Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water in plant and algal cells. They use the ATP and NADPH to make organic molecules from carbon dioxide in a process known as the Calvin cycle. Chloroplasts carry out a number of other functions, including fatty acid synthesis, much amino acid synthesis, the immune response in plants; the number of chloroplasts per cell varies from one, in unicellular algae, up to 100 in plants like Arabidopsis and wheat. A chloroplast is a type of organelle known as a plastid, characterized by its two membranes and a high concentration of chlorophyll. Other plastid types, such as the leucoplast and the chromoplast, contain little chlorophyll and do not carry out photosynthesis. Chloroplasts are dynamic—they circulate and are moved around within plant cells, pinch in two to reproduce.
Their behavior is influenced by environmental factors like light color and intensity. Chloroplasts, like mitochondria, contain their own DNA, thought to be inherited from their ancestor—a photosynthetic cyanobacterium, engulfed by an early eukaryotic cell. Chloroplasts cannot be made by the plant cell and must be inherited by each daughter cell during cell division. With one exception, all chloroplasts can be traced back to a single endosymbiotic event, when a cyanobacterium was engulfed by the eukaryote. Despite this, chloroplasts can be found in an wide set of organisms, some not directly related to each other—a consequence of many secondary and tertiary endosymbiotic events; the word chloroplast is derived from the Greek words chloros, which means green, plastes, which means "the one who forms". The first definitive description of a chloroplast was given by Hugo von Mohl in 1837 as discrete bodies within the green plant cell. In 1883, A. F. W. Schimper would name these bodies as "chloroplastids".
In 1884, Eduard Strasburger adopted the term "chloroplasts". Chloroplasts are one of many types of organelles in the plant cell, they are considered to have originated from cyanobacteria through endosymbiosis—when a eukaryotic cell engulfed a photosynthesizing cyanobacterium that became a permanent resident in the cell. Mitochondria are thought to have come from a similar event, where an aerobic prokaryote was engulfed; this origin of chloroplasts was first suggested by the Russian biologist Konstantin Mereschkowski in 1905 after Andreas Schimper observed in 1883 that chloroplasts resemble cyanobacteria. Chloroplasts are only found in plants and the amoeboid Paulinella chromatophora. Cyanobacteria are considered the ancestors of chloroplasts, they are sometimes called blue-green algae though they are prokaryotes. They are a diverse phylum of bacteria capable of carrying out photosynthesis, are gram-negative, meaning that they have two cell membranes. Cyanobacteria contain a peptidoglycan cell wall, thicker than in other gram-negative bacteria, and, located between their two cell membranes.
Like chloroplasts, they have thylakoids within. On the thylakoid membranes are photosynthetic pigments, including chlorophyll a. Phycobilins are common cyanobacterial pigments organized into hemispherical phycobilisomes attached to the outside of the thylakoid membranes. Somewhere around 1 to 2 billion years ago, a free-living cyanobacterium entered an early eukaryotic cell, either as food or as an internal parasite, but managed to escape the phagocytic vacuole it was contained in; the two innermost lipid-bilayer membranes that surround all chloroplasts correspond to the outer and inner membranes of the ancestral cyanobacterium's gram negative cell wall, not the phagosomal membrane from the host, lost. The new cellular resident became an advantage, providing food for the eukaryotic host, which allowed it to live within it. Over time, the cyanobacterium was assimilated, many of its genes were lost or transferred to the nucleus of the host. From genomes that originally contained over 3000 genes only about 130 genes remain in the chloroplasts of contemporary plants.
Some of its proteins were synthesized in the cytoplasm of the host cell, imported back into the chloroplast. Separately, somewhere around 500 million years ago, it happened again and led to the amoeboid Paulinella chromatophora; this event is called endosymbiosis, or "cell living inside another cell with a mutual benefit for both". The external cell is referred to as the host while the internal cell is called the endosymbiont. Chloroplasts are believed to have arisen after mitochondria, since all eukaryotes contain mitochondria, but not all have chloroplasts; this is called serial endosymbiosis—an early eukaryote engulfing the mitochondrion ancestor, some descendants of it engulfing the chloroplast ancestor, creating a cell with both chloroplasts and mitochondria. Whether or not primary chloroplasts came from a single endosymbiotic event, or many independent engulfments across various eukaryotic lineages, has long been debated, it is now held that organisms with primary chloroplasts share a single ancestor that took in a cyanobacterium 600–2000 million years ago.
