Gastropoda
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 Cypraea chinensis
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Eogastropoda (True Limpets and relatives) |
The gastropods, gasteropods, or univalves, are the largest and most successful class of mollusks. 60,000-75,000 known living species belong to it. Many of them live in marine or freshwater, but many on land too. Some of these species are: snails and slugs, abalone, limpets, cowries and conch. Most of the other animals that produce seashells belong to them as well.
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[change] Description
Snails are distinguished by torsion, a process where the body coils to one side during development.
They typically have a well-defined head with two or four sensory tentacles. They also have a ventral, 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 [1]. The larval shell of a gastropod is called a protoconch.
Most members have a shell, which is in one piece and typically coiled or spiraled that usually opens on the right hand side (as viewed with the shell apex pointing upward). Several species have an operculum that operates as a trapdoor to close the shell. This is usually made of a horn like material, but in some molluscs it is calcareous. In some members, the slugs, the shell is reduced or absent, and the body is streamlined so its torsion is relatively inconspicuous.
The best-known gastropods are terrestrial, but more than two thirds of all species live in a marine environment. Marine gastropods include herbivores, detritus feeders, carnivores and a few ciliary feeders, in which the radula is reduced or absent. The radula is usually adapted to the food that a species eats. The simplest gastropods are the limpets and abalones, both herbivores that use their hard radulas to rasp at seaweeds on rocks. Many marine gastropods are burrowers and have siphons or tubes that extend from the mantle and sometimes the shell. These 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 detect prey from a distance. These gastropods breathe with gills. Some freshwater species and almost all terrestric species have developed lungs. While the gastropods with lungs all belong to one group, Pulmonata, the gastropods with gills belong to another, paraphyletic.
Sea slugs are often flamboyantly coloured, either as a warning if they are poisonous or to camouflage them on the corals and seaweeds on which many of the species are found. Their gills are often in a form of feathery plumes on their backs. This is what gives them their other name, nudibranchs. Nudibranchs with smooth or warty backs have no visible gill mechanisms and respiration may take place directly through the skin. A few of the sea slugs are herbivores and some are carnivores. Many have distinct dietary preferences and regularly occur in association with certain species.
[change] Geological history
The first gastropods were exclusively marine. The first of the group appeared in the Late Cambrian (Chippewaella, Strepsodiscus). Early Cambrian forms like Helcionella and Scenella are no longer considered gastropods. The tiny coiled Aldanella of earliest Cambrian time is probably not even a mollusc. By the Ordovician period the gastropods were a varied group present in a few 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 that era belong to primitive groups. A few of these groups still survive today. By the Carboniferous period many of the shapes seen in living gastropods can be matched in the fossil record. 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. Relatives of the modern land snails are rare before the Cretaceous period. The familiar Helix first appeared in that period.
In rocks of the Mesozoic era gastropods are slightly more common as fossils and their shell is often well preserved. Their fossils occur in beds of 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 have very large numbers of gastropod fossils in them. Many of these fossils are 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 which may be mistaken for 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.
[change] Taxonomy
The taxonomy of the Gastropoda is under constant revision. More and more of the old taxonomy is being abandoned. Nevertheless terms as "opisthobranch" and "prosobranch" are still used. 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. :
- Prosobranchia (gills in front of the heart).
- Opisthobranchia (gills to the right and behind the heart).
- Gymnomorpha (no shell)
- Pulmonata (with lungs instead of gills)
According to the newest insights (Ponder & Lindberg, 1997), the taxonomy of the Gastropoda should be rewritten in terms of strictly monophyletic groups. It will be challenging to integrating these findings into a working taxonomy. 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.
[change] References
- ↑ 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.
- 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
[change] Other websites
- Conchology.be consult over 2,100,000 figured gastropods and other shells
- Taxonomy
- Gastropod Classification compiled by Paul Jeffery
- International Code of Zoological Nomenclature 4th edition, 2000
- Gastropods in captivity
- Reconstructions of fossil gastropods at www.emilydamstra.com
