85,000 recognized living species.
Cornu aspersum (formerly Helix aspersa) – a common land snail
Mollusca is a large phylum of invertebrate animals whose members are
known as molluscs or mollusks[Note 1] (/ˈmɒləsk/). Around 85,000
extant species of molluscs are recognized. The number of fossil
species is estimated between 60,000 and 100,000 additional species.
Molluscs are the largest marine phylum, comprising about 23% of all
the named marine organisms. Numerous molluscs also live in freshwater
and terrestrial habitats. They are highly diverse, not just in size
and in anatomical structure, but also in behaviour and in habitat. The
phylum is typically divided into 9 or 10 taxonomic classes, of which
two are entirely 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 classified
species, and account for 80% of the total.
The three most universal features defining modern molluscs are a
mantle with a significant cavity used for breathing and excretion, the
presence of a radula (except for bivalves), and the structure of the
nervous system. Other than these common elements, molluscs express
great morphological diversity, so many textbooks base their
descriptions on a "hypothetical ancestral mollusc" (see image below).
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 covering the whole upper surface. The underside of the
animal consists of a single muscular "foot". Although molluscs are
coelomates, the coelom tends to be small. The main body cavity is a
hemocoel through which blood circulates; as such, their circulatory
systems are mainly open. The "generalized" mollusc's feeding system
consists of a rasping "tongue", the radula, and a complex digestive
system in which exuded mucus and microscopic, muscle-powered "hairs"
called cilia play various important roles. The generalized mollusc has
two paired nerve cords, or three in bivalves. The brain, in species
that have one, encircles the esophagus. Most molluscs have eyes, and
all have sensors to detect chemicals, vibrations, and touch. The
simplest type of molluscan reproductive system relies on external
fertilization, but more complex variations occur. All produce eggs,
from which may emerge trochophore larvae, more complex veliger larvae,
or miniature adults.
Good evidence exists for the appearance of gastropods, cephalopods and
bivalves in the
Cambrian period 541 to 485.4 million years
ago[dead link]. 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. There is a risk of food poisoning from
toxins which can accumulate in certain molluscs under specific
conditions, however, and because of this, many countries have
regulations to reduce this risk. Molluscs have, for centuries, also
been the source of important luxury goods, notably pearls, mother of
Tyrian purple dye, and sea silk. Their shells have also been
used as money in some preindustrial societies.
Mollusc species can also represent hazards or pests for human
activities. The bite of the blue-ringed octopus is often fatal, and
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 sophisticated, though 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 million
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.
4 Hypothetical ancestral mollusc
4.1 Mantle and mantle cavity
4.4 Circulatory system
4.6 Eating, digestion, and excretion
4.7 Nervous system
8 Human interaction
8.1 Uses by humans
8.2 Harmful to humans
8.2.1 Stings and bites
8.2.2 Disease vectors
12 Further reading
13 External links
The words mollusc and mollusk are both derived from the French
mollusque, which originated from the
Latin molluscus, from mollis,
soft. Molluscus was itself an adaptation of Aristotle's τα
μαλακά (ta malaká), "the soft things", which he applied to
cuttlefish. The scientific study of molluscs is accordingly called
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 it is now known these groups have no
relation to molluscs, and very little to one another, 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
Molluscs have developed such a varied range of body structures, it is
difficult to find synapomorphies (defining characteristics) to apply
to all modern groups. The most general characteristic of molluscs
is they are unsegmented and bilaterally symmetrical. The following
are present in all modern molluscs:
The dorsal part of the body wall is a mantle (or pallium) 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 two pairs of main nerve cords.
Other characteristics that commonly appear in textbooks have
Whether characteristic is found in these classes of Molluscs
Supposed universal Molluscan characteristic
Radula, a rasping "tongue" with chitinous teeth
Absent in 20% of Neomeniomorpha
Internal, cannot extend beyond body
Broad, muscular foot
Reduced or absent
Modified into arms
Small, only at "front" end
Dorsal concentration of internal organs (visceral mass)
Large digestive ceca
No ceca in some Aplacophora
Large complex metanephridia ("kidneys")
One or more valves/ shells
Primitive forms, yes; modern forms, no
Snails, yes; slugs, mostly yes (internal vestigial)
Octopuses, no; cuttlefish, nautilus, squid, yes
About 80% of all known mollusc species are gastropods (snails and
slugs), including this cowry (a sea snail).
