Mollusca is aphylum ofprotostomicinvertebrateanimals, whose members are known asmolluscs ormollusks[a] (/ˈmɒləsks/). Whether the serial features found in some molluscs are ancestral or derived is consideredcontroversial, leaving a question on their origin, according to D K Jacobs, C G Wray, etc. Around 76,000 extantspecies of molluscs are recognized, making it the second-largestanimal phylum afterArthropoda.[5] The number of additionalfossil species is estimated between 60,000 and 100,000,[6] and the proportion of undescribed species is very high. Many taxa remain poorly studied.[7]
The four most universal features defining modern molluscs are a soft body composed almost entirely ofmuscle, amantle with a significant cavity used for breathing andexcretion, the presence of aradula (except forbivalves), and the structure of thenervous 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 ofproteins andchitin reinforced withcalcium 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 arecoelomates, thecoelom tends to be small.The main body cavity is ahemocoel through whichblood circulates; as such, theircirculatory systems are mainlyopen. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exudedmucus and microscopic, muscle-powered "hairs" calledcilia play various important roles. The generalized mollusc has two pairednerve cords, or three inbivalves. Thebrain, in species that have one, encircles theesophagus. Most molluscs haveeyes, and all have sensors to detect chemicals, vibrations, andtouch. The simplest type of molluscanreproductive system relies onexternal fertilization, but more complex variations occur. Nearly all produceeggs, from which may emergetrochophorelarvae, more complexveliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.
Good evidence exists for the appearance of gastropods,cephalopods, and bivalves in theCambrian period, 541–485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestralLophotrochozoa and of their diversification into the well-known living andfossil forms are still subjects of vigorous debate among scientists.
Molluscs have been and still are an important food source for humans. Toxins that can accumulate in certain molluscs under specific conditions create a risk of food poisoning, and many jurisdictions have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notablypearls,mother of pearl,Tyrian purple dye, andsea silk. Their shells have also beenused asmoney in some preindustrial societies.
A handful of mollusc species are sometimes considered hazards or pests for human activities. The bite of theblue-ringed octopus is often fatal, and that ofEnteroctopus dofleini causesinflammation that can last over a month. Stings from a few species of large tropicalcone shells of the family Conidae can also kill, but their sophisticated, though easily produced, venoms have become important tools inneurological research.Schistosomiasis (also known as bilharzia, bilharziosis, or snail fever) is transmitted to humans by 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 someecosystems.
The wordsmollusc andmollusk are both derived from the Frenchmollusque, which originated from the post-classicalLatinmollusca, frommollis, soft, first used by J. Jonston (Historiæ Naturalis, 1650) to describe a group comprising cephalopods.[9]Molluscus is used in classical Latin as an adjective only withnux (nut) to describe a particular type of soft nut. The use ofmollusca in biological taxonomy by Jonston and laterLinnaeus may have been influenced byAristotle'sτὰ μαλάκιαta malákia (the soft ones; <μαλακόςmalakós "soft"), which he appliedinter aliatocuttlefish.[10][11] The scientific study of molluscs is accordingly calledmalacology.[12]
The nameMolluscoida was formerly used to denote a division of the animal kingdom containing thebrachiopods,bryozoans, andtunicates, the members of the three groups having been supposed to somewhat resemble the molluscs. As now known, Brachipoda, Bryozoa and Mollusca are all part of theLophotrochozoa but have very little relation to the Tunicata, so the name Molluscoida has been abandoned.[13]
The most universal features of the body structure of molluscs are amantle with a significant body cavity used forbreathing andexcretion, and the organization of the nervous system. Many have acalcareous shell.[14]
Molluscs have developed such a varied range of body structures, findingsynapomorphies (defining characteristics) to apply to all modern groups is difficult.[15] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical.[16] The following are present in all modern molluscs:[17][19]
Thedorsal part of the body wall is a mantle (or pallium) whichsecretes calcareousspicules, plates or shells. It overlaps the body with enough spare room to form amantle cavity.
