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Molluscs are a clade of organisms that all have soft bodies which typically have a "head" and a "foot" region. Often their bodies are covered by a hard exoskeleton, as in the shells of snails and clams or the plates of chitons.
A part of almost every ecosystem in the world, molluscs are extremely important members of many ecological communities. They range in distribution from terrestrial mountain tops to the hot vents and cold seeps of the deep sea, and range in size from 20-meter-long giant squid to microscopic aplacophorans, a millimeter or less in length, that live between sand grains.
These creatures have been important to humans throughout history as a source of food, jewelry, tools, and even pets. For example, on the Pacific coast of California, Native Americans consumed large quantities of abalone and especially owl limpets. However, the impact of Native Americans on these molluscan communities pales by comparison to the overharvesting of some molluscan taxa by the United States in the 1960s and 1970s. Species whose members once numbered in the millions, now teeter on the verge of extinction. For example, fewer than 100 white abalone remain after several million individuals were captured and sold as meat in the 1970s. Besides having yummy soft parts, molluscs often have desirable hard parts. The shells of some molluscs are considered quite beautiful and valuable. Molluscs can also be nuisances, such as the common garden snail; and molluscs make up a major component of fouling communities both on docks and on the hulls of ships.
They also have a very long and rich fossil record going back more than 550 million years, making them one of the most common types of organism used by paleontologists to study the history of life.
However, new types of data and much larger and more sophisticated analyses continue to be performed. The resolved relationships shown (such as cephalopods, scaphopods, and gastropods) are recent discoveries.
The buccal cavity, at the anterior of the mollusc, contains a radula (lost in bivalves) a ribbon of teeth supported by an odontophore, a muscular structure. The radula is generally used for feeding. The ventral foot is used in locomotion. This foot propels the mollusc by utilizing muscular waves and/or cilia in combination with mucus.
Typically, at least in the more primitive members of each group, there are one or more pairs of gills (called ctenidia) which lie in a posterior cavity (the pallial cavity) or in a posterolateral groove surrounding the foot. The pallial cavity typically contains a pair of sensory osphradia (for smelling) and is the space into which the kidneys, gonads, and anus open.
Molluscs are coelomate, although the coelom is reduced and represented by the kidneys, gonads, and pericardium, the main body cavity which surrounds the heart.
Life history and ecology
Large concentrations of gastropods and bivalves are found at hydrothermal vents in the deep sea. Living in these or other dysoxic habitats appears to be a plesiomorphic condition for the Mollusca and several outgroups. For example, the fauna of Palaeozoic hydrothermal vent communities includes the molluscan groups Bivalvia, Monoplacophora and Gastropoda as well as the outgroups Brachiopoda and Annelida.
The adoption of different feeding habits appears to have had a profound influence on molluscan evolution. The change from grazing to other forms of food acquisition is one of the major features in the radiation of the group. Based on our current understanding of relationships, the earliest molluscs grazed on encrusting animals and detritus. Such feeding may have been selective or indiscriminate and will have encompassed algal, diatom, or cyanobacterial films and mats, or encrusting colonial animals. Truly herbivorous grazers are relatively rare and are limited to some polyplacophorans and a few gastropod groups. Most chaetodermomorph aplacophorans, monoplacophorans and scaphopods feed on protists and/or bacteria while neomeniomorph aplacophorans graze on cnidarians. Cephalopods are mainly active predators as are some gastropods, while a few chitons and septibranch bivalves capture microcrustaceans. Most bivalves are either suspension or deposit feeders that indiscriminately take in particles, but then elaborately sort them based on size and weight, typically assimilating bacteria, protists, and diatoms.
The fossil record
After their initial appearance, molluscan taxonomic diversity tended to remain low until the Ordovician, when gastropods, bivalves, and cephalopods show large increases in diversity. For bivalves and gastropods this diversification increases throughout the Phanerozoic, with relatively small losses at the end-Permian and end-Cretaceous extinction events. Cephalopod diversity is much more variable through the Phanerozoic, whereas the remaining groups (monoplacophorans, rostroconchs, polyplacophorans, and scaphopods) maintain low diversity over the entire Phanerozoic or became extinct.
Original text by Paul Bunje, 2003. Photos of cuttlefish, California Trivia, Giant Squid, Sinistral Pond Snail radula and eggs, all © Larry Jon Friesen. Photos of Inoceramus and Turritella andersoni by Sarah Rieboldt, © UCMP. Mollusca phylogeny based on Sigwart, J.D., and M.D. Sutton. 2007. Deep molluscan phylogeny: synthesis of palaeontological and neontological data. Proceedings of The Royal Society B 274(1624):2413-2419; and suggestions from Gonzalo Giribet, Harvard University.
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