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Cephalopod

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The cephalopods ("head-foot") are the mollusk 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, squids, 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 the world and at all depths. None of them can tolerate freshwater, but a few species do tolerate more or less brackish water.

Contents

Nervous system and behavior

They are 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 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. Many species can see polarization of light. They are probably colorblind, yet they distinguish a vast number of tones.

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 inefficiency of jet propulsion worsens with larger animals. This is probably the reason 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 by the motto "live fast, die young". 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. Early cephalopods were at the top of the food chain. The ancient (cohort Belemnoidea) and modern Coleoidea (cohort Neocoleoidea) diverged from the external shelled Nautiloidea around 425 million years ago. Unlike most modern cephalopods, 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.

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

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

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

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.

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.

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