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Mollusca

 
Sci-Tech Dictionary: Mollusca
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(mə′ləs·kə)

(invertebrate zoology) One of the divisions of phyla of the animal kingdom containing snails, slugs, octopuses, squids, clams, mussels, and oysters; characterized by a shell-secreting organ, the mantle, and a radula, a food-rasping organ located in the forward area of the mouth.

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Sci-Tech Encyclopedia: Mollusca
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A major phylum of the animal kingdom comprising an extreme diversity of external body forms (oysters, clams, chitons, snails, slugs, squid, and octopuses among others), all based on a remarkably uniform basic plan of structure and function. The phylum name is derived from mollis, meaning soft, referring to the soft body within a hard calcareous shell, which is usually diagnostic. Soft-bodied mollusks make extensive use of ciliary and mucous mechanisms in feeding, locomotion, and reproduction. Most molluscan species are readily recognizable as such.

The Mollusca constitute a successful phylum; there are probably over 110,000 living species of mollusks, a number second only to that of the phylum Arthropoda, and more than double the number of vertebrate species. More than 99% of living molluscan species belong to two classes: Gastropoda (snails) and Bivalvia. Ecologically, these two classes can make up a dominant fraction of the animal biomass in many natural communities, both marine and fresh-water.

Classification

The phylum Mollusca is divided into seven distinct extant classes, three of which (Gastropoda, Bivalvia, and Cephalopoda) are of major significance in terms both of species numbers and of ecological bioenergetics, and one extinct class. An outline of their classification follows.

Class Monoplacophora (mainly fossil; but one living genus Neopilina)

Class Aplacophora

     Subclass Neomeniomorpha

     Subclass Chaetodermomorpha

Class Polyplacophora

Class Scaphopoda

Class Rostroconchia (fossil only)

Class Gastropoda

     Subclass Prosobranchia

          Order: Archaeogastropoda

                    Mesogastropoda

                    Neogastropoda

     Subclass Opisthobranchia

          Order: Bullomorpha (or Cephalaspidea)

                    Aplysiomorpha (or Anaspidea)

                    Thecosomata

                    Gymnosomata

                    Pleurobranchomorpha (or Notaspidea)

                    Acochlidiacea

                    Sacoglossa

                    Nudibranchia (or Acoela)

     Subclass Pulmonata

          Order: Systellommatophora

                    Basommatophora

                    Stylommatophora

Class Bivalvia (or Pelecypoda)

     Subclass Protobranchia

     Subclass Lamellibranchia

          Order: Taxodonta

                    Anisomyaria

                    Heterodonta

                    Schizodonta

                    Adapedonta

                    Anomalodesmata

     Subclass Septibranchia

Class Cephalopoda

     Subclass Nautiloidea

     Subclass Ammonoidea (fossil only)

     Subclass Coleoidea

          Order: Belemnoidea

                    Sepioidea

                    Teuthoidea

                    Vampyromorpha

                    Octopoda

Functional morphology

The unique basic plan of the Mollusca involves the different modes of growth and of functioning of the three distinct regions of the molluscan body (see illustration). These are the head-foot with some nerve concentrations, most of the sense organs, and all the locomotory organs; the visceral mass (or hump) containing organs of digestion, reproduction, and excretion; and the mantle (or pallium) hanging from the visceral mass and enfolding it and secreting the shell. In its development and growth, the head-foot shows a bilateral symmetry with an anterioposterior axis of growth. Over and around the visceral mass, however, the mantle-shell shows a biradial symmetry, and always grows by marginal increment around a dorsoventral axis. It is of considerable functional importance that a space is left between the mantle-shell and the visceral mass forming a semi-internal cavity; this is the mantle cavity or pallial chamber within which the typical gills of the mollusk, the ctenidia, develop. This mantle cavity is almost diagnostic of the phylum; it is primarily a respiratory chamber housing the ctenidia, but with alimentary, excretory, and genital systems all discharging into it.

Generalized model of a stem mollusk (or archetype) in side view. There are three distinct regions in the molluscan body: head-foot, visceral mass, and mantle-shell. Water circulation through the mantle cavity, gills (ctenidia), and pallial complex is from ventral inhalant to dorsal exhalant. (<i>After W. D. Russell-Hunter</i>, <i>A Life of Invertebrates</i>, <i>Macmillan</i>, <i>1979</i>)
Generalized model of a stem mollusk (or archetype) in side view. There are three distinct regions in the molluscan body: head-foot, visceral mass, and mantle-shell. Water circulation through the mantle cavity, gills (ctenidia), and pallial complex is from ventral inhalant to dorsal exhalant. (After W. D. Russell-Hunter, A Life of Invertebrates, Macmillan, 1979)

In looking at any mollusk, it is important to realize that whatever the shape of the shell, it is always underlain by the mantle, a fleshy fold of tissues which has secreted it. The detailed structure of the shell and of the mantle edge (with three functionally distinct lobes) is also consistent throughout the Mollusca. The shell is made up of calcium carbonate crystals enclosed in a meshwork of tanned proteins. It is always in three layers: the outer periostracum, the prismatic layer and the innermost or nacreous layer.

