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Diversity[1]

85,000 recognized living species.

Cornu aspersum
Cornu aspersum
(formerly Helix aspersa) – a common land snail

Mollusca
Mollusca
is a large phylum of invertebrate animals whose members are known as molluscs or mollusks[Note 1] (/ˈmɒləsk/). Around 85,000 extant species of molluscs are recognized.[2] The number of fossil species is estimated between 60,000 and 100,000 additional species.[3] Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct. Cephalopod
Cephalopod
molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small. The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian
Cambrian
period 541 to 485.4 million years ago[dead link]. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa
Lophotrochozoa
and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists. Molluscs have been and still are an important food source for anatomically modern humans. There is a risk of food poisoning from toxins which can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple
Tyrian purple
dye, and sea silk. Their shells have also been used as money in some preindustrial societies. Mollusc
Mollusc
species can also represent hazards or pests for human activities. The bite of the blue-ringed octopus is often fatal, and that of Octopus
Octopus
apollyon causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems.

Contents

1 Etymology 2 Definition 3 Diversity 4 Hypothetical ancestral mollusc

4.1 Mantle and mantle cavity 4.2 Shell 4.3 Foot 4.4 Circulatory system 4.5 Respiration 4.6 Eating, digestion, and excretion 4.7 Nervous system 4.8 Reproduction

5 Ecology

5.1 Feeding

6 Classification 7 Evolution

7.1 Fossil
Fossil
record 7.2 Phylogeny

8 Human interaction

8.1 Uses by humans

8.1.1 Bioindicators

8.2 Harmful to humans

8.2.1 Stings and bites 8.2.2 Disease vectors 8.2.3 Pests

9 Notes 10 References 11 Bibliography 12 Further reading 13 External links

Etymology[edit] The words mollusc and mollusk are both derived from the French mollusque, which originated from the Latin
Latin
molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τα μαλακά (ta malaká), "the soft things", which he applied to cuttlefish. The scientific study of molluscs is accordingly called malacology.[4] The name Molluscoida was formerly used to denote a division of the animal kingdom containing the brachiopods, bryozoans, and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs. As it is now known these groups have no relation to molluscs, and very little to one another, the name Molluscoida has been abandoned.[5] Definition[edit] The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. Many have a calcareous shell.[6] Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies (defining characteristics) to apply to all modern groups.[7] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical.[8] The following are present in all modern molluscs:[9][10]

The dorsal part of the body wall is a mantle (or pallium) which secretes calcareous spicules, plates or shells. It overlaps the body with enough spare room to form a mantle cavity. The anus and genitals open into the mantle cavity. There are two pairs of main nerve cords.[10]

Other characteristics that commonly appear in textbooks have significant exceptions:

  Whether characteristic is found in these classes of Molluscs

Supposed universal Molluscan characteristic[9] Aplacophora[11] Polyplacophora[12] Monoplacophora[13] Gastropoda[14] Cephalopoda[15] Bivalvia[16] Scaphopoda[17]

Radula, a rasping "tongue" with chitinous teeth Absent in 20% of Neomeniomorpha Yes Yes Yes Yes No Internal, cannot extend beyond body

Broad, muscular foot Reduced or absent Yes Yes Yes Modified into arms Yes Small, only at "front" end

Dorsal concentration of internal organs (visceral mass) Not obvious Yes Yes Yes Yes Yes Yes

Large digestive ceca No ceca in some Aplacophora Yes Yes Yes Yes Yes No

Large complex metanephridia ("kidneys") None Yes Yes Yes Yes Yes Small, simple

One or more valves/ shells Primitive forms, yes; modern forms, no Yes Yes Snails, yes; slugs, mostly yes (internal vestigial) Octopuses, no; cuttlefish, nautilus, squid, yes Yes Yes

Odontophore Yes Yes Yes Yes Yes No Yes

Diversity[edit]

About 80% of all known mollusc species are gastropods (snails and slugs), including this cowry (a sea snail).[18]

Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.[1] In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were about 12,000 fresh-water gastropods and 35,000 terrestrial. The Bivalvia
Bivalvia
would comprise about 14% of the total and the other five classes less than 2% of the living molluscs.[19] In 2009, Chapman estimated the number of described living species at 85,000.[1] Haszprunar in 2001 estimated about 93,000 named species,[20] which include 23% of all named marine organisms.[21] Molluscs are second only to arthropods in numbers of living animal species[18]—far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000.[22] About 200,000 living species in total are estimated,[1][23] and 70,000 fossil species,[9] although the total number of mollusc species ever to have existed, whether or not preserved, must be many times greater than the number alive today.[24] Molluscs have more varied forms than any other animal phylum. They include snails, slugs and other gastropods; clams and other bivalves; squids and other cephalopods; and other lesser-known but similarly distinctive subgroups. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems. Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes.[7] About 80% of all known mollusc species are gastropods.[18] Cephalopoda
Cephalopoda
such as squid, cuttlefish, and octopuses are among the neurologically most advanced of all invertebrates.[25] The giant squid, which until recently had not been observed alive in its adult form,[26] is one of the largest invertebrates, but a recently caught specimen of the colossal squid, 10 m (33 ft) long and weighing 500 kg (1,100 lb), may have overtaken it.[27] Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of nonmarine molluscs vary widely, partly because many regions have not been thoroughly surveyed. There is also a shortage of specialists who can identify all the animals in any one area to species. However, in 2004 the IUCN Red List of Threatened Species
Species
included nearly 2,000 endangered nonmarine molluscs. For comparison, the great majority of mollusc species are marine, but only 41 of these appeared on the 2004 Red List. About 42% of recorded extinctions since the year 1500 are of molluscs, consisting almost entirely of nonmarine species.[28] Hypothetical ancestral mollusc[edit] Further information: Mollusc
Mollusc
shell

Anatomical
Anatomical
diagram of a hypothetical ancestral mollusc

Because of the great range of anatomical diversity among molluscs, many textbooks start the subject of molluscan anatomy by describing what is called an archi-mollusc, hypothetical generalized mollusc, or hypothetical ancestral mollusc (HAM) to illustrate the most common features found within the phylum. The depiction is visually rather similar to modern monoplacophorans, and some suggest it may resemble very early molluscs.[7][10][13][29] The generalized mollusc is bilaterally symmetrical and has a single, "limpet-like" shell on top. The shell is secreted by a mantle covering the upper surface. The underside consists of a single muscular "foot".[10] The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs.[8] Mantle and mantle cavity[edit] The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according to habitat, to sea, fresh water or air. The cavity was at the rear in the earliest molluscs, but its position now varies from group to group. The anus, a pair of osphradia (chemical sensors) in the incoming "lane", the hindmost pair of gills and the exit openings of the nephridia ("kidneys") and gonads (reproductive organs) are in the mantle cavity.[10] The whole soft body of bivalves lies within an enlarged mantle cavity.[8] Shell[edit] Main article: Mollusc
Mollusc
shell The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs[8]) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate),[10][30] except the outermost layer, which in almost all cases is all conchiolin (see periostracum).[10] Molluscs never use phosphate to construct their hard parts,[31] with the questionable exception of Cobcrephora.[32] While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells.[33] The shell consists of three layers: the outer layer (the periostracum) made of organic matter, a middle layer made of columnar calcite, and an inner layer consisting of laminated calcite, often nacreous.[8] Foot[edit]

Play media

A 50-second video of snails (most likely Natica chemnitzi
Natica chemnitzi
and Cerithium
Cerithium
stercusmuscaram) feeding on the sea floor in the Gulf of California, Puerto Peñasco, Mexico

The underside consists of a muscular foot, which has adapted to different purposes in different classes.[34]:4 The foot carries a pair of statocysts, which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement. In forms having only a top shell, such as limpets, the foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell.[10] In bivalves, the foot is adapted for burrowing into the sediment;[34]:4 in cephalopods it is used for jet propulsion,[34]:4 and the tentacles and arms are derived from the foot.[35] Circulatory system[edit] Molluscs' circulatory systems are mainly open. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton.[8] The blood contains the respiratory pigment hemocyanin as an oxygen-carrier. The heart consists of one or more pairs of atria (auricles), which receive oxygenated blood from the gills and pump it to the ventricle, which pumps it into the aorta (main artery), which is fairly short and opens into the hemocoel.[10] The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coelom as urine. A pair of nephridia ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.[10] Respiration[edit] Most molluscs have only one pair of gills, or even only a singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart.[10]

Eating, digestion, and excretion[edit]

Snail
Snail
radula at work   = Food       = Radula   = Muscles   = Odontophore
Odontophore
"belt"

Members of the mollusc family use intracellular digestion to function. Most molluscs have muscular mouths with radulae, "tongues", bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. The radula primarily functions to scrape bacteria and algae off rocks, and is associated with the odontophore, a cartilaginous supporting organ.[8] The radula is unique to the molluscs and has no equivalent in any other animal. Molluscs' mouths also contain glands that secrete slimy mucus, to which the food sticks. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so the mucus forms a long string called a "food string".[10] At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone of feces and mucus, which is rotated by further cilia so it acts as a bobbin, winding the mucus string onto itself. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it.[10] The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum (a pouch with no other exit) to be digested. The sorting process is by no means perfect.[10] Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large. The anus, in the part of the mantle cavity, is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems.[10] As the head has largely disappeared in bivalves, the mouth has been equipped with labial palps (two on each side of the mouth) to collect the detritus from its mucus.[8] Nervous system[edit]

