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.
Number of species
There are still discoveries of new species of cephalopods:
- 1998 - 703 recent species
- 2001 - 786 recent species
- 2004 - approximate guess, from 1000 to 1200 species
There are many more fossil species. It is estimated there are around 11,000 extinct taxa.
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.
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. Evidence of color vision has been found in only one species, the Sparkling Enope Squid.
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.
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.
Locomotion
Cephalopods move primarily 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.(Campbell, Reece, & Mitchell, p.612)
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.
Small shelly fossils such as
Tommotia were previously interpreted as early cephalopods, but today these tiny fossils are recognized as
sclerites of larger animals. Hence, the earliest cephalopod known is
Plectronoceras from the Late Cambrian Period. 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. 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. 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
- Subclass Ellesmeroceratoidea Flower 1950
- Order Plectronocerida
- Order Protactinocerida
- Order Yanhecerida
- Order Ellesmerocerida
- Subclass Endoceratoidea Teichert, 1933
- Order Endocerida
- Order Intejocerida
- Subclass Actinoceratoidea Teichert, 1933
- Subclass Nautiloidea Agassiz, 1847
- Order Basslerocerida
- Order Tarphycerida
- Order Lituitida
- Order Discosorida
- Order Oncocerida
- Order Nautilida
- Subclass Orthoceratoidea Kuhn, 1940
- Order Orthocerida
- Order Ascocerida
- Order Dissidocerida
- Order Bajkalocerida
- Subclass Bactritoidea Shimansky, 1951
- Subclass Ammonoidea Zittel, 1884
- Subclass Coleoidea Bather, 1888
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
See also
Notes
References
- 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/
- Campbell, Neil A., Reece, Jane B., and Mitchell, Lawrence G.: Biology, fifth edition. Addison Wesley Longman, Inc. Menlo Park, California. 1999 ISBN 0-8053-6566-4
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