AskDefine | Define gastropod

Dictionary Definition

gastropod n : a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes [syn: univalve]

User Contributed Dictionary

English

Noun

  1. any member of a class of mollusks (Gastropoda) that includes snails and slugs; univalve mollusk

Extensive Definition

The class Gastropoda or the gastropods, also previously known as gasteropods, or univalves, and more commonly known as snails, are the most diversified class belonging to the phylum of mollusks, with 60,000-75,000 known living species. This class of animals is second only to insects in its number of known species.
The class Gastropoda is striking in its extraordinary diversification of habitats. Representatives live in gardens, in woodland, in deserts, and on mountains; in small ditches, great rivers and lakes; in estuaries, mudflats, the rocky intertidal, the sandy subtidal, in the abyssal depths of the oceans, and numerous other ecological niches, including parasitic ones.
This class includes very large numbers of species of marine snails and sea slugs, as well as freshwater snails and freshwater limpets, and the terrestrial (land) snails and slugs.
Although the name "snail" can be, and often is, applied to all the members of this class, very commonly the word "snail" is restricted to those species which have an external shell large enough that the soft parts can withdraw completely into it. Those gastropods without a shell, and those which have only a very reduced or internal shell, are often known as slugs.
The marine shelled species of gastropod include edible species such as abalone, conches, periwinkles, whelks, and numerous other sea snails which have coiled seashells. There are also a number of families of species such as all the various limpets, where the shell is coiled only in the larval stage, and is a simple conical structure after that.

Distribution

The gastropods have a worldwide distribution, in the seas and oceans, in brackish water, in freshwater and on land, from the near Arctic and Antarctic zones to the tropics.

Habitat

The gastropods have become adapted to almost every kind of existence on earth, having colonized every medium available except the air. In habitats where there is not enough calcium carbonate to build a really solid shell, such as on some acidic soils on land, one can find various species of slugs, and also some snails which have a thin translucent shell, mostly or entirely composed of protein.
Snails such as Sphincterochila boissieri and Trochoidea seetzenii have adapted to desert conditions, others to an existence in ditches, near deepwater hydrothermal vents, the pounding surf of rocky shores, caves, and many other diverse areas.

Description

4. upper commissura 5. osphradium 6. gills 7. ? ganglion 8. atrium of heart 9. visceral ganglion 10. ventricle 11. foot 12. operculum 13. brain 14. mouth 15. tentacle 16. eye 17. tentacle 18. ? 19. pedal ganglion 20. lower commissura 21. ? 22. pallial cavity / mantle cavity / respiratory cavity 23. parietal ganglion 24. anus 25. hepatopancreas 26. vas deferens 27. rectum 28. nephridium]]
Snails are distinguished by torsion, a process where the visceral mass of the animal rotates 180º to one side during development, such that the anus is situated more or less above the head. (This process is unrelated to the coiling of the shell, which is a separate phenomenon.) Torsion is present in all gastropods, but the opisthobranch gastropods are secondarily de-torted. However, this "rotation hypothesis" is being challenged by the "asymmetry hypothesis" in which the gastropod mantle cavity originated from one side only of a bilateral set of mantle cavities
Gastropods typically have a well-defined head with two or four sensory tentacles, and a ventral foot, which gives them their name (Greek gaster, stomach, and poda, feet). The eyes that may be present at the tip of the tentacles range from simple ocelli that cannot project an image (simply distinguishing light and dark), to more complex pit and even lens eyes . The larval shell of a gastropod is called a protoconch.
Most members have a shell, which is in one piece and is typically coiled or spiraled. This coiled shell usually opens on the right hand side (as viewed with the shell apex pointing upward). Several species have an operculum which in many species is a sort of a trapdoor to close the shell. This is usually made of a horn-like material, but in some molluscs it is calcareous. In the land slugs, the shell is reduced or absent, and the body is streamlined.
Some of the more familiar and better-known gastropods are terrestrial (the land snails and slugs), but more than two thirds of all named species live in a marine environment. Marine gastropods include some that are herbivores, detritus feeders, predatory carnivores, scavengers, parasites, and also a few ciliary feeders, in which the radula is reduced or absent. In some species which have evolved into endoparasites, such as Parenteroxenos doglieli, many of the standard gastropod features are strongly reduced or absent.
The radula of a gastropod is usually adapted to the food that a species eats. The simplest gastropods are the limpets and abalones, herbivores that use their hard radulas to rasp at seaweeds on rocks.
Many marine gastropods are burrowers, and have soft siphons or tubes that extend from the mantle. Sometimes the shell has a siphonal canal to accommodate this structure. A siphon enables the animal to draw a small flow of water into their bodies. The siphon is used primarily to "taste" the water, in order to detect prey from a distance. Gastropods with siphons tend to be either predators or scavengers.
Almost all marine gastropods breathe with gills, but many freshwater species, and the majority of terrestrial species, have a pallial lung. The gastropods which have a lung all belong to one group with common descent, the Pulmonata, however, the gastropods with gills are paraphyletic. The respiratory protein in almost all gastropods is hemocyanin, but a pulmonate family Planorbidae have hemoglobin as respiratory protein.
Some sea slugs are brightly coloured, either as a warning, if they are poisonous or contain stinging cells, or to camouflage them on the hydroids, sponges and seaweeds on which many of the species are found.
In one large group of sea slugs, the gills are arranged as a rosette of feathery plumes on their backs, which gives rise to their other name, nudibranchs. Some nudibranchs have smooth or warty backs and have no visible gill mechanism, such that respiration may likely take place directly through the skin.
A few sea slugs are herbivores and some are carnivores. Many have distinct dietary preferences and regularly occur in close association with their food species.