Diversity and variability of shells of molluscs on display About 80% of all known mollusc species aregastropods (snails andslugs), including thiscowry (a sea snail).[20]
Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.[3] The total number of described species is difficult to estimate because of unresolvedsynonymy. 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 would comprise about 14% of the total and the other five classes less than 2% of the living molluscs.[21] In 2009, Chapman estimated the number of described living mollusc species at 85,000.[3] Haszprunar in 2001 estimated about 93,000 named species,[22] which include 23% of all named marine organisms.[23] Molluscs are second only toarthropods in numbers of living animal species[20]—far behind the arthropods' 1,113,000 but well ahead ofchordates' 52,000.[18]: Front endpaper About 200,000 living species in total are estimated,[3][24] and 70,000 fossil species,[17] 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.[25]
Freshwater andterrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of non-marine 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 theIUCN Red List of Threatened Species included nearly 2,000 endangered non-marine 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 non-marine species.[29]
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 anarchi-mollusc,hypothetical generalized mollusc, orhypothetical ancestral mollusc (HAM) to illustrate the most common features found within the phylum. The depiction is visually rather similar to modernmonoplacophorans.[15][19][30]
The generalized mollusc is an unsegmented, bilaterally symmetrical animal 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".[19] The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs.[16]
The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according tohabitat, 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. Theanus, a pair ofosphradia (chemical sensors) in the incoming "lane", the hindmost pair ofgills and the exit openings of thenephridia (kidneys) known as "Organs of bojanus" andgonads (reproductive organs) are in the mantle cavity.[19] The whole soft body of bivalves lies within an enlarged mantle cavity.[16]
The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as thenudibranchs[16]) that consists of mainlychitin andconchiolin (aprotein hardened withcalcium carbonate),[19][31] except the outermost layer, which in almost all cases is all conchiolin (seeperiostracum).[19] Molluscs never use phosphate to construct their hard parts,[32] with the questionable exception ofCobcrephora.[33]While most mollusc shells are composed mainly ofaragonite, those gastropods that lay eggs with a hard shell usecalcite (sometimes with traces of aragonite) to construct the eggshells.[34]
The shell consists of three layers: the outer layer (theperiostracum) made of organic matter, a middle layer made of columnarcalcite, and an inner layer consisting of laminated calcite, oftennacreous.[16]
In some forms the shell contains openings. Inabalone there are holes in the shell used for respiration and the release of egg and sperm, in thenautilus a string of tissue called thesiphuncle goes through all the chambers, and the eight plates that make up the shell ofchitons are penetrated with living tissue with nerves and sensory structures.[35] The earliest-derived living mollusca, the Polyplacophora (chitons) and shell-less vermiform Aplacophora, remains contentious despite many developmental and molecular studies of these organisms.[36][37] Both studies investigated molluscan phylogeny through synthesis of paleontological and neontological data[38], and the other being large and small-subunit nuclear rRna sequences.[37] Another article describing the chitons, with their shells, this time, is one by D K Jacobs and others, where shell evolution of molluscs and chitons are examined to visualize the evolution of the chiton, from the Polyplacophora, to the Aplacophora, and others.[39]
The body of a mollusc has a ventral muscular foot, which is adapted to different purposes (locomotion, grasping the substratum, burrowing or feeding) in different classes.[40] The foot carries a pair ofstatocysts, which act as balance sensors. In gastropods, it secretesmucus as a lubricant to aid movement. In forms having only a top shell, such aslimpets, 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.[19] In bivalves, the foot is adapted for burrowing into the sediment;[40] in cephalopods it is used for jet propulsion,[40] and the tentacles and arms are derived from the foot.[41]
Most molluscs'circulatory systems are mainlyopen, except forcephalopods, whose circulatory systems areclosed. Although molluscs arecoelomates, theircoeloms are reduced to fairly small spaces enclosing theheart and gonads. The main body cavity is ahemocoel through whichblood andcoelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficienthydrostatic skeleton.[16] The blood of these molluscs contains therespiratory pigmenthemocyanin as anoxygen-carrier. The heart consists of one or more pairs of atria (auricles), which receive oxygenated blood from the gills and pump it to theventricle, which pumps it into theaorta (mainartery), which is fairly short and opens into the hemocoel.[19] The atria of the heart also function as part of theexcretory system by filtering waste products out of the blood and dumping it into the coelom asurine. A pair of metanephridia ("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.[19]
Exceptions to the above are the molluscsPlanorbidae or ram's horn snails, which are air-breathing snails that use iron-basedhemoglobin instead of the copper-based hemocyanin to carry oxygen through their blood.
