Mollusca

Taxobox
name = Molluscs
fossil_range = Ediacaran or Cambrian – Recent

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image_width = 200px
image_caption = Caribbean Reef Squid, "Sepioteuthis sepioidea"
regnum = Animalia
superphylum = Lophotrochozoa
phylum = Mollusca
phylum_authority = Linnaeus, 1758
subdivision_ranks = Classes
subdivision =
Aplacophora
Bivalvia
Caudofoveata
Cephalopoda
Gastropoda † Helcionelloida
Monoplacophora
Polyplacophora † Rostroconchia
Scaphopoda † Tentaculita
diversity = c.250,000 species [Ponder, Winston F. and Lindberg, David R. (Eds.) (2008) [http://www.ucpress.edu/books/pages/10802.php "Phylogeny and Evolution of the Mollusca"] . Berkeley: University of California Press. 481 pp. ISBN 978-0520250925.]
diversity_link = Mollusca

MolluscsCommonly spelt mollusk in the USA; however the spelling "mollusc" is preferred for the reasons given by bruscabrusca.] are animals belonging to the phylum Mollusca. There are around 250,000 extant species within the phylum, with an estimated 70,000 extinct species. It is an extraordinarily diverse group, and representatives of the phylum live in a wide range of environments, from the deep-sea to fairly moist terrestrial habitats.

The phylum Mollusca is typically divided into nine or ten taxonomic classes, of which two are extinct. 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 named species, accounting for 80% of the total. Gastropods are the only molluscan class which has been able to adapt to the land and freshwater as well as the oceans.

Molluscs have such a varied range of body structures that it is difficult to find defining characteristics that apply to all modern groups; the most universal feature is a mantle with a significant cavity used for breathing and excretion, and the structure of the nervous system. As a result of these wide variations, many textbooks base their descriptions on a hypothetical "generalized mollusc". 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 that covers the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom is very small, and the main body cavity is a hemocoel through which blood circulates – molluscs' circulatory systems are mainly open. The "generalized" mollusc feeding system consists of a rasping "tongue" called a radula and a complex digestive system in which slimy mucus and fine, muscle-powered "hairs" called cilia play various important roles.

A striking feature of molluscs is the use of the same organ for multiple functions: the gills both "breathe" and (in bivalves) produce a water current in the mantle cavity which is important for excretion and reproduction; the heart and nephridia ("kidneys") are important parts of the reproductive system as well as of the circulatory and excretory systems respectively.

The "generalized mollusc" has two paired nerve cords, or three in cephalopods. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors that detect chemicals, vibrations and touch. The simplest type of molluscan reproductive system relies on external fertilization, but there are more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults.

There is good evidence for the appearance of gastropods, cephalopods and bivalves in the Cambrian period ma|Cambrian|period end|Cambrian. 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 anatomically modern humans. However there is a risk of food-poisoning from toxins that accumulate in molluscs in certain conditions, and many countries have regulations that aim to minimize this risk. Molluscs have for centuries also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. On the other hand, the bite of the blue-ringed octopus is often fatal, and that of "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 sophsticated yet 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 miliion 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.