Simplified diagram of the mollusc nervous system

The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control the foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar.[10] The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. The bivalves have only three pairs of ganglia— cerebral, pedal, and visceral— with the visceral as the largest and most important of the three functioning as the principal center of "thinking". Some such as the scallops have eyes around the edges of their shells which connect to a pair of looped nerves and which provide the ability to distinguish between light and shadow. Reproduction[edit] See also: Reproductive system
Reproductive system
of gastropods and Reproductive system
Reproductive system
of cephalopods

Apical tuft (cilia) Prototroch (cilia) Stomach Mouth Metatroch (cilia) Mesoderm Anus /// = cilia

Trochophore
Trochophore
larva[36]

The simplest molluscan reproductive system relies on external fertilization, but with more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. Two gonads sit next to the coelom, a small cavity that surrounds the heart, into which they shed ova or sperm. The nephridia extract the gametes from the coelom and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization. Some molluscs use internal fertilization and/or are hermaphrodites, functioning as both sexes; both of these methods require more complex reproductive systems.[10] The most basic molluscan larva is a trochophore, which is planktonic and feeds on floating food particles by using the two bands of cilia around its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. New tissue grows in the bands of mesoderm in the interior, so the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by a veliger stage in which the prototroch, the "equatorial" band of cilia nearest the apical tuft, develops into the velum ("veil"), a pair of cilia-bearing lobes with which the larva swims. Eventually, the larva sinks to the seafloor and metamorphoses into the adult form. While metamorphosis is the usual state in molluscs, the cephalopods differ in exhibiting direct development: the hatchling is a 'miniaturized' form of the adult.[37] Ecology[edit] Feeding[edit] Most molluscs are herbivorous, grazing on algae or filter feeders. For those grazing, two feeding strategies are predominant. Some feed on microscopic, filamentous algae, often using their radula as a 'rake' to comb up filaments from the sea floor. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. To employ this strategy, the plant has to be large enough for the mollusc to 'sit' on, so smaller macroscopic plants are not as often eaten as their larger counterparts.[38] Filter feeders
Filter feeders
are molluscs that feed by straining suspended matter and food particle from water, typically by passing the water over their gills. Most bivalves are filter feeders. Cephalopods
Cephalopods
are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. The monoplacophoran Neopilina uses its radula in the usual fashion, but its diet includes protists such as the xenophyophore Stannophyllum.[39] Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls,[40] whereas dorid nudibranchs and some Vetigastropoda
Vetigastropoda
feed on sponges[41][42] and others feed on hydroids.[43] (An extensive list of molluscs with unusual feeding habits is available in the appendix of GRAHAM, A. (1955). "Molluscan diets". Journal of Molluscan Studies. 31 (3–4): 144. .) Classification[edit] See also: List of mollusc orders Opinions vary about the number of classes of molluscs; for example, the table below shows eight living classes,[20] and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata
Caudofoveata
and solenogasters into one class, the Aplacophora.[11][29] Two of the commonly recognized "classes" are known only from fossils.[18]

Class Major organisms Described living species[20] Distribution

Gastropoda[44] All the snails and slugs including abalone, limpets, conch, nudibranchs, sea hares, sea butterfly 70,000 marine, freshwater, land

Bivalvia[45] clams, oysters, scallops, geoducks, mussels 20,000 marine, freshwater

Polyplacophora[12] chitons 1,000 rocky tidal zone and seabed

Cephalopoda[46] squid, octopus, cuttlefish, nautilus, spirula 900 marine

Scaphopoda[17] tusk shells 500 marine 6–7,000 metres (20–22,966 ft)

Solenogastres[11] worm-like organisms 200 seabed 200–3,000 metres (660–9,840 ft)

Caudofoveata[11] worm-like organisms 120 seabed 200–3,000 metres (660–9,840 ft)

Monoplacophora[13] An ancient lineage of molluscs with cap-like shells 31 seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200 metres (660 ft)

Rostroconchia
Rostroconchia
†[47] fossils; probable ancestors of bivalves extinct marine

Helcionelloida
Helcionelloida
†[48] fossils; snail-like organisms such as Latouchella extinct marine

Classification into higher taxa for these groups has been and remains problematic. A phylogenetic study suggests the Polyplacophora
Polyplacophora
form a clade with a monophyletic Aplacophora.[49] Additionally, it suggests a sister taxon relationship exists between the Bivalvia
Bivalvia
and the Gastropoda. Tentaculita
Tentaculita
may also be in Mollusca
Mollusca
(see Tentaculites). Evolution[edit] Main article: Evolution of molluscs See also: Evolution of cephalopods

The use of love darts by the land snail Monachoides vicinus
Monachoides vicinus
is a form of sexual selection