Each of the eight classes of the Mollusca has a characteristic body form and shell shape. Two classes are enormous (Gastropoda and Bivalvia), one of moderate extent (Cephalopoda), the others being minor by comparison. The Gastropoda constitute a diverse group with the shell usually in one piece. This shell may be coiled as in typical snails—that is, helicoid or turbinate—or it may form a flattened spiral, or a short cone as in the limpets, or it may be secondarily absent as in the slugs. Most gastropods are marine, but many are found in fresh waters and on land; in fact, they are the only successful nonmarine mollusks.

The Bivalvia are a more uniform group, with the shell in the form of twocalcareous valves united by an elastic hinge ligament. Mussels, clams, andoysters are familiar bivalves. The group is mainly marine with a few genera inestuaries and in fresh waters. There can be no land bivalves since their basicfunctional organization is as filter feeders. The third major group, theCephalopoda, includes the most active and most specialized mollusks. There is achambered, coiled shell in Nautilus and in many fossil forms; thisbecomes an internal structure in cuttlefish and squids, and is usually entirelyabsent in octopods.

A diversity of gill patterns have evolved in the major molluscan groups,paralleling the evolution of the mantle-shell patterns. The more advanced gastropods show reduction from a pair of aspidobranchctenidia to a single one, and from that to a one-sided pectinibranch ctenidium(or comb gill), and subsequently to no gill at all in the pulmonate snails. Thebivalves show enlargement of gill leaflets to longer filaments and their subsequent folding into the true lamellibranch condition, used in filter feeding. The gills in the cephalopods, while still structurally homologous, are modified with new skeletal elements to resist the stresses of water pumping by muscles.

Besides the gills, the other organs of the mantle cavity (termed collectively the pallial complex) again show morphological and functional consistency throughout the main groups of the Mollusca. The ctenidia form a curtain functionally dividing the mantle cavity into an inhalant part (usually ventral) containing the osphradia (pallial sense organs which sample the incoming water), and an exhalant part (usually dorsal) containing hypobranchial glands and both the anus and the openings of the kidney and genital ducts.

The cardiac structures of mollusks are also closely linked to the pallial complex. If there is a symmetrical pair of ctenidia, there will be a symmetrical pair of auricles on either side of the muscular ventricle of the heart; if one ctenidium, one auricle; if four ctenidia, four auricles. Note that body fluids in mollusks are almost all blood, just as body cavities are almost all hemocoel.

The respiratory pigment is usually hemocyanin in solution, so that neither circulatory efficiency nor blood oxygen-carrying capacity is high. However, mollusks are mostly sluggish animals with low metabolic (and hence respiratory) rates.

Uniquely molluscan is the use of cilia in “sorting surfaces,” which can segregate particles into different size categories and send them to be disposed of in different ways in several parts of the organism. In a simpler type of sorting surface, the epithelium is thrown into a series of ridges and grooves, the cilia in the grooves beating along them and the cilia on the crests of the ridges beating across them. Thus, fine particles impinging on the surface can be carried in the direction of the grooves, while larger particles are carried at right angles. Such sorting surfaces occur both externally on the feeding organs and internally in the gut of many mollusks. For example, on the labial palps of bivalves, they are used to separate the larger sand grains (which are rejected) from the smaller microorganisms which then pass to the mouth.

The range in levels of complexity of molluscan nervous systems is comparable to that found in the phylum Chordata. The four-strand nervous system with one pair of tiny ganglia found in chitons is not dissimilar to the neural plan in turbellarian flatworms. In contrast, the nervous system and sense organs of a cephalopod like an octopus are equaled and exceeded only by those of some birds and mammals. In the majority of mollusks the nervous system is in an intermediate condition. In mollusks other than cephalopods, the main effectors controlled by the nervous system are cilia and mucous glands. In fact, apart from the muscles which withdraw it into its shell, the typical mollusk is a slow-working animal with little fast nervous control or quick reflexes. In thebrain of modern cephalopods, paired ganglia have been fused into a massive structure, with over 300 million neurons and extensive “association” centers providing considerable mnemic and learning capacities.

