BIVALVIA
BIVALVIA
Origins: 521 million years ago (early Cambrian period, Stage 2)
Extinction: Still extant
Bivalves are a class of marine and freshwater dwelling mollusks, characterized for laterally compressed bodies enclosed by a shell consisted of two hinged parts, a bivalve shell. Most bivalves hide under sediment to evade predators. The shell of a bivalve is composed of calcium carbonate, and each valve is usually similar to the other. Bivalve shells, depending on the species, can range from fractions of a millimetre to over a meter in length (in some extinct species even reaching about 3 meters), but most species don't exceed 10 centimeters of shell length. They are eaten today by humans, though the lifestyle of these mollusks leads them to be toxin accumulators, so food poisoning can occur, though with time, humans have perfected their methods of food preservation and treatment, to allow for a safer consumption of bivalves. The shells of bivalves are also used by humans to produce jewelry and other crafts. Bivalves first evolved in the Cambrian period. Since then they evolved into over 1200 extant genera situated in over 100 families. Most living bivalves are marine, with seven families of mostly freshwater ones, the largest being the Unionidae family, which consists of about 700 living species.
Bivalves have bilateral symmetry and laterally flattened bodies, with a blade-shaped foot, a vestigial head and lack a radula. The shell is composed of two calcareous valves held together by a ligament. The shell can be made of calcite, aragonite or both. Anterior and posterior adductor muscles attached to the shell extend and contract to permit the shell to open and close. In sedentary bivalves, the anterior adductor muscle is lost and only the posterior one remains. Bivalves possess specialized ganglia to coordinate parts of their body, such as the pedal ganglia, that coordinate the foot, and the visceral ganglia, located under the posterior adductor muscle. These ganglia are connected to the cerebropleural ganglia by nerve fibres. The sensory organs of bivalves are located in the posterior margins of the mantle and are usually mechanoreceptors or chemoreceptors, sometimes located in 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. Although many bivalves lack eyes, some others in a collection of clades have simple eyes in the margin of the mantle. Scallops have the most complex eyes with a lens, a two-layered retina and a concave mirror. With or without eyes, all bivalves have photosensitive cells that can detect shadow. Gas exhange in bivalves can occur in many ways, for example, some freshwater species may gape the shell slightly to exchange gases when exposed to air. Obtaining food varies in approach among bivalves, with the basal protobranchs, for example, holding onto the bottom with a pair of tentacles at the edge of the mouth, each of which with a single palp, trapping small food particles that are later consumed. Some more advanced bivalves may have two retractable siphons, one for exhalant and another for inhalant streams of water. Some bivalves can use these siphons to capture prey and bringing it towards the mouth. At least one unusual genus of extant bivalve lives as an endosymbiotic resident of the oesophagus of sea cucumbers. It uses byssal threads to attach itself to the host's throat, feeding on particles that pass through the oesophagus of the echinoderm. Advanced filter feeding bivalves have elongated rod of solidified mucus referred as the "crystaline style", and cilia cause the style to rotate, creating a stream of food-containing mucus that goes towards a sorting region that selects smaller particles into the digestive glands and heavier particles into the intestine. Bivalves have two nephridia, each consists of a long, looped, glandular tube, which opens into the pericardium, and a bladder to store urine. Reproduction in bivalves is very varied; a few species can even brood their young in the mantle cavity, eventually releasing them into the water column as veliger larvae or as juveniles. Other bivalves produce eggs that store a high energy content, so much that the larvae hatch without the need to eat, surviving on energy reserves. Many bivalve larvae permit a high proliferation of their species, as larvae remain planktonic for a longer time, establishing in places far away from their parents. Freshwater bivalves tend to have a different life cycle than marine ones. Larvae of freshwater bivalves attach themselves parasitically to the gills or fins of a fish host. Some unionid bivalves protrude the mantle out of the shell in a shape that resembles a fish, and some fish may see this protuberance as a potential prey or as a conspecific, drawing them close to bivalve, and allowing the bivalve to shoot the parasitic larvae to it.
Another group of aquatic animals that resemble bivalves are the brachiopods, which are of similar size, and also have shells that are divided in two hinged parts. Brachiopods were once very common in Earth's oceans, but now they're much less common, in comparison to the far more abundant bivalves, which, unlike brachiopods, are mollusks.
