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Sunday, January 20, 2013

the phylum mollusca

The Phylum Mollusca

Etymology:- From the Latin Molluscus meaning soft of body.

Characteristics of Mollusca:-
1)Bilaterally symmetrical.
2)Body has more than two cell layers, tissues and organs.
3)Body without cavity.
4)Body possesses a through gut with mouth and anus.
5)Body monomeric and highly variable in form, may possess a dorsal or lateral shells of protein and calcareous spicules.
6)Has a nervous system with a circum-oesophagal ring, ganglia and paired nerve chords.
7)Has an open circulatory system with a heart and an aorta. 

8)Has gaseous exchange organs called ctenidial gills.
9)Has a pair of kidneys.
10)Reproduction normally sexual and gonochoristic.
11)Feed a wide range of material.
12)Live in most environments.


After the Arthropods the Molluscs are the most successful of the animal phyla in terms of numbers of species. There are about 110,000 species known to science most of which are marine. They occupy a vast range of habitats however both aquatic and terrestrial, from the arctic seas to small tropical streams and from valleys to mountainsides 7,000 metres high, there are a few adapted to live in deserts and some are parasitic. They also exhibit an enormous range in size, from species which are almost microscopic to the largest of all invertebrates the giant squid which can weighs 270 kg and measures up to 12 metres long in the body, with tentacles as much as another 50 metres in length. Many species are common and many more a beautiful. Most species secrete a shell of some sort, these shells are long lasting and have been collected by human beings for thousands of years, some of these shells, and the pearls which come from oysters, which are also molluscs may be among the earliest forms of money.

Most molluscs are marine. Molluscs are very ancient organisms believed to have evolved from a flatworm like ancestor during the Precambrium about 650 million years ago. Because many species secrete a shell of some sort the fossil record is good. Different classes of molluscs have been predominant in the past and the Ammonites represent a group of Cephalopods which were extremely abundant for millions of years before they became extinct. There close relatives the Nautiloid cephalopods were also once very successful but are now only represented in the world by one species, Nautilus.
Molluscs, because of their ease of capture, edibility and beauty have long been important to mankind. Molluscs of many sorts are eaten by humans Abilone, Clams, Cockles, Muscles, Octopus, Oysters, Periwinkles, Scallops, Snails, Squid, Whelks, Winkles and many more are all molluscs and all make there contribution to the human diet. Mankind has been deliberately culturing molluscs as food for a long time and the earliest known records of someone farming molluscs for food come from Rome where one Sergius Orata bred oysters.
Mollusc shells have also had a long history of usage by mankind, many have been used as decorations, or as a substance to carve into cameos and buttons. In North America Tusk shells on the west coast and Cockles on the East supplied the basis of a system of money, in many tropical countries the shells of coweries were until recent times used extensively in trade. Pearls, which arise in oysters as a result of the oysters attempts to cover up a grain of sand within its mantle, have been and still are much sort after. The 'mother of pearl' used to make pearl buttons comes from bivalve shells and so great was the market for it that the Mississippi and Missouri river basins have been seriously over collected and the bivalves are now quite scarce. In ancient times the city of Tyre was famous for its purple dye, this dye was made from a marine mollusc called Murex sp. while Sepia, a brown pigment used by artists was, perhaps still is, made from the ink of Cuttlefish. Not all the interactions between man and molluscs are to man's benefit however, slugs and snails are, in some places, serious pests of of crops, and are often a nuisance in peoples gardens. Wooden ships and wharves can be destroyed by burrowing bivalves such as Teredo navalis, known as ship worms, which weaken the timbers until they collapse or fall apart.