It has been proposed. The exception is the amoeboid Paulinella chromatophora, which descends from an ancestor that took in a Prochlorococcus cyanobacterium 90–500 million years ago; these chloroplasts
Phaeocystis is a genus of algae belonging to the Prymnesiophyte class and to the larger division of Haptophyta. It is a widespread marine phytoplankton and can function at a wide range of temperatures and salinities. Members of this genus live in the open ocean, as well as in sea ice, it has a polymorphic life cycle, ranging from free-living cells to large colonies. The ability to form a floating colony is one of the unique attributes of Phaeocystis – hundreds of cells are embedded in a polysaccharide gel matrix, which can increase massively in size during blooms; the largest Phaeocystis blooms form in the polar seas: P. pouchetii in the north and P. antarctica in the south. This intense Phaeocystis productivity persists for about a three-month period, spanning most of the summer in the Southern Hemisphere. Phaeocystis-abundant ecosystems are associated with commercially important stocks of crustaceans, molluscs and mammals. Phaeocystis may have negative effects on higher trophic levels in the marine ecosystem, consequent impacts on human activities, by forming odorous foams on beaches during the wane of a bloom.
The ability to form large blooms and its ubiquity make Phaeocystis an important contributor to the ocean carbon cycle. In addition, Phaeocystis produces a key player in the sulfur cycle. Free-living forms of Phaeocystis are globally distributed and occur in a variety of marine habitats, including coastal oceans, open oceans, polar seas and sea ice. Six species are assigned to the genus: P. antarctica, P. jahnii, P. globosa, P. pouchetti, P. scrobiculata and P. cordata. Three species are associated with bloom formation in nutrient-rich areas, which can occur either or due to anthropogenic inputs. P. globosa blooms in temperate and tropical waters, whereas P. pouchetii and P. antarctica are better adjusted to the cold temperatures prevailing in Arctic and Antarctic waters, respectively. However, P. pouchetii tolerates warmer temperatures and has been seen in temperate waters. Genome comparison has shown that the RUBISCO spacer region is conserved among related colonial Phaeocystis species and identical in P. antarctica, P. pouchetii and two warm-temperate strains of P. globosa, with a single base substitution in two cold-temperate strains of P. globosa.
Phaeocystis can exist as either free-living colonies. Free-living cells can show a variety of morphologies, depending on the species. All species can exist as scaly flagellates, this is the only form, observed for P. scrobiculata and P. cordata. Three species have been observed as colonies and these can exist as a flagellate devoid of scales and filaments. In colonies of Phaeocystis, the colony skin may provide protection against smaller zooplankton grazers and viruses. While suspected in other species, a haploid-diploid life cycle has only been observed in P. globosa. In this cycle, sexual reproduction is dominant in colony bloom formation/termination, two types of vegetative reproduction exist; the genus Phaeocystis is a major producer of 3-dimethylsulphoniopropionate, the precursor of dimethyl sulfide. Biogenic DMS contributes 1.5×1013 g sulfur to the atmosphere annually and plays a major part in the global sulfur cycle, which can affect cloud formation and climate regulation
Phytoplankton are the autotrophic components of the plankton community and a key part of oceans and freshwater basin ecosystems. The name comes from the Greek words φυτόν, meaning "plant", πλαγκτός, meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in high enough numbers, some varieties may be noticeable as colored patches on the water surface due to the presence of chlorophyll within their cells and accessory pigments in some species. Phytoplankton are photosynthesizing microscopic biotic organisms that inhabit the upper sunlit layer of all oceans and bodies of fresh water on Earth, they are agents for "primary production", the creation of organic compounds from carbon dioxide dissolved in the water, a process that sustains the aquatic food web. Phytoplankton obtain energy through the process of photosynthesis and must therefore live in the well-lit surface layer of an ocean, lake, or other body of water. Phytoplankton account for about half of all photosynthetic activity on Earth.
