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108-416: The negative δC excursion representing the onset of OAE1a was rapid, taking only 22,000-47,000 years. The recovery of the global climate from the injection of large amounts of isotopically light carbon lasted for over a million years. The end of OAE1a is characterised by a positive δC excursion, which had a magnitude of +4 to +5%. The OAE lasted for about 1.1 to 1.3 Myr in total; one high-precision estimate put

216-410: A bivalve is composed of calcium carbonate , and consists of two, usually similar, parts called valves . These valves are for feeding and for disposal of waste. These are joined together along one edge (the hinge line ) by a flexible ligament that, usually in conjunction with interlocking "teeth" on each of the valves, forms the hinge . This arrangement allows the shell to be opened and closed without

324-420: A different way, scraping detritus from the seabed, and this may be the original mode of feeding used by all bivalves before the gills became adapted for filter feeding. These primitive bivalves hold on to the bottom with a pair of tentacles at the edge of the mouth, each of which has a single palp , or flap. The tentacles are covered in mucus , which traps the food, and cilia, which transport the particles back to

432-528: A feature shared with two other major groups of marine invertebrates, the bryozoans and the phoronids . Some brachiopod shells are made of calcium phosphate but most are calcium carbonate in the form of the biomineral calcite , whereas bivalve shells are always composed entirely of calcium carbonate, often in the form of the biomineral aragonite . The Cambrian explosion took place around 540 to 520 million years ago (Mya). In this geologically brief period, most major animal phyla diverged including some of

540-460: A hinged shell in two parts. However, brachiopods evolved from a very different ancestral line, and the resemblance to bivalves only arose because they occupy similar ecological niches . The differences between the two groups are due to their separate ancestral origins. Different initial structures have been adapted to solve the same problems, a case of convergent evolution . In modern times, brachiopods are not as common as bivalves. Both groups have

648-546: A negative feedback on global warming. Overall, the biotic effects of OAE1a were comparatively minor relative to other LIP-driven extinction events. Nannoconids that were highly calcified suffered significant decline during OAE1a, likely as a consequence of ocean acidification, although this causal relationship is disputed by other authors. The opportunistic, oyster-like bivalve genus Chondrodonta thrived during OAE1a because of its ability to survive in stressed environments where its competitors could not, and its spike in abundance

756-420: A number of families that live in freshwater. Majority of the class are benthic filter feeders that bury themselves in sediment, where they are relatively safe from predation . Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as scallops and file shells , can swim . Shipworms bore into wood, clay, or stone and live inside these substances. The shell of

864-499: A pair of nephridia . Each of these consists of a long, looped, glandular tube, which opens into the pericardium , and a bladder to store urine. They also have pericardial glands either line the auricles of the heart or attach to the pericardium, and serve as extra filtration organs. Metabolic waste is voided from the bladders through a nephridiopore near the front of the upper part of the mantle cavity and excreted. The sexes are usually separate in bivalves but some hermaphroditism

972-553: A peak in a 5-6 Myr periodicity cycle in the accumulation of phosphorus in marine sediments. During such peaks, the short-term positive feedback loop of increased biological productivity caused by an abundance of phosphorus that caused decreased oxygenation of seawater that then caused increased regeneration of phosphorus from marine sediments dominated, but it was eventually mitigated by a long-term negative feedback loop caused by an increase in atmospheric oxygen that resulted in enhanced wildfire activity and diminished phosphorus input into

1080-484: A relatively small dispersal potential before settling out. The common mussel ( Mytilus edulis ) produces 10 times as many eggs that hatch into larvae and soon need to feed to survive and grow. They can disperse more widely as they remain planktonic for a much longer time. Freshwater bivalves have different lifecycle. Sperm is drawn into a female's gills with the inhalant water and internal fertilization takes place. The eggs hatch into glochidia larvae that develop within

1188-644: A resting state, even when they are permanently submerged. In oysters, for example, their behaviour follows very strict circatidal and circadian rhythms according to the relative positions of the moon and sun. During neap tides, they exhibit much longer closing periods than during spring tides. Although many non-sessile bivalves use their muscular foot to move around, or to dig, members of the freshwater family Sphaeriidae are exceptional in that these small clams climb about quite nimbly on weeds using their long and flexible foot. The European fingernail clam ( Sphaerium corneum ), for example, climbs around on water weeds at

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1296-424: A shadow falling over the animal. Bivalves have an open circulatory system that bathes the organs in blood ( hemolymph ). The heart has three chambers: two auricles receiving blood from the gills, and a single ventricle . The ventricle is muscular and pumps hemolymph into the aorta , and then to the rest of the body. Some bivalves have a single aorta, but most also have a second, usually smaller, aorta serving

1404-436: A shell consisting of two valves, but the organization of the shell is quite different in the two groups. In brachiopods, the two valves are positioned on the dorsal and ventral surfaces of the body, while in bivalves, the valves are on the left and right sides of the body, and are, in most cases, mirror images of one other. Brachiopods have a lophophore , a coiled, rigid cartilaginous internal apparatus adapted for filter feeding,

1512-424: A steady pull. Paired pedal protractor and retractor muscles operate the animal's foot. The sedentary habits of the bivalves have meant that in general the nervous system is less complex than in most other molluscs. The animals have no brain ; the nervous system consists of a nerve network and a series of paired ganglia . In all but the most primitive bivalves, two cerebropleural ganglia are on either side of