Fossil
Fossil
record[edit] Good evidence exists for the appearance of gastropods (e.g. Aldanella), cephalopods (e.g. Plectronoceras, ?Nectocaris) and bivalves (Pojetaia, Fordilla) towards the middle of the Cambrian period, c. 500 million years ago, though arguably each of these may belong only to the stem lineage of their respective classes.[50] However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated. Debate occurs about whether some Ediacaran
Ediacaran
and Early Cambrian
Cambrian
fossils really are molluscs. Kimberella, from about 555 million years ago, has been described by some paleontologists as "mollusc-like",[51][52] but others are unwilling to go further than "probable bilaterian",[53][54] if that.[55] There is an even sharper debate about whether Wiwaxia, from about 505 million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms.[53][56] Nicholas Butterfield, who opposes the idea that Wiwaxia
Wiwaxia
was a mollusc, has written that earlier microfossils from 515 to 510 million years ago are fragments of a genuinely mollusc-like radula.[57] This appears to contradict the concept that the ancestral molluscan radula was mineralized.[58]

The tiny Helcionellid
Helcionellid
fossil Yochelcionella
Yochelcionella
is thought to be an early mollusc[48]

Spirally coiled shells appear in many gastropods.[14]

However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian
Cambrian
rocks from Siberia
Siberia
and China,[59][60] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites.[48] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults.[61] Some analyses of helcionellids concluded these were the earliest gastropods.[62] However, other scientists are not convinced these Early Cambrian
Cambrian
fossils show clear signs of the torsion that identifies modern gastropods twists the internal organs so the anus lies above the head.[14][63][64]

  = Septa   = Siphuncle

Septa and siphuncle in nautiloid shell

Volborthella, some fossils of which predate 530 million years ago, was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus
Nautilus
are. Volborthella's classification is uncertain.[65] The Late Cambrian
Cambrian
fossil Plectronoceras
Plectronoceras
is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus
Nautilus
uses to remove water from compartments it has vacated as it grows, and which is also visible in fossil ammonite shells. However, Plectronoceras
Plectronoceras
and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface.[66] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous
Cretaceous
period 65 million years ago.[67] However, the shell-less Coleoidea
Coleoidea
(squid, octopus, cuttlefish) are abundant today.[68] The Early Cambrian
Cambrian
fossils Fordilla
Fordilla
and Pojetaia are regarded as bivalves.[69][70][71][72] "Modern-looking" bivalves appeared in the Ordovician
Ordovician
period, 488 to 443 million years ago.[73] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event.[74] Even so, bivalves remain abundant and diverse. The Hyolitha
Hyolitha
are a class of extinct animals with a shell and operculum that may be molluscs. Authors who suggest they deserve their own phylum do not comment on the position of this phylum in the tree of life.[75] Phylogeny[edit]

Lophotrochozoa

Brachiopods

Mollusca

Bivalves

Monoplacophorans ("limpet-like", "living fossils")

Gastropods (snails, slugs, limpets, sea hares)

Cephalopods (nautiloids, ammonites, squid, etc.)

Scaphopods (tusk shells)

Aplacophorans (spicule-covered, worm-like)

Polyplacophorans (chitons)

Halwaxiids

Wiwaxia

Halkieria

Orthrozanclus

Odontogriphus

A possible "family tree" of molluscs (2007).[76][77] Does not include annelid worms as the analysis concentrated on fossilizable "hard" features.[76]

The phylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether Kimberella
Kimberella
and any of the "halwaxiids" were molluscs or closely related to molluscs,[52][53][56][57] debates arise about the relationships between the classes of living molluscs.[54] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.[78] Molluscs are generally regarded members of the Lophotrochozoa,[76] a group defined by having trochophore larvae and, in the case of living Lophophorata, a feeding structure called a lophophore. The other members of the Lophotrochozoa
Lophotrochozoa
are the annelid worms and seven marine phyla.[79] The diagram on the right summarizes a phylogeny presented in 2007. Because the relationships between the members of the family tree are uncertain, it is difficult to identify the features inherited from the last common ancestor of all molluscs.[80] For example, it is uncertain whether the ancestral mollusc was metameric (composed of repeating units)—if it was, that would suggest an origin from an annelid-like worm.[81] Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments,[7] while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized.[54] In one particular branch of the family tree, the shell of conchiferans is thought to have evolved from the spicules (small spines) of aplacophorans; but this is difficult to reconcile with the embryological origins of spicules.[80] The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into a cuticle. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills.[48] Eventually, the cuticle would have become mineralized,[48] using the same genetic machinery (engrailed) as most other bilaterian skeletons.[81] The first mollusc shell almost certainly was reinforced with the mineral aragonite.[30] The evolutionary relationships 'within' the molluscs are also debated, and the diagrams below show two widely supported reconstructions:

