A gastropod is any member of Gastropoda, the largest and most diverse class of the phylum Mollusca, with about 75,000 known living species. Gastropods are also referred to as univalves since most have a single shell, or valve, which is characteristically coiled or spiraled, as in snails, limpets, abalones, cowries, whelks, and conches. But Gastropoda is very diverse and many, such as slugs and sea slugs (nudibranches), lack shells; some even have shells with two halves, appearing as if bivalves.
This class is very successful and occupies almost every niche on earth. There are marine, freshwater, and terrestrial species, and they range in the ocean from the intertidal zone to the deepest trenches, on land from deserts to high mountains, and in all types of freshwater ecosystems. They include herbivores, carnivores, detritivores, and parasites. Along the rocky shore of an estuary, for example, the many different types of gastropods may include those that scavenge for dead fish or other organic debris while others eat small algae or bacteria and still others at the top of the food chain eat mollusks and other gastropods lower on the food chain.
As with all mollusks, gastropods are characterized by having a true coelom, a fluid-filled and fully-lined body cavity; a body divided into the three parts of head, visceral mass, and muscular foot; and organ systems for circulation, respiration, digestion, excretion, nerve conduction, and reproduction (Towle 1989). However, the gastropod body plan involves a torsion or twisting during larval development whereby the visceral mass twists 180 degrees in relation to the head, bringing the mantle cavity to the anterior of the animal. Thus, the gills, and renal and anal openings are now near the front of the animal. Gastropoda (meaning "stomach-foot") is typified by a large, ventral, muscular foot for locomotion, and a distinct head that has eyes and sensory tentacles.
The varied forms and colors of gastropod shells appeal to the inner aspect of humans; even many gastropods without shells, such as nudibranches, can be brightly colored and beautiful. Some gastropods, such as abalone, also serve as a food delicacy, and the abalone is a source of mother of pearl for decorative use.
Gastropod classification follows a number of systems, but today is typically divided into two subclasses: Orthogastropoda and Eogastropoda (Ponder and Lindberg 1997).
The gastropod body plan is believed to be based on that of ancestral mollusks, with the difference that most of the body behind the head (mantle, mantle cavity, visceral mass) underwent torsion, twisting it 180 degrees, and bringing the mantle cavity, gills, and renal and anal openings forward near the head. Some species are twisted counterclockwise and some clockwise. Similarly, the shell is coiled asymmetrically, with reach coil beneath the preceding. The gastropod is able to bring its head into the mantle cavity when endangered (Towle 1989).
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 (Götting 1994). The larval shell of a gastropod is called a protoconch.
The feet of some gastropods are modified. The pteropods, or "sea butterflies," have a winglike flap, which allows them to swim on the surface of the sea (Towle 1989).
Gastropods have an open circulatory system, meaning that the blood is not enclosed entirely within blood vessels, but is released into spaces in tissues. It is collected in the gills or heart, and then pumped through a heart back into the blood-filled space, called a hemocoel or blood cavity.
Most gastropods utilize for feeding a radula, which is a tongue-like ribbon of tissue with small teeth located back of the mouth, which act like a saw in scraping food from surfaces. The radula is usually adapted to the food that a species eats. Aquatic snails use it to scrape algae and terrestrial snails to saw off leaves of garden plants (Towle 1989). The simplest gastropods are the limpets and abalones, both herbivores that use their hard radulas to rasp at seaweeds on rocks. Carnivorous oyster drills use the radula to bore through oyster shells to feed on the inner tissue. Not all gastropods have radulas. Marine gastropods include herbivores, detritus feeders, carnivores, and a few ciliary feeders in which the radula is reduced or absent.
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 mollusks 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. 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 terrestrial species have developed lungs. Gastropods with lungs all belong to one group, Pulmonata.
Sea slugs are often flamboyantly colored, 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.
The first gastropods are considered to have been 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.
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.