In all primitive mollusks, the sexes are separate, and external fertilization follows the spawning of eggs and sperm into the sea.

In more advanced mollusks, eggs are larger (and fewer), fertilization may become internal (with complex courtship and copulatory procedures), and larval stages may be sequentially suppressed. A remarkably large number of mollusks (including many higher snails) are hermaphroditic. Although some are truly simultaneous hermaphrodites, many more show various kinds of consecutive sexuality. Most often the male phase occurs first, and these species are said to show protandric hermaphroditism.

Distributional ecology

Mollusks are largely marine. The extensive use of ciliary and mucous mechanisms in feeding, locomotion, reproduction, and other functions demands a marine environment for the majority of molluscan stocks. Apart from a small number of bivalve genera living in brackish and fresh waters, all nonmarine mollusks are gastropods.

Despite the soft, hydraulically moved bodies and relatively permeable skinstypical of all mollusks, some snails are relatively successful as land animals,although they are largely limited to more humid habitats. The primaryphysiological requirements for life on land concern water control, conversion to air breathing, and temperature regulation.

In the sea, all classes of mollusks are found, and all habitats have mollusks. Protobranchiate bivalves are found at depths of over 30,000 ft (9000 m). Although ecologically cephalopod mollusks are limited to the sea, there are sound reasons for claiming modern cephalopods as the most highly organized invertebrate animals. The functional efficiencies of jet propulsion and of massive brains in squid, cuttlefish, and octopuses have not been paralleled in their other physiological systems.

In addition to the extreme diversity of external body form exhibited by different mollusks, they show a remarkable diversity in their ecological distribution and life styles. However, the basic molluscan plan of structure and function always remains recognizable. See also Aplacophora; Bivalvia; Cephalopoda; Gastropoda; Lamellibranchia; Monoplacophora; Polyplacophora; Scaphopoda; Snail.

 
Columbia Encyclopedia: Mollusca
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Mollusca (məlŭs'kə), taxonomic name for the one of the largest phyla of invertebrate animals (Arthropoda is the largest) comprising more than 50,000 living mollusk species and about 35,000 fossil species dating back to the Cambrian period. Mollusks are soft-bodied, and most have a prominent shell. The members of this highly successful and diverse phylum are mostly aquatic and include the familiar scallop, clam, oyster, mussel, snail, slug, squid, cuttlefish, octopus, chiton, and a variety of others. Mollusks occupy habitats ranging from the deep ocean to shallow waters to moist terrestrial niches. Certain mollusks, such as clams, squids, and scallops, constitute important food staples, and molluskan shells are highly valued by collectors. In times past these shells were used as money and today are used ornamentally for such items as buttons and jewelry. There are six classes of mollusks.

Anatomical Features

Although highly diverse, all members of the phylum share certain general features. Most have a well-developed head, which may bear sensory tentacles; in some, like the clam, the head is very reduced.

The Body Wall

All mollusks possess a flexible body wall, which surrounds a body cavity containing the internal organs. The wall, which varies greatly in shape in different species, is usually folded to form a structure called the mantle, which is attached at the top of the body and surrounds it like a tent; the shell is formed on the outside of the mantle. On the underside of the body the wall is usually stretched out to form a thickened mass called the foot. The wall is covered by an outer epidermis and an underlying dermis. The epidermis usually contains gland cells that secrete mucus, which in mollusks has a variety of important uses, such as locomotion, food entrapment, and prevention of water loss. Muscle tissue is found in the body wall, and is particularly plentiful in the foot, which is used for locomotion in most mollusks (although some swim and some are sedentary), and in the mantle in species with reduced shells.

The Shell

The shell is formed by secretions of glandular cells in the mantle. Except in the chitons, the shells of all mollusks are basically similar, differing only in certain mineralogical details. The shell is composed of an outer, prismatic layer containing densely packed cells of calcareous material secreted by the edge of the mantle; and an inner, nacreous layer of thin, laminated plates of calcareous material laid down by the entire mantle surface. When very thin, the nacreous lining of the shell is pearly and iridescent. Layers of this material may form around a grain of sand or other irritant that lodges between the mantle and the shell; this process eventually forms a pearl. Pearl oysters of the genus Pinctada are the most commercially important pearl formers.

The Digestive Tract

The digestive tract of the Mollusca is complex. The foregut region consists of an esophagus and a mouth cavity, which contains a toothed belt called the radula, found in almost all mollusks and peculiar to the phylum. The radula is usually used for scraping food, such as algae, from surfaces. The number and form of radula teeth are highly variable; some species have a single radula tooth while others may have several hundred thousand. In some the teeth are hollow and poison-containing and are used as weapons; other radula modifications exist. The stomachs of mollusks are generally complex, and these, too, differ with the species and according to the feeding habits of the animal.