General Anatomy

Although the original ancestor of the molluscs is lost in the dawn of time scientists have theorised that the original mollusc arose from a flatworm (Platyhelminth) like organism, the similarities are listed in table 1.
Table 1. Comparison of similarities of Molluscs and Turbellarian Platyhelminthes
CharacteristicMolluscsTurbellarian Platyhelminth
Externally ciliated.YesYes
Movement by cillial gliding or ventral muscular wave.YesYes
Possession of mucous glands.YesNo, but Rhabdites very similar
Intracellular digestion.YesYes
Cuticle absent.YesYes
Setae absent.YesYes
Though the modern molluscs show quite a wide degree of adaptable variability in form, there are several basic anatomical characteristics that can be found in all or most of them. The body is divided into two functional regions, the head-foot and the visceral lump. The head-foot is the part you see most easily in slugs and snails. It is mostly a muscular organ covered in cilia and rich in mucous cells, which the mollusc uses to move around, it normally tapers to a tail at one end and has a head incorporated in the front. The head includes a mouth, eyes and tentacles, the last two may be much reduced or even absent. In those species with shells the head-foot can be drawn into the shell. The rest of the body is the visceral mass, this is entirely nonmuscular and contains the organs of digestion and reproduction, it includes the gonads, the kidney, the heart and the digestive diverticulum.
Attached to the dorsal surface of the visceral mass is and hanging freely down the sides of it is the mantle, often called the skirt or pallium. There is a space between the mantle and the viseral mass, this space is greatest towards the rear of the animal where it is called the mantle cavity or the pallial cavity. The mantle cavity generally contains the gills or ctenidia, a water current, generated by beating cillia, enters the mantle cavity at the sides, passes over the gills and departs centrally, i.e. the outward bound current runs out between the two inward bound currents. Near the head, just behind the mouth is a pair or more of ganglia and a nerve ring from which two nerve chords arise that reach out through the body. Molluscs are true coelomic animals, though the coelom they have is small, enclosing only the gonads and the heart where it is called the gonodial cavity and the pericardial cavity respectively.
This then is the plan of a basic unevolved mollusc. This basic plan is changed and adapted, for the requirements of different lifestyles, almost beyond recognition in some of the 6 classes of Mollusca. Hopefully I will get something written about each class in the not too distant future.
The Classes of Phylum Mollusca
AmphineuraNeopilina galatheae
GastropodaCowries, Limpets, Slugs and Snails
ScaphopodaTusk Shells
BivalviaBivalves = Muscles, Clams etc.
CephalopodaNautilus, Octopus and Squid


  • Free living aquatic forms (freshwater or marine), some amphibious.
  • Body is soft and unsegmented enclosed in a glandular mantle covered by usually a shell.
  • Coelom is reduced to a pericardial cavity around the heart.
  • Digestive system is complete.
  • Respiratory organs are in the form of gills called ctenidia.
  • Locomotor structure is represented by muscular foot.
  • They have soft-body, which is differentiated into three regions - head, visceral hump and foot.
  • Visceral hump is covered by a thin, fleshy fold called mantle.
  • Mantle secretes a calcarious shell, which may be external or internal.
  • Reproduction is sexual. Either unisexual or bisexual, development is direct.
            fig.10.18 - Examples of Mollusca
The phylum is divided into six classes
 Class Habitat Nature of shell Nature of foot Example
 Scaphopoda Marine long, cylindrical, open at both the ends Reduced dopeted for digging Dentalium
 Monoplacophora Marine Made up of only one large plate Short, reduced Neopilina
 Polyplaphora Marine Made up of many overlapping plates Elongated ventrial in position Chilton
 GastropodaMarine or fresh water Made up of a single valve, coiled Ventralin position Pila Limnaea
 Pelecypoda Marine or fresh water Made up of two equal valves Wedge shaped Unio Pearl oyster
 Cephalopoda Marine External or internal or absent Modified into tentacles Sepia Octopus