Estimates of accepted described living species of molluscs vary from
50,000 to a maximum of 120,000 species. In 1969 David Nicol
estimated the probable total number of living mollusc species at
107,000 of which were about 12,000 fresh-water gastropods and 35,000
Bivalvia would comprise about 14% of the total and
the other five classes less than 2% of the living molluscs. In
2009, Chapman estimated the number of described living species at
85,000. Haszprunar in 2001 estimated about 93,000 named
species, which include 23% of all named marine organisms.
Molluscs are second only to arthropods in numbers of living animal
species—far behind the arthropods' 1,113,000 but well ahead of
chordates' 52,000. About 200,000 living species in total are
estimated, and 70,000 fossil species, although the total
number of mollusc species ever to have existed, whether or not
preserved, must be many times greater than the number alive today.
Molluscs have more varied forms than any other animal phylum. They
include snails, slugs and other gastropods; clams and other bivalves;
squids and other cephalopods; and other lesser-known but similarly
distinctive subgroups. The majority of species still live in the
oceans, from the seashores to the abyssal zone, but some form a
significant part of the freshwater fauna and the terrestrial
ecosystems. Molluscs 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 octopuses are among the neurologically most advanced
of all invertebrates. The giant squid, which until recently had
not been observed alive in its adult form, is one of the largest
invertebrates, but a recently caught specimen of the colossal squid,
10 m (33 ft) long and weighing 500 kg (1,100 lb),
may have overtaken it.
Freshwater and terrestrial molluscs appear exceptionally vulnerable to
extinction. Estimates of the numbers of nonmarine molluscs vary
widely, partly because many regions have not been thoroughly surveyed.
There is also a shortage of specialists who can identify all the
animals in any one area to species. However, in 2004 the IUCN Red List
Species included nearly 2,000 endangered nonmarine
molluscs. For comparison, the great majority of mollusc species are
marine, but only 41 of these appeared on the 2004 Red List. About 42%
of recorded extinctions since the year 1500 are of molluscs,
consisting almost entirely of nonmarine species.
Hypothetical ancestral mollusc
Anatomical diagram of a 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, and some suggest it may resemble
very early molluscs.
The generalized mollusc is bilaterally symmetrical 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
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
habitat, to sea, fresh water or air. The cavity was at the rear in the
earliest molluscs, but its position now varies from group to group.
The anus, a pair of osphradia (chemical sensors) in the incoming
"lane", the hindmost pair of gills and the exit openings of the
nephridia ("kidneys") and gonads (reproductive organs) are in the
mantle cavity. The whole soft body of bivalves lies within an
enlarged mantle cavity.
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
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.
A 50-second video of snails (most likely
Natica chemnitzi and
Cerithium stercusmuscaram) feeding on the sea floor in the Gulf of
California, Puerto Peñasco, Mexico
The underside consists of a muscular foot, which has adapted to
different purposes in different classes.:4 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;:4 in cephalopods it is
used for jet propulsion,:4 and the tentacles and arms are derived
from the foot.
Molluscs' circulatory systems are mainly open. Although molluscs are
coelomates, their coeloms are reduced to fairly small spaces enclosing
the heart and gonads. The main body cavity is a hemocoel through which
blood and coelomic fluid circulate and which encloses most of the
other internal organs. These hemocoelic spaces act as an efficient
hydrostatic skeleton. The blood contains the respiratory pigment
hemocyanin as an oxygen-carrier. The heart consists of one or more
pairs of atria (auricles), which receive oxygenated blood from the
gills and pump it to the ventricle, which pumps it into the aorta
(main artery), which is fairly short and opens into the hemocoel.
The atria of the heart also function as part of the excretory system
by filtering waste products out of the blood and dumping it into the
coelom as urine. A pair of nephridia ("little kidneys") to the rear of
and connected to the coelom extracts any re-usable materials from the
urine and dumps additional waste products into it, and then ejects it
via tubes that discharge into the mantle cavity.
