Many hard-shelled marine organisms construct their shells or skeletons from calcium carbonate. This mineral occurs naturally in a couple of different crystal structures, aragonite and calcite. Mollusk shells rely chiefly on aragonite, possibly because this was the crystal more easily precipitated from seawater at the time when mollusks first started calcifying their shells (Porter, 2007). Aragonite is also used by scleractinian corals for their skeletons, so it’s not surprising that sand in many productive coastal regions consists largely of aragonite; it’s mostly the broken, ground up shells and skeletons of corals and mollusks past.
Echinoderms, by contrast, use calcite to construct their skeletons (Raup, 1959). The hard parts of a sea urchin are relatively obvious, and both the spines and the test enclosing the body rely on calcite. The softer-looking echinoderms use it too. Starfish, brittle stars and feather stars have more flexible appendages, but these are all supported by many short segments of calcite skeleton. Even sea cucumbers have calcite ossicles embedded in their body wall.
Bryozoans (Taylor, 2012) and Calcareous sponges (Stanley and Hardie, 1998) use both calcite and aragonite, and at least some species show flexibility in which crystal they use, depending on which is favored by ambient water chemistry.
Many species of algae build with calcium carbonate too. Most red calcareous algae build calcite inside their cell membranes, while calcareous green algae usually build aragonite on the outside (Granier, 2012). These algae can be the most important habitat builders in many areas outside of the tropics (Basso, 2012).
Their aragonite tendencies may leave corals, green algae and mollusks especially vulnerable to ocean acidification. At its present concentration of carbon dioxide (CO2), the ocean is still well-supplied with the minerals needed for all organisms that use calcium carbonate to build their shells or skeletons. It has been estimated that by the year 2050, rising CO2 levels will begin to deplete the available ions below optimal levels for aragonite building (Orr et al, 2005), essentially making aragonite more soluble in seawater. This effect has also been measured in the lab on pteropod mollusks. When raised in seawater with the predicted CO2 concentration for the year 2100, theSea butterfly Limacina helicina's calcification rate fell 28% (Comeau, 2009). Recently, samples of this Antarctic species from a region with depressed aragonite levels were found to have significant shell dissolution already, in the wild (Bednaršek et al, 2012).
Different calcareous organisms are affected in different ways by changes in ocean acidification. The geological record shows a number of events in the past 300 million years when sudden very large changes in species richness occurred in some groups of calcium-carbonate builders, which is likely to be related to acidity changes in seawater (Hönisch et al, 2012). Different groups were affected to a greater or lesser degree and it appears that several factors including habitat and physiology influence which groups are more sensitive to rising acidity. For instance, some calcite-builders like sea urchins and calcareous sponges will be slightly less sensitive, since calcite crystal formation is not affected as quickly by increased CO2. However, organisms that build calcite structures with a significant dose of Magnesium ions (High Magnesium Calcite or HMC) like some red algae do, will be the most quickly affected, as HMC is even more soluble than aragonite (Basso, 2012).
There is a lot of uncertainty about how reduced ocean calcification will feedback on the changing carbon cycle globally. The process of dissolving calcium carbonate (or reducing calcification) actually uses up carbon dioxide, shifting the seawater equilibrium toward bicarbonate ion (see Encyclopedia of Earth, 2010, for review). However, the greater impact of the changes will be in the total productivity of the communities that rely on calcification and the habitat it constructs (The Royal Society, 2005). If a reef habitat is lost, the question is, what will take its place? If the new community is equally productive, it may continue to sequester organic carbon, as dead tissue sinking to the deep ocean, just as fast as the original habitat did. Of course for that scenario it should be borne in mind that natural productivity of coral reef communities is extremely high, so an equally productive one succeeding them would be very unlikely.
Mollusks: The Survival Game Video and Lesson Plans
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The phylum Mollusca contains some of the most familiar invertebrates, including snails, slugs, clams, mussels, and octopuses. In contrast to these well-known molluscs, however, others are almost never seen, such as the aplacophorans and monoplacophorans, the latter of which which were only known from Paleozoic fossils until the first live specimen was discovered in the deep sea in 1952 (UCMP 2008).
