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Overview

Brief Summary

The phylum Mollusca contains some of the most familiar invertebrates, including snails, slugs, clams, mussels, and octopuses. In contrast to these well-known molluscs, however, others are almost never seen, such as the aplacophorans and monoplacophorans, the latter of which which were only known from Paleozoic fossils until the first live specimen was discovered in the deep sea in 1952 (UCMP 2008).

Except for the aplacophorans, most molluscs have a well-developed, muscular foot. This structure is used in a multitude of ways, for example: locomotion, clinging to surfaces, burrowing, anchoring in sediment, swimming, and grasping (modified into prehensile tentacles in octopuses). The vast diversity of foot adaptations exemplifies the huge morphological diversity of the mollusc form.

A layer of epidermal tissue called the mantle surrounds the body of molluscs. Specialized glands in the mantle are responsible for the extracellular excretions that form shell structures. In all molluscan groups the shell is produced in layers of (usually) calcium carbonate, either in calcite or aragonite form.

Molluscs have adapted to terrestrial, marine and freshwater habitats all over the globe, although most molluscs are marine. Nearly 100,000 mollusc species are known (excluding the large number of extinct species known only as fossils) and it is clear that many thousands of species of extant species remain undescribed. Around 80% of known molluscs are gastropods (snails and slugs).

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Physical Description

Morphology

Despite the highly diverse forms of the members of this taxon, molluscs share a recognizable and characteristic generalized general body plan, made up of a head, a foot, and viscera contained in a central body. They are generally considered unsegmented, although primitive forms (aplacophorans and polyplacophorans) with repeated body features show intriguing potential for a possibly segmented mollusc-annelid ancestor (e.g. Jacobs et al 2000).

The mollusc head can house various combinations of sensory structures: tentacles, photoreceptors, statocysts, chemoreceptors. In some molluscs these sensory systems can be very well developed (the complex cephalopod eye is a prime example). Also found on the head is a feature unique to molluscs: the radula. Found in the buccal (mouth) cavity, the radula usually exists as a tongue-like plate covered with “teeth” used by herbivores, carnivores and scavengers to scrape food particles into the mouth. Depending on diet and use, tooth number, shape, arrangement, makeup, and growth have adapted diversely. Especially in the gastropods, number and shape of radular teeth are important taxonomic characters. The radula has also been adapted for diverse feeding methods. Some gastropods and cephalopods have a drill-like radula used to bore holes in the shell of prey, sometimes with the aid of acids secreted from an adjacent boring gland. In cone snails the radula is set on the end of a retractable proboscis and is slung out like a harpoon, to inject toxins into the prey, delivered through piercing, hollow teeth. In some cases these toxins are powerful neurotoxins, deathly to humans. Several lineages of molluscs have evolved suspension feeding, especially in the gastropods and bivalves. The radula in these cases is either highly reduced or lost altogether, and in most cases food particles are caught by ctinidia (gills) and moved to the mouth by cilia.

Except for the aplacophorans, most molluscs have a well-developed, muscular foot. This structure is used in a multitude of ways, for example: locomotion, clinging to surfaces, burrowing, anchoring in sediment, swimming, modified into prehensile tentacles (octopus); the vast diversity of foot adaptations exemplifies the huge morphological diversity of the mollusc form.

A layer of epidermal tissue called the mantle surrounds the body of molluscs. Specialized glands in the mantle are responsible for the extracellular excretions that form shell structures. The ancestral mollusc is thought to have one shell capped over the body like a limpet, and from that a diverse number of shell arrangements have evolved. Molluscs may have have one, two, or eight (in chitons) shells. Aplacophorans have no shell, but have instead minute aragonite spicules imbedded within the mantle. Secondary loss or much reduced shell vestiges have also occurred independently in multiple mollusc lineages (for example nudibranchs, slugs, cephalopods). Shells usually provide external protection, but there have been several independent internalizations within cephalopods and opisthobranchia. In all molluscan groups the shell is produced in layers of (usually) calcium carbonate, either in calcite or aragonite form. The wide range of pigmentation, shape, size, sculpturing, and twisting of sea shells is, of course, well known. There is much recent developmental work describing gene expression in shell formation, and the roles of highly conserved regulatory genes such as engrailed and Hox genes have been examined (e.g. Jacobs et al 2000, Samadi and Steiner 2009).