Their cumulative energy fixation in carbon compounds is the basis for the vast majority of oceanic and many freshwater food webs. While all phytoplankton species are obligate photoautotrophs, there are some that are mixotrophic and other, non-pigmented species that are heterotrophic. Of these, the best known are dinoflagellate genera such as Noctiluca and Dinophysis, that obtain organic carbon by ingesting other organisms or detrital material. Phytoplankton nutrients from the water to produce their own food. In the process of photosynthesis, phytoplankton release molecular oxygen into the water, it is estimated that between 50% and 85% of the world's oxygen is produced via phytoplankton photosynthesis. The rest is produced via photosynthesis on land by plants. Furthermore, phytoplankton photosynthesis has controlled the atmospheric CO2/O2 balance since the early Precambrian Eon. Phytoplankton are crucially dependent on minerals; these are macronutrients such as nitrate, phosphate or silicic acid, whose availability is governed by the balance between the so-called biological pump and upwelling of deep, nutrient-rich waters.
Phytoplankton nutrient composition drives and is driven by the Redfield ratio of macronutrients available throughout the surface oceans. However, across large regions of the World Ocean such as the Southern Ocean, phytoplankton are limited by the lack of the micronutrient iron; this has led to some scientists advocating iron fertilization as a means to counteract the accumulation of human-produced carbon dioxide in the atmosphere. Large-scale experiments have added iron to the oceans to promote phytoplankton growth and draw atmospheric CO2 into the ocean. However, controversy about manipulating the ecosystem and the efficiency of iron fertilization has slowed such experiments. Phytoplankton depend on Vitamin B for survival. Areas in the ocean have been identified as having a major lack of Vitamin B, correspondingly, phytoplankton; the effects of anthropogenic warming on the global population of phytoplankton is an area of active research. Changes in the vertical stratification of the water column, the rate of temperature-dependent biological reactions, the atmospheric supply of nutrients are expected to have important effects on future phytoplankton productivity.
The effects of anthropogenic ocean acidification on phytoplankton growth and community structure has received considerable attention. Phytoplankton such as coccolithophores contain calcium carbonate cell walls that are sensitive to ocean acidification; because of their short generation times, evidence suggests some phytoplankton can adapt to changes in pH induced by increased carbon dioxide on rapid time-scales. Phytoplankton serve as the base of the aquatic food web, providing an essential ecological function for all aquatic life. Under future conditions of anthropogenic warming and ocean acidification, changes in phytoplankton mortality may be significant. One remarkable Of the many food chains in the ocean – remarkable due to the small number of links – is that of phytoplankton sustaining krill, which in turn sustain baleen whales; the term phytoplankton encompasses all photoautotrophic microorganisms in aquatic food webs. However, unlike terrestrial communities, where most autotrophs are plants, phytoplankton are a diverse group, incorporating protistan eukaryotes and both eubacterial and archaebacterial prokaryotes.
There are about 5,000 known species of marine phytoplankton. How such diversity evolved despite scarce resources is unclear. In terms of numbers, the most important groups of phytoplankton include the diatoms and dinoflagellates, although many other groups of algae are represented. One group, the coccolithophorids, is responsible for the release of significant amounts of dimethyl sulfide into the atmosphere. DMS is oxidized to form sulfate which, in areas where ambient aerosol particle concentrations are low, can contribute to the population of cloud condensation nuclei leading to increased cloud cover and cloud albedo according to the so-called CLAW Hypothesis. Different types of phytoplankton support different trophic levels within varying ecosystems. In oligotrophic oceanic regions such as the Sargasso Sea or the South Pacific Gyre, phytoplankton is dominated by the small sized cells, called picoplankton a
Coccoliths are individual plates of calcium carbonate formed by coccolithophores which are arranged around them in a coccosphere. Coccoliths are formed within the cell in vesicles derived from the golgi body; when the coccolith is complete these vesicles fuse with the cell wall and the coccolith is exocytosed and incorporated in the coccosphere. The coccoliths are either dispersed following death and breakup of the coccosphere, or are shed continually by some species, they sink through the water column to form an important part of the deep-sea sediments. Thomas Huxley was the first person to observe these forms in modern marine sediments and he gave them the name'coccoliths' in a report published in 1858. Coccoliths are composed of calcium carbonate as the mineral calcite and are the main constituent of chalk deposits such as the white cliffs of Dover, in which they were first described by Henry Clifton Sorby in 1861. There are two main types of coccoliths and holococcoliths. Heterococcoliths are formed of a radial array of elaborately shaped crystal units.