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The Gastropoda
Snails and slugs, limpets, and sea hares
No. of described species: 62,000
First appearance: Early Cambrian
Habitats: everywhere on Earth
Shapes: everything you can think of
Feeding types: they'll eat it all
Partula taeniata
Partula taeniata, a tree snail from Moorea, French Polynesia.
Gastropods are one of the most diverse groups of animals, both in form, habit, and habitat. They are by far the largest group of molluscs, with more than 62,000 described living species, and they comprise about 80% of living molluscs. Estimates of total extant species range from 40,000 to over 100,000, but there may be as many as 150,000 species! There are about about 13,000 named genera for both Recent and fossil gastropods. They have a long and rich fossil record from the Early Cambrian that shows periodic extinctions of subclades, followed by diversification of new groups.
Gastropods have figured prominently in paleobiological and biological studies, and have served as study organisms in numerous evolutionary, biomechanical, ecological, physiological, and behavioral investigations.
They are extremely diverse in size, body and shell morphology, and habits and occupy the widest range of ecological niches of all molluscs, being the only group to have invaded the land.
Fossil record
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Life history & ecology
Gastropods live in every conceivable habitat on Earth. They occupy all marine habitats ranging from the deepest ocean basins to the supralittoral, as well as freshwater habitats, and other inland aquatic habitats including salt lakes. They are also the only terrestrial molluscs, being found in virtually all habitats ranging from high mountains to deserts and rainforest, and from the tropics to high latitudes.
Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior. They include grazers, browsers, suspension feeders, scavengers, detritivores, and carnivores. Carnivory in some taxa may simply involve grazing on colonial animals, while others engage in hunting their prey. Some gastropod carnivores drill holes in their shelled prey, this method of entry having been acquired independently in several groups, as is also the case with carnivory itself. Some gastropods feed suctorially and have lost the radula.
Variation in shell morphology in some marine gastropods
Variation in shell morphology in some marine gastropods: Clockwise from top left, Calliostoma antonii from off the Pacific coast of Panama; the dovesnail Columbella strombiformis from offshore near San Carlos, Mexico; the moonsnail Natica elenae from the Gulf of Panama; Oliva peruviana from shallow water near Iquique, Chile; the turbinid Guildfordia triumphans from Wakayama, Japan; the muricid Vokesimurex elenensis from the Pacific coast of Mexico; a wentletrap, Epitonium scalare, from the Phillippines; and another muricid, Pteropurpura trialata, from offshore near Los Angeles, California.
Most aquatic gastropods are benthic and mainly epifaunal but some are planktonic. A few such as the violet snails (Janthinidae) and the sea lizards (Glaucus) drift on the surface of the ocean where they feed on floating siphonophores, while others (heteropods and Gymnosomata) are active predators swimming in the plankton. Some snails (such as the whelk Syrinx aruanus) reach about 600 mm in length. There is also a very large (and poorly known) fauna of microgastropods that live in marine, freshwater and terrestrial environments. It is amongst these tiny snails (0.5-4 mm) where many of the undescribed species lie.
Most gastropods have separate sexes but some groups (mainly the Heterobranchia) are hermaphroditic. Most hermaphroditic forms do not normally engage in self-fertilization. Basal gastropods release their gametes into the water column where they undergo development; derived gastropods use a penis to copulate or exchange spermatophores and produce eggs surrounded by protective capsules or jelly (see Busycon spiratus photo below).
The first gastropod larval stage is typically a trochophore that transforms into a veliger and then settles and undergoes metamorphosis to form a juvenile snail. While many marine species undergo larval development, there are also numerous marine taxa that have direct development, this mode being the norm in freshwater and terrestrial taxa. Brooding of developing embryos is widely distributed throughout the gastropods, as are sporadic occurrences of hermaphrodism in the non-heterobranch taxa.
The basal groups have non-feeding larvae while veligers of many neritopsines, caenogastropods, and heterobranchs are planktotrophic. Egg size is reflected in the initial size of the juvenile shell or protoconch and this feature has been useful in distinguishing feeding and non-feeding taxa in both Recent and fossil taxa.
More on morphology
Gastropods are characterized by the possession of a single (often coiled) shell, although this is lost in some slug groups, and a body that has undergone torsion so that the pallial cavity faces forwards. They have a well-developed head bearing a pair of cephalic tentacles and eyes that are primitively situated near the outer bases of the tentacles. In some taxa the eyes are located on short to long eye stalks. The mantle edge in some taxa is extended anteriorly to form an inhalant siphon and this is sometimes associated with an elongation of the shell opening (aperture) — this is shown in the photo of the caenogastropod Conus bullatus below. The foot is usually rather large and is typically used for crawling. It can be modified for burrowing, leaping (as in conchs, Strombidae), swimming, or clamping (as in limpets). The foot typically bears an operculum that seals the shell opening (aperture) when the head-foot is retracted into the shell (see photos below). While this structure is present in all gastropod veliger larvae, it is absent in the embryos of some direct developing taxa and in the juveniles and adults of many heterobranchs. The nervous and circulatory systems are well developed with the concentration of nerve ganglia being a common evolutionary theme.
The shell is typically coiled, usually dextrally, the axis of coiling being around a central columella to which a large retractor muscle is attached. The uppermost part of the shell is formed from the larval shell (the protoconch). The shell is partly or entirely lost in the juveniles or adults of some groups, with total loss occurring in several groups of land slugs and sea slugs (nudibranchs).
Gastropod opercula, egg capsules, and protoconch
From left, a whelk, Busycon spiratus, almost entirely out of its shell — the yellowish disc is the operculum; another Busycon spiratus individual, entirely withdrawn inside its shell, with the operculum sealing off the aperture; egg capsules being deposited on the sand by Busycon spiratus; a larval harp shell, Morum oniscus, begins building its shell (protoconch).
Externally, gastropods appear to be bilaterally symmetrical. However, they are one of the most successful clades of asymmetric organisms known. The ancestral state of this group is clearly bilateral symmetry (e.g., chitons, cephalopods, bivalves), but gastropod molluscs twist their organ systems into figure-eights, differentially develop or lose organs on either side of their midline, and generate shells that coil to the right or left. The best documented source of gastropod asymmetry is the developmental process known as torsion.
There is still controversy about the phylogenetic position of some gastropod clades. Though the clades discussed below are well supported in many modern analyses, their relationships to each other remain somewhat unclear.
The neritopsines Nerita fulgurans (left) and Nerita versicolor (right).
