Gastropoda

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Gastropod
Fossil range: Late Cambrian - Recent
Marine gastropod Cypraea chinensis
Marine gastropod Cypraea chinensis
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Cuvier, 1797
Subclasses

Eogastropoda (True Limpets and relatives)
Orthogastropoda

The gastropods, also previously known as gasteropods, or univalves, and more commonly known as snails and slugs, are the largest and most successful class of mollusks, with 60,000-75,000 known living species. This class of animals is second only to insects in its size and diversity.

The class Gastropoda includes very large numbers of marine snails and sea slugs, as well as the freshwater snails, and the terrestrial (land) snails and slugs.

Although the word snail can be, and often is, applied to all the members of this class, very commonly the word "snail" is restricted only to those species which have an external shell. Those without a shell, or with only a very reduced or internal shell, are often known as slugs.

The marine shelled species of gastropod include abalone, cowries, conches, periwinkles, whelks, so the word snail can be used to apply to them and the majority of other sea snails which have coiled seashells. There are also a number of families of species where the shell is coiled only in the larval stage, such as all the various limpets.

Contents

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.

The gastropods have become adapted to almost every kind of existence on earth, having colonized every medium available except the air. Where there is not enough calcium to build a really strong shell, such as on some acidic soils on land, there are still many species of slugs and there are snails which have a thin translucent shell mostly composed of protein.

Snails have adapted to desert conditions, to life in ditches, to life near hydrothermal vents, to the pounding surf of rocky shores, to life in caves, and so on.

Drawing of a male Prosobranchia gastropodlight yellow - bodybrown - shell and operculumgreen - digestive systemlight violet - gillsyellow - osphradiumred - heartpink - dark violet - 1. foot 2. pleural ganglion 3. pneumostome 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
Drawing of a male Prosobranchia gastropod
light yellow - body
brown - shell and operculum
green - digestive system
light violet - gills
yellow - osphradium
red - heart
pink -
dark violet -
1. foot
2. pleural ganglion
3. pneumostome
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
Main article: Gastropod shell

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 opistobranch gastropods are secondarily de-torted. 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 [1]

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 [2]. The larval shell of a gastropod is called a protoconch.

The shell of Zonitoides nitidus, a small land snail, has dextral coiling, which is typical of gastropod shells, but which is not universally found. Upper image: dorsal view of the shell, showing the apex Central image: lateral view showing the spire and aperture of the shell Lower image: basal view showing the umbilicus
The shell of Zonitoides nitidus, a small land snail, has dextral coiling, which is typical of gastropod shells, but which is not universally found.
Upper image: dorsal view of the shell, showing the apex
Central image: lateral view showing the spire and aperture of the shell
Lower image: basal view showing the umbilicus

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. These siphons act as snorkels, enabling the animal to continue to draw in a water current containing oxygen and food into their bodies. The siphons are also used to "taste" the water, in order to detect prey from a distance.

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. Respiratory protein in all gastropods is hemocyanin, but in a family Planorbidae there is 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.

Fossil gastropod and attached mytilid bivalves on a Jurassic limestone bedding plane in southern Israel.
Fossil gastropod and attached mytilid bivalves on a Jurassic limestone bedding plane in southern Israel.
Helix aspersa: a European pulmonate land snail which has also commonly been accidentally introduced in many countries throughout the world.
Helix aspersa: a European pulmonate land snail which has also commonly been accidentally introduced in many countries throughout the world.

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.

Cepaea nemoralis: another European pulmonate land snail which has also been introduced to other countries.
Cepaea nemoralis: another European pulmonate land snail which has also been introduced to other countries.

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.

The taxonomy of the Gastropoda is under constant revision, and more and more of the old taxonomy is being abandoned. Nevertheless terms as "opisthobranch" and "prosobranch" are still being used in a descriptive way. In a sense, we can speak of a taxonomic jungle when we go down to the lower taxonomic levels. The taxonomy of the Gastropoda can be different from author to author. But with the arrival of DNA-sequencing, further revisions of the higher taxonomic levels are to be expected in the near future.

According to the traditional classification there are four subclasses. :

According to the newest insights (Ponder & Lindberg, 1997), the taxonomy of the Gastropoda should be rewritten in terms of strictly monophyletic groups. Integrating these findings into a working taxonomy will be a true 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, observed at especially high frequency in the Gastropods, may account for the observed differences between phylogenies obtained from morphological data and more recent studies based on gene sequences.