Respiration

Respiration is through gills called ctenidia (sing. ctenidium), located in the mantle cavity (the space between the mantle and the body wall proper) and varies with the species and with the type of habitat. For example, intertidal marine mollusks are exposed to air and water alternately and must be able to respire in both conditions; terrestrial species have lost their ctenidia, replacing them with lungs that can function in both water and air. Excretion of wastes is through structures called metanephridia and through the body and gill surfaces.

Circulatory and Nervous Systems

The blood circulates through the gill filaments, where exchange of carbon dioxide and oxygen occurs between the blood and the water flowing over the gill surface. Most molluskan blood contains a respiratory pigment called hemocyanin, a copper compound. When oxygenated such blood is bluish in color; when deoxygenated the blood is colorless. Only a few mollusks have hemoglobin in their blood. Blood circulation is variable within the phylum, but is generally mediated by a muscular heart, which distributes the blood to the tissues. Most mollusks possess well-developed sensory organs. The highest degree of development of the nervous system is found in the class Cephalopoda (octopuses, squids, and nautiluses).

Reproduction

Reproduction is sexual and may be simple or highly complex. The fertilized egg develops into a swimming form called a trochophore larva, which is seen also in the development of annelids; this then elongates to become a veliger larva, characteristic of mollusks, and differing in form in the different classes.

Class Amphineura

This class contains two very different kinds of mollusk. The subclass Polyplacophora contains about 600 species of sedentary animals commonly known as chitons, marine forms found from shallow waters to depths of about 1,300 ft (400 m). A chiton has a broad foot and a shell consisting of eight overlapping plates. The subclass Aplacophora contains about 100 species of wormlike, deepwater marine mollusks.

Class Monoplacophora

This class was created for the genus Neopilina, a mollusk discovered in 1952, when specimens were dredged from a deep trench off the Pacific coast of Central America. Neopilina displays primitive molluskan characteristics; it is the only mollusk with a segmented internal structure and is thought to show a relationship between mollusks and annelids. The animal is about 1 in. (2.5 cm) long and has characteristics of both chitons and gastropods, but does not quite fit into either class.

Class Gastropoda

This class, containing over 35,000 living and 15,000 fossil gastropod species, comprises the largest class of Mollusca, and includes the limpets, top shells, periwinkles, slipper shells, snails, slugs, sea hares, abalones, nudibranches, or sea slugs, and sea butterflies. Gastropods are primarily marine, but freshwater and terrestrial forms occur. When present, the typical gastropod shell is a three-layered, spiral whorl of calcium carbonate, which varies in color, shape, ornamentation, and size according to the species. Within this shell is the tall, coiled body mass. Some forms, such as slugs, are shell-less and do not have a tall body mass. Gastropod larvae undergo a twisting, or torsion, that brings the rear of the body (mantle cavity, gills, and anus) to a position near the head and results in the twisting of internal organ systems. In many this twisted form is retained by the adult; in others it is partially lost.

There are three subclasses: the Prosobranchia, which contains the majority of gastropods; the Pulmonata, which contains the land snails; and the Opisthobranchia, which includes the sea hares and sea slugs. The latter subclass consists of animals with reduced shells or none at all. Most gastropods are motile, but some, e.g., the slipper shell (Crepidula), are sedentary. Some, such as the sea butterflies, swim, and others, including the terrestrial snails, move by means of a well-developed foot.

Many gastropods are herbivores, or plant eaters, with multitoothed radulas for scraping algae from various substrata. Among the carnivorous, or animal-eating, species is the conch, which feeds on smaller mollusks, and the cone shells (Conus), which feed on fish and annelid worms that they first paralyze with poison contained in their hollow radula teeth. The poison is also toxic to humans, causing paralysis and sometimes death. Gastropods have a complex nervous system with ganglia.

Reproduction is variable, but most gastropods have separate sexes. Fertilization of the egg occurs in seawater. Some gastropods are hermaphrodites (having both sexes in the same individual) and some are protandric hermaphrodites, i.e., they are male first and become female as they age.

Gastropods are economically valuable as food for many animals, including humans. Some gastropods are serious pests; the common slug, for example, causes much garden damage.