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
Snail radula at work
= Food = Radula
Members of the mollusc family use intracellular digestion to function.
Most molluscs have muscular mouths with radulae, "tongues", bearing
many rows of chitinous teeth, which are replaced from the rear as they
wear out. The radula primarily functions to scrape bacteria and algae
off rocks, and is associated with the odontophore, a cartilaginous
supporting organ. The radula is unique to the molluscs and has no
equivalent in any other animal.
Molluscs' mouths also contain glands that secrete slimy mucus, to
which the food sticks. Beating cilia (tiny "hairs") drive the mucus
towards the stomach, so the mucus forms a long string called a "food
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.
As the head has largely disappeared in bivalves, the mouth has been
equipped with labial palps (two on each side of the mouth) to collect
the detritus from its mucus.
Simplified diagram of the mollusc nervous system
The cephalic molluscs have two pairs of main nerve cords organized
around a number of paired ganglia, the visceral cords serving the
internal organs and the pedal ones serving the foot. Most pairs of
corresponding ganglia on both sides of the body are linked by
commissures (relatively large bundles of nerves). The ganglia above
the gut are the cerebral, the pleural, and the visceral, which are
located above the esophagus (gullet). The pedal ganglia, which control
the foot, are below the esophagus and their commissure and connectives
to the cerebral and pleural ganglia surround the esophagus in a
circumesophageal nerve ring or nerve collar.
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". Some such as the scallops have eyes around the
edges of their shells which connect to a pair of looped nerves and
which provide the ability to distinguish between light and shadow.
Reproductive system of gastropods and
Reproductive system of
Apical tuft (cilia)
/// = cilia
The simplest molluscan reproductive system relies on external
fertilization, but with more complex variations. All produce eggs,
from which may emerge trochophore larvae, more complex veliger larvae,
or miniature adults. Two gonads sit next to the coelom, a small cavity
that surrounds the heart, into which they shed ova or sperm. The
nephridia extract the gametes from the coelom and emit them into the
mantle cavity. Molluscs that use such a system remain of one sex all
their lives and rely on external fertilization. Some molluscs use
internal fertilization and/or are hermaphrodites, functioning as both
sexes; both of these methods require more complex reproductive
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.
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.
Filter feeders are molluscs that feed
by straining suspended matter and food particle from water, typically
by passing the water over their gills. Most bivalves are filter
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 xenophyophore
Stannophyllum. Sacoglossan sea-slugs suck the sap from algae,
using their one-row radula to pierce the cell walls, whereas dorid
nudibranchs and some
Vetigastropoda feed on sponges and others
feed on hydroids. (An extensive list of molluscs with unusual
feeding habits is available in the appendix of GRAHAM, A. (1955).
"Molluscan diets". Journal of Molluscan Studies. 31 (3–4):
See also: List of mollusc orders
Opinions vary about the number of classes of molluscs; for example,
the table below shows eight living classes, and two extinct ones.
Although they are unlikely to form a clade, some older works combine
Caudofoveata and solenogasters into one class, the
Aplacophora. Two of the commonly recognized "classes" are
known only from fossils.
Described living species
All the snails and slugs including abalone, limpets, conch,
nudibranchs, sea hares, sea butterfly
marine, freshwater, land
clams, oysters, scallops, geoducks, mussels
rocky tidal zone and seabed
squid, octopus, cuttlefish, nautilus, spirula
marine 6–7,000 metres (20–22,966 ft)
seabed 200–3,000 metres (660–9,840 ft)
seabed 200–3,000 metres (660–9,840 ft)
An ancient lineage of molluscs with cap-like shells
seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200
metres (660 ft)
fossils; probable ancestors of bivalves
fossils; snail-like organisms such as Latouchella
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
Tentaculita may also be in
Mollusca (see Tentaculites).
Main article: Evolution of molluscs
See also: Evolution of cephalopods
The use of love darts by the land snail
Monachoides vicinus is a form
of sexual selection
Good evidence exists for the appearance of gastropods (e.g.
Aldanella), cephalopods (e.g. Plectronoceras, ?Nectocaris) and
bivalves (Pojetaia, Fordilla) towards the middle of the Cambrian
period, c. 500 million years ago, though arguably each of these
may belong only to the stem lineage of their respective classes.