1620-534: A strontium isotope minimum, demarcating another peak in silicate weathering. This weathering may have buffered the warming effects of large igneous province volcanism and helped to cool the Earth back to its pre-OAE1a state. Sea levels initially fell during OAE1a as the world warmed and later rose as global cooling occurred, indicating the dominance of aquifer-eustasy in controlling sea level change during this anoxic event. Organic carbon burial increased during OAE1a and

1728-476: A very high rate of carbon dioxide discharge. Despite a much smaller methane clathrate reservoir relative to the present day, the degassing of methane clathrate deposits may have nonetheless significantly exacerbated volcanic warming. Following OAE1a, δO values increased, indicating a drop in temperatures that coincided with a δC org decline, which began in the C4 isotopic phase of the interval. OAE1a coincided with

1836-589: Is called Tethys Sea, Western Tethys Ocean, or Paratethys or Alpine Tethys Ocean. The Black , Caspian , and Aral seas are thought to be its crustal remains, though the Black Sea may, in fact, be a remnant of the older Paleo-Tethys Ocean . The Western Tethys was not simply a single open ocean. It covered many small plates, Cretaceous island arcs , and microcontinents . Many small oceanic basins ( Valais Ocean , Piemont-Liguria Ocean , Meliata Ocean ) were separated from each other by continental terranes on

1944-432: Is composed of two calcareous valves held together by a ligament. The valves are made of either calcite , as is the case in oysters, or both calcite and aragonite . Sometimes, the aragonite forms an inner, nacreous layer, as is the case in the order Pteriida . In other taxa , alternate layers of calcite and aragonite are laid down. The ligament and byssus, if calcified, are composed of aragonite. The outermost layer of

2052-557: Is known. The gonads either open into the nephridia or through a separate pore into a chamber over the gills. The ripe gonads of males and females release sperm and eggs into the water column . Spawning may take place continually or be triggered by environmental factors such as day length, water temperature, or the presence of sperm in the water. Some species are "dribble spawners", releasing gametes during protracted period that can extend for weeks. Others are mass spawners and release their gametes in batches or all at once. Fertilization

2160-578: Is often used as a biostratigraphic indicator of the onset of OAE1a. Tethys Ocean The Tethys Ocean ( / ˈ t iː θ ɪ s , ˈ t ɛ -/ TEETH -iss, TETH - ; Greek : Τηθύς Tēthús ), also called the Tethys Sea or the Neo-Tethys , was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era . It was the predecessor to the modern Indian Ocean ,

2268-408: Is secreted by the mantle edge, and the valves themselves thicken as more material is secreted from the general mantle surface. Calcareous matter comes from both its diet and the surrounding seawater. Concentric rings on the exterior of a valve are commonly used to age bivalves. For some groups, a more precise method for determining the age of a shell is by cutting a cross section through it and examining

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2376-472: Is the ventral or underside region. The anterior or front of the shell is where the byssus (when present) and foot are located, and the posterior of the shell is where the siphons are located. With the hinge uppermost and with the anterior edge of the animal towards the viewer's left, the valve facing the viewer is the left valve and the opposing valve the right. Many bivalves such as clams, which appear upright, are evolutionarily lying on their side. The shell

2484-432: Is usually external. Typically, a short stage lasts a few hours or days before the eggs hatch into trochophore larvae. These later develop into veliger larvae which settle on the seabed and undergo metamorphosis into adults. In some species, such as those in the genus Lasaea , females draw water containing sperm in through their inhalant siphons and fertilization takes place inside the female. These species then brood

2592-701: Is what is thought to have allowed for upwelling in the Arabian Sea and led to the establishment of the modern South Asian Monsoon . It also caused major modifications to the functioning of the AMOC and ACC . During the Oligocene (33.9 to 23 Mya), large parts of central and eastern Europe were covered by a northern branch of the Tethys Ocean, called the Paratethys . The Paratethys was separated from

2700-714: The Alboran , Iberian , and Apulian plates. The high sea level in the Mesozoic flooded most of these continental domains, forming shallow seas. During the early Cenozoic, the Tethys Ocean could be divided into three sections: the Mediterranean Tethys (the direct predecessor to the Mediterranean Sea), the Peri-Tethys (a vast inland sea that covered much of eastern Europe and central Asia, and

2808-824: The Alpide belt (including the Alps , Himalayas , Zagros , and Caucasus Mountains ). All of these geological events, in addition to a drop in sea level rise from Antarctic glaciation, brought an end to the Tethys as it previously existed, fragmenting it into the Indian Ocean, the Mediterranean Sea, and the Paratethys . It was preceded by the Paleo-Tethys Ocean , which lasted between the Cambrian and

2916-612: The Early Silurian , the gills were becoming adapted for filter feeding, and during the Devonian and Carboniferous periods, siphons first appeared, which, with the newly developed muscular foot, allowed the animals to bury themselves deep in the sediment. By the middle of the Paleozoic , around 400 Mya, the brachiopods were among the most abundant filter feeders in the ocean, and over 12,000 fossil species are recognized. By

