Gastropoda

Gastropoda Fossil range: Early Cambrian–Recent[1] PreЄ Є O S D C P T J K Pg N
Land gastropod Helix pomatia, the Roman snail
Scientific classification
Kingdom: Animalia Phylum: Mollusca Class: Gastropoda Cuvier, 1797
Clades
"Paleozoic uncertain …" "Basal taxa …" clade Patellogastropoda clade Vetigastropoda clade Cocculiniformia clade Neritimorpha clade Caenogastropoda clade Heterobranchia

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.

This is the most diversified class in the phylum, with 60,000 to 80,000^[2][3] living species.

There are 409 recent families of gastropods. Fossil gastropods represent another 202 families.^[3] This class of animals is second only to insects in its number of known species.

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 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.

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 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

Gastropods have a worldwide distribution, in the seas and oceans (about 30,000), in brackish water, in freshwater (about 5,000) and on land (about 30,000) , 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, 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.

Anatomy

The anatomy of a common air-breathing land snail such as Helix aspersa

The anatomy of an aquatic snail with a gill, a male prosobranch gastropod
Light yellow - body
Brown - shell and operculum
Green - digestive system
Light violet - gills
Yellow - osphradium
Red - heart
Pink -
Dark violet -
1. foot
2. cerebral ganglion
3. pneumostome
4. upper commissura
5. osphradium
6. gills
7. pleural ganglion
8. atrium of heart
9. visceral ganglion
10. ventricle
11. foot
12. operculum
13. brain
14. mouth
15. tentacle (chemosensory, 2 or 4)
16. eye
17. penis (everted, normally internal)
18. esophageal nerve ring
19. pedal ganglion
20. lower commissura
21. vas deferens
22. pallial cavity / mantle cavity / respiratory cavity
23. parietal ganglion
24. anus
25. hepatopancreas
26. gonad
27. rectum
28. nephridium

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^[4].

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.

The shell

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 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.

Body wall

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.

Lateral outgrowths on the body of nudibranchs are called cerata. These contain a part of digestive gland, which is called the diverticula.

Digestive system

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 radula to rasp at seaweeds on rocks.

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.

Respiratory system

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.

Circulatory system

Gastropods have open circulatory system and the transport fluid is hemolymph. Hemocyanin is present in the hemolymph as the respiratory pigment.

Excretory system

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.

Sensory organs and nervous system

The upper pair of tentacles on the head of Helix pomatia have eyes, but the main sensory organs of the snail are sensory receptors for olfaction which are situated in the epithelium of the tentacles.

Sensory organs of gastropods include olfactory organs, eyes, statocysts and mechanoreceptors.^[5] Gastropods have no hearing.^[5]

The olfactory organs, located on the tips of the 4 tentacles, are the most important sensory organ in terrestrial gastropods ^[5], The chemosensory organs of opisthobranch marine gastropods are called rhinophores.

The eye spots that are present at the tip of the tentacles or instead at the base 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.^[6] Vision is not the most important requirement in terrestrial gastropods, because they are mainly nocturnal animals.^[5]

Nervous system of gastropods include peripheral nervous system and central nervous system. Central nervous system consist of ganglia connected by nerve cells. It include paired ganglia: cerebral ganglia, pedal ganglia, osphradial ganglia, pleural ganglia, parietal ganglia and visceral ganglia. There are sometimes also buccal ganglia.^[5]

Reproductive system

Courtship is a part of mating behavior in some gastropods. In some land snails, features of the reproductive system of gastropods include the presence and utilization of love darts.

In many marine gastropods there are separate sexes; most land gastropods however are hermaphrodites.

Life cycle

A 9-hour-old trochophore of Haliotis asinina
sf - shell field

mating behaviour of Elysia timida

The main aspects of the life cycle of gastropods include:

• egg hatching and eggs of gastropods
• embryonic development of gastropods
• larval stadium: some gastropods may be trochophore and/or veliger
• estivation and hibernation (each of these are present in some gastropods only)
• growth of gastropods
• courtship of gastropods and mating of gastropods: fertilisation is internal or external according to the species. External fertilisation is common in marine gastropods.

Feeding behaviour

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.

Some predatory carnivorous gastropods include, for example: Cone shells, Testacella, Daudebardia, Ghost slug and others.

Geological history

Fossil gastropod and attached mytilid bivalves on a Jurassic limestone bedding plane of the Matmor Formation in southern Israel.

Helix aspersa: a European pulmonate land snail which has 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 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.

Cepaea nemoralis: another European pulmonate land snail which has been introduced to many 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.

Taxonomy

Turritella carinata from the Pliocene of Cyprus.

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^[7]:

• Opisthobranchia (gills to the right and behind the heart).
• Gymnomorpha (no shell)
• Prosobranchia (gills in front of the heart).
• Pulmonata (with a lung instead of gills)

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.

New changes in systematics have been made by Bouchet & Rocroi (2005)^[3][8], resulting in a new taxonomy that is a step closer to the evolutionary history of the phylum.

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^[9] 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.

In 2004 Brian Simison and David R. Lindberg showed possible diphyletic origins of the Gastropoda based on mitochondrial gene order and amino acid sequence analyses of complete genes. ^[10]

References

• Shelagh M. Smith - Key to the British Marine Gastropoda, Contains 44 pages plus line drawings about A4 in size. Published by the Royal Scottish Museum in their Information Series (UK) in 1974 with no ISBN.

External links

Wikimedia Commons has media related to: Gastropoda

• Gastropods in captivity
• Gastropod reproductive behavior
• Reconstructions of fossil gastropods
• 2004 Linnean taxonomy of gastropods