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 possiblysediment 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.[19]
Molluscs useintracellular digestion. Most molluscs have muscular mouths withradulae, "tongues", bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. The radula primarily functions to scrapebacteria andalgae off rocks, and is associated with theodontophore, a cartilaginous supporting organ.[16] The radula is unique to the molluscs and has no equivalent in any other animal.
Molluscs' mouths also containglands that secrete slimymucus, to which the food sticks. Beatingcilia (tiny "hairs") drive the mucus towards the stomach, so the mucus forms a long string called a "food string".[19]
At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone offeces 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.[19]
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'scecum (a pouch with no other exit) to be digested. The sorting process is by no means perfect.[19]
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.[19]
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 thedetritus from its mucus.[16]
The cephalic molluscs have two pairs of mainnerve cords organized around a number of paired ganglia, thevisceral 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 bycommissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above theesophagus (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 acircumesophageal nerve ring ornerve collar.[18]: 284–291
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".[42][43] Some such as thescallops 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.
The simplest molluscan reproductive system relies onexternal fertilization, but with more complex variations. All produce eggs, from which may emergetrochophore larvae, more complexveliger larvae, or miniature adults. Twogonads sit next to thecoelom, a small cavity that surrounds the heart, into which they shedova orsperm. 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 onexternal fertilization. Some molluscs useinternal fertilization and/or arehermaphrodites, functioning as both sexes; both of these methods require more complex reproductive systems.[19]C. obtusus is an endemicsnail species of theEastern Alps. There is strong evidence forself-fertilization in the easternmost snail populations of this species.[45]
The most basic molluscanlarva is atrochophore, which isplanktonic 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 ofmesoderm in the interior, so the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by aveliger stage in which theprototroch, 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 andmetamorphoses 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.[46] The development of molluscs is of particular interest in the field ofocean acidification as environmental stress is recognized to affect the settlement, metamorphosis, and survival of larvae.[47]
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.[48]Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing the water over their gills. Most bivalves are filter feeders, which can be measured through clearance rates. Research has demonstrated that environmental stress can affect the feeding of bivalves by altering the energy budget of organisms.[47]
Cephalopods are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. The monoplacophoranNeopilina uses its radula in the usual fashion, but its diet includesprotists such as thexenophyophoreStannophyllum.[49]Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls,[50] whereasdoridnudibranchs and someVetigastropoda feed onsponges[51][52] and others feed onhydroids.[53] (An extensive list of molluscs with unusual feeding habits is available in the appendix ofGRAHAM, A. (1955)."Molluscan diets".Journal of Molluscan Studies.31 (3–4): 144.)
Opinions vary about the number ofclasses of molluscs; for example, the table below shows seven living classes,[22] and two extinct ones. Although they are unlikely to form a clade, some older works combine theCaudofoveata andSolenogasters into one class, theAplacophora.[30][18]: 291–292 Two of the commonly recognized "classes" are known only from fossils.[20]
A possible "family tree" of molluscs (2007).[57][58] Does not includeannelid worms as the analysis concentrated on fossilizable "hard" features.[57]
Thephylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whetherKimberella and any of the "halwaxiids" were molluscs or closely related to molluscs,[59][60][61][62] debates arise about the relationships between the classes of living molluscs.[63] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.[64]
Molluscs are generally regarded members of theLophotrochozoa,[57] a group defined by havingtrochophore larvae and, in the case of livingLophophorata, a feeding structure called alophophore. The other members of the Lophotrochozoa are theannelid worms and seven marinephyla.[65] The diagram on the right summarizes a phylogeny presented in 2007 without the annelid worms.
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.[66] For example, it is uncertain whether the ancestral mollusc wasmetameric (composed of repeating units)—if it was, that would suggest an origin from anannelid-like worm.[67] Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments,[15] while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized.[63] In one particular branch of the family tree, the shell ofconchiferans is thought to have evolved from thespicules (small spines) ofaplacophorans; but this is difficult to reconcile with theembryological origins of spicules.[66]
The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into acuticle. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills.[55] Eventually, the cuticle would have become mineralized,[55]using the same genetic machinery (engrailed) as most other bilaterianskeletons.[67] The first mollusc shell almost certainly was reinforced with the mineralaragonite.[31]
Classification into higher taxa for molluscan classes has been and remains problematic. Numerous different clades have been proposed but few have received strong support. Traditionally, Mollusca is split into two subphyla,Conchifera andAculifera, based on the presence of a shell. The "Testaria" hypothesis is similar, but includes chitons with the rest of the conchiferans. Some studies completely reject the proposal, instead favoring a "Serialia" hypothesis which classifies chitons and monoplacophorans as closely related.
Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses,[72] although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.[73]
However, an analysis in 2009 using bothmorphological andmolecular phylogenetics comparisons concluded the molluscs are notmonophyletic; in particular,Scaphopoda andBivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca ispolyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded.[64] A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated.[74] 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.[75] Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question.[76]
The enigmaticKimberella quadrata (fossil pictured) from theEdiacaran has been described as being "mollusc-like" because of its features which are shared with modern day molluscs.
Good evidence exists for the appearance ofgastropods (e.g.,Aldanella),cephalopods (e.g.,Plectronoceras,Nectocaris?) andbivalves (Pojetaia,Fordilla) towards the middle of theCambrian period, c. 500 million years ago, though arguably each of these may belong only to the stem lineage of their respective classes.[77] However, the evolutionary history both of the emergence of molluscs from the ancestral groupLophotrochozoa, and of their diversification into the well-known living andfossil forms, is still vigorously debated.
Debate occurs about whether someEdiacaran and EarlyCambrian fossils really are molluscs.[78]Kimberella, from about555 million years ago, has been described by some paleontologists as "mollusc-like",[79][59] but others are unwilling to go further than "probablebilaterian",[60][63] if that.[80]
There is an even sharper debate about whetherWiwaxia, from about505 million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type ofradula or more similar to that of somepolychaete worms.[60][61] Nicholas Butterfield, who opposes the idea thatWiwaxia was a mollusc, has written that earliermicrofossils from515 to 510 million years ago are fragments of a genuinely mollusc-like radula.[62] This appears to contradict the concept that the ancestral molluscan radula was mineralized.[81]
Spirally coiled shells appear in manygastropods.[18]: 300–343
However, thehelcionellids, which first appear over540 million years ago in Early Cambrian rocks fromSiberia and China,[82][83] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliesttrilobites.[55] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with morelimpet-like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults.[84]
Some analyses of helcionellids concluded these were the earliestgastropods.[85] However, other scientists are not convinced these Early Cambrian fossils show clear signs of thetorsion that identifies modern gastropods twists the internal organs so the anus lies above the head.[18]: 300–343 [86][87]
Volborthella, some fossils of which predate530 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 mineralsilicon dioxide (silica), and it was not divided into a series of compartments bysepta as those of fossil shelled cephalopods and the livingNautilus are.Volborthella's classification is uncertain.[88] The Middle Cambrian fossilNectocaris is often interpreted as a cephalopod with 2 arms and no shell, but the Late Cambrian fossilPlectronoceras is now thought to be the earliest undisputed cephalopod fossil, as its shell had septa and asiphuncle, a strand of tissue thatNautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossilammonite shells. However,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.[89] All cephalopods with external shells except thenautiloids became extinct by the end of theCretaceous period65 million years ago.[90] However, the shell-lessColeoidea (squid,octopus,cuttlefish) are abundant today.[91]
TheHyolitha are a class of extinct animals with a shell andoperculum that may be molluscs. Authors who suggest they deserve their ownphylum do not comment on the position of this phylum in the tree of life.[98]
For millennia, molluscs have been a source of food for humans, as well as important luxury goods, notablypearls,mother of pearl,Tyrian purple dye,sea silk, and chemical compounds. Their shells have also been used as a form ofcurrency in some preindustrial societies. Some species of molluscs can bite or sting humans, and some have become agricultural pests.
Molluscs, especially bivalves such asclams andmussels, have been an important food source since at least the advent ofanatomically modern humans, and this has often resulted in overfishing.[99] Other commonly eaten molluscs includeoctopuses andsquids,whelks,oysters, andscallops.[100] 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.[101] Some countries regulate importation and handling of molluscs and otherseafood, mainly to minimize the poison risk fromtoxins that can sometimes accumulate in the animals.[102]
Most molluscs with shells can produce pearls, but only the pearls ofbivalves and somegastropods, whose shells are lined withnacre, are valuable.[18]: 300–343, 367–403 The best natural pearls are produced by marinepearl oysters,Pinctada margaritifera andPinctada mertensi, which live in thetropical andsubtropical waters of thePacific Ocean. Natural pearls form when a small foreign object gets stuck between themantle and shell.