3024-797: The Early Triassic , while the Neotethys formed during the Late Triassic and lasted in some form up to the Oligocene – Miocene boundary (about 24–21 million years ago) when it completely closed. A portion known as the Paratethys was isolated during the Oligocene (34 million years ago) and lasted up to the Pliocene (about 5 million years ago), when it largely dried out. The modern inland seas of Europe and Western Asia, namely

3132-668: The Lamellibranchiata and Pelecypoda , is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consisting of a hinged pair of half- shells known as valves . As a group, bivalves have no head and lack some typical molluscan organs such as the radula and the odontophore . Their gills have evolved into ctenidia , specialised organs for feeding and breathing. Common bivalves include clams , oysters , cockles , mussels , scallops , and numerous other families that live in saltwater, as well as

3240-721: The Mediterranean Sea , and the Eurasian inland marine basins (primarily represented today by the Black Sea and Caspian Sea ). During the early Mesozoic, as Pangaea broke up, the Tethys Ocean was defined as the ocean located between the ancient continents of Gondwana and Laurasia . After the opening of the Indian and Atlantic oceans during the Cretaceous Period and the breakup of these continents over

3348-513: The Paratethys when the Alpine front was still 100 km (62 mi) farther south. In 1885, the Austrian palaeontologist Melchior Neumayr deduced the existence of the Tethys Ocean from Mesozoic marine sediments and their distribution, calling his concept Zentrales Mittelmeer ( lit.   ' Central Mediterranean Sea ' ) and described it as a Jurassic seaway, which extended from

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3456-726: The Permian–Triassic extinction event 250 Mya, bivalves were undergoing a huge radiation of diversity. The bivalves were hard hit by this event, but re-established themselves and thrived during the Triassic period that followed. In contrast, the brachiopods lost 95% of their species diversity . The ability of some bivalves to burrow and thus avoid predators may have been a major factor in their success. Other new adaptations within various families allowed species to occupy previously unused evolutionary niches. These included increasing relative buoyancy in soft sediments by developing spines on

3564-533: The Triassic , a new ocean began forming in the southern end of the Paleo-Tethys Ocean. A rift formed along the northern continental shelf of Southern Pangaea (Gondwana). Over the next 60 million years, that piece of shelf, known as Cimmeria , traveled north, pushing the floor of the Paleo-Tethys Ocean under the eastern end of northern Pangaea (early / proto- Laurasia ). The Neo-Tethys Ocean formed between Cimmeria and Gondwana, directly over where

3672-587: The Veneridae , with more than 680 species and the Tellinidae and Lucinidae , each with over 500 species. The freshwater bivalves include seven families, the largest of which are the Unionidae , with about 700 species. The taxonomic term Bivalvia was first used by Linnaeus in the 10th edition of his Systema Naturae in 1758 to refer to animals having shells composed of two valves . More recently,

3780-597: The intertidal and sublittoral zones of the oceans. A sandy sea beach may superficially appear to be devoid of life, but often a very large number of bivalves and other invertebrates are living beneath the surface of the sand. On a large beach in South Wales , careful sampling produced an estimate of 1.44 million cockles ( Cerastoderma edule ) per acre of beach. Bivalves inhabit the tropics, as well as temperate and boreal waters. A number of species can survive and even flourish in extreme conditions. They are abundant in

3888-517: The oesophagus . The cerebral ganglia control the sensory organs, while the pleural ganglia supply nerves to the mantle cavity. The pedal ganglia, which control the foot, are at its base, and the visceral ganglia, which can be quite large in swimming bivalves, are under the posterior adductor muscle. These ganglia are both connected to the cerebropleural ganglia by nerve fibres . Bivalves with long siphons may also have siphonal ganglia to control them. The sensory organs of bivalves are largely located on

3996-400: The " crystalline style " projected into the stomach from an associated sac. Cilia in the sac cause the style to rotate, winding in a stream of food-containing mucus from the mouth, and churning the stomach contents. This constant motion propels food particles into a sorting region at the rear of the stomach, which distributes smaller particles into the digestive glands, and heavier particles into

4104-490: The 1960s, "fixist" geologists, however, regarded Tethys as a composite trough, which evolved through a series of orogenic cycles. They used the terms 'Paleotethys', 'Mesotethys', and 'Neotethys' for the Caledonian , Variscan , and Alpine orogenies, respectively. In the 1970s and 1980s, these terms and 'Proto-Tethys', were used in different senses by various authors, but the concept of a single ocean wedging into Pangea from

4212-873: The Aralo-Caspian Formation extending from close to the Danube delta across Crimea, up the east side of the Volga river to Samara, then south of the Urals to beyond the Aral Sea. Brackish and upper freshwater components (OSM) of the Miocene are now known to extend through the North Alpine foreland basin and onto the Swabian Jura with thickness of up to 250 m (820 ft); these were deposited in

4320-458: The Arctic, about 140 species being known from that zone. The Antarctic scallop, Adamussium colbecki , lives under the sea ice at the other end of the globe, where the subzero temperatures mean that growth rates are very slow. The giant mussel, Bathymodiolus thermophilus , and the giant white clam, Calyptogena magnifica , both live clustered around hydrothermal vents at abyssal depths in