In particular, the Neritopsina are placed below the Vetigastropoda in some analyses (thus they become the sister group of Vetigastropoda + Caenogastropoda + Heterobranchia). Also, the enigmatic taxon Cocculinidae is still uncertain. It may actually be a member of the Neritopsina clade, as some characters indicate. Most likely it is the sister clade to Neritopsina, though.
Neritopsina contains several families which have marine, freshwater, and terrestrial members. The largest family, Neritidae, includes many marine, brackish, and freshwater lineages. This family alone has probably invaded freshwater habitats at least six times (Holthuis 1995). The two terrestrial families, Helicinidae and Hydrocenidae, can be found as far back as the Devonian.
Neritopsines come in all shapes and sizes and can have coiled to limpet-shaped shells, with one species (Titiscania) being a slug. This group was previously included within the "Archaeogastropoda." The shell is never nacreous and an operculum is present in adults. The radula has many teeth in each row.
The Vetigastropoda is a diverse group that includes the keyhole and slit-limpets (Fissurellidae), abalones (Haliotiidae), slit shells (Pleurotomariidae), the top shells (trochids), and about 10 other families.
All are marine, and have coiled to limpet-shaped shells. This group was also previously included within the "Archaeogastropoda." The shell is nacreous in many of these taxa and an operculum is usually present. The radula has many teeth in each row.
Assorted vetigastropods
Assorted vetigastropods: from left, abalone; Puncturella longifissa, a keyhole limpet — note the hole or "keyhole" just above the apex of the shell through which water is expelled; Margarites marginatus, a trochid; and the radula of the vetigastropod snail Sinezona rimuloides, greatly magnified.
Caenogastropoda is a very large, diverse group containing about 100 mostly marine families. Familiar groups include the littorines (Littorinidae), cowries (Cypraeidae), creepers (Cerithiidae, Batellariidae, and Potamididae), worm snails (Vermetidae), moon snails (Naticidae), frog shells (Ranellidae and Bursidae), apple snails (Ampullariidae) and a large, almost entirely marine group of about 20 families that are all carnivores belonging to the clade Neogastropoda. These neogastropods include whelks (Buccinidae), muricids (Muricidae), volutes (Volutidae), harps (Harpidae), cones (Conidae), and augers (Terebridae). Caenogastropod shells are typically coiled, a few being limpet-like (e.g., the slipper limpets, Calyptraeidae). One family (Vermetidae) has shells resembling worm-tubes. While most caenogastropods possess a shell that encloses the animal, it is reduced in some and has become a small internal remnant in the slug-like Lamellariidae. Eulimidae are all parasitic on echinoderms, most being shelled ectoparasites but some have become shell-less, worm-like internal parasites. Some groups have invaded freshwater, the most important being the Viviparidae, Ampullariidae, Thiaridae (and several closely related families), and smaller-sized snails belong to the very diverse families Hydrobiidae, Bithyniidae, and Pomatiopsidae. There are a few terrestrial taxa, the cyclophorids being the most significant family.
Caenogastropods were previously comprised of the "Mesogastropoda" and "Neogastropoda" within the "Prosobranchia." Of these two groups only the Neogastropoda remains as a monophyletic group. The shell is never nacreous and an operculum is present in adults. Apart from members of the Neogastropoda, the radula usually has only seven teeth in each row. The radula of neogastropods has five to one tooth in each row and is absent in some species.
Assorted caenogastropods
Assorted caenogastropods: clockwise from top left, the cowrie Cypraea leucodon from the Phillippines, about 7.3 cm in length; the moon snail Bulbus fragilis from off the Aleutian Islands; the ranellid Charonia lampas from the Canary Islands; the apple snail Pomacea gigas, 14 cm in width, from a riverbank in Peru; the conid Conus bullatus from the Marquesas Islands; the volute Arctomelon tamikoae and the muricid Boreotrophon alaskanus, both from off the Aleutian Islands; and the whelk Busycon sinistrum, 17 cm long, from off the northeast coast of Florida.
Heterobranchia is a very large group that has only recently been recognized as a clade within Gastropoda. Several marine and one freshwater group (Valvatidae) that were previously included in the "Mesogastropoda" and two very large groups previously given subclass status, the Opisthobranchia and Pulmonata (collectively the Euthyneura), were found to be related lineages in a recent phylogenetic analysis. The more basal members comprise about a dozen families that are mostly small-sized, poorly-known operculate groups.
The opisthobranchs comprise about 25 families and 2000 species of the bubble shells (many families) and the sea slugs (many families) as well as the sea hares (Aplysiidae). Virtually all opisthobranchs are marine with the majority showing shell reduction or shell loss and only some of the "primitive" shell-bearing taxa having an operculum as adults.
The pulmonates comprise the majority of land snails and slugs, a very diverse group comprising many families and about 20,000 species. A few marine pulmonates (including the limpet-shaped Siphonariidae) comprise groups that mostly inhabit estuaries. A basal group of mainly estuarine air breathing slugs (Onchidiidae) also has terrestrial relatives (Veronicellidae, Rathouisiidae). Some important groups of freshwater snails are also included here — the Lymnaeidae, Planorbidae, Physidae and Ancylidae. The operculum is absent in all pulmonates except the estuarine Amphibolidae and the freshwater Glacidorbidae. The shells of heterobranchs are never nacreous.
Assorted heterobranchs
Assorted heterobranchs: clockwise from top left, the bubble shell Hydatina physis from the Canary Islands; the Clown Nudibranch, Triopha catalinae, from off the Santa Barbara coast; the Ragged Sea Hare, Bursatella leachii, from off the Florida coast; the siphonariid Physella heterostropha from northeastern Florida; Euglandina rosea, a carnivorous terrestrial spiraxid pulmonate from Florida; the Florida Leatherleaf, Leidyula floridana, a veronicellid from Florida, about 8 cm long; and the pond snail Lymnaea stagnalis.
The File Limpet, Lottia limulata (left), and a group of Colisella (Lottia?) sp. limpets, from the intertidal off California.
A visit to almost any rocky intertidal habitat in the world will reveal these wonderful, cap-shaped gastropods, the true limpets. Tenaciously clinging to the rocks with their hard shells to protect them, they have many different behaviors in their environments associated with their feeding strategies. But the true limpets are not restricted to the intertidal, they can be found beneath the waves, in the deep sea associated with hydrothermal vent habitats, and there are even some species which live exclusively on drift-wood that has sunk to the bottom of the ocean.
All are marine and limpet-shaped and many live in the intertidal zone. This group was previously included within the "Archaeogastropoda." The shell is nacreous in some taxa and the operculum is absent in adults. Their radula has several teeth in each row, some of which are strengthened by the incorporation of metallic ions such as iron.
Cocculinids are a group of simple white limpets that occur on waterlogged wood and other organic substrates in the deep sea.
The relationships of Cocculinidae are unclear. Several recent phylogenetic analyses place them as closely related to the Neritopsina, or as the sister group to the clade that includes Caenogastropoda and Neritopsina. Some authors believe, however, that they are members of the Neritopsina. Further systematic research is needed to clarify the relationships of this enigmatic group.

