Nacre known as mother of pearl, is an organic-inorganic composite material produced by some molluscs as an inner shell layer. It is strong and iridescent. Nacre is found in some of the most ancient lineages of bivalves and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, this results in a non-iridescent shine, or more in non-nacreous iridescence such as flame structure as is found in conch pearls; the outer layer of pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae. Nacre is composed of hexagonal platelets of aragonite 10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina. Depending on the species, the shape of the tablets differ. Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules.
These layers are separated by sheets of organic matrix composed of elastic biopolymers. This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa. Strength and resilience are likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation; this structure, at multiple length sizes increases its toughness, making it as strong as silicon. The statistical variation of the platelets has a negative effect on the mechanical performance because statistical variation precipitates localization of deformation. However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening. On the other hand, the fracture toughness of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned. But, higher statistical variations generates weak regions which allows the crack to propagate without much resistance causing the fracture toughness decreases.
Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours; the crystallographic c-axis points perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have different c-axis orientation randomly oriented within ~20° of vertical. In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis, this way inclined; the interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness. In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers. Nacre formation is not understood; the initial onset assembly, as observed in Pinna nobilis, is driven by the aggregation of nanoparticles within an organic matrix that arrange in fibre-like polycrystalline configurations.
The particle number increases successively and, when critical packing is reached, they merge into early-nacre platelets. Nacre growth is mediated by organics, controlling the onset and form of crystal growth. Individual aragonite "bricks" are believed to grow to the full height of the nacreous layer, expand until they abut adjacent bricks; this produces the hexagonal close-packing characteristic of nacre. Bricks may nucleate on randomly dispersed elements within the organic layer, well-defined arrangements of proteins, or may grow epitaxially from mineral bridges extending from the underlying tablet. Nacre differs from fibrous aragonite – a brittle mineral of the same form – in that the growth in the c-axis is slow in nacre, fast in fibrous aragonite. Nacre is secreted by the epithelial cells of the mantle tissue of various molluscs; the nacre is continuously deposited onto the inner surface of the shell, the iridescent nacreous layer known as mother of pearl. The layers of nacre smooth the shell surface and help defend the soft tissues against parasites and damaging debris by entombing them in successive layers of nacre, forming either a blister pearl attached to the interior of the shell, or a free pearl within the mantle tissues.
The process is called encystation and it continues as long as the mollusc lives. The form of nacre varies from group to group. In bivalves, the nacre layer is formed of single crystals in a hexagonal close packing. In gastropods, crystals are twinned, in cephalopods, they are pseudohexagonal monocrystals, which are twinned; the main commercial sources of mother of pearl have been the pearl oyster, freshwater pearl mussels, to a lesser extent the abalone, popular for their sturdiness and beauty in the latter half of the 19th century. Used for pearl buttons during the 1900s, were the shells of the great green turban snail Turbo marmoratus and the large top snail, Tectus niloticus; the international trade in mother of pearl is governed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an agreement signed by more than 170 countries. Nacre has been used for centuries for a variety o
Wikispecies is a wiki-based online project supported by the Wikimedia Foundation. Its aim is to create a comprehensive free content catalogue of all species. Jimmy Wales stated that editors are not required to fax in their degrees, but that submissions will have to pass muster with a technical audience. Wikispecies is available under the GNU Free Documentation License and CC BY-SA 3.0. Started in September 2004, with biologists across the world invited to contribute, the project had grown a framework encompassing the Linnaean taxonomy with links to Wikipedia articles on individual species by April 2005. Benedikt Mandl co-ordinated the efforts of several people who are interested in getting involved with the project and contacted potential supporters in early summer 2004. Databases were evaluated and the administrators contacted, some of them have agreed on providing their data for Wikispecies. Mandl defined two major tasks: Figure out how the contents of the data base would need to be presented—by asking experts, potential non-professional users and comparing that with existing databases Figure out how to do the software, which hardware is required and how to cover the costs—by asking experts, looking for fellow volunteers and potential sponsorsAdvantages and disadvantages were discussed by the wikimedia-I mailing list.