Between the mantle and the body proper is the mantle cavity, which may be organized as one or two separate spaces or grooves. Many important functions occur in the mantle cavity: the ctenidia (gills) are positioned here and the body systems, namely the nephridia (kidney like organs), the gut and the reproductive organs open up into this space. In aquatic molluscs cilia on the surface of the mantle and organs maintain water flow through the mantle cavity to take away wastes and bring in oxygenated water (and food particles for those suspension feeding molluscs). Molluscs have an open circulatory system with a full heart (with the exception of the cephalopods, which have a closed circulatory system). Their nervous system is well developed, usually consisting of a dorsal ganglion, a ring of nerves around the esophagus, and two pairs of lateral nerve cords running the length of the body, which are connected transversely in a ladder-like arrangement. There is an enormous range of nervous system development in the molluscs, from the poorly developed ganglia of the aplacophorans to the extreme cephalization of the cephalopods. Important work in the fields of neurobiology has been carried out on the squid Doryteuthis pealeii (formerly Loligo pealeii) and on Aplysia sea slugs.

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Size

Molluscs range in size from almost microscopic to animals 20 meters long (giant squid) or weighing 450 pounds (giant clams).

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Ecology

Habitat

Molluscs have adapted to terrestrial, marine and freshwater habitats all over the globe, although most molluscs are marine.

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Associations

Animal / predator
adult of Muricidae is predator of Mollusca

Animal / parasite
larva of Sarcophaga melanura parasitises Mollusca
Other: minor host/prey

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Known predators

Mollusca is prey of:
mixed-food consumers
secondary carnivores
Callinectes
Actinopterygii
Alburnus alburnus
bleak
Geococcyx californianus
Pollicipes polymerus
Huso huso
Oncorhynchus tshawytscha
Pseudodoras niger
Lates niloticus
Lepomis megalotis
Coris aygula
Ambystoma annulatum
Caretta caretta
Thamnophis butleri
Diadophis punctatus
Gavia stellata
Diomedea epomophora
Sula dactylatra
Egretta thula
Egretta tricolor
Mycteria americana
Eudocimus ruber
Cygnus olor
Anas fulvigula
Anas strepera
Anas cyanoptera
Anas americana
Aix sponsa
Aythya americana
Pandion haliaetus
Coturnix delegorguei
Actitis macularia
Larus canus
Fratercula cirrhata
Anodorhynchus hyacinthinus
Passerella iliaca
Corvus caurinus
Sorex gaspensis
Neurotrichus gibbsii
Peromyscus gossypinus
Lagenorhynchus australis
Lagenorhynchus cruciger
Feresa attenuata
Phocoenoides dalli
Delphinapterus leucas
Monodon monoceros
Mesoplodon europaeus
Mesoplodon carlhubbsi
Mesoplodon layardii
Enhydra lutris
Zalophus californianus
Neophoca cinerea
Callorhinus ursinus
Arctocephalus australis
Arctocephalus philippii
Arctocephalus townsendi
Phoca largha
Monachus tropicalis
Mirounga leonina
Mirounga angustirostris
Cephalophus niger
Alligator mississippiensis
Puma concolor
Prionailurus viverrinus
Mesoplodon peruvianus
Coturnix adansonii
Eremophila alpestris
Euoticus elegantulus
Cebus olivaceus
Hydromys chrysogaster
Ambystoma mexicanum
Amblonyx cinereus
Lontra provocax
Lutrogale perspicillata
Melogale personata
Martes melampus
Arvicola terrestris
Solenodon paradoxus
Potamogale velox

Based on studies in:
unknown: Black Sea (Marine)
Mexico: Guerrero (Coastal)
Uganda (Lake or pond)
England, River Thames (River)