Molluscs

Aculifera

Solenogastres

Caudofoveata

Polyplacophorans

Conchifera

Monoplacophorans

Bivalves

Scaphopods

Gastropods

Cephalopods

The "Aculifera" hypothesis[76]

Molluscs

Solenogastres

Caudofoveata

Testaria

Polyplacophorans

Monoplacophorans

Bivalves

Scaphopods

Gastropods

Cephalopods

The "Testaria" hypothesis[76]

Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses,[82] although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.[83] However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic; in particular, Scaphopoda
Scaphopoda
and Bivalvia
Bivalvia
are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca
Mollusca
is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded.[78] A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated.[84] Current molecular data are insufficient to constrain the molluscan phylogeny, and since the methods used to determine the confidence in clades are prone to overestimation, it is risky to place too much emphasis even on the areas of which different studies agree.[85] Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question.[86] Human interaction[edit] Main article: Molluscs in culture For millennia, molluscs have been a source of food for humans, as well as important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, and chemical compounds. Their shells have also been used as a form of currency in some preindustrial societies. A number of species of molluscs can bite or sting humans, and some have become agricultural pests. Uses by humans[edit] Further information: Seashell
Seashell
and List of edible molluscs Molluscs, especially bivalves such as clams and mussels, have been an important food source since at least the advent of anatomically modern humans, and this has often resulted in overfishing.[87] Other commonly eaten molluscs include octopuses and squids, whelks, oysters, and scallops.[88] In 2005, China accounted for 80% of the global mollusc catch, netting almost 11,000,000 tonnes (11,000,000 long tons; 12,000,000 short tons). Within Europe, France remained the industry leader.[89] Some countries regulate importation and handling of molluscs and other seafood, mainly to minimize the poison risk from toxins that can sometimes accumulate in the animals.[90]

Saltwater pearl oyster farm in Seram, Indonesia

Most molluscs with shells can produce pearls, but only the pearls of bivalves and some gastropods, whose shells are lined with nacre, are valuable.[14][16] The best natural pearls are produced by marine pearl oysters, Pinctada
Pinctada
margaritifera and Pinctada
Pinctada
mertensi, which live in the tropical and subtropical waters of the Pacific Ocean. Natural pearls form when a small foreign object gets stuck between the mantle and shell. The two methods of culturing pearls insert either "seeds" or beads into oysters. The "seed" method uses grains of ground shell from freshwater mussels, and overharvesting for this purpose has endangered several freshwater mussel species in the southeastern United States.[16] The pearl industry is so important in some areas, significant sums of money are spent on monitoring the health of farmed molluscs.[91]

Byzantine
Byzantine
Emperor Justinian I
Justinian I
clad in Tyrian purple
Tyrian purple
and wearing numerous pearls

Other luxury and high-status products were made from molluscs. Tyrian purple, made from the ink glands of murex shells, "... fetched its weight in silver" in the fourth century BC, according to Theopompus.[92] The discovery of large numbers of Murex
Murex
shells on Crete
Crete
suggests the Minoans
Minoans
may have pioneered the extraction of "imperial purple" during the Middle Minoan period in the 20th–18th centuries BC, centuries before the Tyrians.[93][94] Sea silk
Sea silk
is a fine, rare, and valuable fabric produced from the long silky threads (byssus) secreted by several bivalve molluscs, particularly Pinna nobilis, to attach themselves to the sea bed.[95] Procopius, writing on the Persian wars circa 550 CE, "stated that the five hereditary satraps (governors) of Armenia
Armenia
who received their insignia from the Roman Emperor were given chlamys (or cloaks) made from lana pinna. Apparently, only the ruling classes were allowed to wear these chlamys."[96] Mollusc
Mollusc
shells, including those of cowries, were used as a kind of money (shell money) in several preindustrial societies. However, these "currencies" generally differed in important ways from the standardized government-backed and -controlled money familiar to industrial societies. Some shell "currencies" were not used for commercial transactions, but mainly as social status displays at important occasions, such as weddings.[97] When used for commercial transactions, they functioned as commodity money, as a tradable commodity whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurable inflation if more efficient transport or "goldrush" behavior appeared.[98] Bioindicators[edit] Bivalve
Bivalve
molluscs are used as bioindicators to monitor the health of aquatic environments in both fresh water and the marine environments. Their population status or structure, physiology, behaviour or the level of contamination with elements or compounds can indicate the state of contamination status of the ecosystem. They are particularly useful since they are sessile so that they are representative of the environment where they are sampled or placed.[99] Potamopyrgus antipodarum is used by some water treatment plants to test for estrogen-mimicking pollutants from industrial agriculture. Harmful to humans[edit] Stings and bites[edit]