The board of directors of the Wikimedia Foundation voted by 4 to 0 in favor of the establishment of a Wikispecies. The project is hosted at species.wikimedia.org. It was merged to a sister project of Wikimedia Foundation on September 14, 2004. On October 10, 2006, the project exceeded 75,000 articles. On May 20, 2007, the project exceeded 100,000 articles with a total of 5,495 registered users. On September 8, 2008, the project exceeded 150,000 articles with a total of 9,224 registered users. On October 23, 2011, the project reached 300,000 articles. On June 16, 2014, the project reached 400,000 articles. On January 7, 2017, the project reached 500,000 articles. On October 30, 2018, the project reached 600,000 articles, a total of 1.12 million pages. Wikispecies comprises taxon pages, additionally pages about synonyms, taxon authorities, taxonomical publications, institutions or repositories holding type specimen. Wikispecies asks users to use images from Wikimedia Commons. Wikispecies does not allow the use of content.
All Species Foundation Catalogue of Life Encyclopedia of Life Tree of Life Web Project List of online encyclopedias The Plant List Wikispecies, The free species directory that anyone can edit Species Community Portal The Wikispecies Charter, written by Wales
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Freshwater pearl mussel
The freshwater pearl mussel is an endangered species of freshwater mussel, an aquatic bivalve mollusc in the family Margaritiferidae. Although the name "freshwater pearl mussel" is used for this species, other freshwater mussel species can create pearls and some can be used as a source of mother of pearl. In fact, most cultured pearls today come from Hyriopsis species in Asia, or Amblema species in North America, both members of the related family Unionidae; the interior of the shell of Margaritifera margaritifera has thick nacre. This species is capable of making fine-quality pearls, was exploited in the search for pearls from wild sources. In recent times, the Russian malacologist Valeriy Zyuganov received worldwide reputation after he discovered that the pearl mussel exhibited negligible senescence and he determined that it had a maximum lifespan of 210–250 years; the data of V. V. Zyuganov gained general acceptance. Subspecies within the species Margaritifera magaritifera include: Margaritifera margaritifera margaritifera Margaritifera margaritifera parvula Margaritifera margaritifera durrovensis Phillips, 1928 - critically endangered subspecies in Ireland.
Synonym: Margaritifera durrovensis. This subspecies is mentioned in annexes V of Habitats Directive as Margaritifera durrovensis; the freshwater pearl mussel is one of the longest-living invertebrates in existence. The oldest known specimen in Europe was caught in 1993 in Estonia. Like all bivalve molluscs, the freshwater pearl mussel has a shell consisting of two parts that are hinged together, which can be closed to protect the animal's soft body within; the shell is large and elongated yellowish-brown in colour when young and becoming darker with age. Older parts of the shell appear corroded, an identifying feature of this mussel species; the inner surface of the shell is sometimes tinged with attractive iridescent colours. Like all molluscs, the freshwater pearl mussel has a muscular'foot'; the native distribution of this species is Holarctic. The freshwater pearl mussel can be found on both sides of the Atlantic, from the Arctic and temperate regions of western Russia, through Europe to northeastern North America.
North America: eastern Canada and New England in the United States' Northeast Europe, including: Austria - estimated total population of 70 000 individuals in Mühlviertel and in Waldviertel, in the states of Upper and Lower Austria, respectively. Belgium Czech Republic - critically endangered. In Bohemia locally extinct in Moravia. Listed in Decree for implementation, No. 395/1992 Sb. as Critically Threatened species. Its Conservation status in 2004-2006 is bad in report for European commission in accordance with Habitats Directive. Denmark Estonia Fennoscandia - vulnerable in Finland and Norway, endangered in Sweden. Rare in southern Finland, more common in the north. Widespread but not common in Norway. Rare in Sweden. In Kola Peninsula and Karelia. France Germany - critically endangered. Listed as protected species in annex 1 in Bundesartenschutzverordnung. Great Britain. More than half the world's recruiting population exists in Scotland with populations in more than 50 rivers in the Highlands, although illegal harvesting has affected their survival.
