It has been suggested that this article should be
Hypothetical ancestral mollusc. (discuss) (May 2023)
|85,000 recognized living species.|
Mollusca is the second-largest
Molluscs are the largest
The three most universal features defining modern molluscs are a
Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541–485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral 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
A handful of mollusc species are sometimes considered hazards or pests for human activities. The bite of the
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 now known, these groups have no relation to molluscs, and very little to one another, so the name Molluscoida has been abandoned.
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.
Molluscs have developed such a varied range of body structures, finding
- The mantle cavity.
- The genitalsopen into the mantle cavity.
- There are two pairs of main nerve cords.
Other characteristics that commonly appear in textbooks have significant exceptions:
|Whether characteristic is found in these classes of Molluscs|
|Supposed universal Molluscan characteristic||Aplacophora: 291–292||
|Monoplacophora: 298–300||Gastropoda: 300–343||
|Radula, a rasping "tongue" with chitinous teeth||Absent in 20% of
|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|
|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|
Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.
Molluscs have more varied forms than any other animal
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.
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". The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs.
Mantle and mantle cavity
The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according to
The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate), except the outermost layer, which in almost all cases is all conchiolin (see periostracum). Molluscs never use phosphate to construct their hard parts, with the questionable exception of Cobcrephora. 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.
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.
In some forms the shell contains openings. In abalones there are holes in the shell used for respiration and the release of egg and sperm, in the nautilus a string of tissue called the siphuncle goes through all the chambers, and the eight plates that make up the shell of chitons are penetrated with living tissue with nerves and sensory structures.
The underside consists of a muscular foot, which has adapted to different purposes (locomotion, grasping the substratum, burrowing or feeding) in different classes. 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. In bivalves, the foot is adapted for burrowing into the sediment; in cephalopods it is used for jet propulsion, and the tentacles and arms are derived from the foot.
Exceptions to the above are the molluscs Planorbidae or ram's horn snails, which are air-breathing snails that use iron-based hemoglobin 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 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.
Eating, digestion, and excretion
Molluscs' mouths also contain
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.
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.
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.
The cephalic molluscs have two pairs of main
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".[
The simplest molluscan reproductive system relies on
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. The development of molluscs is of particular interest in the field of ocean acidification as environmental stress is recognized to affect the settlement, metamorphosis, and survival of larvae.
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.
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
Opinions vary about the number of
|Class||Major organisms||Described living species||Distribution|
|Gastropoda : 300||all
sea hares, sea butterflies
|70,000||marine, freshwater, land|
|Bivalvia : 367||clams, oysters, scallops, geoducks, mussels, rudists†||20,000||marine, freshwater|
Polyplacophora : 292–298
|chitons||1,000||rocky tidal zone and seabed|
|Cephalopoda : 343||squid, octopuses, cuttlefish, nautiluses, Spirula, belemnites†, ammonites†||900||marine|
Scaphopoda : 403–407
|tusk shells||500||marine 6–7,000 metres (20–22,966 ft)|
|† Cricoconarida||extinct|
|Aplacophora : 291–292||worm-like molluscs||320||seabed 200–3,000 metres (660–9,840 ft)|
|Monoplacophora : 298–300||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†||fossils; probable ancestors of bivalves||
|Helcionelloida†||fossils; snail-like molluscs such as Latouchella||extinct||marine|
Classification into higher taxa for these groups has been and remains problematic. A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora. Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda. Tentaculita may also be in Mollusca (see Tentaculites).
Good evidence exists for the appearance of
Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. Kimberella, from about , has been described by some paleontologists as "mollusc-like", but others are unwilling to go further than "probable bilaterian", if that.
There is an even sharper debate about whether Wiwaxia, from about , 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. Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from are fragments of a genuinely mollusc-like radula. This appears to contradict the concept that the ancestral molluscan radula was mineralized.
However, the Helcionellids, which first appear over in Early Cambrian rocks from Siberia and China, are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites. 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.
Some analyses of helcionellids concluded these were the earliest
The Early Cambrian fossils
The 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.
Molluscs are generally regarded members of the Lophotrochozoa, 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 are the annelid worms and seven marine phyla. 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. 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. Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments, while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized. 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.
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. Eventually, the cuticle would have become mineralized, using the same genetic machinery (engrailed) as most other bilaterian skeletons. The first mollusc shell almost certainly was reinforced with the mineral aragonite.
The evolutionary relationships within the molluscs are also debated, and the diagrams below show two widely supported reconstructions:
Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses, although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.
However, an analysis in 2009 using both
For millennia, molluscs have been a source of food for humans, as well as important luxury goods, notably
Uses by humans
Molluscs, especially bivalves such as
Most molluscs with shells can produce pearls, but only the pearls of
The two methods of culturing
Other luxury and high-
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. 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.
Bivalve molluscs are used as
Harmful to humans
Stings and bites
All octopuses are venomous,
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. 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.
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." 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.
Some species of molluscs, particularly certain snails and
- The formerly dominant U.K. spelling mollusk is still used in the U.S.—see the reasons given by Gary Rosenberg (1996). For the spelling mollusc, see the reasons given in: Brusca & Brusca. Invertebrates (2nd ed.)..
