group was dominant through Paleozoic times, and the ammonites flourished in the Mesozoic era.

Cephalopods Fossil range: Late Cambrian - Recent
A variety of cephalopod forms from Ernst Haeckel's 1904 Kunstformen der Natur.
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) ?†Boletzkyida Sepiida Sepiolida Spirulida Teuthida Octopoda Vampyromorphida

The cephalopods (Greek plural Κεφαλόποδα (kephalópoda); "head-foot") 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. There are around 786 distinct living species of Cephalopods. 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.

Number of species

There are still discoveries of new species of cephalopods:

• 1998 - 703 recent species ^[1]
• 2001 - 786 recent species ^[2]
• 2004 - approximate guess, from 1000 to 1200 species ^[3]

There are many more fossil species. It is estimated there are around 11,000 extinct taxa.^[4]

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. Except nautiluses, cephalopods have special skin cells called chromatophores that change color and are used for communication and camouflage. The nervous system of cephalopods is the most complex of the invertebrates. 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]

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.^[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]

Locomotion

Cephalopods move primary 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 why many species prefer to use their fins or arms for locomotion if possible. 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.

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.

Evolution

The class developed during the late Cambrian, and were during the Paleozoic and Mesozoic dominant and diverse marine life forms. 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 diverged from the external shelled nautiloid during the middle Paleozoic Era, between 450 and 300 million years ago. 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, and others died off. 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.

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 (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
    • (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
• 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).

See also

• Cephalopod intelligence
• Cephalopod size
• Kraken

Notes

References

• 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

• 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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