Bivalves emerged in the aftermath of the Cambrian explosion. Brachiopods would have also evolved around this time, and both groups of invertebrates left their impressions in the fossil record, in the form of their fossilized shells. Bivalve fossils are formed when the sediment that surrounds their shell is turned into solid rock. Despite bivalves being present in the Paleozoic fossil record, brachiopods remained the most abundant of both groups in the middle of the era, with over 12 000 species recognized in mid Paleozoic fossils. After the end of the Paleozoic, however, bivalves adopted a myriad of techniques to diversify further and occupy previously unoccupied niches. This shift from a brachiopod to a bivalve dominated world was believed to stem from a greater abbility of bivalves to colonize the vastness of aquatic niches. Bivalves outcompeting brachiopods was a thing that was heavily agreed upon, but currently the agreement lies more on just chance, and that both groups have been coexisting with each other and weren't really competing to a wide degree, and that upon the aftermath of extinction events, one group randomly diversified more than the other.
Bivalves are a successful aquatic group found nearly everywhere in the world that has sustainable bodies of water. In littoral areas they can be abundant to an incredible degree, with one acre being enough to harbour almost one and a half million individual bivalves. Some bivalves grow in polar sea ice, surviving in subzero temperatures, with incredibly slow growth rates. Some species can be found in the hadopelagic zone, over 7.5 kilometers deep in the ocean. Freshwater bivalves tend to have much more restricted ranges, however.
Most bivalves adopt a sedentary or sessile lifestyle, spending their whole lives in the place they settled since the juvenile stage. Some bivalves attach themselves to hard surfaces using tough byssus made of collagen and elastic proteins. Bivalves filter large amounts of water to feed and and breathe, but they're not always with their shell open. They often shut down their shell to rest, even when submerged. Tidal and circadian cycles can regulate the period of closure for bivalves. Typically the round and thick shell of bivalves makes them impervious to most predators. Despite that, several predators have indeed specialized to involve bivalves in their diet. Many demersal fish rely on bivalves to feed on. Some ornithodirans have specialized mouths to pry open bivalves. Marine mammals are known to have adapted extensively for a diet of bivalves. Invertebrates such as crustaceans, echinoderms and cephalopods also prey on bivalves. Crustaceans can crack their shells with their pincers, and starfish can use their water vascular system to force open their shells and insert their stomach between the valves to digest the mollusk's body. Curiously, it seems both crustaceans and starfish prefer bivalves that are attached by byssus threads than ones that are cemented to the substrate, likely because its easier to manipulate the bivalve in the former case. Some snails may bore bivalve shells with their radula and then suck the contents of the bivalve's soft body through their proboscis. To evade these predators, some bivalves use their eyes to see where their predators are, and shut their shells in a fast movement to move them through the water and swim away. Sometimes the siphons of a bivalve, if located in the exterior, can be snapped off by a predator, though these can perfectly regenerate with time. Other bivalves can produce noxious substances when stressed.
Humans raise and harvest bivalves for consumption. They can be farmed, with juveniles being reared in suspended rafts or cemented in ropes. The volumes of this rearing can be sustainable enough for large scale commerce. Severel breeding programmes have improved the quality of the stock that is placed in the market.
Several types of bivalves are eaten, either raw or cooked. However, because many bivalves are filter feeders, a lot if impurities go through them, include microbial pathogens, so they can be a health issue. Typhoid outbreaks have been linked before to the consumption of raw bivalves. The first report of such kind in the United States was in 1894. The Norwalk virus has also been transmitted through this medium of consumption, and has caused historical damages to oyster farming, for example. In the late 1980s, an outbreak of hepatitis A associated with the consumption of inadequately cooked clams took place in Shanghai, China, infecting nearly 300 thousand people, and killing 47 people. Paralytic shellfish poisoning (PSP) can also be caused by consuming bivalves that have a high concentration of toxic dinoflagellates. In the most severe cases, it can lead to paralysis and even death. Similarly, high concentrations of certain diatoms in raw bivalves can cause intoxication. Those diatoms have domoic acid, which can destroy brain cells causing memory loss, gastroenteritis, long-term neurological problems or death.