4428-517: The Black Sea and Caspian Sea, are remnants of the Paratethys Sea. The sea is named after Tethys , who, in ancient Greek mythology, is a water goddess, a sister and consort of Oceanus , mother of the Oceanid sea nymphs and of the world's great rivers, lakes and fountains. The eastern part of the Tethys Ocean is sometimes referred to as Eastern Tethys. The western part of the Tethys Ocean

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4536-750: The Caribbean to the Himalayas. In 1893, the Austrian geologist Eduard Suess proposed the hypothesis that an ancient and extinct inland sea had once existed between Laurasia and the continents which formed Gondwana II. He named it the Tethys Sea after the Greek sea goddess Tethys. He provided evidence for his theory using fossil records from the Alps and Africa. He proposed the concept of Tethys in his four-volume work Das Antlitz der Erde ( The Face of

4644-425: The Earth ). In the following decades during the 20th century, " mobilist " geologists such as Uhlig (1911), Diener (1925), and Daque (1926) regarded Tethys as a large trough between two supercontinents which lasted from the late Palaeozoic until continental fragments derived from Gondwana obliterated it. After World War II , Tethys was described as a triangular ocean with a wide eastern end. From 1920s to

4752-527: The OAE, occurring in the C3 isotopic interval, is believed to reflect volcanic release of carbon dioxide into the atmosphere and its consequent warming of the Earth. Enrichments in unradiogenic osmium , which is primarily derived from alteration of oceanic crust by hydrothermal volcanism, further bolster volcanism as the driver of OAE1a. Multiple LIPs have been implicated as causes of the rapid global warming responsible for

4860-464: The Pacific Ocean. They have chemosymbiotic bacteria in their gills that oxidise hydrogen sulphide , and the molluscs absorb nutrients synthesized by these bacteria. Some species are found in the hadal zone , like Vesicomya sergeevi, which occurs at depths of 7600–9530 meters. The saddle oyster, Enigmonia aenigmatica , is a marine species that could be considered amphibious . It lives above

4968-605: The Paleo-Tethys formerly rested. During the Jurassic period about 150 Mya, Cimmeria finally collided with Laurasia and stalled, so the ocean floor behind it buckled under , forming the Tethys Trench . Water levels rose, and the western Tethys shallowly covered significant portions of Europe, forming the first Tethys Sea. Around the same time, Laurasia and Gondwana began drifting apart , opening an extension of

5076-502: The Romans, and mariculture has more recently become an important source of bivalves for food. Modern knowledge of molluscan reproductive cycles has led to the development of hatcheries and new culture techniques. A better understanding of the potential hazards of eating raw or undercooked shellfish has led to improved storage and processing. Pearl oysters (the common name of two very different families in salt water and fresh water) are

5184-636: The Tethys Sea between them which today is the part of the Atlantic Ocean between the Mediterranean and the Caribbean . As North and South America were still attached to the rest of Laurasia and Gondwana, respectively, the Tethys Ocean in its widest extension was part of a continuous oceanic belt running around the Earth between about latitude 30°N and the Equator . Thus, ocean currents at

5292-655: The Tethys oceans should be confused with the Rheic Ocean , which existed to the west of them in the Silurian Period. To the north of the Tethys, the then-land mass is called Angaraland and to the south of it, it is called Gondwanaland . From the Ediacaran (600  Mya ) into the Devonian (360 Mya ), the Proto-Tethys Ocean existed and was situated between Baltica and Laurentia to

5400-601: The Tethys were eventually closed off in what is now the Middle East during the Miocene , as a consequence of the northern migration of Africa/Arabia and global sea levels falling due to the concurrent formation of the Antarctic Ice Sheet . This decoupling occurred in two steps, first around 20 Mya and another around 14 Mya. The complete closure of the Tethys led to a global reorganization of currents, and

5508-469: The Tethys with the formation of the Alps, Carpathians , Dinarides , Taurus , and Elburz mountains during the Alpine orogeny . During the late Miocene , the Paratethys gradually disappeared, and became an isolated inland sea. Separation from the wider Tethys during the early Miocene initially led to a boost in primary productivity for the Paratethys, but this gave way to a total ecosystem collapse during

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5616-472: The air, can gape the shell slightly and gas exchange can take place. Oysters, including the Pacific oyster ( Magallana gigas ), are recognized as having varying metabolic responses to environmental stress, with changes in respiration rate being frequently observed. Most bivalves are filter feeders , using their gills to capture particulate food such as phytoplankton from the water. Protobranchs feed in

5724-572: The class was known as Pelecypoda, meaning " axe -foot" (based on the shape of the foot of the animal when extended). The name "bivalve" is derived from the Latin bis , meaning 'two', and valvae , meaning 'leaves of a door'. ("Leaf" is an older word for the main, movable part of a door. We normally consider this the door itself.) Paired shells have evolved independently several times among animals that are not bivalves; other animals with paired valves include certain gastropods (small sea snails in

5832-906: The direct predecessor to the Paratethys Sea), and the Indian Tethys (the direct predecessor to the Indian Ocean). The Turgai Strait extended out of the Peri-Tethys, connecting the Tethys with the Arctic Ocean . As theories have improved, scientists have extended the "Tethys" name to refer to three similar oceans that preceded it, separating the continental terranes: in Asia, the Paleo-Tethys (Devonian–Triassic), Meso-Tethys (late Early Permian –Late Cretaceous), and Ceno-Tethys (Late-Triassic–Cenozoic) are recognized. None of