Diversity

The molluscs, with 250,000 species, are the second only to arthropods in numbers of living animal species citation | editor=Ponder, W.F. and Lindberg, D.R.
date=2008 | title=Phylogeny and Evolution of the Mollusca
publisher=Berkeley: University of California Press | pages=481 | isbn=978-0520250925] – far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000.cite book
author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | page=Front endpaper 1] There have also been an estimated 70,000 extinct species.cite book
author=Brusca, R.C., and Brusca, G.J. | title=Invertebrates | edition=2 | date=2003 | page=702
publisher=Sinauer Associates | isbn=0878930973] Molluscs have more varied forms than any other animal phylum – snails and other gastropods, clams and other bivalves, squids and other cephalopods, and other less well-known but similarly distinctive sub-groups. The majority of species still live in the oceans, from the sea-shores to the abyssal zone, but are also significant members of freshwater and terrestrial ecosystems. They are extremely diverse in tropical and temperate regions but can be found at all latitudes. About 80% of all known mollusc species are gastropods. Cephalopoda such as squid, cuttlefish and octopus are among the most neurologically-advanced of all invertebrates. cite book
author=Barnes, R.S.K., Calow, P., Olive, P.J.W., Golding, D.W. and Spicer, J.I. | date=2001
title=The Invertebrates, A Synthesis | edition=3| publisher=Blackwell Science | location =UK ] The giant squid, which until recently had not been observed alive in its adult form, [Kubodera, T. & Mori, K. (2005) PDFlink|1= [http://www.journals.royalsoc.ac.uk/openurl.asp?genre=article&issn=0962-8452&volume=272&issue=1581&spage=2583 First-ever observations of a live giant squid in the wild.] "Proceedings of the Royal Society B: Biological Sciences" 272 (1581), 2583-2586.] is one of the largest invertebrates. However a recently-caught specimen of the colossal squid, convert|10|m|ft long and weighing convert|500|kg|ton, may have overtaken it. [cite web
author=Richard Black | title=Colossal squid out of the freezer
publisher=BBC News A date=April 26, 2008
url=http://news.bbc.co.uk/1/hi/sci/tech/7367774.stm | accessdate=2008-10-01 ]

General description

Definition

The word ' is derived from the French "mollusque", which originated from the Latin "molluscus", from ', soft. "Molluscus" was itself an adaptation of Aristotle's τᾲ μαλάκια, "the soft things", which he applied to cuttlefish. [cite book
contribution=Mollusca | title=Shorter Oxford English Dictionary
editor=Little, L., Fowler, H.W., Coulson, J., and Onions, C.T.
publisher=Oxford University press | date=1964 ] The scientific study of molluscs is known as malacology. [cite book
contribution=Malacology | title=Shorter Oxford English Dictionary
editor=Little, L., Fowler, H.W., Coulson, J., and Onions, C.T.
publisher=Oxford University press | date=1964 ]

Molluscs have developed such a varied range of body structures that it is difficult to find synapomorphies (defining characteristics) that apply to all modern groups.cite journal
author=Giribet, G., Okusu, A., Lindgren, A.R., Huff, S.W., Schrödl, M., and Nishiguchi, M.K.
title=Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons
journal=Proceedings of the National Academy of Sciences of the United States of America
date=May 2006 | volume=103 | issue=20 | pages=7723-7728 | doi=10.1073/pnas.0602578103 |url=http://www.pnas.org/content/103/20/7723.full | accessdate=2008-09-30 ] The following are present in all modern molluscs:cite book
author=Brusca, R.C., and Brusca, G.J. | title=Invertebrates | edition=2 | date=2003 | page=702
publisher=Sinauer Associates | isbn=0878930973] cite book
author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=284-291]
*The dorsal part of the body wall is a mantle 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 at least two pairs of main nerve cords (three in bivalvescite book
author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=367-403] )