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The Mollusca
Sea slugs, squid, snails, and scallops

An introduction
A cuttlefish, a coleoid cephalopod, moves primarily by undulating its body fins.
Mollusca is one of the most diverse groups of animals on the planet, with at least 50,000 living species (and more likely around 200,000). It includes such familiar organisms as snails, octopuses, squid, clams, scallops, oysters, and chitons. Mollusca also includes some lesser known groups like the monoplacophorans, a group once thought to be extinct for millions of years until one was found in 1952 in the deep ocean off the coast of Costa Rica.
Molluscs are a clade of organisms that all have soft bodies which typically have a "head" and a "foot" region. Often their bodies are covered by a hard exoskeleton, as in the shells of snails and clams or the plates of chitons.
A part of almost every ecosystem in the world, molluscs are extremely important members of many ecological communities. They range in distribution from terrestrial mountain tops to the hot vents and cold seeps of the deep sea, and range in size from 20-meter-long giant squid to microscopic aplacophorans, a millimeter or less in length, that live between sand grains.
These creatures have been important to humans throughout history as a source of food, jewelry, tools, and even pets. For example, on the Pacific coast of California, Native Americans consumed large quantities of abalone and especially owl limpets. However, the impact of Native Americans on these molluscan communities pales by comparison to the overharvesting of some molluscan taxa by the United States in the 1960s and 1970s. Species whose members once numbered in the millions, now teeter on the verge of extinction. For example, fewer than 100 white abalone remain after several million individuals were captured and sold as meat in the 1970s. Besides having yummy soft parts, molluscs often have desirable hard parts. The shells of some molluscs are considered quite beautiful and valuable. Molluscs can also be nuisances, such as the common garden snail; and molluscs make up a major component of fouling communities both on docks and on the hulls of ships.