Holococcoliths are formed of minute calcite rhombohedra, arranged in continuous arrays. The two coccolith types were thought to be produced by different families of coccolithophores. Now, however, it is known through a mix of observations on field samples and laboratory cultures, that the two coccolith types are produced by the same species but at different life cycle phases. Heterococcoliths are produced in the diploid life-cycle phase and holococcoliths in the haploid phase. Coccoliths are classified depending on shape. Common shapes include: Calyptrolith – basket-shaped with openings near the base Caneolith – disc- or bowl-shaped Ceratolith – horseshoe or wishbone shaped Cribrilith – disc-shaped, with numerous perforations in the central area Cyrtolith – convex disc shaped, may with a projecting central process Discolith – ellipsoidal with a raised rim, in some cases the high rim forms a vase or cup-like structure Helicolith – a placolith with a spiral margin Lopadolith – basket or cup-shaped with a high rim, opening distally Pentalith – pentagonal shape composed of five-four sided crystals Placolith – rim composed of two plates stacked on top of one another Prismatolith – polygonal, may have perforations Rhabdolith – a single plate with a club-shaped central process Scapholith – rhombohedral, with parallel lines in center Although coccoliths are remarkably elaborate structures whose formation is a complex product of cellular processes, their function is unclear.
Hypotheses include defence against grazing by zooplankton or infection by viruses. Because coccoliths are formed of low-Mg calcite, the most stable form of calcium carbonate, they are fossilised, they are found in sediments together with similar microfossils of uncertain affinities from the Upper Triassic to recent. Coccoliths and related fossils are referred to as calcareous calcareous nanoplankton; the EHUX website - site dedicated to Emiliania huxleyi, containing essays on blooms, coccolith function, etc. International Nannoplankton Association site - includes an illustrated guide to coccolith terminology and several image galleries. Nannotax - illustrated guide to the taxonomy of coccolithophores and other nannofossils. Cocco Express - Coccolithophorids Expressed Sequence Tags & Microarray Database Possible functions of Coccoliths
Chrysochromulina is a genus of haptophytes. This phytoplankton is distributed globally in brackish and marine waters across 60 known species. All Chrysochromulina species are phototrophic, however some have been shown to be mixotrophic, including exhibiting phagotrophy under certain environmental conditions; the cells are small, characterized by having scales, observed using electron microscopy. Some species, under certain environmental conditions have been shown to produce toxic compounds that are harmful to larger marine life including fish. Individuals of the genus are known to grow between 3.0 and 13.0 µm in length, with the largest being those of the Chrysochromulina polylepis species. The cell surface is covered with plate-like scales, with additional layers of different scale types overlaid; as is characteristic of all haptophytes, members of the genus Chrysochromulina possess two flagella and a unique flagella-like organelle known as the haptonema. The haptonema can vary in length, reaching upwards of 60 µm, functions in cell attachment and feeding but differs from flagella in terms of microtubule arrangement, Chrysochromulina, as one genus of haptophytes, holds an essential role in global carbon sequestration and toxic bloom formation in world’s ocean.
Most haptophytes are photosynthetic micro-alga. Haptophytes can live in both marine water systems; this combined lifestyle makes haptophytes efficient organisms in global carbon fixation, they occupy 30% to 50% photosynthetic biomass in the ocean. Haptophytes have an evolutionary history around 1.2 billion years long. The evidence from fossils support this statement. In 2014, The draft genome sequence of Chrysochromulina tobin has been posted by researchers from University of Washington. C. tobin belongs to the taxon prymnesiales. As the first complete genome graph in this taxon, it can provide a broad understanding of haptophytes’ evolutionary history and the diversity of this clade of algae. Furthermore, it promoted the study about certain genomes and proteins which are responsible for the toxic formation and chemical release; some species, such as Chrysochromulina polylepis, have been identified to produce a carbon-heavy membrane damaging toxin. Research has suggested a correlation between marine nitrogen/phosphorus compositions and toxin production levels of these haptophytes.