When living in polluted waters, bivalves can accumulate heavy metals and organic pollutants in their tissues. Because of that, bivalves are used as bioindicators of the health of an ecosystem, though they're not perfect in that task. Because their inabbility of proccessing some pollutants that get preserved in their tissues, they may be better indicators of past levels of pollution than present ones, for example. Despite that, they're still widely reliable. Crushed bivalve shells can remove heavy metals from the water by swapping the calcium in their aragonite for the heavy metal, containing the pollutants in solid form. Bivalves have been used to reduce the pollutant contents of contaminated water.
Humans have found bivalve shells to be rather fascinating for their overall beauty, selling them, purchasing them and using them as decorations. The extent of these decorations is very diverse. They can be used as embellishments, or cemented into pathways to decorate them. Bivalve shells have had ceremonial and even monetary uses too. The Winnebago Tribe from Wisconsin incorporated powdered freshwater bivalve shell into clay to temper into pottery vessels. They also used the shells as scoops for gouging out fired logs when building canoes and they drilled holes in them and fitted wooden handles for tilling the ground. Bivalve shells are also used as a complement in the diet of poultry. Bivalve shells have also been traditionally inlaid in furniture. Because of the necessity of harvesting mollusk shells and pearls, regulations are also reinforced into the trade of such. Pearls are also a valuable and very commercialized product, and those are produced by many bivalves. Although in the nature, finding pearls is a work of luck, currently pearls are easier to produce in bivalve farms, using specific substances to facilitate their formation. Bivalves are also a vehicle of symbolism, with the Roman goddess of love, Venus, often depicted rising from the sea, in a scallop shell. Because of that, many bivalve shells have turned into a symbol of fertility.
The taxonomy of bivalves has had its history, and many attempts have been given to employ a classification system that considers the shape of the shell, its microstructures and hinger configuration to decipher relationships. R.C. Moore, in 1952, divided bivalves in two groups, one of them being the Prionodesmacea, characterized for having a prismatic and nacreous shell structure, poorly developed siphons, and other characters, and the other was Teleodesmacea, described for having a porcelanous and partly nacreous shell structure. In 1935, Johannes Thiele introduced his own take on a mollusk taxonomy, based on the 1909 work by Cossmann and Peyrot. He divided bivalves in three orders: Taxodonta, Anisomyaria and Eulamellibranchiata. Taxonomies in the 21st century take into account molecular, anatomical, morphological, biogeographic, paleontological and stratigraphic data, that recognize a much clearer taxonomic picture of over 300 families, over 200 of those entirely extinct, and over 100 more that occur in the recent past.
main source: Wikipedia
Phylogeny
- Bivalvia
- Bositra buchii
- Pinna hartmanni
NAME: Bositra buchii
SIZE: 1 - 4 centimeters wide
DESCRIBED BY: Roemer, 1836
CLOSEST LIVING RELATIVES: Other pterioid bivalves
DEPICTED IN: The Life of a Temnodontosaurus episode 17
Bositra buchii is a species from the extinct Posidoniidae family. It is the type fossil of the Posidonia Shale. It was originally placed in their own genus, named Posidonia, hence why this formation is called that way. It was later realized that the genus should be considered a junior synonym of the genus Bositra. Despite this, the Posidonia Shale retains this name. The habitat and mode of life of these bivalves have been a matter of debate. It has been interpreted as a pseudoplanktonic organism, a benthic animal, an hybrid between both lifestyles, or even a chemosymbiotic dweller associated with the crinoid rafts. It was eventually concluded in a large study that these bivalves likely had a bottom dwelling lifestyle, aggregating in huge numbers that would result in their fossil abundance.
NAME: Pinna hartmanni
SIZE: Up to 90 centimeters tall
DESCRIBED BY: Zieten, 1833
CLOSEST LIVING RELATIVES: Modern Pinna species
DEPICTED IN: The Life of a Temnodontosaurus episode 17 and 20 and The Life of a Palaeopleurosaurus
Pinna hartmanni is an extinct member of the genus Pinna, which still survives in the modern day, and its the namesake of the Pinnidae family. Colloquially named pen shells, extant members of this genus were historically important for the harvest of sea silk.
Pinna is a temporally and geographically widespread genus. It dates back to the Carboniferous period, and survived the Permian-Triassic extinction, being well represented in the Jurassic and Cretaceous periods, surviving the K-Pg extinction event, and being found in our present day oceans.
Today 27 living species from the genus Pinna are known, and several other species are known to have gone extinct, including Pinna hartmanni from the early Jurassic seas of the Posidonia Shale.