Geological history

The first gastropods were exclusively marine, with the earliest representatives of the group appearing in the Late Cambrian (Chippewaella, Strepsodiscus). Early Cambrian forms like Helcionella and Scenella are no longer considered gastropods, and the tiny coiled Aldanella of earliest Cambrian time is probably not even a mollusk. By the Ordovician period the gastropods were a varied group present in a range of aquatic habitats. Commonly, fossil gastropods from the rocks of the early Palaeozoic era are too poorly preserved for accurate identification. Still, the Silurian genus Poleumita contains fifteen identified species. Fossil gastropods are less common during the Palaeozoic era than bivalves.
Most of the gastropods of the Palaeozoic era belong to primitive groups, a few of which still survive today. By the Carboniferous period many of the shapes we see in living gastropods can be matched in the fossil record, but despite these similarities in appearance the majority of these older forms are not directly related to living forms. It was during the Mesozoic era that the ancestors of many of the living gastropods evolved.
One of the earliest known terrestrial (land-dwelling) gastropods is Maturipupa which is found in the Coal Measures of the Carboniferous period in Europe, but relatives of the modern land snails are rare before the Cretaceous period when the familiar Helix first appeared.
In rocks of the Mesozoic era gastropods are slightly more common as fossils, their shells are often well preserved. Their fossils occur in beds which were deposited in both freshwater and marine environments. The "Purbeck Marble" of the Jurassic period and the "Sussex Marble" of the early Cretaceous period which both occur in southern England are limestones containing the tightly packed remains of the pond snail Viviparus.
Rocks of the Cenozoic era yield very large numbers of gastropod fossils, many of these fossils being closely related to modern living forms. The diversity of the gastropods increased markedly at the beginning of this era, along with that of the bivalves.
Certain trail-like markings preserved in ancient sedimentary rocks are thought to have been made by gastropods crawling over the soft mud and sand. Although these trails are of debatable origin, some of them do resemble the trails made by living gastropods today.
Gastropod fossils may sometimes be confused with ammonites or other shelled cephalopods. An example of this is Bellerophon from the limestones of the Carboniferous period in Europe, the shell of which is planispirally coiled and can be mistaken for the shell of a cephalopod.
Gastropods are one of the groups that record the changes in fauna caused by the advance and retreat of the Ice Sheets during the Pleistocene epoch.

Taxonomy

The taxonomy of the Gastropoda is under constant revision, and more and more of the old taxonomy is being abandoned as the results of DNA studies slowly become clearer. Nevertheless some of the older terms such as "opisthobranch" and "prosobranch" are still being used in a descriptive way.
The taxonomy of the Gastropoda as shown in various texts can differ in major ways, and on-going revisions of the higher taxonomic levels are to be expected in the near future.
In the older classification there were four subclasses:
According to newer insights based on DNA sequencing, (Ponder & Lindberg, 1997), the taxonomy of the Gastropoda must be rewritten in terms of strictly monophyletic groups. Integrating these findings into a working taxonomy will continue to be a challenge in the coming years. At present, it is impossible to give a classification of the Gastropoda that has consistent ranks and also reflects current usage.
Convergent evolution, which appears to exist at especially high frequency within the class Gastropoda, may account for the observed differences between the phylogenies which are obtained from morphological data and the more recent studies based on gene sequences.
New changes in systematics have been made by (Bouchet & Rocroi, 2005)., resulting in a new taxonomy that is a step closer to the evolutionary history of phyla.
Malacologists currently make do with classifications that are hybrids of the latest existing taxonomy and later revisions published in scientific journals. In the past, the taxonomy of gastropods was largely based on morphological characters of the taxa. The recent advances are more based on molecular characters through research of DNA and RNA. This has made the taxonomical ranks and their hierarchy controversial. The debate about these issues is not likely to end soon.
This new taxonomy has tried to reconcile these recent advances by using unranked clades for taxa above the rank of superfamily (replacing the ranks suborder, order, superorder and subclass), while using the traditional Linnaean approach for all taxa below the rank of superfamily. Whenever monophyly has not been tested or is known to be paraphyletic or polyphyletic, the term "group" or "informal group" has been used. The classification of families into subfamilies is often not well resolved and should be regarded as the best possible hypothesis.

Proposed classification

For an even more recent taxonomic scheme please see Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
Class Gastropoda Cuvier, 1797 Incertæ sedis Subclass Eogastropoda Ponder & Lindberg, 1996 (earlier: Prosobranchia) Subclass Orthogastropoda Ponder & Lindberg, 1996 (earlier Prosobranchia, Opisthobranchia) Incertæ sedis Superorder Cocculiniformia Haszprunar, 1987 Superorder ‘Hot Vent Taxa' Ponder & Lindberg, 1997 Superorder Vetigastropoda Salvini-Plawen, 1989 (limpets) Superorder Neritaemorphi Koken, 1896 Superorder Caenogastropoda Cox, 1960 Superorder Heterobranchia J.E. Gray, 1840 Other extant classes of the Mollusca are Bivalvia, Scaphopoda, Aplacophora, Polyplacophora, Monoplacophora and Cephalopoda.

Footnotes

Bibliographic References

  • Shelagh M. Smith - Key to the British Marine Gastropoda, Contains 44 pages plus line drawings about A4 in size.
  • Paul Jeffery. Suprageneric classification of class GASTROPODA. The Natural History Museum, London, 2001
  • Ponder & Lindberg, Towards a phylogeny of gastropod molluscs; an analysis using morphological characters. Zoological Journal of the Linnean Society, 119 83-2651; 1997
  • ">http://www.bolinfonet.org/pdf/MPEVsnailpaper.pdf}}

External links

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