New changes in systematics are made by (Bouchet & Rocroi, 2005).[3][4]

Class Gastropoda (Cuvier, 1797)
Incertæ sedis

Subclass Eogastropoda (Ponder & Lindberg, 1996) (earlier: Prosobrachia)

  • Order Euomphalida de Koninck 1881 (fossil)
      • Superfamily Macluritoidea
      • Superfamily Euomphaloidea
      • Superfamily Platyceratoidea
  • Order Patellogastropoda Lindberg, 1986 (true limpets)
Limpets in the intertidal zone in Cornwall, England.
Limpets in the intertidal zone in Cornwall, England.
    • Suborder Patellina Van Ihering, 1876
    • Suborder Nacellina Lindberg, 1988
      • Superfamily Acmaeoidea Carpenter, 1857
      • Superfamily Nacelloidea Thiele, 1891
    • Suborder Lepetopsina McLean, 1990
      • Superfamily Lepetopsoidea McLean, 1990

Subclass Orthogastropoda Ponder & Lindberg, 1996 (earlier Prosobrancanahia, Opisthobranchia)
Incertæ sedis

  • Order Murchisoniina Cox & Knight, 1960 (fossil)
      • Superfamily Murchisonioidea Koken, 1889
      • Superfamily Loxonematoidea Koken, 1889
      • Superfamily Lophospiroidea Wenz, 1938
      • Superfamily Straparollinoidea
    • Grade Subulitoidea Lindström, 1884

Superorder Cocculiniformia Haszprunar, 1987

      • Superfamily Cocculinoidea Dall, 1882
      • Superfamily Lepetelloidea Dall, 1882 (deep sea limpets)

Superorder ‘Hot Vent Taxa' Ponder & Lindberg, 1997

  • Order Neomphaloida Sitnikova & Starobogatov, 1983
      • Superfamily Neomphaloidea McLean, 1981 (hydrothermal vents limpets)
      • Superfamily Peltospiroidea McLean, 1989

Superorder Vetigastropoda Salvini-Plawen, 1989 (limpets)

Archaeogastropod from the Pliocene of Cyprus. A serpulid worm is attached.
Archaeogastropod from the Pliocene of Cyprus. A serpulid worm is attached.
      • Superfamily Fissurelloidea Fleming, 1822 (keyhole limpets)
      • Superfamily Haliotoidea Rafinesque, 1815 (abalones)
      • Superfamily Lepetodriloidea McLean, 1988 (hydrothermal vent limpets)
      • Superfamily Pleurotomarioidea Swainson, 1840 (slit shells)
      • Superfamily Seguenzioidea Verrill, 1884
      • Superfamily Trochoidea Rafinesque, 1815 (top shells)

Superorder Neritaemorphi Koken, 1896

  • Order Cyrtoneritomorpha (fossil)
  • Order Neritopsina Cox & Knight, 1960

Superorder Caenogastropoda Cox, 1960

  • Order Architaenioglossa Haller, 1890
      • Superfamily Ampullarioidea J.E. Gray, 1824
      • Superfamily Cyclophoroidea J.E. Gray, 1847 (terrestrials)
  • Order Sorbeoconcha Ponder & Lindberg, 1997
    • Suborder Discopoda P. Fischer, 1884
      • Superfamily Campaniloidea Douvillé, 1904
      • Superfamily Cerithioidea Férussac, 1822
    • Suborder Hypsogastropoda Ponder & Lindberg, 1997
    • Infraorder Littorinimorpha Golikov & Starobogatov, 1975
      • Superfamily Calyptraeoidea Lamarck, 1809
      • Superfamily Capuloidea J. Fleming, 1822
      • Superfamily Carinarioidea Blainville, 1818 (formerly called Heteropoda)
      • Superfamily Cingulopsoidea Fretter & Patil, 1958
      • Superfamily Cypraeoidea Rafinesque, 1815 (cowries)
      • Superfamily Ficoidea Meek, 1864
      • Superfamily Laubierinoidea Warén & Bouchet, 1990
      • Superfamily Littorinoidea (Children), 1834 (periwinkles)
      • Superfamily Naticoidea Forbes, 1838 (moon shells)
      • Superfamily Rissooidea J.E. Gray, 1847 (Risso shells) (includes genus Oncomelania, schistosomiasis transmission vector)
      • Superfamily Stromboidea Rafinesque, 1815 (true conchs)
      • Superfamily Tonnoidea Suter, 1913
      • Superfamily Trivioidea Troschel, 1863
      • Superfamily Vanikoroidea J.E. Gray, 1840
      • Superfamily Velutinoidea J.E. Gray, 1840
      • Superfamily Vermetoidea Rafinesque, 1815 (worm shells)
      • Superfamily Xenophoroidea Troschel, 1852 (carrier shells)
    • Infraorder Ptenoglossa J.E. Gray, 1853
      • Superfamily Eulimoidea Philippi, 1853
      • Superfamily Janthinoidea Lamarck, 1812
      • Superfamily Triphoroidea J.E. Gray, 1847
    • Infraorder Neogastropoda Thiele, 1929