This list may not be complete but is based on published studies.
  • T. S. Petipa, E. V. Pavlova, G. N. Mironov, The food web structure, utilization transport of energy by trophic levels in the planktonic communities. In: Marine Food Chains, J. H. Steele, Ed. (Oliver and Boyd, Edinburgh, 1970), 142-167, from p. 154.
  • T. S. Petipa, E. V. Pavlova, G. N. Mironov, The food web structure, utilization transport of energy by trophic levels in the planktonic communities. In: Marine Food Chains, J. H. Steele, Ed. (Oliver and Boyd, Edinburgh, 1970), 142-167 from p. 155.
  • A. Yanez-Arancibia, Taxonomia, ecologia y estructura de las comunidades de peces en lagunas costeras con bocas efimeras del Pacifico de Mexico.
  • K. H. Mann, R. H. Britton, A. Kowalczewski, T. J. Lack, C. P. Mathews and I. McDonald, Productivity and energy flow at all trophic levels in the River Thames, England. In: Productivity Problems of Freshwaters, Z. Kajak and A. Hillbricht-Ilkowska, Eds. (P
  • M. J. Burgis, I. G. Dunn, G. G. Ganf, L. M. McGowan and A. B. Viner, Lake George, Uganda: Studies on a tropical freshwater ecosystem. In: Productivity Problems of Freshwaters, Z. Kajak and A. Hillbricht-Ilkowska, Eds. (Polish Scientific, Warsaw, 1972), p
  • K. H. Mann, Case history: The River Thames. In: River Ecology and Man (R. T. Oglesby, C. A. Carlson, J. A. McCann, Eds.), Academic Press, New York and London, pp. 215-232 (1972), from p. 224.
  • Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web (online). Accessed February 16, 2011 at http://animaldiversity.org. http://www.animaldiversity.org
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Known prey organisms

Mollusca preys on:
detritus
phytoplankton
saprophagous plankton
algae
zooplankton
seston

Based on studies in:
unknown: Black Sea (Marine)
Mexico: Guerrero (Coastal)
England, River Thames (River)
Uganda (Lake or pond)

This list may not be complete but is based on published studies.
  • T. S. Petipa, E. V. Pavlova, G. N. Mironov, The food web structure, utilization transport of energy by trophic levels in the planktonic communities. In: Marine Food Chains, J. H. Steele, Ed. (Oliver and Boyd, Edinburgh, 1970), 142-167, from p. 154.
  • T. S. Petipa, E. V. Pavlova, G. N. Mironov, The food web structure, utilization transport of energy by trophic levels in the planktonic communities. In: Marine Food Chains, J. H. Steele, Ed. (Oliver and Boyd, Edinburgh, 1970), 142-167 from p. 155.
  • A. Yanez-Arancibia, Taxonomia, ecologia y estructura de las comunidades de peces en lagunas costeras con bocas efimeras del Pacifico de Mexico.
  • K. H. Mann, R. H. Britton, A. Kowalczewski, T. J. Lack, C. P. Mathews and I. McDonald, Productivity and energy flow at all trophic levels in the River Thames, England. In: Productivity Problems of Freshwaters, Z. Kajak and A. Hillbricht-Ilkowska, Eds. (P
  • M. J. Burgis, I. G. Dunn, G. G. Ganf, L. M. McGowan and A. B. Viner, Lake George, Uganda: Studies on a tropical freshwater ecosystem. In: Productivity Problems of Freshwaters, Z. Kajak and A. Hillbricht-Ilkowska, Eds. (Polish Scientific, Warsaw, 1972), p
  • K. H. Mann, Case history: The River Thames. In: River Ecology and Man (R. T. Oglesby, C. A. Carlson, J. A. McCann, Eds.), Academic Press, New York and London, pp. 215-232 (1972), from p. 224.
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Life History and Behavior

Reproduction

Like other systems, reproduction is highly variable among molluscs. Molluscs can be dioecious or simultaneously or sequentially hermaphroditic. Gametes are freely spawned in some groups, others have internal fertilization and complex mating behaviors, many produce egg capsules, egg cases, or brood chambers.