Except for the aplacophorans, most molluscs have a well-developed, muscular foot. This structure is used in a multitude of ways, for example: locomotion, clinging to surfaces, burrowing, anchoring in sediment, swimming, and grasping (modified into prehensile tentacles in octopuses). The vast diversity of foot adaptations exemplifies the huge morphological diversity of the mollusc form.
A layer of epidermal tissue called the mantle surrounds the body of molluscs. Specialized glands in the mantle are responsible for the extracellular excretions that form shell structures. In all molluscan groups the shell is produced in layers of (usually) calcium carbonate, either in calcite or aragonite form.
Molluscs have adapted to terrestrial, marine and freshwater habitats all over the globe, although most molluscs are marine. Nearly 100,000 mollusc species are known (excluding the large number of extinct species known only as fossils) and it is clear that many thousands of species of extant species remain undescribed. Around 80% of known molluscs are gastropods (snails and slugs).
Most molluscs undergo spiral cleavage. Development can be direct (proceed right to settling into a juvenile form) or indirect, going through the swimming trochophore larval stage. The trochophore is very similar to the annelid trochophore. Before settling, many groups then go onto a second larval stage which is unique to molluscs: the feeding (usually) and swimming veliger larvae. Molluscs go through the uniquely molluscan process of torsion, usually during the veliger stage of development. Torsion involves counterclockwise rotation of the visceral mass up to 180 degrees with respect to the head and foot, to profoundly change the relative location of the body regions. Many groups then “detort” to some degree later in development or adulthood. Theories as to the evolutionary significance of torsion abound but this phenomenon is not well understood (Brusca and Brusca 2003). In the long run, torsion has allowed for much morphological diversification over the course of evolution.
Molluscs have adapted to terrestrial, marine and freshwater habitats all over the globe, although most molluscs are marine.
Despite the highly diverse forms of the members of this taxon, molluscs share a recognizable and characteristic generalized general body plan, made up of a head, a foot, and viscera contained in a central body. They are generally considered unsegmented, although primitive forms (aplacophorans and polyplacophorans) with repeated body features show intriguing potential for a possibly segmented mollusc-annelid ancestor (e.g. Jacobs et al 2000).
The mollusc head can house various combinations of sensory structures: tentacles, photoreceptors, statocysts, chemoreceptors. In some molluscs these sensory systems can be very well developed (the complex cephalopod eye is a prime example). Also found on the head is a feature unique to molluscs: the radula. Found in the buccal (mouth) cavity, the radula usually exists as a tongue-like plate covered with “teeth” used by herbivores, carnivores and scavengers to scrape food particles into the mouth. Depending on diet and use, tooth number, shape, arrangement, makeup, and growth have adapted diversely. Especially in the gastropods, number and shape of radular teeth are important taxonomic characters. The radula has also been adapted for diverse feeding methods. Some gastropods and cephalopods have a drill-like radula used to bore holes in the shell of prey, sometimes with the aid of acids secreted from an adjacent boring gland. In cone snails the radula is set on the end of a retractable proboscis and is slung out like a harpoon, to inject toxins into the prey, delivered through piercing, hollow teeth. In some cases these toxins are powerful neurotoxins, deathly to humans. Several lineages of molluscs have evolved suspension feeding, especially in the gastropods and bivalves. The radula in these cases is either highly reduced or lost altogether, and in most cases food particles are caught by ctinidia (gills) and moved to the mouth by cilia.
Except for the aplacophorans, most molluscs have a well-developed, muscular foot. This structure is used in a multitude of ways, for example: locomotion, clinging to surfaces, burrowing, anchoring in sediment, swimming, modified into prehensile tentacles (octopus); the vast diversity of foot adaptations exemplifies the huge morphological diversity of the mollusc form.
