An introduction
A cuttlefish, a coleoid cephalopod, moves generally by undulating its body fins.

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Mollusca is one of the most varied groups of pets on the planet, v at the very least 50,000 living species (and more likely about 200,000). It contains such familiar organisms together snails, octopuses, squid, clams, scallops, oysters, and also chitons. Mollusca additionally includes some lesser known groups like the monoplacophorans, a group once assumed to be extinct for countless years until one was discovered in 1952 in the deep ocean off the coastline of Costa Rica.

Molluscs space a clade the organisms that all have soft body which typically have a "head" and also a "foot" region. Regularly their bodies space covered by a tough exoskeleton, as in the shells of snails and clams or the plates of chitons.

A component of nearly every ecosystem in the world, molluscs are extremely essential members of plenty of ecological communities. They variety in distribution from terrestrial mountain tops to the warm vents and also cold seeps of the deep sea, and range in dimension from 20-meter-long huge squid to microscopic aplacophorans, a millimeter or less in length, that live in between sand grains.

These creatures have actually been vital to human beings throughout background as a source of food, jewelry, tools, and even pets. For example, ~ above the Pacific coast of California, aboriginal Americans consumed big quantities that abalone and especially owl limpets. However, the influence of native Americans on these molluscan communities pales by comparison to the overharvesting of some molluscan taxa by the United says in the 1960s and also 1970s. Species whose members once numbered in the millions, currently teeter ~ above the verge the extinction. For example, fewer than 100 white abalone remain after numerous million people were captured and sold as meat in the 1970s. Besides having actually yummy soft parts, molluscs frequently have preferable hard parts. The shells of some molluscs are thought about quite beautiful and also valuable. Molluscs can likewise be nuisances, such as the typical garden snail; and molluscs consist of a major component of fouling communities both on docks and also on the hulls the ships.

On the left is a maritime snail, the California Trivia (Trivia californiana). Here the mantle covers lot of the shell. Note how a part of the mantle is rolled into a tube form to form the siphon just over the head. At the right is a repair of among the largest of all molluscs, the giant Squid (Architeuthis).

They also have a an extremely long and rich fossil document going back much more than 550 million years, making them one of the most common species of organism supplied by paleontologists to study the background of life.

SystematicsMolluscan systematics are still in flux. Together you have the right to see from the cladogram below, there is quiet no commitment on several of the significant relationships. The polytomies presented indicate the the question of which molluscs space the most carefully related is tho a issue of debate.

However, brand-new types the data and much larger and more sophisticated analyses continue to it is in performed. The resolved relationships presented (such together cephalopods, scaphopods, and also gastropods) are recent discoveries.


Visit the mollusca pages ~ above the Tree of Life for an ext on molluscan systematics.

MorphologyDespite their amazing diversity, every molluscs share part unique qualities that specify their human body plan. The body has a head, a foot and also a visceral mass. This is all spanned with a mantle (also recognized as a pallium) that commonly secretes the shell. In part groups, prefer slugs and octopuses, the mantle is secondarily lost, while in others, it is offered for various other activities, such as respiration.

The freshwater Sinistral Pond Snail (Physella sp.) scrapes birds from the glass with its radula, the two "toothy" arcs you have the right to see lining the mouth. Click on the photograph for a closer look.

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

Typically, at least in the an ext primitive members of every group, there space one or much more pairs of gills (called ctenidia) i m sorry lie in a posterior cavity (the pallial cavity) or in a posterolateral groove surrounding the foot. The pallial cavity commonly contains a pair the sensory osphradia (for smelling) and is the space into i beg your pardon the kidneys, gonads, and also anus open.

Molluscs room coelomate, back the coelom is reduced and also represented through the kidneys, gonads, and pericardium, the key body cavity which surrounds the heart.

Life history and ecologyMolluscs occur in nearly every habitat discovered on Earth, wherein they are often the most conspicuous organisms. While many are found in the marine environment, prolonging from the intertidal to the deepest oceans, several significant gastropod clades live mostly in freshwater or terrestrial habitats. Remarkably, one research found about 3000 species within a single locality in ~ a coral reef in brand-new Caledonia. In terrestrial communities, gastropods can achieve reasonably high diversity and also abundance: as numerous as 60-70 types may coexist in a solitary habitat and abundance in leaf litter can exceed an ext than 500 people in 4 liters the litter.