75% of sites surveyed in 2010 had suffered "significant and lasting criminal damage" and in response the police and Scottish Natural Heritage have launched a campaign to protect the species. This species has been protected in the United Kingdom under the Wildlife and Countryside Act 1981 since 1998 and protected according to section 9 since 1991. Iberian Peninsula Ireland; the Cladagh river contains one of the largest populations surviving in northern Ireland, estimated minimum 10,000, confined to a 6 km stretch of undisturbed river in the middle section. Luxembourg Latvia Lithuania - extinct Poland - extinct Russian Federation - in the rivers of the White Sea basin of the Arkhangelsk and Murmansk Regions, it is east border of the area of distribution M. margaritifera. Clean, fast-flowing streams and rivers are required for the freshwater pearl mussel, where it lives buried or buried in fine gravel and coarse sand in water at depths between 0.5 and 2 metres, but sometimes at greater depths. Clean gravel and sand is essential for juvenile freshwater pearl mussels, for if the stream or river bottom becomes clogged with silt, they cannot obtain oxygen and will die.
Essential is the presence of a healthy population of salmonids, a group of fish including salmon and trout, on which the freshwater pearl mussel relies for part of its life cycle. Capable of living for up to 130 years, the freshwater pearl mussel begins life as a tiny larva, measuring just 0.6 to 0.7 millimetres long, ejected into the water from an adult mussel in a mass of one to four million other larvae. This remarkable event takes place over just one to two days, sometime between September; the larvae, known as glochidia, resemble tiny mussels, but their minute shells are held open until they snap shut on a suitable host. The host of freshwater pearl mussel larvae are juvenile fish fro
Mollusca is the second largest phylum of invertebrate animals. The members are known as mollusks. Around 85,000 extant species of molluscs are recognized; the number of fossil species is estimated between 100,000 additional species. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs live in freshwater and terrestrial habitats, they are diverse, not just in size and in anatomical structure, but in behaviour and in habitat. The phylum is divided into 8 or 9 taxonomic classes, of which two are extinct. Cephalopod molluscs, such as squid and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species; the gastropods 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, 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". This has a single, "limpet-like" shell on top, made of proteins and chitin reinforced with calcium carbonate, 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. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles; the generalized mollusc has three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, all have sensors to detect chemicals and touch; the simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur.
All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced, they have kidney-like organs for excretion. Good evidence exists for the appearance of gastropods and bivalves in the Cambrian period, 541 to 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 still are an important food source for anatomically modern humans. There is a risk of food poisoning from toxins which can accumulate in certain molluscs under specific conditions and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, their shells have been used as money in some preindustrial societies. Mollusc species can represent hazards or pests for human activities.
The bite of the blue-ringed octopus is fatal, that of Octopus apollyon causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can kill, but their sophisticated, though produced, venoms have become important tools in neurological research. Schistosomiasis is transmitted to humans via water snail hosts, affects about 200 million people. Snails and slugs can be serious agricultural pests, accidental or deliberate introduction of some snail species into new environments has 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, which he applied inter alia to cuttlefish; the scientific study of molluscs is accordingly called malacology. The name Molluscoida was used to denote a division of the animal kingdom containing the brachiopods and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs.
As it is now known these groups have no relation to molluscs, little to one another, the name Molluscoida has been abandoned. The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, the organization of the nervous system. Many have a calcareous shell. Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies to apply to all modern groups; the most general characteristic of molluscs is they are bilaterally symmetrical. The following are present in all modern molluscs: The dorsal part of the body wall is a mantle which secretes calcareous spicules, plates or shells, it overlaps the body with enough spare room to form a mantle cavity. The anus and genitals open into the mantle cavity. There are two pairs of main nerve cords. Other characteristics that 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.
In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were ab
A bivalve shell is part of the body, the exoskeleton or shell, of a bivalve mollusk. In life, the shell of this class of mollusks is composed of valves. Bivalves are common in all aquatic locales, including saltwater, brackish water, freshwater; the shells of bivalves wash up on beaches and along the edges of lakes and streams. Bivalves by definition possess two shells or valves, a "right valve" and a "left valve", that are joined by a ligament; the two valves articulate with one another using structures known as "teeth" which are situated along the hinge line. In many bivalve shells, the two valves are symmetrical along the hinge line— when symmetrical, such an animal is said to be equivalved. If symmetrical front-to-back, the valves are said to be equilateral, are otherwise considered inequilateral; this exoskeleton serves not only for muscle attachment, but for protection from predators and from mechanical damage. The shell has several layers, is made of calcium carbonate precipitated out into an organic matrix.