The two methods of culturingpearls insert either "seeds" or beads into oysters. The "seed" method uses grains of ground shell from freshwatermussels, and overharvesting for this purpose hasendangered several freshwater mussel species in the southeastern United States.[18]: 367–403 The pearl industry is so important in some areas, significant sums of money are spent on monitoring the health of farmed molluscs.[103]
Other luxury and high-status products were made from molluscs.Tyrian purple, made from the ink glands ofmurex shells, "fetched its weight in silver" in the fourth centuryBC, according toTheopompus.[104] The discovery of large numbers ofMurex shells onCrete suggests theMinoans may have pioneered the extraction of "imperial purple" during the Middle Minoan period in the 20th–18th centuries BC, centuries before theTyrians.[105][106]Sea silk is a fine, rare, and valuablefabric produced from the long silky threads (byssus) secreted by several bivalve molluscs, particularlyPinna nobilis, to attach themselves to the sea bed.[107]Procopius, writing on the Persian warscirca 550CE, "stated that the five hereditary satraps (governors) ofArmenia who received their insignia from the Roman Emperor were givenchlamys (or cloaks) made fromlana pinna. Apparently, only the ruling classes were allowed to wear these chlamys."[108]
Mollusc shells, including those ofcowries, were used as a kind ofmoney (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 associal status displays at important occasions, such as weddings.[109] When used for commercial transactions, they functioned as a means of exchange similar to money in ordinarybusiness transactions, a tradable good whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurableinflation if more efficient transport or "goldrush" behavior appeared.[110]
Bivalve molluscs are used asbioindicators 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.[111]Potamopyrgus antipodarum is used by some water treatment plants to test for estrogen-mimicking pollutants from industrial agriculture.Several species of mollusca have been used as bioindicators of environmental stresses that can cause DNA damage. These species include the American oysterCrassostrea virginica,[112] zebra mussels (Dreissena polymorpha)[113][114] and the blue musselMytilus edulis.[115]
Theblue-ringed octopus's rings are a warning signal; this octopus is alarmed, and its bite can kill.[116]
Some molluscs sting or bite, but deaths from mollusc venoms total less than 10% of those fromjellyfish stings.[117]
All octopuses are venomous,[118] but only a few species pose a significant threat to humans.Blue-ringed octopuses in the genusHapalochlaena, which live around Australia and New Guinea, bite humans only if severely provoked,[116] but their venom kills 25% of human victims. Another tropical species,Octopus apollyon, causes severeinflammation that can last for over a month even if treated correctly,[119] and the bite ofOctopus rubescens can causenecrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated.[120]
All species ofcone 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 oftoxins, some fast-acting and others slower but deadlier.[121][117][122] The effects of individual cone-shell toxins on victims' nervous systems are so precise as to be useful tools for research inneurology, and the small size of theirmolecules makes it easy to synthesize them.[121][123]
Schistosomiasis (also known as bilharzia, bilharziosis or snail fever), a disease caused by the fluke wormSchistosoma, 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."[124] 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 asintermediate hosts.[125]
Some species of molluscs, particularly certain snails andslugs, can be serious crop pests,[126] and when introduced into new environments, can unbalance localecosystems. One such pest, the giant African snailAchatina fulica, has been introduced to many parts of Asia, as well as to many islands in theIndian Ocean andPacific Ocean. In the 1990s, this species reached theWest Indies. Attempts to control it by introducing the predatory snailEuglandina rosea proved disastrous, as the predator ignoredAchatina fulica and went on to extirpate several native snail species instead.[127]
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^Rosenberg, Gary (2014). "A new critical estimate of named species-level diversity of the recent mollusca".American Malacological Bulletin.32 (2):308–322.doi:10.4003/006.032.0204.S2CID86761029.
^abcdPonder, W.F.; Lindberg, D.R., eds. (2008).Phylogeny and Evolution of the Mollusca. Berkeley, CA: University of California Press. p. 481.ISBN978-0-520-25092-5.