Class Pelecypoda (Bivalvia)

This class contains the mollusks known as bivalves, including the mussels, oysters, scallops, and clams. All have shells composed of two pieces known as valves. In most, the valves are of similar size, but in some sedentary species, such as the oysters, the upper valve, which covers the left side of the body, is larger than the lower valve, which covers the right side and is attached to the substratum. Two large muscles, called adductors, hold the valves together at the top of the body. Pelecypod shells vary greatly in size, color, and ornamentation. The freshwater seed shells are among the smallest known, being less than .1 in. (c.2 mm) in length, while the shell of the giant clam may exceed 4 ft (120.4 cm) in length.

The foot of pelecypods is adapted for burrowing in all species except the sedentary ones, where it is reduced in size. Some species, e.g., the cockles, use the foot to hop about from place to place. Pelecypods have a greatly reduced head and no radula. Most have a single pair of large gills used for respiration and for trapping minute food particles. Members of the order Protobranchia use another structure, the proboscis, to feed on bottom detritus. The order Septibranchia contains animals that have lost their gills; they are carnivores or scavengers. Pelecypods have a relatively simple nervous system with three pairs of ganglia and two pairs of long nerve cords. An organ of equilibrium, called a statocyst, is present in most. Fertilization normally occurs in surrounding seawater, and most pelecypods have separate sexes. All are aquatic, and they constitute an important food source for many animals, including humans.

Class Scaphopoda

This small class of marine mollusks includes 200 species of burrowing animals commonly known as the tusk, or tooth, shells. The shell is long, cylindrical and tooth- or tusk-shaped, and open at both ends. The foot and the small head project from the larger end. Threadlike tentacles hang from the head and are used for gathering the microscopic organisms on which tusk shells feed. Most scaphopods are tiny, usually only several inches (about 6 cm) long. They are found in both shallow and deep waters; they burrow into the bottom, with only the upper opening protruding.

Class Cephalopoda

This class contains the cephalopods, animals commonly known as squid, cuttlefish, octopus, and nautilus. The giant squid is the largest of all mollusks. Most cephalopods are highly adapted for swimming. The body mass is very tall. There is no foot; the lower part of the body wall is drawn out to form a ring of arms, or tentacles, around the head. Among living cephalopods, only the nautilus (subclass Nautiloidea) has a complete external shell; extinct members of the subclass and the extinct ammonites (subclass Ammonoidea) had similar spiral shells. Members of the subclass Coleoidea (the squid, cuttlefish, and octopus), have an internal shell or no shell at all.

All cephalopods are carnivorous and possess a radula and powerful beaks. The nervous system and the sense of vision are highly developed. In most cephalopods the sexes are separate and reproduction requires copulation. Fertilization may occur inside or outside the mantle cavity. Cephalopods are worldwide in distribution and are found in all depths of the ocean. They are an important food staple for many animals, including humans.

WordNet: Mollusca
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Note: click on a word meaning below to see its connections and related words.

The noun has one meaning:

Meaning #1: gastropods; bivalves; cephalopods; chitons
  Synonym: phylum Mollusca

Wikipedia: Mollusca
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Mollusca
Fossil range: Cambrian–Recent
Caribbean reef squid, Sepioteuthis sepioidea
Scientific classification
Kingdom: Animalia
Superphylum: Lophotrochozoa
Phylum: Mollusca
Linnaeus, 1758
Classes

Aplacophora
Bivalvia
Caudofoveata
Cephalopoda
Gastropoda
† Helcionelloida
Monoplacophora
Polyplacophora
† Rostroconchia
Scaphopoda
Diversity
about 93,000 recognized living species[1]

Molluscs[note 1] are animals belonging to the phylum Mollusca. There are around 93,000 recognized extant species, making it the largest marine phylum with about 23% of all named marine organisms. Representatives of the phylum live in a huge range of habitats including marine, freshwater, and terrestrial environments. Molluscs are a highly diverse group, in size, in anatomical structure, in behaviour and in habitat.

The phylum is typically divided into nine or ten taxonomic classes, of which two are entirely extinct. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total number of classified molluscan species. Cephalopod molluscs such as squid, cuttlefish and octopus are among the most neurologically advanced of all invertebrates. Either the giant squid or the colossal squid is the largest known invertebrate species.

Molluscs have such a varied range of body structures that it is difficult to find defining characteristics that apply to all modern groups. The two most universal features are a mantle with a significant cavity used for breathing and excretion, and the structure of the nervous system. As a result of this wide diversity, 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 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 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. A striking feature of molluscs is the use of the same organ for multiple functions. For example: the heart and nephridia ("kidneys") are important parts of the reproductive system as well as the circulatory and excretory systems; in bivalves, the gills both "breathe" and produce a water current in the mantle cavity which is important for excretion and reproduction.

There is good evidence for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 542 to 488.3 million years ago. 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 under 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. Their shells have also been used as a money in some pre-industrial societies.

