Mollusca

. 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. The most abundant metallic element in molluscs is calcium.

Molluscs have for many centuries 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 pre-industrial societies. Some molluscs contain toxic compounds.

Distinguishing features

The two 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. Because of the great range of anatomical diversity, many textbooks base their descriptions on a hypothetical "generalized mollusc", with features common to many but not all classes within the Mollusca.

Diversity

s (snails and slugs), including the cowry (a sea snail) pictured here. Haszprunar in 2001 estimated about 93,000 named species, Molluscs are second only to arthropods in numbers of living animal species—far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000. It has been estimated that there are about 200,000 living species in total, and 70,000 fossil species,

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 sub-groups. 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. The giant squid, which until recently had not been observed alive in its adult form, is one of the largest invertebrates. However a recently caught specimen of the colossal squid, long and weighing , may have overtaken it.

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. 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 non-marine molluscs. For comparison, the great majority of mollusc species are marine but only 41 of these appeared on the 2004 Red List. 42% of recorded extinctions since the year 1500 are of molluscs, almost entirely non-marine species.

Definition

The words and mollusk are both derived from the French mollusque, which originated from the Latin molluscus, from , soft. Molluscus was itself an adaptation of Aristotle's τᾲ μαλάκια, "the soft things", which he applied to cuttlefish. The scientific study of molluscs is known as malacology.

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. The most general characteristic of molluscs is that they are unsegmented and bilaterally symmetrical. The following are present in all modern molluscs:

The dorsal part of the body wall is a mantle (or pallium) which secretes calcareous spicules, plates or shells. It overlaps the body with enough spare room to form a mantle cavity.

The anus and genitals open into the mantle cavity.

There are two pairs of main nerve cords. !Polyplacophora !Monoplacophora !Gastropoda !Cephalopoda !Bivalvia !Scaphopoda |- ! style="text-align:left" | Radula, a rasping "tongue" with chitinous teeth | Absent in 20% of Neomeniomorpha || Yes || Yes || Yes || Yes || No || Internal, cannot extend beyond body |- ! style="text-align:left" | Broad, muscular foot | Reduced or absent || Yes || Yes || Yes || Modified into arms || Yes || Small, only at "front" end |- ! style="text-align:left" | Dorsal concentration of internal organs (visceral mass) | Not obvious || Yes || Yes || Yes || Yes || Yes || Yes |- ! style="text-align:left" | Large digestive ceca | No ceca in some aplacophora || Yes || Yes || Yes || Yes || Yes || No |- ! style="text-align:left" | Large complex metanephridia ("kidneys") | None || Yes || Yes || Yes || Yes || Yes || Small, simple |}

A "generalized mollusc"

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.

The generalized mollusc has a single, "limpet-like" shell on top. The shell is secreted by a mantle that covers the upper surface. The underside consists of a single muscular "foot". except that the outermost layer in almost all cases is all conchiolin (see periostracum). with the questionable exception of Cobcrephora. While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells.

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, that is often nacreous. in cephalopods it is used for jet propulsion,

Circulation

Molluscs' circulatory systems are mainly open. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton.

Ecology

Feeding

Most molluscs are herbivorous, grazing on algae. 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 itself with its radula. To employ this strategy, the plant has to be large enough for the mollusc to 'sit' on; therefore smaller macroscopic plants enjoy less molluscan herbivory than their larger counterparts. Naturally, there are exceptions; the cephalopods are primarily (perhaps entirely) predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. The monoplacophoran Neopilina uses its radula in the usual fashion, but its diet includes protists such as the xenophyophore Stannophyllum. Sacoglossan nudibranchs suck the sap from algae, using their one-row radula to pierce the cell walls, whereas dorid nudibranchs and some Vetigastrpods feed on sponges and others feed on hydroids. (An extensive list of molluscs with unusual feeding habits is available in the appendix of .)

Classification

Opinions vary about the number of classes of molluscs—for example the table below shows eight living classes, and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and solenogasters into one class, the Aplacophora. || All the snails and slugs including abalone, limpets, conch, nudibranchs, sea hares, sea butterfly || align="center" | 70,000 || marine, freshwater, land |- | Cephalopoda || squid, octopus, cuttlefish, nautilus || align="center" | 900 || marine |- | Bivalvia || clams, oysters, scallops, geoducks, mussels || align="center" | 20,000 || marine, freshwater |- | Scaphopoda || fossils; probable ancestors of bivalves || align="center" | extinct || marine |- | Helcionelloida † but others are unwilling to go further than "probable bilaterian". Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from are fragments of a genuinely mollusc-like radula. This appears to contradict the concept that the ancestral molluscan radula was mineralized.

{| align="right" |- valign="top" | fossil Yochelcionella is thought to be an early mollusc are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites. Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. There have been suggestions that the tiny specimens were juveniles and the larger ones adults.

Some analyses of helcionellids concluded that these were the earliest gastropods. 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. For a long time it was thought that Volborthella, some fossils of which pre-date , 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. 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. All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period . However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.

The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves. "Modern-looking" bivalves appeared in the Ordovician period, . One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous-Tertiary extinction. Even so, bivalves remain abundant and diverse.