Many maritime molluscs emerge from your eggs together planktonic trochophore larvae, however, Sinistral Pond Snails (Physella sp.) arise from their eggs as young snails. The whitish, jellybean-shaped organisms space ostracodes (crustaceans).
Marine molluscs happen on a big variety the substrates including rocky shores, coral reefs, dirt flats, and also sandy beaches. Gastropods and also chitons are characteristic of these hard substrates, and bivalves space commonly connected with softer substrates wherein they burrow into the sediment. However, over there are many exceptions: the largest living bivalve, Tridacna gigas, lives on coral reefs, and also many bivalves (e.g., mussels and also oysters) connect themselves to tough substrates. Some microscopic gastropods even live interstitially in between sand grains.

Large concentration of gastropods and bivalves are found at hydrothermal vents in the deep sea. Living in this or other dysoxic habitats appears to be a plesiomorphic condition for the Mollusca and several outgroups. For example, the fauna of Palaeozoic hydrothermal vent areas includes the molluscan groups Bivalvia, Monoplacophora and also Gastropoda as well as the outgroups Brachiopoda and Annelida.

The fostering of various feeding habits shows up to have had a profound influence on molluscan evolution. The adjust from grazing to other creates of food acquisition is just one of the major features in the radiation that the group. Based upon our existing understanding of relationships, the earliest molluscs grazed on encrusting animals and detritus. Such feeding may have actually been selective or indiscriminate and will have incorporated algal, diatom, or cyanobacterial films and also mats, or encrusting early american animals. Important herbivorous grazers are reasonably rare and are minimal to part polyplacophorans and also a couple of gastropod groups. Most chaetodermomorph aplacophorans, monoplacophorans and also scaphopods feed on protists and/or bacteria if neomeniomorph aplacophorans graze top top cnidarians. Cephalopods room mainly active predators as space some gastropods, while a few chitons and also septibranch bivalves capture microcrustaceans. Many bivalves room either suspension or deposit feeders the indiscriminately take in particles, yet then elaborately type them based upon size and weight, frequently assimilating bacteria, protists, and diatoms.

The fossil recordThe Mollusca include some of the earliest metazoans known. So late Precambrian rocks of southern Australia and also the White Sea an ar in north Russia save bilaterally symmetrical, benthic pets with a univalved covering (Kimberella) the resembles those the molluscs. The earliest unequivocal molluscs are helcionelloid molluscs that date from so late Ediacaran (Vendian) rocks. In the early on Cambrian the Coeloscleritophora are additionally present. Most of the familiar groups, consisting of gastropods, bivalves, monoplacophorans, and also rostroconchs, all day from the early on Cambrian, vice versa, cephalopods are very first found in the middle Cambrian, polyplacophorans in the so late Cambrian, and also the Scaphopoda in the middle Ordovician. Most of these early taxa tend to be small (‹10 mm in length). The so late Vendian-Early Cambrian taxa bear small resemblance come the Cambrian-Ordovician taxa (most the which continue to be extant today).

On the left is Inoceramus sp., a bivalve indigenous the Cretaceous that Alameda County, CA. At appropriate is Turritella andersoni, a gastropod native the Eocene that Ventura County, CA.

After their initial appearance, molluscan taxonomic diversity had tendency to stay low until the Ordovician, once gastropods, bivalves, and also cephalopods show large increases in diversity. For bivalves and also gastropods this diversification boosts throughout the Phanerozoic, with fairly small losses at the end-Permian and also end-Cretaceous extinction events. Cephalopod diversity is much an ext variable through the Phanerozoic, vice versa, the remaining teams (monoplacophorans, rostroconchs, polyplacophorans, and scaphopods) preserve low diversity over the entire Phanerozoic or came to be extinct.

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Original text by Paul Bunje, 2003. Image of cuttlefish, California Trivia, huge Squid, Sinistral Pond Snail radula and eggs, all © Larry Jon Friesen. Photos of Inoceramus and Turritella andersoni by buy it Rieboldt, © betterworld2016.org. Mollusca phylogeny based on Sigwart, J.D., and M.D. Sutton. 2007. Deep molluscan phylogeny: synthesis of palaeontological and neontological data. Proceedings that The Royal society B 274(1624):2413-2419; and suggestions from gonzalo Giribet, Harvard University.