^abHealy, J.M. (2001). "The Mollusca". In Anderson, D.T. (ed.).Invertebrate Zoology (2 ed.). Oxford University Press. pp. 120–171.ISBN978-0-19-551368-4.
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^abcWilbur, Karl M.; Trueman, E.R.; Clarke, M.R., eds. (1985),The Mollusca, vol. 11. Form and Function, New York: Academic Press,ISBN0-12-728702-7 page 4
^Shigeno, S.; Sasaki, T.; Moritaki, T.; Kasugai, T.; Vecchione, M.; Agata, K. (January 2008). "Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus embryonic development".Journal of Morphology.269 (1):1–17.doi:10.1002/jmor.10564.PMID17654542.S2CID13109195.
^Jensen, K. R. (February 1993). "Morphological adaptations and plasticity of radular teeth of the Sacoglossa (= Ascoglossa) (Mollusca: Opisthobranchia) in relation to their food plants".Biological Journal of the Linnean Society.48 (2):135–155.doi:10.1111/j.1095-8312.1993.tb00883.x.
^Budd GE, Jensen S (May 2000). "A critical reappraisal of the fossil record of the bilaterian phyla".Biol Rev Camb Philos Soc.75 (2):253–95.doi:10.1017/s000632310000548x (inactive 3 December 2024).PMID10881389.{{cite journal}}: CS1 maint: DOI inactive as of December 2024 (link)
^Steiner, M.; Li, G.; Qian, Y.; Zhu, M.; Erdtmann, B.D. (2007). "Neoproterozoic to Early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China)".Palaeogeography, Palaeoclimatology, Palaeoecology.254 (1–2): 67.Bibcode:2007PPP...254...67S.doi:10.1016/j.palaeo.2007.03.046.
^Mus, M.M.; Palacios, T.; Jensen, S. (2008). "Size of the earliest mollusks: Did small helcionellids grow to become large adults?".Geology.36 (2): 175.Bibcode:2008Geo....36..175M.doi:10.1130/G24218A.1.
^Frýda, J.; Nützel, A.; Wagner, P.J. (2008)."Paleozoic Gastropoda". In Ponder, W.F.; Lindberg, D.R. (eds.).Phylogeny and evolution of the Mollusca. California Press. pp. 239–264.ISBN978-0-520-25092-5.
^Zong-Jie, F. (2006). "An introduction to Ordovician bivalves of southern China, with a discussion of the early evolution of the Bivalvia".Geological Journal.41 (3–4):303–328.Bibcode:2006GeolJ..41..303Z.doi:10.1002/gj.1048.S2CID129430674.
^Mannino, M.A.; Thomas, K.D. (2002). "Depletion of a resource? The impact of prehistoric human foraging on intertidal mollusc communities and its significance for human settlement, mobility and dispersal".World Archaeology.33 (3):452–474.doi:10.1080/00438240120107477.JSTOR827879.S2CID161085658.
^Garrow, J.S.; Ralph, A.; James, W.P.T. (2000).Human Nutrition and Dietetics. Elsevier Health Sciences. p. 370.ISBN978-0-443-05627-7.
^The fourth-centuryBC historianTheopompus, cited by Athenaeus (12:526) around 200 BC; according toGulick, C.B. (1941).Athenaeus, The Deipnosophists. Cambridge, Massachusetts: Harvard University Press.ISBN978-0-674-99380-8.
^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.S2CID129588313.
^Hogendorn, J. & Johnson, M. (2003).The Shell Money of the Slave Trade. Cambridge University Press.ISBN978-0521541107. 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)
Sturm, C.; Pearce, T.A.; Valdes, A. (2006).The Mollusks: A Guide to Their Study, Collection, and Preservation. Universal.ISBN1-58112-930-0.OCLC69028066.
Trigo, J.E.; Díaz Agras, G.J.; García-Álvarez, O.L.; Guerra, A.; Moreira, J.; Pérez, J.; Rolán, E.; Troncoso, J.S.; Urgorri, V. (2018). Troncoso, J.S.; Trigo, J.E.; Rolán, E. (eds.).Guía de los Moluscos Marinos de Galicia (in Spanish). Vigo: Servicio de Publicacións da Universidade de Vigo.ISBN978-84-8158-787-6.OCLC1107124658.
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