Unionida is a monophyletic order of freshwater mussels, aquatic bivalve molluscs. The order includes most including the freshwater pearl mussels; the most common families are the Margaritiferidae. All have in common a larval stage, temporarily parasitic on fish, nacreous shells, high in organic matter, that may crack upon drying out, siphons too short to permit the animal to live buried in sediment; the shells of these mussels are variable in shape, but equivalve and elongate. They have solid, nacreous valves with a pearly interior, radial sculpture, an entire pallial line. Families and species in the order Unionida are found on six continents, where they are restricted to freshwater rivers, streams and some lakes. There are 900 species worldwide. Around 300 species of these freshwater mussels are endemic to North America. Unlike other bivalve orders, Unionida has no marine species, although one species tolerates brackish water; this widespread trait and its global distribution suggests the group has inhabited freshwater throughout its geologic history.
Unionida burrow into the substrate in clean, fast flowing freshwater water rivers and creeks, with their posterior margins exposed. They pump water through the incurrent aperture, obtaining oxygen and filtering food from the water column. Freshwater mussels are some of the longest-living invertebrates in existence; these clams have, like all bivalve mollusks, a shell consisting of two parts that are hinged together, which can be closed to protect the animal's soft body within. Like all mollusks, the freshwater mussels have a muscular "foot", which enables the mussel to move and bury itself within the bottom substrate of its freshwater habitat. Unionida have a complex life cycle involving parasitic larvae; this larval form used to be described as "parasitic worms" on the fish host, the larvae are not "worms" and do not harm fish under normal circumstances. Most of these freshwater mussel species have separate sexes; the sperm is ejected from the mantle cavity through the male's excurrent aperture and taken into the female's mantle cavity through the incurrent aperture.
Fertilised eggs move from the gonads to the gills where they further ripen and metamorph into glochidia, the first larval stage. Mature glochidia are released by the female and attach to the gills, fins or skin of a host fish; the freshwater mussel larvae have hooks, which enable the individual to attach itself to fish. Some freshwater mussels release their glochidia in mucilaginous packets called conglutinates; the conglutinate has a sticky filament that allows it to adhere to the substrate so it is not washed away. There is an more specialized way of dispersal known as a super-conglutinate; the super-conglutinate resembles an aquatic fly larva or a fish egg, complete with a dark area that looks like an eyespot, it is appetizing to fish. When a fish consumes it, it breaks up. Mussels that produce conglutinates and super-conglutinates are gill parasites, the glochidia attaching to the fish gills to continue their development into juveniles. A cyst is formed around the glochidia, they stay on the fish for several weeks or months before they fall off as juvenile freshwater mussels which bury themselves in the sediment.
This unique life cycle allows Unionida freshwater mussels to move upstream with the fish host species. Many of these freshwater mussel species face conservation issues due to habitat degradation and in some cases due to over-exploitation for the freshwater pearl industry, for the nacre of their shells, used in button manufacturing. Of the North American Unionida about 70% are either extinct, threatened or are listed as species of special concern; these bivalve mollusks were exploited for freshwater pearls, for their nacre, used in the button manufacturing industry in the late 19th and early 20th centuries. The effects of heavy fishing for freshwater mussels in North America in for use in manufacturing buttons put many of these species close to extinction; the "pearl rush" in North America occurred in the mid to late 1800s as people could find freshwater mussels in rivers and streams by "pollywogging" for mussels, some of which had freshwater pearls which they could sell for a significant price.
The art of "pollywogging" involves shuffling one's feet in the mud feeling around for freshwater mussels. Because this was easy to do, an easy way to make money from freshwater selling pearls, this period has been euphemistically called the "pearl rush", some historians have compared it to the gold rush in California. A formal freshwater mussel fishing industry was established in the mid-1850s to take advantage of this natural resource; the "pearl rush" to find freshwater pearls became so intense in some rivers that millions of freshwater mussels were killed in a few years. In some rivers and streams entire freshwater mussel beds were eliminated. Although the negative impact of the "pearl rush" on freshwater mussel populations was significant, in the cold light of history it was minor compared to the over fishing that took place just a few years with the "pearl" button industry. Freshwater pearls from North America come from freshwater mussels in the family Unionidae. About 20 different species of Unionidae are commercially harvested for pearls.