The blue-ringed octopus's rings are a warning signal; this octopus is alarmed, and its bite can kill.[100]

Some molluscs sting or bite, but deaths from mollusc venoms total less than 10% of those from jellyfish stings.[101] All octopuses are venomous,[102] but only a few species pose a significant threat to humans. Blue-ringed octopuses in the genus Hapalochlaena, which live around Australia and New Guinea, bite humans only if severely provoked,[100] but their venom kills 25% of human victims. Another tropical species, Octopus
Octopus
apollyon, causes severe inflammation that can last for over a month even if treated correctly,[103] and the bite of Octopus
Octopus
rubescens can cause necrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated.[104]

Live cone snails can be dangerous to shell collectors, but are useful to neurology researchers.[105]

All species of cone snails are venomous and can sting painfully when handled, although many species are too small to pose much of a risk to humans, and only a few fatalities have been reliably reported. Their venom is a complex mixture of toxins, some fast-acting and others slower but deadlier.[105][101][106] The effects of individual cone-shell toxins on victims' nervous systems are so precise as to be useful tools for research in neurology, and the small size of their molecules makes it easy to synthesize them.[105][107] Disease vectors[edit]

Skin vesicles created by the penetration of Schistosoma. (Source: CDC)

Schistosomiasis
Schistosomiasis
(also known as bilharzia, bilharziosis or snail fever), a disease caused by the fluke worm Schistosoma, is "second only to malaria as the most devastating parasitic disease in tropical countries. An estimated 200 million people in 74 countries are infected with the disease – 100 million in Africa alone."[108] The parasite has 13 known species, two of which infect humans. The parasite itself is not a mollusc, but all the species have freshwater snails as intermediate hosts.[109] Pests[edit] Some species of molluscs, particularly certain snails and slugs, can be serious crop pests,[110] and when introduced into new environments, can unbalance local ecosystems. One such pest, the giant African snail Achatina fulica, has been introduced to many parts of Asia, as well as to many islands in the Indian Ocean
Indian Ocean
and Pacific Ocean. In the 1990s, this species reached the West Indies. Attempts to control it by introducing the predatory snail Euglandina rosea
Euglandina rosea
proved disastrous, as the predator ignored Achatina fulica
Achatina fulica
and went on to extirpate several native snail species, instead.[111] Notes[edit]

^ The formerly dominant spelling mollusk is still used in the U.S. — see the reasons given in Gary Rosenberg's "Mollusckque - Mollusk vs Mollusc". For the spelling mollusc, see the reasons given in: Brusca & Brusca. Invertebrates (2nd ed.). 

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Oysters and Other Molluscs: a Western Australian Perspective". Journal of Shellfish Research. 25 (1): 233–238. doi:10.2983/0730-8000(2006)25[233:DOPOAO]2.0.CO;2. CS1 maint: Date and year (link) ^ The fourth-century BC historian Theopompus, cited by Athenaeus (12:526) around 200 BC ; according to Gulick, C.B. (1941). Athenaeus, The Deipnosophists. Cambridge, Mass.: Harvard University Press. ISBN 0-674-99380-2.  ^ Reese, D.S. (1987). "Palaikastro Shells and Bronze Age Purple-Dye Production in the Mediterranean Basin". Annual of the British School of Archaeology at Athens. 82: 201–6. doi:10.1017/s0068245400020438.  ^ Stieglitz, R.R. (March 1994). "The Minoan Origin of Tyrian Purple". Biblical Archaeologist. 57 (1): 46–54. doi:10.2307/3210395. JSTOR 3210395.  ^ Webster's Third New International Dictionary (Unabridged) 1976. G. & C. Merriam Co., p. 307. ^ Turner, R.D.; Rosewater, J. (June 1958). "The Family Pinnidae in the Western Atlantic". Johnsonia. 3 (38): 294.  ^ Maurer, B. (October 2006). "The Anthropology of Money" (PDF). Annual Review of Anthropology. 35: 15–36. doi:10.1146/annurev.anthro.35.081705.123127. Archived from the original (PDF) on 2007-08-16.  ^ Hogendorn, J. & Johnson, M. (2003). The Shell Money
Money
of the Slave Trade. Cambridge University Press. ISBN 0521541107.  Particularly chapters "Boom and slump for the cowrie trade" (pages 64–79) and "The cowrie as money: transport costs, values and inflation" (pages 125–147) ^ Université Bordeaux; et al. "MolluSCAN eye project". Retrieved 2017-01-28. CS1 maint: Explicit use of et al. (link) ^ a b Alafaci, A. "Blue ringed octopus". Australian Venom Research Unit. Retrieved 2008-10-03.  ^ a b Williamson, J.A.; Fenner, P.J.; Burnett, J.W.; Rifkin, J. (1996). Venomous and Poisonous Marine Animals: A Medical and Biological Handbook. UNSW Press. pp. 65–68. ISBN 0-86840-279-6.  ^ Anderson, R.C. (1995). "Aquarium husbandry of the giant Pacific octopus". Drum and Croaker. 26: 14–23.  ^ Brazzelli, V.; Baldini, F.; Nolli, G.; Borghini, F.; Borroni, G. (March 1999). " Octopus
Octopus
apollyon bite". Contact Dermatitis. 40 (3): 169. doi:10.1111/j.1600-0536.1999.tb06025.x. PMID 10073455.  ^ Anderson, R.C. (1999). "An octopus bite and its treatment". The Festivus. 31: 45–46.  ^ a b c Concar, D. (19 October 1996). "Doctor snail—Lethal to fish and sometimes even humans, cone snail venom contains a pharmacopoeia of precision drugs". New Scientist. Retrieved 2008-10-03.  ^ Livett, B. "Cone Shell Mollusc
Mollusc
Poisoning, with Report of a Fatal Case". Department of Biochemistry and Molecular Biology, University of Melbourne.  ^ Haddad Junior, V.; Paula Neto, J.O.B.D.; Cobo, V.L.J. (September–October 2006). "Venomous mollusks: The risks of human accidents by conus snails (gastropoda: Conidae) in Brazil". Revista da Sociedade Brasileira de Medicina Tropical. 39 (5): 498. doi:10.1590/S0037-86822006000500015. PMID 17160331.  ^ "The Carter Center
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Bibliography[edit]