- Rosenberg, Gary (1996). "Mollusckque – Mollusk vs. Mollusc". Archived from the original on 3 March 2012.
- Taylor, P.D.; Lewis, D.N. (2005). Fossil Invertebrates. Harvard University Press.
- Aristotle. "Book I part 1, Book IV part 1, etc.". History of Animals.
- Little, L.; Fowler, H.W.; Coulson, J.; Onions, C.T., eds. (1964). "Malacology". Shorter Oxford English Dictionary. Oxford University press.
- Chisholm, Hugh, ed. (1911). . Encyclopædia Britannica. Vol. 18 (11th ed.). Cambridge University Press. p. 675.
- Hogan, C. Michael. (2010). "Calcium". In Jorgensen, A.; Cleveland, C. (eds.). Encyclopedia of Earth. National Council for Science and the Environment.
- : 284–291
- Hancock, Rebecca (2008). "Recognising research on molluscs". Australian Museum. Archived from the original on 30 May 2009. Retrieved 9 March 2009.
- Ponder, Winston F. & Lindberg, David R. (2004). "Phylogeny of the Molluscs" (Press release). World Congress of Malacology. Retrieved 9 March 2009.
- Barnes, R.S.K.; Calow, P.; Olive, P.J.W.; Golding, D.W.; Spicer, J.I. (2001). The Invertebrates: A synthesis (3 ed.). UK: Blackwell Science.
- Black, Richard (26 April 2008). "Colossal squid out of the freezer". BBC News. Retrieved 1 October 2008.[permanent dead link]
- Tendal O.S. (1985). "Xenophyophores (Protozoa, Sarcodina) in the diet of Neopilina galatheae (Mollusca, Monoplacophora)" (PDF). Galathea Report. 16: 95–98. Archived from the original (PDF) on 30 November 2012. Retrieved 14 September 2013.
- Publishers, Bentham Science (July 1999). Current Organic Chemistry. Bentham Science Publishers.
- Lambert, W.J. (1 October 1991). "Coexistence of hydroid-eating Nudibranchs: Do feeding biology and habitat use matter?". Archived from the original on 3 December 2021.
- Budd, G. E. & Jensen, S. A critical reappraisal of the fossil record of the bilaterian phyla. Biol. Rev. 75, 253–295 (2000).
- Budd, G. E., and S. Jensen. 2016: The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution. Biological Reviews 7:Online ahead of print.
- Kouchinsky, A. (2000). "Shell microstructures in Early Cambrian molluscs" (PDF). Acta Palaeontologica Polonica. 45 (2): 119–150. Retrieved 4 November 2009.
- Hagadorn, J.W. & Waggoner, B.M. (2002). "The Early Cambrian problematic fossil Volborthella: New insights from the Basin and Range". In Corsetti, F.A. (ed.). Proterozoic-Cambrian of the Great Basin and Beyond, Pacific Section SEPM Book 93 (PDF). SEPM (Society for Sedimentary Geology). pp. 135–150. Archived from the original (PDF) on 11 September 2006.
- Monks, N. "A Broad Brush History of the Cephalopoda". Retrieved 21 March 2009.
- Pojeta, J. (2000). "Cambrian Pelecypoda (Mollusca)". American Malacological Bulletin. 15: 157–166.
- "The Mollusca". University of California Museum of Paleontology. Retrieved 2 October 2008.
- "Introduction to the Lophotrochozoa". University of California Museum of Paleontology. Retrieved 2 October 2008.
- "Importing fishery products or bivalve molluscs". United Kingdom: Food Standards Agency. Archived from the original on 30 October 2012. Retrieved 2 October 2008.
- 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.
- Université Bordeaux; et al. "MolluSCAN eye project". Archived from the original on 13 November 2016. Retrieved 28 January 2017.
- Alafaci, A. (5 June 2018). "Blue ringed octopus". Australian Venom Research Unit. Retrieved 3 October 2008.
- Anderson, R.C. (1995). "Aquarium husbandry of the giant Pacific octopus". Drum and Croaker. 26: 14–23.
- Anderson, R.C. (1999). "An octopus bite and its treatment". The Festivus. 31: 45–46.
- 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 3 October 2008.
- Livett, B. "Cone Shell Mollusc Poisoning, with Report of a Fatal Case". Department of Biochemistry and Molecular Biology, University of Melbourne. Archived from the original on 7 November 2012. Retrieved 3 October 2008.
- "The Carter Center Schistosomiasis Control Program". The Carter Center. Retrieved 3 October 2008.
- Sturm, C.; Pearce, T.A. & Valdes, A. The Mollusks: A Guide to Their Study, Collection, and Preservation. Universal Publishers. 2006. 454 pages. ISBN 1581129300
- 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., ed. Guía de los Moluscos Marinos de Galicia. Vigo: Servicio de Publicacións da Universidade de Vigo. 836 pages. ISBN 978-84-8158-787-6
- "Mollusca" at the 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 Watch Programme
- Online biomonitoring of bivalve activity, 24/7: MolluSCAN eye Archived 2016-11-13 at the Wayback Machine