5940-662: The east, roughly where Suess first proposed it, remained. In the 1960s, the theory of plate tectonics became established, and Suess's "sea" could clearly be seen to have been an ocean. Plate tectonics provided an explanation for the mechanism by which the former ocean disappeared: oceanic crust can subduct under continental crust . Tethys was considered an oceanic plate by Smith (1971); Dewey, Pitman, Ryan and Bonnin (1973); Laubscher and Bernoulli (1973); and Bijou-Duval, Dercourt and Pichon (1977). Bivalvia And see text Bivalvia ( / b aɪ ˈ v æ l v i ə / ) or bivalves , in previous centuries referred to as

6048-436: The edges of lakes and ponds; this enables the clam to find the best position for filter feeding. The thick shell and rounded shape of bivalves make them awkward for potential predators to tackle. Nevertheless, a number of different creatures include them in their diet. Many species of demersal fish feed on them including the common carp ( Cyprinus carpio ), which is being used in the upper Mississippi River to try to control

6156-402: The family Juliidae ), members of the phylum Brachiopoda and the minute crustaceans known as ostracods and conchostracans . Bivalves have bilaterally symmetrical and laterally flattened bodies, with a blade-shaped foot, vestigial head and no radula . At the dorsal or back region of the shell is the hinge point or line, which contain the umbo and beak and the lower, curved margin

6264-421: The female's shell. Later they are released and attach themselves parasitically to the gills or fins of a fish host. After several weeks they drop off their host, undergo metamorphosis and develop into adults on the substrate . Some of the species in the freshwater mussel family, Unionidae , commonly known as pocketbook mussels, have evolved an unusual reproductive strategy. The female's mantle protrudes from

6372-434: The first creatures with mineralized skeletons. Brachiopods and bivalves made their appearance at this time, and left their fossilized remains behind in the rocks. Possible early bivalves include Pojetaia and Fordilla ; these probably lie in the stem rather than crown group. Watsonella and Anabarella are perceived to be (earlier) close relatives of these taxa. Only five genera of supposed Cambrian "bivalves" exist,

6480-434: The fish's gills, where they attach and trigger a tissue response that forms a small cyst around each larva. The larvae then feed by breaking down and digesting the tissue of the fish within the cysts. After a few weeks they release themselves from the cysts and fall to the stream bed as juvenile molluscs. Brachiopods are shelled marine organisms that superficially resemble bivalves in that they are of similar size and have

6588-467: The gills are also much longer than those in more primitive bivalves, and are folded over to create a groove through which food can be transported. The structure of the gills varies considerably, and can serve as a useful means for classifying bivalves into groups. A few bivalves, such as the granular poromya ( Poromya granulata ), are carnivorous , eating much larger prey than the tiny microalgae consumed by other bivalves. Muscles draw water in through

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6696-595: The high tide mark in the tropical Indo-Pacific on the underside of mangrove leaves, on mangrove branches, and on sea walls in the splash zone . Some freshwater bivalves have very restricted ranges. For example, the Ouachita creekshell mussel, Villosa arkansasensis , is known only from the streams of the Ouachita Mountains in Arkansas and Oklahoma, and like several other freshwater mussel species from

6804-434: The hind parts of the animal. The hemolymph usually lacks any respiratory pigment. In the carnivorous genus Poromya , the hemolymph has red amoebocytes containing a haemoglobin pigment. The paired gills are located posteriorly and consist of hollow tube-like filaments with thin walls for gas exchange . The respiratory demands of bivalves are low, due to their relative inactivity. Some freshwater species, when exposed to

6912-411: The historic æra, a vast region of Europe and Asia was covered by a Mediterranean Sea of brackish water, of which the present Caspian is the diminished type. ... To render the distinction between these accumulations and all others clear and unambiguous, we have adopted the term Aralo-Caspian, first applied in a geographical sense, by our great precursor Humboldt, to this region of the globe. ... Judging from

7020-399: The incremental growth bands. The shipworms , in the family Teredinidae have greatly elongated bodies, but their shell valves are much reduced and restricted to the anterior end of the body, where they function as scraping organs that permit the animal to dig tunnels through wood. The main muscular system in bivalves is the posterior and anterior adductor muscles. These muscles connect

7128-526: The inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey. Many bivalves have no eyes, but a few members of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea have simple eyes on the margin of the mantle. These consist of a pit of photosensory cells and a lens . Scallops have more complex eyes with a lens, a two-layered retina , and a concave mirror. All bivalves have light-sensitive cells that can detect

7236-421: The inhalant siphon which is modified into a cowl-shaped organ, sucking in prey. The siphon can be retracted quickly and inverted, bringing the prey within reach of the mouth. The gut is modified so that large food particles can be digested. The unusual genus, Entovalva , is endosymbiotic , being found only in the oesophagus of sea cucumbers . It has mantle folds that completely surround its small valves. When