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Growth

Development

Most molluscs undergo spiral cleavage. Development can be direct (proceed right to settling into a juvenile form) or indirect, going through the swimming trochophore larval stage. The trochophore is very similar to the annelid trochophore. Before settling, many groups then go onto a second larval stage which is unique to molluscs: the feeding (usually) and swimming veliger larvae. Molluscs go through the uniquely molluscan process of torsion, usually during the veliger stage of development. Torsion involves counterclockwise rotation of the visceral mass up to 180 degrees with respect to the head and foot, to profoundly change the relative location of the body regions. Many groups then “detort” to some degree later in development or adulthood. Theories as to the evolutionary significance of torsion abound but this phenomenon is not well understood (Brusca and Brusca 2003). In the long run, torsion has allowed for much morphological diversification over the course of evolution.

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Evolution and Systematics

Evolution

Systematics and Taxonomy

The molluscs demonstrate remarkable morphological diversity, a characteristic that has confused molluscan taxonomy from the group’s inception. The Latin root molluscus means soft, and many soft-bodied invertebrates have been added and removed this group until Cuvier’s modern approximation in 1795 (Brusca and Brusca 2003). Mollusca is the second largest invertebrate phylum after the arthropods. Some 93,000 extant species have been described, but the thinking is this number represents only about half of the living species. 70,000 fossil species are also known. Most classifications recognize ten molluscan classes (two extinct). One class, the gastropods (snails and slugs), contains about 80% of mollusc species.


A very rich molluscan fossil record dates back 500 million year to the Precambrian. The evolutionary origins of molluscs are still disputed, but recent well-respected molecular phylogenetic analyses place the molluscs in the Lophotrochozoa, along with annelids, brachiopods, bryozoans and several other phyla (Halanych et al. 1995). Relationships within the Mollusca are also unclear and disputed; some recent analyses challenge whether this enormous phylum is a natural, monophyletic group (Sigwart and Sutton 2007 and references therein).

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Functional Adaptations

Functional adaptation

Shell protects, supports, and allows for growth: shelled mollusks
 

The shells of many mollusks provide protection and support while accomodating growth due to their conical structure.

           
  "Consider shapes that satisfy the following set of conditions. To provide both support and protection for the organism, the shape must be a hollow one, but an opening must exist somewhere. Growth can occur only by addition to the inner surface or the free edge. And the shape should change only minimally as it grows. A cubic shell with an open face won't work: addition to walls will give more shell relative to its contained volume, and addition to cylinder doesn't meet the conditions--addition to the edge will move it from short and fat to long and (relatively) thin. What will work are cones, whether circular or elliptical. Add to the edge and thicken the walls and one gets a bigger cone, isometric with the original.

With only slight variations of the condition of isometry, all sorts of wild derivatives of cones are possible--and these latter are the shapes in which shelled mollusks occur." (Vogel 2003:88-89)
  Learn more about this functional adaptation.
  • Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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Functional adaptation

Protein plays role in crystal formation: molluscs
 

The conchiolin protein of many molluscs plays a role in shell formation by serving as a major matrix component for crystal formation.

   
  "The shell is secreted by the mantle, the tissue layer under the shell, of the mollusc, and consists of two or three layers. The outermost is the periostracum, made of a tough protein called conchiolin. The periostracum is often brown in colour although it may be so thin that it is virtually transparent: sometimes it is quite furry…Inside the periostracum are one or two layers of argonite or calcite, different crystalline forms of calcium carbonate, more commonly known as chalk. The main central layer is called the prismatic layer: the inner layer is known as the lamellate or nacreous layer. Here the crystals are laid in an overlapping zigzag formation that scatters light and produces the iridescent effect known as 'mother of pearl.'" (Foy and Oxford Scientific Films 1982:115)
  Learn more about this functional adaptation.
  • Foy, Sally; Oxford Scientific Films. 1982. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London. 238 p.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:89,720Public Records:65,897
Specimens with Sequences:74,303Public Species:6,555
Specimens with Barcodes:67,627Public BINs:10,724
Species:9,735         
Species With Barcodes:7,846         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Mollusca

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Genomic DNA is available from 2 specimens with morphological vouchers housed at Museum of Tropical Queensland and Museum Victoria
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Conservation

Threats

Calcification and ocean acidification

Many hard-shelled marine organisms construct their shells or skeletons from calcium carbonate. This mineral occurs naturally in a couple of different crystal structures, aragonite and calcite. Mollusk shells rely chiefly on aragonite, possibly because this was the crystal more easily precipitated from seawater at the time when mollusks first started calcifying their shells (Porter, 2007). Aragonite is also used by scleractinian corals for their skeletons, so it’s not surprising that sand in many productive coastal regions consists largely of aragonite; it’s mostly the broken, ground up shells and skeletons of corals and mollusks past.