Further research has since determined that both low nitrogen or low phosphorus levels in the cells capable of leading to an increase in toxin production, with phosphorus proving to be more influential. Despite this correlation, it unlikely that nitrogen or phosphorus are directly linked to toxin formulation, as the toxins themselves are carbon-based. Additionally, other growth-limiting factors such as light and salinity have been known to increase toxicity, suggesting that the toxins where selective advantage for cell defense during times of low growth; as such, studies support the idea that the metabolic responses to cellular stresses on an environmental and physiological level due to nutrient limitations are responsible for such toxin productions. Many Chrysochromulina species have been found to form algal blooms around the world; some of these blooms in the North Atlantic can produce compounds that are toxic to other marine organisms under the correct environmental conditions. It is common for blooms to be formed between April and August in Scandinavian coastal waters, however the specific Chrysochromulina species present varies from year to year.
In the late spring of 1988 the Chrysochromulina bloom that travelled from the Kattegat to the Skagerrak was made up of only one species, C. polylepis. This particular bloom was toxic to other marine organisms including protozoa, 900 tonnes of farmed fish due to the production of haemolytic compounds by C. polylepis. C. polylepis is not toxic at the concentrations found in the region, however certain environmental conditions such as strong stratification with a warm surface layer and low salinity following a winter featuring high amounts of nitrogen run-off increasing the N:P ratio is believed to have led to the successful C. plylepis bloom. It is thought that the production of these toxic compounds limited grazing of C. polylepis allowing for the bloom to be dominated by a single species. The toxic effects seemed to reverse and the food web was restored by 1993. From April to May in 1992, in the southern Kattegat there was a large bloom made up of many phytoplankton species, with over 90% biomass being Chrysochromulina species.
The most abundant species in the bloom were C. hirta, C. spinifera, C. ericina, C. brevifilum and an undescribed species. C. hirta, C. spinifera, C. ericina are characterized as small cells with long spines protruding to give the overall organisms a 25-76μm diameter, too large for the ciliates present to engulf, one reason that the bloom was so successful. Another reason for the success of the bloom was the low presence of grazers in the bloom, about 5% of the Chrysochromulina species. There was no evidence directly correlating this bloom or the species present to the production of toxins like the C. polylepis bloom in 1988. Two major viruses have been found to infect Chrysochromulina: CpV-BQ1 and CeV-01B. Freshwater samples from Lake Ontario were filtered and analyzed using transmission electron microscopy to identify the CpV-BQ1 virus. CpV-BQ1 is an icosahedral nucleocytoplasmic large DNA virus with a genome size 485kb, it is a member of the Megavirales order with characteristics of phycodnaviridae and mimivirus families.
Concentrations of Chr
The Chrysophyceae called chrysophytes, golden-brown algae or golden algae are a large group of algae, found in freshwater. Golden algae is commonly used to refer to a single species, Prymnesium parvum, which causes fish kills; the Chrysophyceae should not be confused with the Chrysophyta, a more ambiguous taxon. Although "chrysophytes" is the anglicization of "Chrysophyta", it refers to the Chrysophyceae, they were taken to include all such forms of the diatoms and multicellular brown algae, but since they have been divided into several different groups based on pigmentation and cell structure. Some heterotrophic flagellates as the bicosoecids and choanoflagellates were sometimes seen as related to golden algae too, they are now restricted to a core group of related forms, distinguished by the structure of the flagella in motile cells treated as an order Chromulinales. It is possible; the "primary" cell of chrysophytes contains two specialized flagella. The active, "feathered" flagellum is oriented toward the moving direction.