However, the evolutionary history both of the emergence of molluscs
from the ancestral group Lophotrochozoa, and of their diversification
into the well-known living and fossil forms, is still vigorously
Debate occurs about whether some
Ediacaran and Early
really are molluscs. Kimberella, from about 555 million years
ago, 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
505 million years ago, 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 515 to 510 million years ago are fragments
of a genuinely mollusc-like radula. This appears to contradict the
concept that the ancestral molluscan radula was mineralized.
Yochelcionella is thought to be an early
Spirally coiled shells appear in many gastropods.
However, the Helcionellids, which first appear over 540 million
years ago 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
gastropods. However, other scientists are not convinced these
Cambrian fossils show clear signs of the torsion that identifies
modern gastropods twists the internal organs so the anus lies above
Septa and siphuncle in nautiloid shell
Volborthella, some fossils of which predate 530 million years
ago, was long thought to be a cephalopod, but discoveries of more
detailed fossils showed its shell was not secreted, but built from
grains of the mineral silicon dioxide (silica), and 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. The Late
Plectronoceras is now
thought to be the earliest clearly cephalopod fossil, as its shell had
septa and a siphuncle, a strand of tissue that
Nautilus uses to remove
water from compartments 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. All cephalopods with external
shells except the nautiloids became extinct by the end of the
Cretaceous period 65 million years ago. However, the
Coleoidea (squid, octopus, cuttlefish) are abundant
Pojetaia are regarded as
bivalves. "Modern-looking" bivalves appeared in the
Ordovician period, 488 to 443 million years ago. One
bivalve group, the rudists, became major reef-builders in the
Cretaceous, but became extinct in the Cretaceous–Paleogene
extinction event. Even so, bivalves remain abundant and diverse.
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
("limpet-like", "living fossils")
(snails, slugs, limpets, sea hares)
(nautiloids, ammonites, squid, etc.)
Scaphopods (tusk shells)
A possible "family tree" of molluscs (2007). Does not include
annelid worms as the analysis concentrated on fossilizable "hard"
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, debates arise about the
relationships between the classes of living molluscs. In fact,
some groups traditionally classified as molluscs may have to be
redefined as distinct but related.
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
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
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
The evolutionary relationships 'within' the molluscs are also debated,
and the diagrams below show two widely supported reconstructions:
The "Aculifera" hypothesis
The "Testaria" hypothesis
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 morphological and molecular
phylogenetics comparisons concluded the molluscs are not monophyletic;
Bivalvia are both separate, monophyletic
lineages unrelated to the remaining molluscan classes; the traditional
Mollusca is polyphyletic, and it can only be made monophyletic
if scaphopods and bivalves are excluded. A 2010 analysis recovered
the traditional conchiferan and aculiferan groups, and showed molluscs
were monophyletic, demonstrating that available data for solenogastres
was contaminated. Current molecular data are insufficient to
constrain the molluscan phylogeny, and since the methods used to
determine the confidence in clades are prone to overestimation, it is
risky to place too much emphasis even on the areas of which different
studies agree. Rather than eliminating unlikely relationships, the
latest studies add new permutations of internal molluscan
relationships, even bringing the conchiferan hypothesis into
Main article: Molluscs in culture
For millennia, molluscs have been a source of food for humans, as well
as important luxury goods, notably pearls, mother of pearl, Tyrian
purple dye, sea silk, and chemical compounds. Their shells have also
been used as a form of currency in some preindustrial societies. A
number of species of molluscs can bite or sting humans, and some have
become agricultural pests.
Uses by humans
Seashell and List of edible molluscs
Molluscs, especially bivalves such as clams and mussels, have been an
important food source since at least the advent of anatomically modern
humans, and this has often resulted in overfishing. Other commonly
eaten molluscs include octopuses and squids, whelks, oysters, and
scallops. In 2005, China accounted for 80% of the global mollusc
catch, netting almost 11,000,000 tonnes (11,000,000 long tons;
12,000,000 short tons). Within Europe, France remained the industry
leader. Some countries regulate importation and handling of
molluscs and other seafood, mainly to minimize the poison risk from
toxins that can sometimes accumulate in the animals.
