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Gastropoda

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The gastropods, gasteropods, or univalves, are the largest and most successful class of mollusks, with 60,000-75,000 extant species known, comprising the snails and slugs as well as a vast number of marine and freshwater species.

Contents

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, 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 typically coiled or spiralled 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 horny 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.

While the best-known gastropods are terrestrial, 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, but 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 are 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 which gives rise to 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.

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 mollusc. 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 shell 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 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.

Taxonomy

The taxonomy of the Gastropoda is under constant revision, but 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.

Proposed classification, down to the level of superfamily

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
Clausiliidae or Door Snails
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Clausiliidae or Door Snails

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

References

  • 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

External links

 

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