Cephalopod

From the Simple English Wikipedia, the free encyclopedia that anyone can change

Cephalopods

Juvenile cephalopod from plankton
Antarctica

Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda Cuvier, 1797

The cephalopods (Greek plural Κεφαλόποδα (kephalópoda); "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 subclasses which have not died out. 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 the world and at all depths. None of them can tolerate freshwater, but a few species do tolerate more or less brackish water.

1 Well known Cephalopods
2 Number of species
3 Nervous system and behaviour
4 Circulatory system
5 Moving around
6 Reproduction and life cycle
7 Evolution
8 Notes
9 References
10 Other websites

[change] Well known Cephalopods

octopus
squid (4 different groups)
cuttlefish (Sepia)
Nautilus
Ammonites (extinct)
Belemnites (also extinct)

[change] 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]

[change] Nervous system and behaviour

Many scientists believe tha Cephalopods are the most intelligent of the invertebrates. Cephalopods have well developed senses and large brains. Their brains are larger than those 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.

Cephalopod can see very well. 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]

[change] 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]

[change] Moving around

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. 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.

[change] 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.

[change] Evolution

The class developed during the late Cambrian and were dominant and diverse marine life forms during the Paleozoic and Mesozoic . 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. 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.

[change] Notes

↑ [updated 28-Nov-2000] [cit. 12-Dec-2003] http://www.cephbase.dal.ca/spdb/allsp.cfm
↑ [updated 13-Jun-2003] [cit. 27-Feb-2005] http://www.cephbase.utmb.edu/spdb/allsp.cfm
↑ Brune, R., H. (2004) Encyklopedie ulit a lastur. – Rebo Productions, Dobřejovice, 1. vydání, 336 pp., page 16. (in Czech)
↑ Ivanov M., Hrdličková, S. & Gregorová, R. (2001) Encyklopedie zkamenělin. – Rebo Productions, Dobřejovice, 1. vydání, 312 pp., page 139. (in Czech)
↑ ^5.0 ^5.1 Messenger, John B.; Roger T. Hanlon (1998). Cephalopod Behaviour. Cambridge: Cambridge University Press, 17-21. ISBN 0-521-64583-2.
↑ Hanlon and Messenger, 68.
↑ Wells, M.J. (1980). "Nervous control of the heartbeat in octopus". Journal of Experimental Biology 85 (1): 112.
↑ Ghiretti-Magaldi, A., Ghiretti, F. (October 1992). "The Pre-history of Hemocyanin. The Discovery of Copper in the Blood of Molluscs". Cellular and Molecular Life Sciences 48 (10).

[change] 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.