Ruppert, E.E.; Fox, R.S.; Barnes, R.D. (2004). Invertebrate
Invertebrate
Zoology (7 ed.). Brooks / Cole. ISBN 0-03-025982-7. 

Further reading[edit]

Sturm, C., T.A. Pearce, and A. Valdes. The Mollusks: A Guide to their Study, Collection, and Preservation. Universal Publishers. 2006. 454 pages. ISBN 1581129300

External links[edit]

Wikimedia Commons has media related to Mollusca.

The Wikibook Dichotomous Key has a page on the topic of: Mollusca

Data related to Mollusca
Mollusca
at Wikispecies Mollusca
Mollusca
at the Encyclopedia of Life
Encyclopedia of Life
Researchers complete mollusk evolutionary tree; 26 October 2011 Hardy's Internet Guide to Marine Gastropods Rotterdam Natural History Museum Shell Image Gallery Mussel
Mussel
Watch Programme Online biomonitoring of bivalve activity, 24/7: MolluSCAN eye

v t e

Extant Animal
Animal
phyla

Domain Archaea Bacteria Eukaryota (Supergroup Plant Hacrobia Heterokont Alveolata Rhizaria Excavata Amoebozoa Opisthokonta

Animal Fungi)

A n i m a l i a

Porifera (sponges)

Diploblasts (Eumetazoa)

Ctenophora
Ctenophora
(comb jellies)

ParaHoxozoa

Placozoa
Placozoa
(Trichoplax)

Planulozoa

Cnidaria
Cnidaria
(jellyfish and relatives)

Bilateria (Triploblasts)

(see below↓)

Bilateria

Xenacoelomorpha

Xenoturbellida (Xenoturbella) Acoelomorpha

acoels nemertodermatids

N e p h r o z o a

Deuterostomia

Chordata

lancelets tunicates craniates / vertebrates

Ambulacraria

Echinodermata (starfish and relatives) Hemichordata

acorn worms pterobranchs

P r o t o s t o m i a

Ecdysozoa

Scalidophora

Kinorhyncha
Kinorhyncha
(mud dragons) Priapulida
Priapulida
(penis worms)

N+L+P

Nematoida

Nematoda (roundworms) Nematomorpha
Nematomorpha
(horsehair worms)

L+P

Loricifera

Panarthropoda

Arthropoda (arthropods) Tardigrada (waterbears) Onychophora
Onychophora
(velvet worms)

S p i r a l i a

Gnathifera¹

Chaetognatha
Chaetognatha
(arrow worms) Gnathostomulida (jaw worms) Micrognathozoa (Limnognathia) Syndermata

Rotifera Acanthocephala

Platytrochozoa

R+M

Mesozoa

Orthonectida Dicyemida
Dicyemida
or Rhombozoa

Rouphozoa¹

Platyhelminthes (flatworms) Gastrotricha (hairybacks)

Lophotrochozoa

Cycliophora (Symbion) Mollusca
Mollusca
(molluscs)

A+N

Annelida (ringed worms) Nemertea
Nemertea
(ribbon worms)

Lophophorata

Bryozoa

Entoprocta
Entoprocta
or Kamptozoa Ectoprocta (moss animals)

Brachiozoa

Brachiopoda (lamp shells) Phoronida (horseshoe worms)