California Trivia and Giant Squid
On the left is a marine snail, the California Trivia (Trivia californiana). Here the mantle covers much of the shell. Note how a portion of the mantle is rolled into a tube shape to form the siphon just above the head. At the right is a restoration of one of the largest of all molluscs, the Giant Squid (Architeuthis).

They also have a very long and rich fossil record going back more than 550 million years, making them one of the most common types of organism used by paleontologists to study the history of life.

Molluscan systematics are still in flux. As you can see from the cladogram below, there is still no agreement on some of the major relationships. The polytomies shown indicate that the question of which molluscs are the most closely related is still a matter of debate.
However, new types of data and much larger and more sophisticated analyses continue to be performed. The resolved relationships shown (such as cephalopods, scaphopods, and gastropods) are recent discoveries.
Visit the mollusca pages on the Tree of Life for more on molluscan systematics.
Despite their amazing diversity, all molluscs share some unique characteristics that define their body plan. The body has a head, a foot and a visceral mass. This is all covered with a mantle (also known as a pallium) that typically secretes the shell. In some groups, like slugs and octopuses, the mantle is secondarily lost, while in others, it is used for other activities, such as respiration.

Radula of the Sinistral Pond Snail
The freshwater Sinistral Pond Snail (Physellasp.) scrapes algae from the glass with its radula, the two "toothy" arcs you can see lining the mouth. Click on the photo for a closer look.

The buccal cavity, at the anterior of the mollusc, contains a radula (lost in bivalves) — a ribbon of teeth supported by an odontophore, a muscular structure. The radula is generally used for feeding. The ventral foot is used in locomotion. This foot propels the mollusc by utilizing muscular waves and/or cilia in combination with mucus.