The smooth passive flagellum, oriented toward the opposite direction, may be present only in rudimentary form in some species. An important characteristic used to identify members of the class Chrysophyceae is the presence of a siliceous cyst, formed endogenously. Called statospore, stomatocyst or statocyst, this structure is globose and contains a single pore; the surface of mature cysts may be ornamented with different structural elements and are useful to distinguish species. Most members are unicellular flagellates, with either two visible flagella, as in Ochromonas, or sometimes one, as in Chromulina; the Chromulinales as first defined by Pascher in 1910 included only the latter type, with the former treated as the order Ochromonadales. However, structural studies have revealed that a short second flagellum, or at least a second basal body, is always present, so this is no longer considered a valid distinction. Most of these have no cell covering; some have loricae or shells, such as Dinobryon, sessile and grows in branched colonies.
Most forms with silicaceous scales are now considered a separate group, the synurids, but a few belong among the Chromulinales proper, such as Paraphysomonas. Some members are amoeboid, with long branching cell extensions, though they pass through flagellate stages as well. Chrysamoeba and Rhizochrysis are typical of these. There is one species, Myxochrysis paradoxa, which has a complex life cycle involving a multinucleate plasmodial stage, similar to those found in slime molds; these were treated as the order Chrysamoebales. The superficially similar Rhizochromulina was once included here, but is now given its own order based on differences in the structure of the flagellate stage. Other members are non-motile. Cells may be naked and embedded in mucilage, such as Chrysosaccus, or coccoid and surrounded by a cell wall, as in Chrysosphaera. A few are filamentous or parenchymatous in organization, such as Phaeoplaca; these were included in various older orders, most of the members of which are now included in separate groups.
Hydrurus and its allies, freshwater genera which form branched gelatinous filaments, are placed in the separate order Hydrurales, but may belong here. Classification of the class Chrysophyceae according to Pascher: Division Chrysophyta Class Chrysophyceae Order Chrysomonadales Order Chrysocapsales Order Chrysosphaerales Order Chrysotrichales Class Heterokontae Class Diatomeae According to Smith: Class Chrysophyceae Order Chrysomonadales Suborder Cromulinae Suborder Isochrysidineae Suborder Ochromonadineae Order Rhizochrysidales Order Chrysocapsales Order Chrysotrichales Order Chrysosphaerales According to Bourrely: Class Chrysophyceae Order Phaeoplacales Order Stichogloeales Order Phaeothamniales Order Chrysapionales Order Thallochrysidales Order Chrysosphaerales Order Chrysosaccales Order Rhizochrysidales Order Ochromonadales Order Isochrysidales Order Silicoflagellales Order Craspedomonadales Order Chromulinales According to Starmach: Class Chrysophyceae Subclass Heterochrysophycidae Order Chromulinales Order Ochromonadales Subclass Acontochrysophycidae Order Chrysarachniales Order Stylococcales Order Chrysosaccales Order Phaeoplacales Subclass Craspedomonadophycidae Order Monosigales Classification of the class Chrysophyceae and splinter groups according to Kristiansen: Class ChrysophyceaeOrder Ochromonadales Order Mallomonadales Order Chrysamoebales Order Chrysocapsales Order Hydrurales Order Chrysosphaerales Order Phaeothamniales Order SarcinochrysidalesClass PedinellophyceaeOrder PedinellalesClass DictyochophyceaeOrder Dictyochales Classification of the phylum Chrysophyta according to Margulis et al.: Phylum Chrysophyta Class Chrysophyceae Class Pedinellophyceae Class Dictyochophyceae According to van den Hoek and Jahns: Class Chrysophyceae Order Ochromonadales Order Mallomonadales Order Pedinellales Order Chrysamoebidales Order Chrysocapsales Order Chrysosphaerales Order Phaeothamniales Classification of the class Chrysophyceae and splinter groups according to Preisig: Class ChrysophyceaeOrder Bicosoecales Order Chromulinales Order Hibberdiales Ord
Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species the valves are calcified, many are somewhat irregular in shape. Many, but not all, oysters are in the superfamily Ostreoidea; some kinds of oysters are consumed cooked or raw and are regarded as a delicacy. Some kinds of pearl oysters are harvested for the pearl produced within the mantle. Windowpane oysters are harvested for their translucent shells, which are used to make various kinds of decorative objects. First attested in English during the 14th century, the word "oyster" comes from Old French oistre, in turn from Latin ostrea, the feminine form of ostreum, the latinisation of the Greek ὄστρεον, "oyster". Compare ὀστέον, "bone". True oysters are members of the family Ostreidae; this family includes the edible oysters, which belong to the genera Ostrea, Ostreola and Saccostrea. Examples include the Belon oyster, eastern oyster, Olympia oyster, Pacific oyster, the Sydney rock oyster.