7344-443: The intestine. Waste material is consolidated in the rectum and voided as pellets into the exhalent water stream through an anal pore. Feeding and digestion are synchronized with diurnal and tidal cycles. Carnivorous bivalves generally have reduced crystalline styles and the stomach has thick, muscular walls, extensive cuticular linings and diminished sorting areas and gastric chamber sections. The excretory organs of bivalves are

7452-494: The invasive zebra mussel ( Dreissena polymorpha ). Birds such as the Eurasian oystercatcher ( Haematopus ostralegus ) have specially adapted beaks which can pry open their shells. The herring gull ( Larus argentatus ) sometimes drops heavy shells onto rocks in order to crack them open. Sea otters feed on a variety of bivalve species and have been observed to use stones balanced on their chests as anvils on which to crack open

7560-485: The largest living bivalve is the giant clam Tridacna gigas , which can grow to a length of 1,200 mm (47 in) and a weight of more than 200 kg (441 lb). The largest known extinct bivalve is a species of Platyceramus whose fossils measure up to 3,000 mm (118 in) in length. In his 2010 treatise, Compendium of Bivalves , Markus Huber gives the total number of living bivalve species as about 9,200 combined in 106 families. Huber states that

7668-515: The late Miocene as a result of rapid dissolution of carbonate . In Chapter 13 of his 1845 book, Roderick Murchison described a distinctive formation extending from the Black Sea to the Aral Sea in which the creatures differed from those of the purely marine period that preceded them. The Miocene deposits of Crimea and Taman (south of the Sea of Azov ) are identical with formations surrounding

7776-544: The length of OAE1a at 1.157 Myr. OAE1a ensued during a hot climatic interval, with the global average temperature being around 21.5 °C. The Tethys Ocean experienced an increase in humidity at the beginning of OAE1a, while conditions around the Boreal Ocean were initially dry and only humidified later on during the OAE. The increase in global temperatures that caused OAE1a was most likely driven by large igneous province (LIP) volcanism. The negative δC excursion preceding

7884-418: The mantle lobes secrete the valves, and the mantle crest secretes the whole hinge mechanism consisting of ligament , byssus threads (where present), and teeth . The posterior mantle edge may have two elongated extensions known as siphons , through one of which water is inhaled, and the other expelled. The siphons retract into a cavity, known as the pallial sinus . The shell grows larger when more material

7992-452: The mantle though the valves. In sedentary or recumbent bivalves that lie on one valve, such as the oysters and scallops, the anterior adductor muscle has been lost and the posterior muscle is positioned centrally. In species that can swim by flapping their valves, a single, central adductor muscle occurs. These muscles are composed of two types of muscle fibres, striated muscle bundles for fast actions and smooth muscle bundles for maintaining

8100-457: The masses of water now separated from each other, from the Aral to the Black Sea inclusive, were formerly united in this vast pre-historical Mediterranean ; which (even if we restrict its limits to the boundaries we already know, and do not extend them eastward, amid low regions untrodden by geologists) must have exceeded in size the present Mediterranean! On the accompanying map, Murchison shows

8208-626: The most common source of natural pearls . The shells of bivalves are used in craftwork, and the manufacture of jewellery and buttons. Bivalves have also been used in the biocontrol of pollution. Bivalves appear in the fossil record first in the early Cambrian more than 500 million years ago. The total number of known living species is about 9,200. These species are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water and estuarine species) represent about 8,000 species, combined in four subclasses and 99 families with 1,100 genera. The largest recent marine families are

8316-420: The negative δC excursion and lasted for approximately 0.85 Myr. The drop in seawater pH was associated with the acme of the carbonate crisis. δLi measurements indicate an enrichment in isotopically light lithium coeval with the negative δC excursion, signifying an increase in silicate weathering amidst the volcanically induced global warming of OAE1a. A second negative δLi excursion occurred synchronously with

8424-651: The north and Gondwana to the south. From the Silurian (440 Mya ) through the Jurassic periods, the Paleo-Tethys Ocean existed between the Hunic terranes and Gondwana. Over a period of 400 million years, continental terranes intermittently separated from Gondwana in the Southern Hemisphere to migrate northward to form Asia in the Northern Hemisphere. About 250 Mya, during

8532-422: The number of 20,000 living species, often encountered in literature, could not be verified and presents the following table to illustrate the known diversity: The bivalves are a highly successful class of invertebrates found in aquatic habitats throughout the world. Most are infaunal and live buried in sediment on the seabed, or in the sediment in freshwater habitats. A large number of bivalve species are found in

8640-576: The oceans. An increase in the ratios of organic carbon to reactive phosphorus species and of total nitrogen to reactive phosphorus confirms leakage of sedimentary phosphorus back into the water column occurred during OAE1a, with this process likely being accelerated by the increased global temperatures of the time. Marine productivity increased. The productivity spike was likely driven by an increase in iron availability. Increased sulphate flux from volcanism caused an increase in hydrogen sulphide production, which in turn increased phosphorus availability in

8748-758: The onset of OAE1a, including the High Arctic Large Igneous Province (HALIP), the Kerguelen Plateau , and the Ontong Java Plateau . The rate of greenhouse gas emissions leading up to OAE1a was relatively slow, causing the anoxic event to only generate a minor extinction event, in contrast to the severe LIP-induced Capitanian , Permian-Triassic , and Triassic-Jurassic mass extinctions and the ongoing Holocene extinction caused in part by anthropogenic greenhouse gas release, each of which were or are characterised by