The common names of the most prolific pearl-bearing species include: the
The periostracum is a thin organic coating or "skin", the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs it is seen in snails and clams, i.e. in gastropods and bivalves, but it is found in cephalopods such as Allonautilus scrobiculatus. Periostracum is an integral part of the shell, it forms as the shell forms, along with the other shell layers. Periostracum is visible as the outer layer of the shell of many molluscan species from terrestrial and marine habitats, may be seen in land snails, river mussels and other kinds of freshwater bivalves, as well as in many kinds of marine shelled molluscs; the word "periostracum" means "around the shell", meaning that the periostracum is wrapped around what is the more calcareous part of the shell. Technically the calcareous part of the shell can be referred to as the "ostracum", but that term is only rarely used; this shell layer is composed of a type of protein known as conchiolin. Conchiolin is composed of quinone-tanned proteins, which are similar to those found in the epidermal cuticle.
The formation of a shell requires certain biological machinery. The shell is deposited within a small compartment, the extrapallial space, sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs; this caps off the extrapallial space, bounded on its other surfaces by the existing shell and the mantle. The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur; the accumulation of ions is driven by ion pumps packed within the calcifying epithelium. The organic matrix forms the scaffold that directs crystallization, the deposition and rate of crystals is controlled by hormones produced by the mollusc; the periostracum was essential in allowing early molluscs to obtain large size with a single valve. The periostracum is secreted from a groove in the mantle, termed the periostracal groove.
When secreted, it consists of the soluble protein periostracin. Periostracum is yellowish or brownish in color. In some species it is black; the periostracum is often a different color than the underlying layer of the shell. In the shells of species which have periostracum, this shell layer is quite physically worn away or chemically eroded in the parts of the shell that are older, thus it may only still be visible in the more formed areas of the shell. Periostracum can in some cases be quite thin, smooth and transparent, such that it looks like a thin yellow varnish, or it can be thicker and more or less opaque; when it is thick it is relatively rough in texture and dull. In some species the periostracum is tufted, or forms hair-like growths which in some cases can give the fresh shell a velvety feel, see. In some species the periostracum adheres tightly to the underlying shell surface, in others the periostracum is less attached. In certain marine species, such as for example certain species of cone snails, a heavy periostracum obscures the color patterns that exist on the calcareous layer of the shell.
In many aquatic species, once a shell has been removed from the water and has had time to dry out the periostracum may become brittle and start to flake or peel off of the surface of the shell. It is not uncommon for shell collectors to deliberately remove a periostracum layer if they feel that a shell is more attractive without it; however the periostracum is an important part of the shell, is of interest to malacologists. Details of the periostracum can sometimes be helpful in identifying a species. Haired shells occur in gastropods in several species of the Stylommatophoran families Polygyridae and Hygromiidae; these families are only distantly related, suggesting that this features has evolved several times independently. Haired shells are exclusively observed in species living in moist microhabitats, like layers of fallen leaves, broad-leaved vegetation, damp meadows or wet scree; such a correlation suggests an adaptive significance of the trait in such a habitat. These hairs can reach varying lengths.
In some cases hardly visible, they confer an furry impression to the shell in others. These semi-rigid structures are part of the periostracum, a thin protein layer secreted by the snail to cover the calcareous shell. Building hairs requires the snail to have specialised glandular tissue and complex strategies to form them; this trait can be assumed to be costly and should thus present a selective advantage to its bearers in order to be conserved. Experiments by Pfenninger et al. on genus Trochulus showed an increased adherence of haired shells to wet surfaces. Haired shells appeared to be the ancestral character state, a feature most lost three times independently; the possession of hairs facilitates the adherence of the snails to their herbaceous food plants during foraging when humidity levels are high. The absence of hairs in some Trochulus species could thus be explained as a loss of the potential adaptive function linked to habitat shifts; the periostracum of brachiopods is made of chitin.
New cells on the edges of the brachiopod mantle secrete material that extends the periostracum, but are displace
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
Constantine Samuel Rafinesque
Constantine Samuel Rafinesque-Schmaltz, as he is known in Europe, was a nineteenth-century polymath born near Constantinople in the Ottoman Empire and self-educated in France. He traveled as a young man in the United States settling in Ohio in 1815, where he made notable contributions to botany and the study of prehistoric earthworks in North America, he contributed to the study of ancient Mesoamerican linguistics, in addition to work he had completed in Europe. Rafinesque was eccentric, is portrayed as an erratic genius, he was an autodidact who excelled in various fields of knowledge, as a zoologist, botanist and polyglot. He wrote prolifically on such diverse topics as anthropology, biology and linguistics, but was honored in none of these fields during his lifetime. Among his theories were that ancestors of Native Americans had migrated by the Bering Sea from Asia to North America, that the Americas were populated by numerous black indigenous peoples at the time of European contact. Rafinesque was born on October 1783 in Galata, a suburb of Constantinople.