Other characteristics that commonly appear in textbooks have significant exceptions:There is debate about whether some Ediacaran and Early Cambrian fossils really are molluscs. "Kimberella", from about ma|555, has been described as "mollusc-like",cite journal
author = Fedonkin, M.A. | coauthors = Waggoner, B.M. | year = 1997
title = The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
journal = Nature | volume = 388 | issue = 6645 | pages = 868 | doi = 10.1038/42242] cite journal | title=New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications | author=Fedonkin, M.A., Simonetta, A. and Ivantsov, A.Y. | journal=Geological Society, London, Special Publications | date=2007 | volume-286 | pages=157-179 | doi=10.1144/SP286.12 | url=http://www.geosci.monash.edu.au/precsite/docs/workshop/prato04/abstracts/fedonkin2.pdf | accessdate=2008-07-10 ] but others are unwilling to go further than "probable bilaterian".cite journal
author = Butterfield, N.J. | year = 2006
title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
journal = Bioessays | volume = 28 | issue = 12 | pages = 1161–6 | doi = 10.1002/bies.20507
accessdate = 2007-05-21] There is an even sharper debate about whether "Wiwaxia", from about ma|505 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.cite journal
author = Caron, J.B. | coauthors = Scheltema, A., Schander, C., and Rudkin, D. | year = 2006
date=2006-07-13
title = A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale
journal = Nature | volume = 442 | issue = 7099 | pages = 159–163 | issn = | doi = 10.1038/nature04894
url = http://www.nature.com/nature/journal/v442/n7099/pdf/nature04894.pdf | accessdate = 2008-08-07 ] Nicholas Butterfield, who opposes the idea that "Wiwaxia" was a mollusc, has written that earlier microfossils from ma|515|510 are fragments of a genuinely mollusc-like radula. cite journal
author=Butterfield, N.J. | title=An Early Cambrian Radula
journal=Journal of Paleontology | date=May 2008 | volume=82 | issue=3 | pages=543-554
doi=10.1666/07-066.1
url=http://findarticles.com/p/articles/mi_qa3790/is_200805/ai_n25501673/pg_1?tag=artBody;col1
accessdate=2008-08-20]

However the Helcionellids, which first appear over ma|535 in the Early Cambrian, are thought to be early molluscs with rather snail-like shells, and possibly the ancestors of the modern conchiferans, a group that includes all the well-known modern families – gastropods, cephalopods and bivalves. cite journal
author=Runnegar, B. and Pojeta, J. | date=1974
title=Molluscan phylogeny: the paleontological viewpoint
journal=Science | volume=186 | issue=4161 | pages=311-7 | doi=10.1126/science.186.4161.311
url=http://www.sciencemag.org/cgi/content/abstract/186/4161/311 | accessdate=2008-07-30 ] [ citation
author=Peel, J.S. | date=1991 | contribution=Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca | editor=Simonetta, A.M. and Conway Morris, S
title=The Early Evolution of Metazoa and the Significance of Problematic Taxa | isbn=0521402425
publisher=Cambridge University Press |pages=157-177 ] [cite journal
author=Gubanov, A.P., and Peel, J.S.
title=The early Cambrian helcionelloid mollusc "Anabarella" Vostokova
journal=Palaeontology | volume=46 | issue=5 | pages=1073-1087 | date=November 2003
doi=10.1111/1475-4983.00334 ] Although most Helcionellid fossils are only a few millimeters long, the discovery of larger specimens in 2008 has led to suggestions that the tiny specimens were juveniles and that adults were a few centimeters long, like most modern snails.citation
author = Mus, M. M.; Palacios, T.; Jensen, S. | year = 2008
title = Size of the earliest mollusks: Did small helcionellids grow to become large adults?
journal = Geology | volume = 36 | issue = 2 | pages = 175 | doi = 10.1130/G24218A.1
url = http://geology.geoscienceworld.org/cgi/content/abstract/36/2/175 | accessdate=2008-10-01] Fossil gastropods, with their characteristic twisted shells, have been reported from "Latest Early Cambrian" rocks in Canada – unfortunately it is impossible to give a numerical date for these rocks. [cite journal
author=Landing, E., Geyer, G., and Bartowski, K.E.
title=Latest Early Cambrian Small Shelly Fossils, Trilobites, and Hatch Hill Dysaerobic Interval onthe Quebec Continental Slope
journal=Journal of Paleontology | volume=76| issue=2 | date=March 2002 | pages=287-305
doi=10.1666/0022-3360(2002)076<0287:LECSSF>2.0.CO;2 ] Annotated image | float=right | caption=Septa and siphuncle in nautiloid shell