A layer of epidermal tissue called the mantle surrounds the body of molluscs. Specialized glands in the mantle are responsible for the extracellular excretions that form shell structures. The ancestral mollusc is thought to have one shell capped over the body like a limpet, and from that a diverse number of shell arrangements have evolved. Molluscs may have have one, two, or eight (in chitons) shells. Aplacophorans have no shell, but have instead minute aragonite spicules imbedded within the mantle. Secondary loss or much reduced shell vestiges have also occurred independently in multiple mollusc lineages (for example nudibranchs, slugs, cephalopods). Shells usually provide external protection, but there have been several independent internalizations within cephalopods and opisthobranchia. In all molluscan groups the shell is produced in layers of (usually) calcium carbonate, either in calcite or aragonite form. The wide range of pigmentation, shape, size, sculpturing, and twisting of sea shells is, of course, well known. There is much recent developmental work describing gene expression in shell formation, and the roles of highly conserved regulatory genes such as engrailed and Hox genes have been examined (e.g. Jacobs et al 2000, Samadi and Steiner 2009).
Between the mantle and the body proper is the mantle cavity, which may be organized as one or two separate spaces or grooves. Many important functions occur in the mantle cavity: the ctenidia (gills) are positioned here and the body systems, namely the nephridia (kidney like organs), the gut and the reproductive organs open up into this space. In aquatic molluscs cilia on the surface of the mantle and organs maintain water flow through the mantle cavity to take away wastes and bring in oxygenated water (and food particles for those suspension feeding molluscs). Molluscs have an open circulatory system with a full heart (with the exception of the cephalopods, which have a closed circulatory system). Their nervous system is well developed, usually consisting of a dorsal ganglion, a ring of nerves around the esophagus, and two pairs of lateral nerve cords running the length of the body, which are connected transversely in a ladder-like arrangement. There is an enormous range of nervous system development in the molluscs, from the poorly developed ganglia of the aplacophorans to the extreme cephalization of the cephalopods. Important work in the fields of neurobiology has been carried out on the squid Doryteuthis pealeii (formerly Loligo pealeii) and on Aplysia sea slugs.
Like other systems, reproduction is highly variable among molluscs. Molluscs can be dioecious or simultaneously or sequentially hermaphroditic. Gametes are freely spawned in some groups, others have internal fertilization and complex mating behaviors, many produce egg capsules, egg cases, or brood chambers.
Molluscs range in size from almost microscopic to animals 20 meters long (giant squid) or weighing 450 pounds (giant clams).
The molluscs demonstrate remarkable morphological diversity, a characteristic that has confused molluscan taxonomy from the group’s inception. The Latin root molluscus means soft, and many soft-bodied invertebrates have been added and removed this group until Cuvier’s modern approximation in 1795 (Brusca and Brusca 2003). Mollusca is the second largest invertebrate phylum after the arthropods. Some 93,000 extant species have been described, but the thinking is this number represents only about half of the living species. 70,000 fossil species are also known. Most classifications recognize ten molluscan classes (two extinct). One class, the gastropods (snails and slugs), contains about 80% of mollusc species.
A very rich molluscan fossil record dates back 500 million year to the Precambrian. The evolutionary origins of molluscs are still disputed, but recent well-respected molecular phylogenetic analyses place the molluscs in the Lophotrochozoa, along with annelids, brachiopods, bryozoans and several other phyla (Halanych et al. 1995). Relationships within the Mollusca are also unclear and disputed; some recent analyses challenge whether this enormous phylum is a natural, monophyletic group (Sigwart and Sutton 2007 and references therein).
Non-marine molluscs appear to have a very high extinction rate. Lydeard et al (2004) list terrestrial and fresh water mollusc extinctions as about 40% of total recorded animal extinctions, far greater than marine molluscs.
Many different molluscs have been integrated into human culture since prehistoric times in a plethora of forms: shell money, jewelry and food, crop pests, and disease carriers (Schistosomiasis is a watersnail-born parasite that effects hundreds of millions of people in the world).
Mollusca is the second-largest phylum of invertebrate animals, after the Arthropoda; members are known as molluscs or mollusks[a] (/ˈmɒləsk/). Around 85,000 extant species of molluscs are recognized.[3] The number of fossil species is estimated between 60,000 and 100,000 additional species.[4] The proportion of undescribed species is very high. Many taxa remain poorly studied.[5]
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 anatomical structure, but also in behaviour and habitat. The phylum is typically divided into 7 or 8[6] taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, 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 and account for 80% of the total classified species.
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. Nearly all produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.
Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541–485.4 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. Toxins that can accumulate in certain molluscs under specific conditions create a risk of food poisoning, and many jurisdictions have regulations to reduce 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 money in some preindustrial societies.