Mollusc species can also represent hazards or pests for human activities. 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 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.

Contents

Diversity

About 80% of all known mollusc species are gastropods.[2]

There are about 93,000 named mollusc species,[1] which include 23% of all named marine organisms.[3] Molluscs are second only to arthropods in numbers of living animal species[2] – far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000.[4] It has been estimated that there are about 200,000 living species in total,[5] and 70,000 fossil species,[6] although the total number of mollusc species that ever existed, whether or not preserved, must be many times greater than the number alive today. [7]

Molluscs have more varied forms than any other animal phylum – snails and other gastropods, clams and other bivalves, squids and other cephalopods, and other lesser-known but similarly distinctive sub-groups. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some are significant members of freshwater and terrestrial ecosystems. They are extremely diverse in tropical and temperate regions but can be found at all latitudes.[8] About 80% of all known mollusc species are gastropods.[2] Cephalopoda such as squid, cuttlefish and octopus are among the most neurologically-advanced of all invertebrates.[9] The giant squid, which until recently had not been observed alive in its adult form,[10] is one of the largest invertebrates. However a recently-caught specimen of the colossal squid, 10 metres (33 ft) long and weighing 500 kilograms (0.49 LT; 0.55 ST), may have overtaken it.[11]

Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of non-marine molluscs vary widely, partly because many regions have not been thoroughly surveyed and there is a shortage of specialists in classifying these animals. However, in 2004 the IUCN Red List of Threatened Species included nearly 2,000 endangered non-marine molluscs. For comparison, the great majority of molluscs species are marine but only 41 of these appeared on the 2004 Red List. 42% of recorded extinctions since 1500 involved molluscs, almost entirely non-marine.[12]

General description

Definition

The word mollusc is derived from the French mollusque, which originated from the Latin molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τᾲ μαλάκια, "the soft things", which he applied to cuttlefish.[13] The scientific study of molluscs is known as malacology.[14]

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.[8] The following are present in all modern molluscs:[6][15]

Other characteristics that commonly appear in textbooks have significant exceptions:

  Class
Characteristic[6] Aplacophora[17] Polyplacophora[18] Monoplacophora[19] Gastropoda[20] Cephalopoda[21] Bivalvia[16] Scaphopoda[22]
Radula, a rasping "tongue" with chitinous teeth Absent in 20% of Neomeniomorpha Yes Yes Yes Yes No Internal, cannot extend beyond body
Broad, muscular foot Reduced or absent Yes Yes Yes Modified into arms Yes Small, only at "front" end
Dorsal concentration of internal organs (visceral mass) Not obvious Yes Yes Yes Yes Yes Yes
Large digestive ceca No ceca in some aplacophora Yes Yes Yes Yes Yes No
Large complex metanephridia ("kidneys") None Yes Yes Yes Yes Yes Small, simple

A "generalized mollusc"

Further information: Mollusc shell
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    Digestive & excretory system
    Circulatory & respiratory
    Central nervous system
    Reproductive system
 1 Radula
 2 Mouth
 3 Shell
 4 Stomach
 5 Gonad
 6 Heart
 7 Coelom
 9 Mantle
10 Mantle cavity
11 Anus
12 Gill
13 Foot
15 Pedal nerve cord
16 Gut
17 Visceral nerve cord
18 Nerve ring
A generalized mollusc[23]

Because of the enormous variations between groups of molluscs, many text books start the subject by describing a "generalized mollusc", which some suggest may resemble very early molluscs and which is rather similar to modern monoplacophorans.[8][15][19][24]

The mantle secretes a shell that is mainly chitin and conchiolin (a protein) hardened with calcium carbonate,[15][25] except that the outermost layer is all conchiolin.[15] Molluscs never use phosphate to construct their hard parts,[26] with the questionable exception of Cobcrephora.[27] The mantle cavity is a fold in the mantle that encloses a significant amount of space, and was probably at the rear in the earliest molluscs but its position now varies from group to group. In the "generalized mollusc" the anus, a pair of osphradia (chemical sensors), the hindmost pair of gills and the exit openings of the nephridia ("kidneys") and gonads (reproductive organs) are in the mantle cavity.[15]

The underside of the body generally consists of a muscular foot, which has been adapted for different purposes in different classes.[28]:4 In gastropods, it secretes mucus as a lubricant to aid movement. In forms that have only a top shell, such as limpets, the foot acts a sucker attaching to 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.[15] In bivalves, the foot is adapted for burrowing into the sediment;[28]:4 in cephalopods it is used for jet propulsion,[28]:4 and the tentacles and arms are derived from the foot.[29]

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 circulates and which encloses most of the other internal organs. 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.[15]

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 coleom 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.[15]

Most molluscs have only one pair of gills, or even only one 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 that 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 connected to the heart.[15]

    = Food
    = Radula
    = Odontophore "belt"
    = Muscles
Snail radula at work.