annotations=For a long time it was thought that "Volborthella", some fossils of which pre-date ma|530, was a cephalopod. However discoveries of more detailed fossils showed that "Volborthella"’s shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living "Nautilus" are. "Volborthella"’s classification is uncertain. [citation
author=Hagadorn, J.W., and Waggoner, B.M. | date= 2002 | pages=135-150
contribution=The Early Cambrian problematic fossil Volborthella: New insights from the Basin and Range
editor=Corsetti, F.A.
title=Proterozoic-Cambrian of the Great Basin and Beyond, Pacific Section SEPM Book 93
publisher=SEPM (Society for Sedimentary Geology)
url=http://www3.amherst.edu/~jwhagadorn/publications/volb.pdf | accessdate=2008-10-01 ] The Late Cambrian fossil "Plectronoceras" is now thought to be the earliest clearly cephalod fossil, as its shell had septa and a siphuncle, a strand of tissue that "Nautilus" uses to remove water from compartments that it has vacated as it grows, and which is also visible in fossil ammonite 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. [cite book
author=Vickers-Rich, P., Fenton, C.L., Fenton, M.A. and Rich, T.H.
title=The Fossil Book: A Record of Prehistoric Life
publisher=Courier Dover Publications | date= 1997 | isbn=0486293718 | pages=269-272 ] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period ma|65.cite journal |author=Marshall C.R., and Ward P.D. |year=1996 |title=Sudden and Gradual Molluscan Extinctions in the Latest Cretaceous of Western European Tethys |journal=Science |volume=274 |issue=5291 |pages=1360–1363 |doi=10.1126/science.274.5291.1360 |pmid=8910273 ] However the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant to-day.

The Early Cambrian fossils "Fordilla" and "Pojetaia" are regarded as bivalves. [cite journal
author=Pojeta, J. | date=2000 | title=Cambran Pelecypoda (Mollusca)
journal=American Malacological Bulletin | volume=15 | pages=157-166 ] [ cite journal
author=Schneider, J.A. | title=Bivalve systematics during the 20th century
journal=Journal of Paleontology | date=November 2001 | volume=75 | issue=6 | pages=1119-1127
doi=10.1666/0022-3360(2001)075<1119:BSDTC>2.0.CO;2
url=http://findarticles.com/p/articles/mi_qa3790/is_200111/ai_n9001371/pg_3 | accessdate=2008-10-05 ] [cite journal
author=Gubanov, A.P., Kouchinsky, A.V. and Peel, J.S.
title=The first evolutionary-adaptive lineage within fossil molluscs
journal=Lethaia | volume=32 | issue=2 | pages=155-157 | doi=10.1111/j.1502-3931.1999.tb00534.x ] [ cite journal
author=Gubanov, A.P., and Peel, J.S.
title=The early Cambrian helcionelloid mollusc "Anabarella" Vostokova
journal=Palaeontology | volume=46 |issue=5 | pages=1073-1087 | doi=10.1111/1475-4983.00334] "Modern-looking" bivalves appeared in the Ordovician period, ma|488|443. [cite journal
author=Zong-Jie, F.
title=An introduction to Ordovician bivalves of southern China, with a discussion of the early evolution of the Bivalvia
journal=Geological Journal | Volume=41 | issue=3-4 | pages 303-328 | doi=10.1002/gj.1048] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous-Tertiary extinction. [cite journal
author=Raup, D.M., and Jablonski, D.
title=Geography of end-Cretaceous marine bivalve extinctions | journal=Science | date=1993
volume=260 | issue=5110 | pages=971–973 |doi=10.1126/science.11537491 | pmid=11537491 ] However bivalves are now abundant and diverse.