A handful of mollusc species are sometimes considered 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 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 by 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.
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 τὰ μαλάκια ta malákia (the soft ones; < μαλακός malakós "soft"), which he applied inter alia to cuttlefish.[7][8] The scientific study of molluscs is accordingly called malacology.[9]
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 now known, these groups have no relation to molluscs, and very little to one another, so the name Molluscoida has been abandoned.[10]
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 shell.[11]
Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult.[12] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical.[13] The following are present in all modern molluscs:[14][16]
Other characteristics that commonly appear in textbooks have significant exceptions:
Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.[1] The total number of described species is difficult to estimate because of unresolved synonymy. 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 terrestrial. The Bivalvia would comprise about 14% of the total and the other five classes less than 2% of the living molluscs.[18] In 2009, Chapman estimated the number of described living mollusc species at 85,000.[1] Haszprunar in 2001 estimated about 93,000 named species,[19] which include 23% of all named marine organisms.[20] Molluscs are second only to arthropods in numbers of living animal species[17] — far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000.[15]: Front endpaper About 200,000 living species in total are estimated,[1][21] and 70,000 fossil species,[14] 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.[22]
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.[12] About 80% of all known mollusc species are gastropods.[17] Cephalopoda such as squid, cuttlefish, and octopuses are among the neurologically most advanced of all invertebrates.[23] The giant squid, which until recently had not been observed alive in its adult form,[24] 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.[25]
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 of Threatened 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.[26]
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.[12][16][27]
The generalized mollusc is an unsegmented, bilaterally symmetrical animal 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".[16] The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs.[13]
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.[16] The whole soft body of bivalves lies within an enlarged mantle cavity.[13]
The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs[13]) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate),[16][28] except the outermost layer, which in almost all cases is all conchiolin (see periostracum).[16] Molluscs never use phosphate to construct their hard parts,[29] with the questionable exception of Cobcrephora.[30] 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 eggshells.[31]
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.[13]
In some forms the shell contains openings. In abalones there are holes in the shell used for respiration and the release of egg and sperm, in the nautilus a string of tissue called the siphuncle goes through all the chambers, and the eight plates that make up the shell of chitons are penetrated with living tissue with nerves and sensory structures.[32]
The underside consists of a muscular foot, which has adapted to different purposes (locomotion, grasping the substratum, burrowing or feeding) in different classes.[33] 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.[16] In bivalves, the foot is adapted for burrowing into the sediment;[33] in cephalopods it is used for jet propulsion,[33] and the tentacles and arms are derived from the foot.[34]
Most molluscs' circulatory systems are mainly open, except for cephalopods, whose circulatory systems are closed. 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.[13] The blood of these molluscs 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.[16] 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.[16]
Exceptions to the above are the molluscs Planorbidae or ram's horn snails, which are air-breathing snails that use iron-based hemoglobin instead of the copper-based hemocyanin to carry oxygen through their blood.
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.[16]
Molluscs use intracellular digestion. 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.[13] 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 string".[16]
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.[16]
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.[16]
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.[16]
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.[13]
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.[16]
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.
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 systems.[16]
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.[36] The development of molluscs is of particular interest in the field of ocean acidification as environmental stress is recognized to affect the settlement, metamorphosis, and survival of larvae.[37]
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.[38] Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing the water over their gills. Most bivalves are filter feeders, which can be measured through clearance rates. Research has demonstrated that environmental stress can affect the feeding of bivalves by altering the energy budget of organisms.[37]
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.[39] Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls,[40] whereas dorid nudibranchs and some Vetigastropoda feed on sponges[41][42] and others feed on hydroids.[43] (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): 144..)
Opinions vary about the number of classes of molluscs; for example, the table below shows seven living classes,[19] and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and Solenogasters into one class, the Aplacophora.[27][15]: 291–292 Two of the commonly recognized "classes" are known only from fossils.[17]
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.[46] Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda. Tentaculita may also be in Mollusca (see Tentaculites).
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.[47] 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 debated.
Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. Kimberella, from about 555 million years ago, has been described by some paleontologists as "mollusc-like",[48][49] but others are unwilling to go further than "probable bilaterian",[50][51] if that.[52]
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.[50][53] 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.[54] This appears to contradict the concept that the ancestral molluscan radula was mineralized.[55]
However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks from Siberia and China,[56][57] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites.[45] 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.[58]
Some analyses of helcionellids concluded these were the earliest gastropods.[59] However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion that identifies modern gastropods twists the internal organs so the anus lies above the head.[15]: 300–343 [60][61]
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.[62] The Middle Cambrian fossil Nectocaris is often interpreted as a cephalopod with 2 arms and no shell, but the Late Cambrian fossil Plectronoceras is now thought to be the earliest undisputed 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.[63] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65 million years ago.[64] However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.[65]
The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves.[66][67][68][69] "Modern-looking" bivalves appeared in the Ordovician period, 488 to 443 million years ago.[70] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event.[71] Even so, bivalves remain abundant and diverse.
The 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 life.[72]
Monoplacophorans
("limpet-like", "living fossils")
Scaphopods (tusk shells)
Gastropods
(snails, slugs, limpets, sea hares)
Cephalopods
(nautiloids, ammonites, octopus, squid, etc.)
Aplacophorans
(spicule-covered, worm-like)
Polyplacophorans (chitons)
† HalwaxiidsThe 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,[49][50][53][54] debates arise about the relationships between the classes of living molluscs.[51] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.[75]
Molluscs are generally regarded members of the Lophotrochozoa,[73] 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.[76] The diagram on the right summarizes a phylogeny presented in 2007 without the annelid worms.
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.[77] 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.[78] Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments,[12] while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized.[51] 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 spicules.[77]
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.[45] Eventually, the cuticle would have become mineralized,[45] using the same genetic machinery (engrailed) as most other bilaterian skeletons.[78] The first mollusc shell almost certainly was reinforced with the mineral aragonite.[28]
The evolutionary relationships within the molluscs are also debated, and the diagrams below show two widely supported reconstructions:
Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses,[79] although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.[80]
However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic; in particular, Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded.[75] A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated.[81] 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.[82] Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question.[83]
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.
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.[84] Other commonly eaten molluscs include octopuses and squids, whelks, oysters, and scallops.[85] 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.[86] 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.[87]
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Most molluscs with shells can produce pearls, but only the pearls of bivalves and some gastropods, whose shells are lined with nacre, are valuable.[15]: 300–343, 367–403 The best natural pearls are produced by marine pearl oysters, 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 and shell.
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.[15]: 367–403 The pearl industry is so important in some areas, significant sums of money are spent on monitoring the health of farmed molluscs.[88]
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.[89] 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.[90][91] 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.[92] 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 chlamys."[93]
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.[94] 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 appeared.[95]
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.[96] Potamopyrgus antipodarum is used by some water treatment plants to test for estrogen-mimicking pollutants from industrial agriculture.
Some molluscs sting or bite, but deaths from mollusc venoms total less than 10% of those from jellyfish stings.[98]
All octopuses are venomous,[99] 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,[97] 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,[100] 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.[101]
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.[102][98][103] 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.[102][104]
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."[105] 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.[106]
Some species of molluscs, particularly certain snails and slugs, can be serious crop pests,[107] 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.[108]
{{cite book}}: CS1 maint: others (link) {{cite journal}}: CS1 maint: unfit URL (link) {{cite book}}: CS1 maint: bot: original URL status unknown (link) Cornu aspersum (formerly Helix aspersa) – a common land snail Mollusca is the second-largest phylum of invertebrate animals, after the Arthropoda; members are known as molluscs or mollusks (/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. The proportion of undescribed species is very high. Many taxa remain poorly studied.
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 anatomical structure, but also in behaviour and habitat. The phylum is typically divided into 7 or 8 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, 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 and account for 80% of the total classified species.
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. Nearly all produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.
Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541–485.4 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.
Fossilized ammonite displayed at the National Museum of the Philippines Molluscs have been and still are an important food source for anatomically modern humans. Toxins that can accumulate in certain molluscs under specific conditions create a risk of food poisoning, and many jurisdictions have regulations to reduce 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 money in some preindustrial societies.
A handful of mollusc species are sometimes considered 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 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 by 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.
