Fossil range: Early Cambrian–Recent
|Air-breathing land gastropod Helix pomatia, the Roman snail|
The class Gastropoda or gastropods (also previously known as univalves and sometimes also spelled Gasteropoda) form a major part of the phylum Mollusca. Gastropods are more commonly known as snails and slugs, and include those that live in the sea, in freshwater and on land. This class of animals is second only to the insects in its number of known species. Its fossil history goes back to the Late Cambrian.
Gastropoda is the most highly diversified class in the phylum Mollusca, with 60,000 to 80,000 living snail and slug species. The anatomy, behavior, feeding and reproductive adaptations of gastropods vary very significantly from one clade or group to another, therefore it is very difficult or impossible to make more than a very few general statements about these topics that will be valid for all of the gastropods.
There are 409 recent families of gastropods. Fossil gastropods represent another 202 families. The gastropods include many thousands of species of marine snails and sea slugs, as well as freshwater snails and freshwater limpets, and the terrestrial (land) snails and slugs.
The class Gastropoda has an 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 including the hydrothermal vents, and numerous other ecological niches, including parasitic ones.
Although the name "snail" can be, and often is, applied to all the members of this class, very commonly this word 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 usually 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.
Gastropods have a worldwide distribution, in the seas and oceans (about 30,000 species), in brackish water, in freshwater (about 5,000 species) and on land (about 24,000 described species, (Chapman, 2009)), 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. In habitats where there is not enough calcium carbonate to build a really solid shell, such as on some acidic soils on land, there are still various species of slugs, and also some snails which have a thin translucent shell, mostly or entirely composed of the protein conchiolin.
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.
Snails such as Sphincterochila boissieri and Xerocrassa seetzeni have adapted to desert conditions, other snails have adapted to an existence in ditches, near deepwater hydrothermal vents, the pounding surf of rocky shores, caves, and many other diverse areas.
Snails are distinguished by an anatomical process known as torsion, 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.
Torsion occurs in two mechanistic stages. The first is muscular, and the second is mutagenetic. The effects of torsion are primarily physiological - the organism develops an asymmetrical nature with the majority of growth occurring on the left side. This leads to the loss of right-paired appendages (e.g. ctenidia (comb-like respiratory apparatus), gonads, nephridia, etc). Furthermore, the anus becomes redirected to the same space as the head. This is speculated to have some evolutionary function, as prior to torsion, when retracting into the shell, first the posterior end would get pulled in, and then the anterior. Now, the front can get be retracted more easily, perhaps suggesting a defensive purpose.
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 with eyes, and a ventral foot, which gives them their name (Greek gaster, stomach, and poda, feet). The larval shell of a gastropod is called a protoconch.
Most shelled gastropods have a shell which is in one piece, and which is typically coiled or spiraled. This coiled shell usually opens on the right-hand side (as viewed with the shell apex pointing upward). Numerous 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 sea slugs are very brightly colored. This serves either as a warning, when they are poisonous or contain stinging cells, or to camouflage them on the brightly-colored hydroids, sponges and seaweeds on which many of the species are found.
Many marine gastropods are burrowers, and have a siphon that extends out from the mantle edge. Sometimes the shell has a siphonal canal to accommodate this structure. A siphon enables the animal to draw a flow of water into their mantle cavity and over the gill. 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 a gill, 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.
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.
The primary organs of excretion in gastropods are nephridia, which produce either ammonia or uric acid as a waste product. The nephridium also plays an important role in maintaining water balance in freshwater and terrestrial species. Additional organs of excretion, at least in some species, include pericardial glands in the body cavity, and digestive glands opening into the stomach.
In terrestrial gastropods (land snails and slugs), the olfactory organs, located on the tips of the 4 tentacles, are the most important sensory organ , The chemosensory organs of opisthobranch marine gastropods are called rhinophores.
In the majority of gastropods, eye spots are present, either at the tip of the tentacles or instead at the base of the tentacles. These "eyes" range from simple ocelli that cannot project an image and which can only distinguish light and dark, to more complex pit eyes and even lens eyes. IN land snails and slugs, vision is not the most important sense, because they are mainly nocturnal animals.
The nervous system of gastropods includes the peripheral nervous system and the central nervous system. The central nervous system consist of ganglia connected by nerve cells. It includes paired ganglia: the cerebral ganglia, pedal ganglia, osphradial ganglia, pleural ganglia, parietal ganglia and the visceral ganglia. There are sometimes also buccal ganglia.
Courtship is a part of mating behavior in some gastropods including some of the Helicidae. Again, in some land snails, an unusual feature of the reproductive system of gastropods is the presence and utilization of love darts.
The main aspects of the life cycle of gastropods include:
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.
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.
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 were 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.
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 a few of the older terms such as "opisthobranch" and "prosobranch" are still sometimes 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, 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.
This new classification system is based partly on the older systems of classification and partly on new cladistic research. In the past, the taxonomy of gastropods was largely based on phenetic 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.
In this new taxonomy, Bouchet, Rocroi et al. have used 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.