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 Paleozoic era are too poorly preserved for accurate identification. Still, the Silurian genus Poleumita contains 15 identified species. Fossil gastropods are less common during the Paleozoic era than bivalves.
|Paleozoic era (542 - 251 mya)|
Most of the gastropods of the Paleozoic 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 considered not to be 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 that 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.
|Mesozoic era (251 - 65 mya)|
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.
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.
In their work, which has become a standard reference in the field, Ponder and Lindberg (1997) present the Orthogastropoda as one of two subclasses of the Gastropoda, the other subclass being the Eogastropoda.
This subclass, which one could call the true snails, is defined most briefly as all those gastropods that are not members of Patellogastropoda, the true limpets, or its ancestors. Included are abalone, snails, whelks, cowries, sea slugs (nudibranches), winkels, cones, and so forth, as well as keyhole limpets.
Orthogastropods form a clade, supported by unambiguous synapomorphies. These synapomorphies (a series of characteristics that appear in its members, but not in the other forms it diverged from) are the identifying characteristics of the clade.
Some of the characteristics are:
True limpets are marine gastropods with flattened, cone-shaped shells in the order Patellogastropoda in the subclass Eogastropoda, the other subclass of Gastropoda, along with Orthogastropoda.
Limpets live throughout the intertidal zone, from the high zone (upper littoral) to the shallow subtidal on the rocky coasts of most oceans. Limpets can be commonly found attached to rocks, looking like little disks or bumps on the rock surface. They attach themselves to the substratum using pedal mucus and a muscular "foot," which enables them to remain attached through dangerous wave action and which also seals against the rock to protect from desiccation during low tide.
Limpets forage by grazing on algae found on rock surfaces. They scrape films of algae from the rock with a radula. Limpets move by rippling the muscles of their foot in a wave-like motion.
Some limpets have a hole at the top, through which gaseous exchange can occur. Most limpets are less than 3 inches (8 centimeters) long, but a West Mexican Limpet grows to be 8 inches (20 centimeters).
Limpets found on sheltered shores (limpets that are less frequently in contact with wave action, and thus less frequently in contact with water) have a greater risk of desiccation due to the effects of sunlight, water evaporation, and the wind. To avoid drying out they will clamp to the rock they inhabit, minimizing water-loss from the rim around their base. As this occurs chemicals are released that promote the vertical growth of the limpet's shell.
Some species of limpets exhibit homing behavior, returning to the same spot on the rock, known as a "home scar," just before the tide recedes. In such species, the shape of their shell often grows to precisely match the contours of the rock surrounding the scar. This behavior presumably allows them to form a better seal to the rock and may help protect from either predation or desiccation. It is still unclear how limpets find their way back to the same spot each time, but it is thought that they follow a mucus trail left as they move, and utilize their tentacles.
Other species, notably Lottia gigantea seem to "garden" a patch of algae around their home scar. They will aggressively push other organisms out of this patch by ramming with their shell, thereby allowing their patch of algae to grow for their own grazing. Also, where the limpets eat the algae off of bare rocks, it causes places where other organisms can grow and thrive.
Limpets are preyed upon by a variety of organisms including starfish, shore-birds, fish, seals, and humans. Limpets exhibit a variety of defenses, such as fleeing or clamping their shells against the substratum. The defense response can be determined by the type of predator, which can often be detected chemically by the limpet.
Limpets can be long lived, with tagged specimens surviving for more than ten years.
The taxonomy of the Gastropoda is under constant revision, but more and more of the old taxonomy is being abandoned. 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. With the arrival of DNA-sequencing, further revisions of the higher taxonomic levels are to be expected in the near future.
The traditional classification recognized four subclasses. :
According to the newest insights (Ponder & Lindberg, 1997), the taxonomy of the Gastropoda should be rewritten in terms of strictly monophyletic groups. They recognize the subclasses of Orthogastropoda and Eogastropoda.
Integrating new 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.
Class Gastropoda (Cuvier, 1797)
Subclass Eogastropoda (Ponder & Lindberg, 1996) (earlier: Prosobranchia)
Subclass Orthogastropoda Ponder & Lindberg, 1996 (earlier Prosobranchia, Opisthobranchia)
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
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