Most molluscs have muscular mouths with radulae, "tongues" bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. This is primarily designed to scrape bacteria and algae off rocks. Their mouths also contain glands that secrete slimy mucus, to which the food sticks. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so that the mucus forms a long string. 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 that 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 that 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 entrance to the hindgut pinch off a piece of the prostyle so that it is excreted, preventing the prostyle from growing too large. The anus is in the part of the mantle cavity that is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems.[15]

In general, molluscs have two pairs of main nerve cords, the visceral cords serving the internal organs and the pedal ones serving the foot. Both pairs run below the level of the gut, and include ganglia as local control centers in important parts of the body. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The only ganglia above the gut are the cerebral ganglia, which sit above the esophagus (gullet) and handle "messages" from and to the eyes. The pedal ganglia, which control the foot, are just below the esophagus and their commissure and connections to the cerebral ganglia encircle the esophagus in a nerve ring.[15]

A typical mollusc has: a pair of tentacles on the head, containing chemical and mechanical sensors; a pair of eyes on the head, a pair of statocysts in the foot which act as balance sensors; and a pair of osphradia, chemical sensors, in the incoming "lane" of the mantle cavity.[15]

Apical tuft (cilia)
Prototroch (cilia)
Stomach
Mouth
Metatroch (cilia)
Mesoderm
Anus
/// = cilia
Trochophore larva[30]

In the simplest molluscan reproductive systems, two gonads sit next to the coelom that surrounds the heart and shed ova or sperm into the coleom, from which the nephridia extract them 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, where an individual can function as both sexes; both of these methods require more complex reproductive systems.[15]

The most basic molluscan larva is a trochophore which is planktonic and feeds on floating food particles by using the two bands of cilia round 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 that 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. In some species the newborn larvae are already veligers, and other species have direct development, in which a miniature adult emerges from the egg.[15]

Classification

Opinions vary about the number of classes of molluscs – for example the table below shows eight living classes,[1] and two extinct ones. However some authors combine the Caudofoveata and solenogasters into one class, the Aplacophora.[17][24] Two of the commonly-recognized classes are known only from fossils[2]

Class Major organisms Described living species[1] Distribution
Caudofoveata[17] worm-like organisms 120 seabed 200–3,000 metres (660–9,800 ft)
Aplacophora[17] solenogasters, worm-like organisms 200 seabed 200–3,000 metres (660–9,800 ft)
Polyplacophora[18] chitons 1,000 rocky tidal zone and seabed
Monoplacophora[19] limpet-like organisms 31 seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200 metres (660 ft)
Gastropoda[31] abalone, limpets, conch, nudibranchs, sea hares, sea butterfly, snails, slugs 70,000 marine, freshwater, land
Cephalopoda[32] squid, octopus, cuttlefish, nautilus 900 marine
Bivalvia[33] clams, oysters, scallops, mussels 20,000 marine, freshwater
Scaphopoda[22] tusk shells 500 marine 6–7,000 metres (20–22,970 ft)
Rostroconchia †[34] fossils; probable ancestors of bivalves extinct marine
Helcionelloida †[35] fossils; snail-like organisms such as Latouchella extinct marine

Evolution

Fossil record

There is debate about whether some Ediacaran and Early Cambrian fossils really are molluscs. Kimberella, from about 555 million years ago, has been described as "mollusc-like",[36][37] but others are unwilling to go further than "probable bilaterian".[38][note 2][39] 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.[38][40] 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.[41]

The tiny Helcionellid fossil Yochelcionella is thought to be an early mollusc[35]
Spirally-coiled shells are appear in many gastropods[20]

However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks from Siberia and China,[42][43] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites.[35] 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. There have been suggestions that the tiny specimens were juveniles and the larger ones adults.[44]

Some analyses of helcionellids concluded that these were the earliest gastropods.[45] However other scientists are not convinced that Early Cambrian fossils show clear signs of the torsion that identifies modern gastropods twists the internal organs so that the anus lies above the head.[46][47][20]

    = Septa
    = Siphuncle
Septa and siphuncle in nautiloid shell

For a long time it was thought that Volborthella, some fossils of which pre-date 530 million years ago, 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.[48] The Late Cambrian fossil 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 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.[49] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65 million years ago.[50] However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.[51]

The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves.[52][53][54][55] "Modern-looking" bivalves appeared in the Ordovician period, 488 to 443 million years ago.[56] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous-Tertiary extinction.[57] However, bivalves are now abundant and diverse.