His father F. G. Rafinesque was a French merchant from Marseilles, his father died in Philadelphia about 1793. Rafinesque spent his youth in Marseilles, was self-educated. By the age of twelve, he had begun collecting plants for a herbarium. By fourteen, he taught himself perfect Greek and Latin because he needed to follow footnotes in the books he was reading in his paternal grandmother's libraries. In 1802, at the age of nineteen, Rafinesque sailed to Philadelphia in the United States with his younger brother, they traveled through Pennsylvania and Delaware, where he made the acquaintance of most of the young nation's few botanists. In 1805 Rafinesque returned to Europe with his collection of botanical specimens, settled in Palermo, where he learned Italian, he became so successful in trade that he retired by age twenty-five and devoted his time to natural history. For a time Rafinesque worked as secretary to the American consul. During his stay in Sicily, he studied fishes, naming many new discovered species of each.
He was elected a Fellow of the American Academy of Arts and Sciences in 1808. Rafinesque had a common-law wife. After their son died in 1815, he returned to the United States; when his ship Union foundered near the coast of Connecticut, he lost all his books and all his specimens. Settling in New York, Rafinesque became a founding member of the newly established "Lyceum of Natural History." In 1817 his book Florula Ludoviciana or A Flora of the State of Louisiana was criticized by fellow botanists, which caused his writings to be ignored. By 1818, he had named more than 250 new species of plants and animals, he was rebuilding his collection of objects from nature. In the summer of 1818, in Henderson, Rafinesque made the acquaintance of fellow naturalist John James Audubon, in fact stayed in Audubon's home for some three weeks. Audubon, although enjoying Rafinesque's company, took advantage of him in practical jokes involving fantastic, made-up species. In 1819 Rafinesque became professor of botany at Transylvania University in Lexington, where he gave private lessons in French and Spanish.
He was loosely associated with John D. Clifford, a merchant, interested in the ancient earthworks which remained throughout the Ohio Valley. Clifford conducted archival research, seeking the origins of these mounds, Rafinesque measured and mapped them; some had been lost to American development. He was elected a member of the American Antiquarian Society in 1820. Rafinesque started recording all the new species of plants and animals he encountered in travels throughout the state, he was considered an erratic student of higher plants. In the spring of 1826, he left the university after quarreling with its president, he traveled and lectured in various places, endeavored to establish a magazine and a botanic garden, but without success. He moved to a center of publishing and research, without employment, he published The Atlantic Journal and Friend of Knowledge, a Cyclopædic Journal and Review, of which only eight issues were printed. He gave public lectures and continued publishing at his own expense.
Rafinesque died of stomach and liver cancer in Philadelphia on September 18, 1840. It has been speculated that the cancer may have been induced by Rafinesque's self-medication years before with a mixture containing maidenhair fern, he was buried in a plot in. In March 1924 what were thought to be his remains were transported to Transylvania University and reinterred in a tomb under a stone inscribed, "Honor to whom honor is overdue." Rafinesque published 6,700 binomial names of plants, many of which have priority over more familiar names. The quantity of new taxa he produced, both plants and animals, has made Rafinesque memorable or notorious among biologists. Rafinesque applied to join the Lewis and Clark Expedition, but was twice turned down by Thomas Jefferson. After studying the specimens collected by the expedition, he assigned scientific names to the black-tailed prairie dog, the white-footed mouse and the mule deer. Rafinesque was one of the first to use the term "evolution" in the context of biological speciation.
Rafinesque proposed a theory of evolution before Charles Darwin. In a letter in 1832, Rafinesque wrote: The truth is that Species and Genera are forming i
Fossilworks is a portal which provides query and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. Fossilworks is housed at Macquarie University, it includes many analysis and data visualization tools included in the Paleobiology Database. "Fossilworks". Retrieved 2010-04-08
The eastern elliptio is a freshwater mussel in the Unionidae family, native to Canada and the United States. Media related to Elliptio complanata at Wikimedia Commons