All shell-bearing mollusks can secrete pearls, yet most are not valuable. Pearls can form in both freshwater environments. Pearl oysters are not related to true oysters, being members of a distinct family, the feathered oysters. Both cultured pearls and natural pearls can be extracted from pearl oysters, though other molluscs, such as the freshwater mussels yield pearls of commercial value; the largest pearl-bearing oyster is the marine Pinctada maxima, the size of a dinner plate. Not all individual oysters produce pearls naturally. In fact, in a harvest of two and a half tons of oysters, only three to four oysters produce what commercial buyers consider to be absolute perfect pearls. In nature, pearl oysters produce pearls by covering a minute invasive object with nacre. Over the years, the irritating object is covered with enough layers of nacre to become a pearl; the many different types and shapes of pearls depend on the natural pigment of the nacre, the shape of the original irritant. Pearl farmers can culture a pearl by placing a nucleus a piece of polished mussel shell, inside the oyster.
In three to seven years, the oyster can produce a perfect pearl. These pearls are not as valuable as natural pearls, but look the same. In fact, since the beginning of the 20th century, when several researchers discovered how to produce artificial pearls, the cultured pearl market has far outgrown the natural pearl market. A number of bivalve molluscs have common names that include the word "oyster" because they either taste like or look somewhat like true oysters, or because they yield noticeable pearls. Examples include: Thorny oysters in the genus Spondylus Pilgrim oyster, another term for a scallop, in reference to the scallop shell of St. James Saddle oysters, members of the Anomiidae family known as jingle shells Dimydarian oysters, members of the family Dimyidae Windowpane oysters In the Philippines, a local thorny oyster species known as Tikod Amo is a favorite seafood source in the southern part of the country; because of its good flavor, it commands high prices. Oysters are filter feeders.
Suspended plankton and particles are trapped in the mucus of a gill, from there are transported to the mouth, where they are eaten and expelled as feces or pseudofeces. Oysters feed most at temperatures above 10 °C. An oyster can filter up to 5 L of water per hour; the Chesapeake Bay's once-flourishing oyster population filtered excess nutrients from the estuary's entire water volume every three to four days. Today, that would take nearly a year. Excess sediment and algae can result in the eutrophication of a body of water. Oyster filtration can mitigate these pollutants. In addition to their gills, oysters can exchange gases across their mantles, which are lined with many small, thin-walled blood vessels. A small, three-chambered heart, lying under the adductor muscle, pumps colorless blood to all parts of the body. At the same time, two kidneys, located on the underside of the muscle, remove waste products from the blood, their nervous system includes three pairs of ganglia. While some oysters have two sexes, their reproductive organs contain sperm.
Because of this, it is technically possible for an oyster to fertilize its own eggs. The gonads surround the digestive organs, are made up of sex cells, branching tubules, connective tissue. Once the female is fertilized, she discharges millions of eggs into the water; the larvae develop in about six hours and exist suspended in the water column as veliger larvae for two to three weeks before settling on a bed and maturing to sexual adulthood within a year. A group of oysters is called a bed or oyster reef; as a keystone species, oysters provide habitat for many marine species. Crassostrea and Saccostrea live in the intertidal zone, while Ostrea is subtidal; the hard surfaces of oyster shells and the nooks between the shells provide places where a host of small animals can live. Hundreds of animals, such as sea anemones and hooked mussels, inhabit oyster reefs. Many of these animals are prey to larger animals, including fish, such as striped bass, black drum and croakers. An oyster reef can increase the surface area of a flat bottom 50-fold.
An oyster's mature shape depends on the type of bottom to which it is attached, but it always orients itself with its outer, flared shell tilted upward. One valve is cupped and t