It is secreted by a part of the molluscan body known as the mantle. The shells of bivalves are equal sides connected by a hinge. Bivalve shells are collected by professional and amateur conchologists and are sometimes harvested for commercial sale in the international shell trade or for use in glue, chalk, or varnish to the detriment of the local ecology; the bivalve shell is composed of two calcareous valves. The mantle, a thin membrane surrounding the body, secretes the shell valves and hinge teeth; the mantle lobes secrete the valves, the mantle crest creates the other parts. The mantle itself is attached to the shell by numerous small mantle retractor muscles, which are arranged in a narrow line along the length of the interior of the shell; the position of this line is quite visible on the inside of each valve of a bivalve shell, as a shiny line, the pallial line, which runs along a small distance in from the outer edge of each valve joining the anterior adductor muscle scar to the posterior adductor muscle scar.
The adductor muscles are. In some bivalves the mantle edges fuse to form siphons, which take in and expel water during suspension feeding. Species which live buried in sediment have long siphons, when the bivalve needs to close its shell, these siphons retract into a pocket-like space in the mantle; this feature of the internal anatomy of a bivalve is indicated on the interior of the shell surface as a pallial sinus, an indentation in the pallial line. The valves of the shell are made of either calcite or both calcite and aragonite with the aragonite forming an inner layer, as is the case with the Pterioida which have this layer in the form of nacre or mother of pearl; the outermost layer of the shell is known as the periostracum and is composed of a horny organic substance. This sometimes forms a brownish "skin" on the outside of the shell; the periostracum may start to peel off of a shell when the shell is allowed to dry out for long periods. The shell is added to, increases in size, in two ways - by increments added to the open edge of the shell, by a gradual thickening throughout the animal's life.
The two shell valves are held together at the animal's dorsum by the ligament, composed of the tensilium and resilium. In life the ligament opens the shell, the adductor muscle or muscles close the shell; when a bivalve dies, its adductor muscle relax and the resilium pushes the valves open. A few groups of bivalves are active swimmers like the scallops. In many species of cemented bivalves, the lower valve is more cupped than the upper valve, which tends to be rather flat. In some groups of cemented bivalves the lower or cemented valve is the left valve, in others it is the right valve; the oldest point of a bivalve shell is called the beak, the raised area around it is known as the umbo. The hinge area is the back of the shell; the lower, curved margin is the ventral side. The anterior or front of the shell is where the byssus and foot are located and the posterior or back of the shell is where the siphon is located. Without being able to view these organs, determining anterior and posterior can be rather more difficult.
In those animals with a siphon, the pallial sinus of the siphon, which will be present on both the left and right valves, will point towards the animal's posterior— such valves are called sinopalliate. Shells without a pallial sinus are termed integripalliate— such animals have a byssal notch present on the anterior end of the right valve, the anterior auricles or "wings" of both valves will be either larger than, or equal to, the posterior ones; such valves may have a distinctive "comb" or ctinoleum within the byssal notch on the right valve. If a valve has neither notch nor comb nor sinus, and
Freshwater bivalves are one kind of freshwater molluscs, along with freshwater snails. They are bivalves which live in freshwater, as opposed to saltwater, the main habitat type for bivalves; the majority of species of bivalve molluscs live in the sea, but in addition, a number of different families live in freshwater. These families belong to two different evolutionary lineages, the two groups are not related. Freshwater bivalves live in many types of habitat, ranging from small ditches and ponds, to lakes, canals and swamps. Species in the two groups vary in size; some of the pea clams have an adult size of only 3 mm. In contrast, one of the largest species of freshwater bivalves is the swan mussel, in the family Unionidae. Freshwater pearl mussels are economically important as a source of freshwater pearls and mother of pearl. Order Unionida The Unionida, of worldwide distribution, are the pearly freshwater mussels. All reproduce by means of a larval stage, parasitic on a fish or salamander. Many species are utilized as sources of mother-of-pearl.
Families: Margaritiferidae Unionidae Hyriidae Etheriidae Mutelidae Mycetopodidae IridinidaeOrder Unionida: pearly freshwater mussels Order Veneroida The Veneroida is a large group of bivalve "clams", most of which are marine. However, several families occur in brackish waters. Families: Corbiculidae Sphaeriidae Dreissenidae Order Veneroida ANSP site Info on Unionida genera at Info on Rhode Island freshwater clams and mussels