Phylogeny

A possible "family tree" of molluscs (2007).[58][59] Does not include annelid worms as the analysis concentrated on fossilizable "hard" features.[58]

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,[37][38][40][41] there are debates about the relationships between the classes of living molluscs.[39] In fact some groups traditionally classifed as molluscs may have to be redefined as distinct but related.[60]

Molluscs are generally regarded members of the Lophotrochozoa,[58] a group defined by having trochophore larvae and, in the case of living Lophorata, a feeding structure called a lophophore. The other members of the Lophotrochozoa are the annelid worms and seven marine phyla.[61] The diagram on the right summarizes a phylogeny presented in 2007.

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.[62] Scientists disagree about this: Giribet and colleagues concluded in 2006 that the repetition of gills and of the foot's retractor muscles were later developments, [8] while in 2007 Sigwart concluded that the ancestral mollusc was metameric, and that it had a foot used for creeping and a "shell" that was mineralized.[39]

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.[35] Eventually, the cuticle would have become mineralized;[35] this mineralization may have happened one or many times,[verification needed] but uses the same genetic machinery (engrailed) as most other bilaterian skeletons.[62] The first mollusc shell almost certainly was reinforced with the mineral aragonite.[63]

The evolutionary relationships within the molluscs are also debated, and the diagrams below show two widely-supported reconstructions:

However, an analysis in 2009 that used both morphological and molecular phylogenetics comparisons concluded that the molluscs are not monophyletic; in particular, that Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes –in other words that the traditional phylum Mollusca is polyphyletic, and that it can only be made monophyletic if scaphopods and bivalves are excluded.[60]

Interactions with humans

Uses by humans

Further information: Seashell
Mollusc output in 2005

Molluscs, 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.[64] Other molluscs commonly eaten include octopuses and squids, whelks, oysters, and scallops.[65] In 2005, China accounted for 80% of the global mollusc catch, netting almost 11 million tonnes. Within Europe, France remained the industry leader.[66] 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.[67]

Saltwater pearl oyster farm in Seram, Indonesia

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.[16][20] 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.[16] The pearl industry is so important in some areas that significant sums of money are spent on monitoring the health of farmed molluscs.[68]

Byzantine Emperor Justinian I clad in Tyrian purple

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.[69] 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–18th century BC, centuries before the Tyrians.[70][71] 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.[72] 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."[73]

Mollusc shells, including those of cowries, were used as a kind of money in several pre-industrial 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.[74] When used for commercial transactions they functioned as commodity money, in other words 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 appeared.[75]

Threats to humans

Stings and bites

The blue-ringed octopus's rings are a warning signal – this octopus is alarmed, and its bite can kill.[76]

When handled alive, a few species of molluscs in the wild can sting or bite, and in the case of an even lesser number of species, this can present a serious risk to the human who is handling the animal. To put this into the correct perspective however, deaths from mollusc venoms are less than 10% of the number of deaths from jellyfish stings.[77]

All octopuses are venomous[78] 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,[76] 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.[79]

Live cone snails can be dangerous to shell-collectors but are useful to neurology researchers[80]

Cone snails, carnivorous gastropods which feed on marine invertebrates (and in the case of larger species on 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.[80] Many painful stings have been reported and a few fatalities, although some of the reported fatalities may be exaggerations.[77] Only the few larger species of cone snail that can capture and kill fish are likely to be seriously dangerous to humans.[81] 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.[80][82]

The traditional belief that a giant clam can trap the leg of a person between its valves, thus drowning them, is a myth.[83]

Pests

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."[84] The parasite has 13 known species, of which two infect humans. The parasite itself is not a mollusc, but all the species have freshwater snails as intermediate hosts.[85]

Some species of molluscs, particularly certain snails and slugs, can be serious crop pests,[86] and snails or slugs 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.[87]

Despite its name, Molluscum contagiosum is a viral disease, and is unrelated to molluscs.[88]

Notes

  1. ^ Spelled mollusk in the USA, see reasons given in Rosenberg's [1]; for the spelling "mollusc" see the reasons given by Brusca & Brusca. Invertebrates (2nd ed.). .
  2. ^ It is possible that Kimberella is more closely related to, for instance, the brachiopods than the molluscs

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Further reading

  • Starr & Taggart (2002). Biology: The Unity and Diversity of Life. Pacific Grove, California: Thomson Learning. 
  • Nunn, J.D., Smith, S.M., Picton, B.E. and McGrath, D. 2002. 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.

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