Phylogeny

The phylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether "Kimberella" and any of the "halwaxiids" were molluscs or closely related to molluscs, there are debates about the relationships between the classes of living molluscs.cite journal
author=Sigwart, J.D., and Sutton, M.D. | date=October 2007
title=Deep molluscan phylogeny: synthesis of palaeontological and neontological data
journal=Proceedings of The Royal Society: Biology | volume=274 | issue=1624 | pages=2413-2419
doi=10.1098/rspb.2007.0701 For a summary, see cite web
title=The Mollusca | publisher=University of Californai Museam of Paleontology
url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php | accessdate=2008-10-02 ]

Molluscs are members of the Lophotrochozoa, a group defined by having trochophore larvae and, in the case of living brachiopods, a feeding structure called a lophophore. The other members of the Lophotrochozoa are the annelid worms and seven marine phyla. [cite web
title=Introduction to the Lophotrochozoa
publisher=University of Californai Museam of Paleontology
url=http://www.ucmp.berkeley.edu/phyla/lophotrochozoa.html | accessdate=2008-10-02 ]

The diagram on the right summarizes a phylogeny presented in 2007. Two other widely-supported reconstructions of the evolutionary relationships "within" the molluscs are:

Interactions with humans

Uses by humans

Mollusks, especially bivalves such as clams and mussels, have been an important food source for many different peoples around the world at least since the appearance of anatomically modern humans – and this has often resulted in over-fishing. [cite journal
author=Mannino, M.A., and Thomas, K.D.
title=Depletion of a resource? The impact of prehistoric human foraging on intertidal mollusc communities and its significance for human settlement, mobility and dispersal
journal=World Archaeology | volume 33 | issue=3 | date=February 2002 | pages=452-474
doi=10.1080/00438240120107477 ] Other molluscs commonly eaten include octopuses and squids, whelks, oysters, and scallops. [cite book
author=Garrow, J.S., Ralph, A., and James, W.P.T. | title=Human Nutrition and Dietetics | page=370
publisher=Elsevier Health Sciences | date=2000 | isbn=0443056277] In 2005, China accounted for 80 per cent of the global mollusc catch, netting almost 11 million tonnes. Within Europe, France remained the industry leader. [cite web
url=http://www.fao.org/figis/servlet/TabLandArea?tb_ds=Capture&tb_mode=TABLE&tb_act=SELECT&tb_grp=COUNTRY | accessdate=2008-10-03
title=China catches almost 11m tonnes of molluscs in 2005 | publisher=FAO ] However some countries have strict regulations about the importation and handling of molluscs and other seafood, mainly to minimize the risk that humans may be poisoned by toxins that have accumulated in the animals. [cite web
title=Importing fishery products or bivalve molluscs | publisher=Food Standards Agency
location=United Kingdom
url=http://www.food.gov.uk/foodindustry/imports/want_to_import/fisheryproducts/
accessdate=2008-10-02]

Most molluscs that have shells can produce pearls, but only the pearls of bivalves and some gastropods whose shells are lined with nacre are valuable. The best natural pearls are produced by the pearl oysters "Pinctada margaritifera" and "Pinctada mertensi", which live in the tropical and sub-tropical waters of the Pacific Ocean. Natural pearls form when a small foreign object gets stuck between the mantle and shell. There are two methods of culturing pearls, by inserting either "seeds" or beads into oysters. The "seed" method uses grains of ground shell from freshwater mussels, and over-harvesting for this purpose has endangered several freshwater mussel species in the southeastern USA. The pearl industry is so important in some areas that significant sums of money are spent on monitoring the health of farmed molluscs. [cite journal
author=Jones, J.B., and Creeper, J.
title=Diseases of Pearl Oysters and Other Molluscs: a Western Australian Perspective
journal=Journal of Shellfish Research | volume=25 | issue=1 | date=April 2006 | pages=233–238
doi=10.2983/0730-8000(2006)25 [233:DOPOAO] 2.0.CO;2 ]