Echinoderms, by contrast, use calcite to construct their skeletons (Raup, 1959). The hard parts of a sea urchin are relatively obvious, and both the spines and the test enclosing the body rely on calcite. The softer-looking echinoderms use it too. Starfish, brittle stars and feather stars have more flexible appendages, but these are all supported by many short segments of calcite skeleton. Even sea cucumbers have calcite ossicles embedded in their body wall.

Bryozoans (Taylor, 2012) and Calcareous sponges (Stanley and Hardie, 1998) use both calcite and aragonite, and at least some species show flexibility in which crystal they use, depending on which is favored by ambient water chemistry.

Many species of algae build with calcium carbonate too. Most red calcareous algae build calcite inside their cell membranes, while calcareous green algae usually build aragonite on the outside (Granier, 2012). These algae can be the most important habitat builders in many areas outside of the tropics (Basso, 2012).

Their aragonite tendencies may leave corals, green algae and mollusks especially vulnerable to ocean acidification. At its present concentration of carbon dioxide (CO2), the ocean is still well-supplied with the minerals needed for all organisms that use calcium carbonate to build their shells or skeletons. It has been estimated that by the year 2050, rising CO2 levels will begin to deplete the available ions below optimal levels for aragonite building (Orr et al, 2005), essentially making aragonite more soluble in seawater. This effect has also been measured in the lab on pteropod mollusks. When raised in seawater with the predicted CO2 concentration for the year 2100, the Sea butterfly Limacina helicina's calcification rate fell 28% (Comeau, 2009). Recently, samples of this Antarctic species from a region with depressed aragonite levels were found to have significant shell dissolution  already, in the wild (Bednaršek et al, 2012).

Different calcareous organisms are affected in different ways by changes in ocean acidification. The geological record shows a number of events in the past 300 million years when sudden very large changes in species richness occurred in some groups of calcium-carbonate builders, which is likely to be related to acidity changes in seawater (Hönisch et al, 2012). Different groups were affected to a greater or lesser degree and it appears that several factors including habitat and physiology influence which groups are more sensitive to rising acidity. For instance, some calcite-builders like sea urchins and calcareous sponges will be slightly less sensitive, since calcite crystal formation is not affected as quickly by increased CO2. However, organisms that build calcite structures with a significant dose of Magnesium ions (High Magnesium Calcite or HMC) like some red algae do, will be the most quickly affected, as HMC is even more soluble than aragonite (Basso, 2012).

There is a lot of uncertainty about how reduced ocean calcification will feedback on the changing carbon cycle globally. The process of dissolving calcium carbonate (or reducing calcification) actually uses up carbon dioxide, shifting the seawater equilibrium toward bicarbonate ion (see Encyclopedia of Earth, 2010, for review). However, the greater impact of the changes will be in the total productivity of the communities that rely on calcification and the habitat it constructs (The Royal Society, 2005). If a reef habitat is lost, the question is, what will take its place? If the new community is equally productive, it may continue to sequester organic carbon, as dead tissue sinking to the deep ocean, just as fast as the original habitat did. Of course for that scenario it should be borne in mind that natural productivity of coral reef communities is extremely high, so an equally productive one succeeding them would be very unlikely.

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Non-marine molluscs appear to have a very high extinction rate. Lydeard et al (2004) list terrestrial and fresh water mollusc extinctions as about 40% of total recorded animal extinctions, far greater than marine molluscs.

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Relevance to Humans and Ecosystems

Benefits

Uses

Many different molluscs have been integrated into human culture since prehistoric times in a plethora of forms: shell money, jewelry and food, crop pests, and disease carriers (Schistosomiasis is a watersnail-born parasite that effects hundreds of millions of people in the world).

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