The Hyolitha is a class of extinct animals with a shell and operculum that may be molluscs. Authors who suggest that they deserve their own phylum do not comment on the position of this phylum in the tree of life

Phylogeny

A possible "family tree" of molluscs (2007). Does not include annelid worms as the analysis concentrated on fossilizable "hard" features. 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, The diagram on the right summarizes a phylogeny presented in 2007.

Because the relationships between the members of the family tree are uncertain, it is difficult to identify the features inherited from the last common ancestor of all molluscs. For example, it is uncertain whether the ancestral mollusc was metameric (composed of repeating units)—if it was, that would suggest an origin from an annelid-like worm. Scientists disagree about this: Giribet and colleagues concluded in 2006 that the repetition of gills and of the foot's retractor muscles were later developments, In one particular one branch of the family tree, the shell of conchiferans is thought to have evolved from the spicules (small spines) of aplacophorans; however this is difficult to reconcile with the embryological origins of spicules.

The evolutionary relationships within the molluscs are also debated, and the diagrams below show two widely supported reconstructions: {| cellspacing="0" cellpadding="0" |- valign="top" |The "Aculifera" hypothesis |The "Testaria" hypothesis |}

Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses, although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.

However, an analysis in 2009 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. A 2010 analysis managed to recover the traditional conchiferan and aculiferan groups, but similarly concluded that the molluscs are not monophyletic, this time suggesting that solenogastres are more closely related to the non-molluscan taxa used as an outgroup than to other molluscs. Current molecular data is insufficient to constrain the molluscan phylogeny, and since the methods used to determine the confidence in clades are prone to over-estimation, it is risky to place too much emphasis even on the areas that different studies agree.

Human interaction

Uses by humans

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 over-fishing. Other commonly eaten molluscs include octopuses and squids, whelks, oysters, and scallops. In 2005, China accounted for 80% of the global mollusc catch, netting almost . Within Europe, France remained the industry leader. Some countries regulate importation and handling of molluscs and other seafood, mainly to minimize the poison risk from toxins that accumulate in the animals.

farm in Seram, Indonesia |alt=Photo of three circular metal cages in shallows, with docks, boathouses and palm trees in background]] 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.

Emperor Justinian I clad in Tyrian purple and wearing numerous pearls|alt=Mosaic of mustachioed, curly-haired man wearing crown and surrounded by halo ]] Other luxury and high-status products were made from molluscs. Tyrian purple, made from the ink glands of murex shells, "... fetched its weight in silver" in the fourth-century BC, according to Theopompus. The discovery of large numbers of Murex shells on Crete suggests 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. Sea silk is a fine, rare and valuable fabric produced from the long silky threads (byssus) secreted by several bivalve molluscs, particularly Pinna nobilis, to attach themselves to the sea bed. Procopius, writing on the Persian wars circa 550 CE, "stated that the five hereditary satraps (governors) of Armenia who received their insignia from the Roman Emperor were given chlamys (or cloaks) made from lana pinna (Pinna "wool," or byssus). Apparently only the ruling classes were allowed to wear these chlamys."

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. 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.

Stings and bites

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. Blue-ringed octopus bites are often fatal, and the bite of other octopuses can cause unpleasant symptoms. Stings from a few species of large tropical cone shells can also kill. However, the sophisticated venoms of these cone snails have become important tools in neurological research and show promise as sources of new medications.

's rings are a warning signal—this octopus is alarmed, and its bite can kill.

All octopuses are venomous but only a few species pose a significant threat to humans. Blue-ringed octopuses in the genus Hapalochlaena, which live around Australia and New Guinea, bite humans only if severely provoked, but their venom kills 25% of human victims. Another tropical species, Octopus apollyon, causes severe inflammation that can last for over a month even if treated correctly, and the bite of Octopus rubescens can cause necrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated.

s can be dangerous to shell-collectors but are useful to neurology researchers Many painful stings have been reported, and a few fatalities, although some of the reported fatalities may be exaggerations. 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.

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

Pests

created by the penetration of Schistosoma. Source: Centers for Disease Control and Prevention]]

Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 million people. A few species of snails and slugs are serious agricultural pests, and in addition, accidental or deliberate introduction of various snail species into new territory has resulted in serious damage to some natural ecosystems.

Schistosomiasis is "second only to malaria as the most devastating parasitic disease in tropical countries. An estimated 200 million people in 74 countries are infected with the disease — 100 million in Africa alone." The parasite has 13 known species, of which two infect humans. The parasite itself is not a mollusc, but all the species have freshwater snails as intermediate hosts.

Some species of molluscs, particularly certain snails and slugs, can be serious crop pests, and when introduced into new environments can unbalance local ecosystems. One such pest, the giant African snail Achatina fulica, has been introduced to many parts of Asia, as well as to many islands in the Indian Ocean and Pacific Ocean. In the 1990s this species reached the West Indies. Attempts to control it by introducing the predatory snail Euglandina rosea proved disastrous, as the predator ignored Achatina fulica and went on to extirpate several native snail species instead.

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

Notes

Footnotes

Citations

Further reading

; http://www.springerlink.com/content/l665628020163255/;

External links

Hardy's Internet Guide to Marine Gastropods

Planktonic mollusca fact sheets

Rotterdam Natural History Museum Shell Image Gallery

Molluscs Category:Phyla