Other luxury and high-status products have been 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. [The fourth-century BC historian Theopompus, cited by Athenaeus (12:526) around 200 BC ; according to cite book
author=Gulick, C.B. | date=1941| title=Athenaeus, The Deipnosophists | location=Cambridge, Mass.
publisher=Harvard University Press ] The discovery of large numbers of Murex shells on Crete suggests that the Minoans may have pioneered the extraction of "Imperial purple" during the Middle Minoan period in the 20th&ndash;18th century BC, centuries before the Tyrians. [cite journal
author=Reese, D.S. | date=1987
title=Palaikastro Shells and Bronze Age Purple-Dye Production in the Mediterranean Basin
journal=Annual of the British School of Archaeology at Athens | volume=82 | pages=201-6 ] [cite journal
author=Stieglitz, R.R. | date=1994 | title=The Minoan Origin of Tyrian Purple
journal=Biblical Archaeologist | volume=57 | pages=46-54 ] Sea silk is a fine, rare and valuable fabric produced from the long silky threads (byssus) secreted by several bivalve mollusks, particularly "Pinna nobilis", to attach themselves to the sea bed. ["Webster's Third New International Dictionary (Unabridged)" 1976. G. & C. Merriam Co., p. 307.] Procopius, writing on the Persian wars circa 550 CE, "stated that the five hereditary satraps (governors) of Armenia who received their insignia from the Roman Emperor were given chlamys (or cloaks) made from "lana pinna" (Pinna "wool," or byssus). Apparently only the ruling classes were allowed to wear these chlamys." [cite journal
author=Turner, R.D., and Rosewater, J. | title=The Family Pinnidae in the Western Atlantic
journal=Johnsonia | volume=3 | issue=38 | date=June 1958 | page=294 ]

Threats to humans

tings and bites

Apart from the risks of food-poisoning or seafood allergies, which can be fatal, a few species of molluscs in the wild can present a serious risk to humans, when handled. To put this into correct perspective, however, deaths from mollusc venoms are less than 10% of the number of deaths from jellyfish stings.cite book
author=Williamson, J.A., Fenner, P.J., Burnett, J.W., and Rifkin, J.
title=Venomous and Poisonous Marine Animals: A Medical and Biological Handbook
publisher=UNSW Press | date=1996 | isbn=0868402796 | pages=65-68
url=http://books.google.co.uk/books?id=YsZ3GryFIzEC&pg=PA75&lpg=PA75&dq=mollusc+venom+fatal&source=web&ots=tBDHW2xdlx&sig=PAvTz2z3hnwUyE4lfU8fvzwNbD4&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA57,M1 | accessdate=2008-10-03]

All octopuses are venomousAnderson, R.C. (1995) Aquarium husbandry of the giant Pacific octopus. "Drum and Croaker" 26:14-23] but only a few species pose a significant threat to humans. The blue-ringed octopuses in the genus "Hapalochlaena", which live around Australia and New Guineau, bite humans only if severely provoked,cite web
title=Blue ringed octopus | author=Alafaci, A. | publisher=Australian Venom Research Unit
http://www.avru.org/compendium/biogs/A000060b.htm | accessdate=2008-10-03 ] but their venom kills 25% of human victims. Another tropical species, "Octopus apollyon", causes severe inflammation that can last for over a month even if treated correctly. [cite journal
author=Brazzelli, V., Baldini, F., Nolli, G., Borghini, F., and Borroni, G.
title="Octopus apollyon" bite
journal=Contact Dermatitis | volume=40 | issue=3 | pages=169-170 | date=1999
doi=10.1111/j.1600-0536.1999.tb06025.x ]