Superorder Heterobranchia J.E. Gray, 1840

Cochlodina laminata from the family Clausiliidae or Door snails, a small land pulmonate which has a sinistral or left-handed shell, on the trunk of a tree, in woodland, England
Cochlodina laminata from the family Clausiliidae or Door snails, a small land pulmonate which has a sinistral or left-handed shell, on the trunk of a tree, in woodland, England
    • Suborder Nudibranchia Blainville, 1814 (nudibranchs)
    • Infraorder Anthobranchia Férussac, 1819
      • Superfamily Doridoidea Rafinesque, 1815
      • Superfamily Doridoxoidea Bergh, 1900
      • Superfamily Onchidoridoidea Alder & Hancock, 1845
      • Superfamily Polyceroidea Alder & Hancock, 1845
    • Infraorder Cladobranchia Willan & Morton, 1984
      • Superfamily Dendronotoidea Allman, 1845
      • Superfamily Arminoidea Rafinesque, 1814
      • Superfamily Metarminoidea Odhner in Franc, 1968
      • Superfamily Aeolidioidea J.E. Gray, 1827
  • Order Pulmonata Cuvier in Blainville, 1814 (pulmonates)
    • Suborder Systellommatophora Pilsbry, 1948
      • Superfamily Onchidioidea Rafinesque, 1815
      • Superfamily Otinoidea H. & A. Adams, 1855
      • Superfamily Rathouisioidea Sarasin, 1889
    • Suborder Basommatophora Keferstein in Bronn, 1864 (freshwater pulmonates, pond snails)
    • Suborder Eupulmonata Haszprunar & Huber, 1990
    • Infraorder Acteophila Dall, 1885 (= formerly Archaeopulmonata)
      • Superfamily Melampoidea Stimpson, 1851
    • Infraorder Trimusculiformes Minichev & Starobogatov, 1975
      • Superfamily Trimusculoidea Zilch, 1959
    • Infraorder Stylommatophora A. Schmidt, 1856 (land snails)
    • Subinfraorder Orthurethra
      • Superfamily Achatinelloidea Gulick, 1873
      • Superfamily Cochlicopoidea Pilsbry, 1900
      • Superfamily Partuloidea Pilsbry, 1900
      • Superfamily Pupilloidea Turton, 1831
    • Subinfraorder Sigmurethra
      • Superfamily Acavoidea Pilsbry, 1895
      • Superfamily Achatinoidea Swainson, 1840
      • Superfamily Aillyoidea Baker, 1960
      • Superfamily Arionoidea J.E. Gray in Turnton, 1840
      • Superfamily Buliminoidea Clessin, 1879
      • Superfamily Camaenoidea Pilsbry, 1895
      • Superfamily Clausilioidea Mörch, 1864
      • Superfamily Dyakioidea Gude & Woodward, 1921
      • Superfamily Gastrodontoidea Tryon, 1866
      • Superfamily Helicoidea Rafinesque, 1815
      • Superfamily Helixarionoidea Bourguignat, 1877
      • Superfamily Limacoidea Rafinesque, 1815
      • Superfamily Oleacinoidea H. & A. Adams, 1855
      • Superfamily Orthalicoidea Albers-Martens, 1860
      • Superfamily Plectopylidoidea Moellendorf, 1900
      • Superfamily Polygyroidea Pilsbry, 1894
      • Superfamily Punctoidea Morse, 1864
      • Superfamily Rhytidoidea Pilsbry, 1893
      • Superfamily Sagdidoidera Pilsbry, 1895
      • Superfamily Staffordioidea Thiele, 1931
      • Superfamily Streptaxoidea J.E. Gray, 1806
      • Superfamily Strophocheiloidea Thiele, 1926
      • Superfamily Trigonochlamydoidea Hese, 1882
      • Superfamily Zonitoidea Mörch, 1864
      • ? Superfamily Athoracophoroidea P. Fischer, 1883 (= Tracheopulmonata)
      • ? Superfamily Succineoidea Beck, 1837 (= Heterurethra)

Other extant classes of the Mollusca are Bivalvia, Scaphopoda, Aplacophora, Polyplacophora, Monoplacophora and Cephalopoda.

  1. ^ Louise R. Page (2006). "Modern insights on gastropod development: Reevaluation of the evolution of a novel body plan". Integrative and Comparative Biology 46 (2). 
  2. ^ Götting, Klaus-Jürgen (1994). "Schnecken", in Becker, U., Ganter, S., Just, C. & Sauermost, R.: Lexikon der Biologie. Heidelberg: Spektrum Akademischer Verlag. ISBN 3-86025-156-2. 
  3. ^ Bouchet, P.; Rocroi, J.-P. (Ed.); Frýda,J.; Hausdorf,B.; Ponder, W.; Valdes, A.; Warén, A. (2005). Classification and nomenclator of gastropod families. Malacologia: International Journal of Malacology, 47(1-2). ConchBooks: Hackenheim, Germany. ISBN 3-925919-72-4. 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278
  4. ^ Poppe G.T. & Tagaro S.P. 2006. The new classification of Gastropods according to Bouchet & Rocroi, 2005. Visaya, février 2006: 10 pp. http://www.journal-malaco.fr/bouchet&rocroi_2005_Visaya.pdf
  5. ^ Published by the Royal Scottish Museum in their Information Series (UK) in 1974 with no ISBN

  • Shelagh M. Smith - Key to the British Marine Gastropoda, Contains 44 pages plus line drawings about A4 in size. [5]
  • 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
  • Elpidio A. Remigio and Paul D.N. Hebert (2003). "Testing the utility of partial COI sequences for phylogenetic (full text on line)". Molecular Phylogenetics and Evolution 29. 

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