Major groups within phyla

Sponges

Calcareous Hexactinellid Demosponge Homoscleromorpha

Cnidarians

Anthozoa
Anthozoa
inc. corals Medusozoa
Medusozoa
inc. jellyfish Myxozoa

Vertebrates

Jawless fish Cartilaginous fish Bony fish Amphibians Reptiles/Birds Mammals

Echinoderms

Sea lilies Asterozoa
Asterozoa
inc. starfish Echinozoa

Nematodes

Chromadorea Enoplea Secernentea

Arthropods

Chelicerates/Arachnids Myriapods Crustaceans Hexapods/Insects

Platyhelminths

Turbellaria Trematoda Monogenea Cestoda

Bryozoans

Phylactolaemata Stenolaemata Gymnolaemata

Annelids

Polychaetes Clitellata Echiura

Molluscs

Gastropods Cephalopods Bivalves Chitons Tusk shells

Phyla with ≥5000 extant species bolded See also Diploblasts Monoblastozoa (nomen dubium)

¹Platyzoa

v t e

Living things in culture

Academic disciplines

Anthrozoology Ethnobiology

Ethnobotany Ethnoecology Ethnoentomology Ethnoherpetology Ethnoichthyology Ethnolichenology Ethnomycology Ethnoornithology Ethnoprimatology Ethnozoology

Groups

Animals

Arthropods

Insects

Topics

Beekeeping Entomophagy Flea
Flea
circus Insects in art Insects in literature Insects in medicine Insects in music Insects in mythology Insects in religion Sericulture

Types

Ant Bee

Mythology

Beetle

Beetlewing

Butterfly Cicada Cricket Dragonfly Flea

Flea
Flea
circus

Fly Grasshopper Ladybird Louse Praying mantis Scarab Termite Wasp Woodworm

Other

Arthropods in film Crab Lobster Scorpion Spider

Arachnophobia Tarantella

Tick

Molluscs

Cephalopods
Cephalopods
in popular culture Conch
Conch
(instrument) Conchology Edible molluscs Octopus Pearl Scallop Seashell Sea silk Shell money Shipworm Tyrian purple Venus shell

Vertebrates

Amphibians

Frog Salamander Toad

Toadstone

Birds

Aviculture Birdwatching Bird
Bird
conservation Birds in culture Cockfighting Falconry Game bird Pigeon racing Poultry Archaeopteryx Barnacle goose Eagle

Fish

Fishing

History

Fish farming Fishkeeping Recreational fishing Shark

Attacks Jaws

Mammals

Topics

Animal
Animal
husbandry Fur farming Hunting In sport In professional wrestling Laboratory animal Livestock Pack animal Working animal

Types

Bat Bear

Baiting Hunting Teddy bear

Cattle Deer Elephant Dolphin Fox Horse

Riding Worship

Leopard Lion Primate

Gorilla Gorilla suit Monkey Orangutan

Seal

hunting

Sheep Whale

Tay Whale Whaling Whale
Whale
watching

Wolf

Werewolf

Reptiles

Crocodile

Attacks Farming Crocodile
Crocodile
tears

Dinosaur

Crystal Palace Dinosaurs Jurassic
Jurassic
Park Stegosaurus Triceratops Tyrannosaurus rex

Dragon Lizard Snake

Caduceus In the Bible Rod of Asclepius Snakebite Snake
Snake
charming Symbolism Worship

Turtle

Bixi World Turtle

Other phyla

Coral Jellyfish Starfish

Other

Aesop's Fables Animal
Animal
epithet Animal
Animal
husbandry In heraldry Lists of legendary creatures Man-eater Zodiac

Plants

Agriculture

History

Botanical illustration Floral design

Ikebana

Gardening Herbalism Fictional plants Magical plants

Mandrake

Medicinal plants Pharmacognosy Plant epithet Sacred grove

In India

Sacred plants

Bodhi Tree Lime tree Sacred lotus Sacred herb

In mythology

Barnacle tree Fig Trees Yggdrasil

Fungi

Medicinal fungi Amanita muscaria Edible mushroom

Agaricus bisporus

Psilocybin mushroom

Microbes

Biological warfare Fermentation

In food processing List of microbes

Microbial art Microbes and Man Pathogen Protein
Protein
production Bacteria

Economic importance Tuberculosis

Protist Virus Yeast

Bread Beer Wine

Related

Anthropomorphism Fossil Legendary creature Lists of fictional species Parasitoid Template:Insects in culture

Animals portal Biology portal

Taxon identifiers

Wd: Q25326 ADW: Mollusca EoL: 2195 EPPO: 1MOLLP Fauna Europaea: 11368 Fossilworks: 7805 ITIS: 69458 NCBI: 6447 WoRMS: 51

Authority control

GND: 41894

.