Cone shells, carnivorous gastropods that feed on marine invertebrates and fish, produce a huge array of toxins, some fast-acting and others slower but deadlier – they can afford to do this because their toxins are relatively cheap to make compared with those of snakes or spiders.cite journal
author=Concar, D. | journal=New Scientist | date=19 October 1996
title=Doctor snail – Lethal to fish and sometimes even humans, cone snail venom contains a pharmacopoeia of precision drugs
http://environment.newscientist.com/article/mg15220523.900-doctor-snail--lethal-to-fish-and-sometimes-even-humans-cone-snail-venom-contains-apharmacopoeia-of-precision-drugs-itdavid-concarit-finds-out-how-the-toxinstarget-nerve-cells.html | accessdate=2008-10-03 ] Many painful stings have been reported and a few fatalities, although some of the reported fatalities may be exaggerations. Only the few species that can kill fish are likely to be seriously dangerous to humans. [cite web
url=http://grimwade.biochem.unimelb.edu.au/cone/deathby.html03
title=Cone Shell Mollusc Poisoning, with Report of a Fatal Case | author=Livett, B.
publisher=Department of Biochemistry and Molecular Biology, University of Melbourne] The effects of individual cone shell toxins on victims' nervous systems are so precise that they are useful tools for research in neurology, and the small size of their molecules makes it easy to synthesize them. [cite journal
author=Haddad, V.(junior), de Paula Neto, J.B., and Cobo, V.J.
title=Venomous mollusks: the risks of human accidents by "Conus" snails (Gastropoda: Conidae) in Brazil
journa=Revista da Sociedade Brasileira de Medicina Tropical | volume=39 | issue=(5) | pages=498-500
date=September-October 2006
url=http://www.scielo.br/pdf/rsbmt/v39n5/a15v39n5.pdf | accessdate=2008-10-03 ]

The traditional belief that a giant clam can trap the leg of a person between its valves, thus drowning them, is a myth. [cite book
author=Cerullo, M.M., Rotman, J.L., and Wertz, M.
title=The Truth about Dangerous Sea Creatures
publisher=Chronicle Books | date=2003 | isbn=0811840506 | page=10
url=http://books.google.co.uk/books?hl=en&lr=&id=1MOxNDmFLd4C&oi=fnd&pg=PA1&dq=giant+clam+trap+foot&ots=lYe_I4NhIO&sig=0KwRdmEoB4rt6zk1lvxPdiH_jkE#PPA10,M1 | accessdate=2008-10-03]

Pests

[

caption= Skin vesicles created by the penetration of Schistosoma. Source: Centers for Disease Control and Prevention ]
Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) "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." [cite web
title=The Carter Center Schistosomiasis Control Program |publisher=The Carter Center
url=http://www.cartercenter.org/health/schistosomiasis/index.html | accessdate=2008-10-03 ] The parasite has 13 known species, of which two infect humans; all the species have freshwater snails as intermediate hosts. [cite book
author=Brown, D.S. | title=Freshwater Snails of Africa and Their Medical Importance
publisher=CRC Press | date=1994 | isbn=0748400265 | page=305]

Despite its name, "Molluscum contagiosum" is a viral disease unrelated to molluscs. [cite web
title=Molluscum (Molluscum Contagiosum): Frequently Asked Questions for Everyone
publisher=Centers for Disease Control and Prevention
url=http://www.cdc.gov/ncidod/dvrd/molluscum/faq/everyone.htm | accessdate=2008-10-03]

Some species of molluscs, particularly certain snails and slugs, can be serious crop pests. [cite book
author=Barker, G.M. | title=Molluscs As Crop Pests | publisher=CABI Publications | date=2002
isbn=0851993206 ] Snails introduced into new environments can unbalance local ecosystems, and 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 and Pacific Oceans. In the 1990s this species reached the West Indies. Attempts to control it by introducing the predatory snail "Euglandina rosea" proved disastrous, as the predator ignored "Achatina fulica" and went on to extirpate several native snail species instead. [cite journal
author=Civeyrel, L., and Simberloff, D.
title=A tale of two snails: is the cure worse than the disease?
journal=Biodiversity and Conservation | volume=5 issue=10 | date=October 1996
doi=10.1007/BF00051574]

Notes

References

Further reading

*
* Nunn, J.D., Smith, S.M., Picton, B.E. and McGrath, D. 202. "Checklist, atlas of distribution and bibliography for the marine mollusca of Ireland." in. Marine Biodiversity in Ireland and Adjacent Waters. Ulster Museum. publication no. 8.

External links

* [http://www.gastropods.com/ Hardy's Internet Guide to Marine Gastropods]
* [http://www.tafi.org.au/zooplankton/imagekey/mollusca/index.html Mollusca Fact Sheet]