Saltwater pearl oyster farm in Seram, Indonesia
Most molluscs with 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 marine pearl
Pinctada margaritifera and
Pinctada mertensi, which live in
the tropical and subtropical waters of the Pacific Ocean. Natural
pearls form when a small foreign object gets stuck between the mantle
The two methods of culturing pearls insert either "seeds" or beads
into oysters. The "seed" method uses grains of ground shell from
freshwater mussels, and overharvesting for this purpose has endangered
several freshwater mussel species in the southeastern United
States. The pearl industry is so important in some areas,
significant sums of money are spent on monitoring the health of farmed
Justinian I clad in
Tyrian purple and wearing
Other luxury and high-status products were 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 discovery of large numbers of
Murex shells on
Crete suggests the
Minoans may have pioneered the extraction of
"imperial purple" during the Middle Minoan period in the 20th–18th
centuries BC, centuries before the Tyrians.
Sea silk is a
fine, rare, and valuable fabric produced from the long silky threads
(byssus) secreted by several bivalve molluscs, particularly Pinna
nobilis, to attach themselves to the sea bed. 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.
Apparently, only the ruling classes were allowed to wear these
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
Bivalve molluscs are used as bioindicators to monitor the health of
aquatic environments in both fresh water and the marine environments.
Their population status or structure, physiology, behaviour or the
level of contamination with elements or compounds can indicate the
state of contamination status of the ecosystem. They are particularly
useful since they are sessile so that they are representative of the
environment where they are sampled or placed. Potamopyrgus
antipodarum is used by some water treatment plants to test for
estrogen-mimicking pollutants from industrial agriculture.
Harmful to humans
Stings and bites
The blue-ringed octopus's rings are a warning signal; this octopus is
alarmed, and its bite can kill.
Some molluscs sting or bite, but deaths from mollusc venoms total less
than 10% of those from jellyfish stings.
All octopuses are venomous, but only a few species pose a
significant threat to humans. Blue-ringed octopuses in the genus
Hapalochlaena, which live around Australia and New Guinea, bite humans
only if severely provoked, 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, and the bite of
Octopus rubescens can cause necrosis
that lasts longer than one month if untreated, and headaches and
weakness persisting for up to a week even if treated.
Live cone snails can be dangerous to shell collectors, but are useful
to neurology researchers.
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.
Skin vesicles created by the penetration of Schistosoma. (Source: CDC)
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 slugs, can
be serious crop pests, and when introduced into new environments,
can unbalance local ecosystems. 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 Ocean and Pacific Ocean. 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.
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Sturm, C., T.A. Pearce, and A. Valdes. The Mollusks: A Guide to their
Study, Collection, and Preservation. Universal Publishers. 2006. 454
pages. ISBN 1581129300
Wikimedia Commons has media related to Mollusca.
The Wikibook Dichotomous Key has a page on the topic of: Mollusca
Data related to
Mollusca at Wikispecies
Mollusca at the
Encyclopedia of Life
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
Ctenophora (comb jellies)
Cnidaria (jellyfish and relatives)
craniates / vertebrates
Echinodermata (starfish and relatives)
Kinorhyncha (mud dragons)
Priapulida (penis worms)
Nematomorpha (horsehair worms)
Onychophora (velvet worms)
Chaetognatha (arrow worms)
Gnathostomulida (jaw worms)
Dicyemida or Rhombozoa
Annelida (ringed worms)
Nemertea (ribbon worms)
Entoprocta or Kamptozoa
Ectoprocta (moss animals)
Brachiopoda (lamp shells)
Phoronida (horseshoe worms)
Anthozoa inc. corals
Medusozoa inc. jellyfish
Asterozoa inc. starfish
Phyla with ≥5000 extant species bolded
Monoblastozoa (nomen dubium)
Living things in culture
Insects in art
Insects in literature
Insects in medicine
Insects in music
Insects in mythology
Insects in religion
Arthropods in film
Cephalopods in popular culture
Birds in culture
In professional wrestling
Crystal Palace Dinosaurs
In the Bible
Rod of Asclepius
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List of microbes
Microbes and Man
Lists of fictional species
Template:Insects in culture
Fauna Europaea: 11368