8856-507: The others being Tuarangia , Camya and Arhouriella and potentially Buluniella . Bivalve fossils can be formed when the sediment in which the shells are buried hardens into rock. Often, the impression made by the valves remains as the fossil rather than the valves. During the Early Ordovician , a great increase in the diversity of bivalve species occurred, and the dysodont, heterodont, and taxodont dentitions evolved. By

8964-400: The palps. These then sort the particles, rejecting those that are unsuitable or too large to digest, and conveying others to the mouth. In more advanced bivalves, water is drawn into the shell from the posterior ventral surface of the animal, passes upwards through the gills, and doubles back to be expelled just above the intake. There may be two elongated, retractable siphons reaching up to

9072-523: The period is before the larva first feeds, the larger the egg and yolk need to be. The reproductive cost of producing these energy-rich eggs is high and they are usually smaller in number. For example, the Baltic tellin ( Macoma balthica ) produces few, high-energy eggs. The larvae hatching out of these rely on the energy reserves and do not feed. After about four days, they become D-stage larvae, when they first develop hinged, D-shaped valves. These larvae have

9180-409: The posterior mantle margins. The organs are usually mechanoreceptors or chemoreceptors , in some cases located on short tentacles . The osphradium is a patch of sensory cells located below the posterior adductor muscle that may serve to taste the water or measure its turbidity . Statocysts within the organism help the bivalve to sense and correct its orientation. In the order Anomalodesmata ,

9288-434: The present Caspian Sea , in which the univalves of freshwater origin are associated with forms of Cardiacae and Mytili that are common to partially saline or brackish waters. This distinctive fauna has been found throughout all the enormously developed Tertiary formations of the southern and south-eastern steppes. ... and leads at once to the conviction, that during long periods antecedent, as will be hereafter explained, to

9396-527: The prominence of modern bivalves over brachiopods seems due to chance disparities in their response to extinction events . The adult maximum size of living species of bivalve ranges from 0.52 mm (0.02 in) in Condylonucula maya , a nut clam, to a length of 1,532 millimetres (60.3 in) in Kuphus polythalamia , an elongated, burrowing shipworm. However, the species generally regarded as

9504-600: The recital of travellers and from specimens of the rock, we have no doubt that it extended to Khivah and the Aral Sea ;; beyond which the low level of the adjacent eastern deserts would lead us to infer, that it spread over wide tracts in Asia now inhabited by the Turkomans and Kirghis , and was bounded only by the mountains of the Hindoo Kusk and Chinese Tartary . ... there can be no sort of doubt, that all

9612-401: The same period, it came to be defined as the ocean bordered by the continents of Africa, Eurasia, India, and Australasia. During the early-mid Cenozoic, the Indian, African, Australian and Arabian plates moved north and collided with the Eurasian plate, which created new borders to the ocean, a land barrier to the flow of currents between the Indian and Mediterranean basins, and the orogenies of

9720-496: The sea cucumber sucks in sediment, the bivalve allows the water to pass over its gills and extracts fine organic particles. To prevent itself from being swept away, it attaches itself with byssal threads to the host 's throat. The sea cucumber is unharmed. The digestive tract of typical bivalves consists of an oesophagus , stomach , and intestine . Protobranch stomachs have a mere sac attached to them while filter-feeding bivalves have elongated rod of solidified mucus referred to as

9828-419: The seabed, one each for the inhalant and exhalant streams of water. The gills of filter-feeding bivalves are known as ctenidia and have become highly modified to increase their ability to capture food. For example, the cilia on the gills, which originally served to remove unwanted sediment, have become adapted to capture food particles, and transport them in a steady stream of mucus to the mouth. The filaments of

9936-469: The sediment remains damp even when the tide is out. When buried in the sediment, burrowing bivalves are protected from the pounding of waves, desiccation, and overheating during low tide, and variations in salinity caused by rainwater. They are also out of the reach of many predators. Their general strategy is to extend their siphons to the surface for feeding and respiration during high tide, but to descend to greater depths or keep their shell tightly shut when

10044-435: The shell and develops into an imitation small fish, complete with fish-like markings and false eyes. This decoy moves in the current and attracts the attention of real fish. Some fish see the decoy as prey, while others see a conspecific . They approach for a closer look and the mussel releases huge numbers of larvae from its gills, dousing the inquisitive fish with its tiny, parasitic young. These glochidia larvae are drawn into

10152-468: The shell is the periostracum , a thin layer composed of horny conchiolin . The periostracum is secreted by the outer mantle and is easily abraded. The outer surface of the valves is often sculpted, with clams often having concentric striations, scallops having radial ribs and oysters a latticework of irregular markings. In all molluscs, the mantle forms a thin membrane that covers the animal's body and extends out from it in flaps or lobes. In bivalves,

10260-571: The shell, gaining the ability to swim, and in a few cases, adopting predatory habits. For a long time, bivalves were thought to be better adapted to aquatic life than brachiopods were, outcompeting and relegating them to minor niches in later ages. These two taxa appeared in textbooks as an example of replacement by competition. Evidence given for this included the fact that bivalves needed less food to subsist because of their energetically efficient ligament-muscle system for opening and closing valves. All this has been broadly disproven, though; rather,