Typically, at least in the more primitive members of each group, there are one or more pairs of gills (called ctenidia) which lie in a posterior cavity (the pallial cavity) or in a posterolateral groove surrounding the foot. The pallial cavity typically contains a pair of sensory osphradia (for smelling) and is the space into which the kidneys, gonads, and anus open.
Molluscs are coelomate, although the coelom is reduced and represented by the kidneys, gonads, and pericardium, the main body cavity which surrounds the heart.
Life history and ecology
Molluscs occur in almost every habitat found on Earth, where they are often the most conspicuous organisms. While most are found in the marine environment, extending from the intertidal to the deepest oceans, several major gastropod clades live predominantly in freshwater or terrestrial habitats. Remarkably, one study found around 3000 species within a single locality at a coral reef in New Caledonia. In terrestrial communities, gastropods can achieve reasonably high diversity and abundance: as many as 60-70 species may coexist in a single habitat and abundance in leaf litter can exceed more than 500 individuals in four liters of litter.

Eggs of the Sinistral Pond Snail
Many marine molluscs emerge from their eggs as planktonic trochophore larvae, however, Sinistral Pond Snails (Physella sp.) emerge from their eggs as young snails. The whitish, jellybean-shaped organisms are ostracodes (crustaceans).
Marine molluscs occur on a large variety of substrates including rocky shores, coral reefs, mud flats, and sandy beaches. Gastropods and chitons are characteristic of these hard substrates, and bivalves are commonly associated with softer substrates where they burrow into the sediment. However, there are many exceptions: the largest living bivalve, Tridacna gigas, lives on coral reefs, and many bivalves (e.g., mussels and oysters) attach themselves to hard substrates. Some microscopic gastropods even live interstitially between sand grains.
Large concentrations of gastropods and bivalves are found at hydrothermal vents in the deep sea. Living in these or other dysoxic habitats appears to be a plesiomorphic condition for the Mollusca and severaloutgroups. For example, the fauna of Palaeozoic hydrothermal vent communities includes the molluscan groups BivalviaMonoplacophora and Gastropoda as well as the outgroups Brachiopoda and Annelida.
The adoption of different feeding habits appears to have had a profound influence on molluscan evolution. The change from grazing to other forms of food acquisition is one of the major features in the radiation of the group. Based on our current understanding of relationships, the earliest molluscs grazed on encrusting animals and detritus. Such feeding may have been selective or indiscriminate and will have encompassed algal, diatom, or cyanobacterial films and mats, or encrusting colonial animals. Truly herbivorous grazers are relatively rare and are limited to some polyplacophorans and a few gastropod groups. Most chaetodermomorph aplacophorans, monoplacophorans and scaphopods feed on protists and/or bacteria while neomeniomorph aplacophorans graze on cnidarians. Cephalopods are mainly active predators as are some gastropods, while a few chitons and septibranch bivalves capture microcrustaceans. Most bivalves are either suspension or deposit feeders that indiscriminately take in particles, but then elaborately sort them based on size and weight, typically assimilating bacteria, protists, and diatoms.
The fossil record
The Mollusca include some of the oldest metazoans known. Late Precambrian rocks of southern Australia and the White Sea region in northern Russia contain bilaterally symmetrical, benthic animals with a univalved shell (Kimberella) that resembles those of molluscs. The earliest unequivocal molluscs are helcionelloid molluscs that date from Late Ediacaran (Vendian) rocks. In the Early Cambrian the Coeloscleritophora are also present. Most of the familiar groups, including gastropods, bivalves, monoplacophorans, and rostroconchs, all date from the Early Cambrian, whereas cephalopods are first found in the Middle Cambrian, polyplacophorans in the Late Cambrian, and the Scaphopoda in the MiddleOrdovician. Most of these early taxa tend to be small (‹10 mm in length). The Late Vendian-Early Cambrian taxa bear little resemblance to the Cambrian-Ordovician taxa (most of which remain extant today).

Inoceramus and Turritella
On the left is Inoceramus sp., a bivalve from the Cretaceous of Alameda County, CA. At right is Turritella andersoni, a gastropod from the Eocene of Ventura County, CA.

After their initial appearance, molluscan taxonomic diversity tended to remain low until the Ordovician, when gastropods, bivalves, and cephalopods show large increases in diversity. For bivalves and gastropods this diversification increases throughout the Phanerozoic, with relatively small losses at the end-Permian and end-Cretaceous extinction events. Cephalopod diversity is much more variable through the Phanerozoic, whereas the remaining groups (monoplacophorans, rostroconchs, polyplacophorans, and scaphopods) maintain low diversity over the entire Phanerozoic or became extinct.

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