10368-420: The shells and open them more easily when they could tackle them from different angles. Octopuses either pull bivalves apart by force, or they bore a hole into the shell and insert a digestive fluid before sucking out the liquified contents. Certain carnivorous gastropod snails such as whelks ( Buccinidae ) and murex snails ( Muricidae ) feed on bivalves by boring into their shells. A dog whelk ( Nucella ) drills

10476-400: The shells with their pincers and starfish use their water vascular system to force the valves apart and then insert part of their stomach between the valves to digest the bivalve's body. It has been found experimentally that both crabs and starfish preferred molluscs that are attached by byssus threads to ones that are cemented to the substrate. This was probably because they could manipulate

10584-740: The shells. The Pacific walrus ( Odobenus rosmarus divergens ) is one of the main predators feeding on bivalves in Arctic waters. Shellfish have formed part of the human diet since prehistoric times, a fact evidenced by the remains of mollusc shells found in ancient middens. Examinations of these deposits in Peru has provided a means of dating long past El Niño events because of the disruption these caused to bivalve shell growth. Further changes in shell development due to environmental stress has also been suggested to cause increased mortality in oysters due to reduced shell strength. Invertebrate predators include crustaceans, starfish and octopuses. Crustaceans crack

10692-584: The southeastern US, it is in danger of extinction. In contrast, a few species of freshwater bivalves, including the golden mussel ( Limnoperna fortunei ), are dramatically increasing their ranges. The golden mussel has spread from Southeast Asia to Argentina, where it has become an invasive species . Another well-travelled freshwater bivalve, the zebra mussel ( Dreissena polymorpha ) originated in southeastern Russia, and has been accidentally introduced to inland waterways in North America and Europe, where

10800-416: The species damages water installations and disrupts local ecosystems . Most bivalves adopt a sedentary or even sessile lifestyle, often spending their whole lives in the area in which they first settled as juveniles. The majority of bivalves are infaunal, living under the seabed, buried in soft substrates such as sand, silt, mud, gravel, or coral fragments. Many of these live in the intertidal zone where

10908-590: The tide goes out. They use their muscular foot to dig into the substrate. To do this, the animal relaxes its adductor muscles and opens its shell wide to anchor itself in position while it extends its foot downwards into the substrate. Then it dilates the tip of its foot, retracts the adductor muscles to close the shell, shortens its foot and draws itself downwards. This series of actions is repeated to dig deeper. Other bivalves, such as mussels , attach themselves to hard surfaces using tough byssus threads made of collagen and elastin proteins. Some species, including

11016-774: The time around the Early Cretaceous ran very differently from the way they do today. Between the Jurassic and the Late Cretaceous , which started about 100 Mya, Gondwana began breaking up, pushing Africa and India north across the Tethys and opening up the Indian Ocean. Throughout the Cenozoic (66 million to the dawn of the Neogene, 23 Mya), the connections between the Atlantic and Indian Oceans across

11124-558: The true oysters, the jewel boxes , the jingle shells , the thorny oysters and the kitten's paws , cement themselves to stones, rock or larger dead shells. In oysters, the lower valve may be almost flat while the upper valve develops layer upon layer of thin horny material reinforced with calcium carbonate. Oysters sometimes occur in dense beds in the neritic zone and, like most bivalves, are filter feeders. Bivalves filter large amounts of water to feed and breathe but they are not permanently open. They regularly shut their valves to enter

11232-406: The two halves detaching. The shell is typically bilaterally symmetrical , with the hinge lying in the sagittal plane . Adult shell sizes of bivalves vary from fractions of a millimetre to over a metre in length, but the majority of species do not exceed 10 cm (4 in). Bivalves have long been a part of the diet of coastal and riparian human populations. Oysters were cultured in ponds by

11340-402: The two valves and contract to close the shell. The valves are also joined dorsally by the hinge ligament , which is an extension of the periostracum. The ligament is responsible for opening the shell, and works against the adductor muscles when the animal opens and closes. Retractor muscles connect the mantle to the edge of the shell, along a line known as the pallial line . These muscles pull

11448-400: The water column by inhibiting its burial on the seafloor and enabled the development of anoxia. The large-scale volcanic release of carbon dioxide caused a drop in the pH of seawater at the start of OAE1a, as much of this excess carbon dioxide was absorbed by the ocean and dissolved as carbonic acid . Seawater carbonate-saturation was severely reduced. Ocean acidification began shortly after

11556-417: The young inside their mantle cavity, eventually releasing them into the water column as veliger larvae or as crawl-away juveniles. Most of the bivalve larvae that hatch from eggs in the water column feed on diatoms or other phytoplankton. In temperate regions, about 25% of species are lecithotrophic , depending on nutrients stored in the yolk of the egg where the main energy source is lipids . The longer

11664-472: Was heightened during intervals of enhanced humidity. OAE1a, as with other OAEs, exhibited widespread deposition of black shales rich in organic matter incapable of being decomposed on the seabed, as the anoxic conditions prohibited habitation of most microbial decomposers. Black shale deposition begins during the C6 stage of OAE1a and lasted for around 0.4 Myr. As with silicate weathering, organic carbon burial acted as

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