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''Pecten maximus'',
common names In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contras ...
the great scallop, king scallop, St James shell or escallop, is a northeast Atlantic
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of
scallop Scallop () is a common name that encompasses various species of marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related famili ...
, an edible saltwater
clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...
, a marine
bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
in the family
Pectinidae Scallop () is a common name that encompasses various species of Marine (ocean), marine bivalve mollusc, molluscs in the Taxonomy (biology), taxonomic Family (biology), family Pectinidae, the scallops. However, the common name "scallop" is also ...
. This is the type species of the genus. This species may be
conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organism ...
with '' Pecten jacobaeus'', the pilgrim's scallop, which has a much more restricted distribution.


Description

The shell of ''Pecten maximus'' is quite robust and is characterised by having "ears" of equal size on either side of the apex. The right, or lower, valve is convex and slightly overlaps the flat left, or upper, valve. Larger specimens have a nearly circular outline and the largest may measure 21 cm in length. The "ears" are prominent and are a minimum of half the width of the shell, with the byssal notch situated in the right anterior ear being slight and not serrated. The sculpture of the valves is distinctive and consists of 12 to 17 wide radiating ribs and numerous concentric lines, which clearly show the scallop's growth history, while the "ears" show a few thin ribs, which radiate from the beaks. The radiating ribs reach the margins of the valves and this creates a crenulated form. The left valve is normally reddish-brown whilst the right valve varies from white through cream to shades of pale brown contrasting with pink, red or pale yellow tints; either valve may show zigzag patterns and may also show bands and spots of red, pink or bright yellow. The colour of the body of ''Pecten maximus'' is pink or red with the mantle marbled brown and white. When young they are attached to the substrate by a
byssus A byssus () is a bundle of filaments secreted by many species of bivalve mollusc that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including pen shells ( Pinnidae), true mussels (Mytili ...
but mature animals are capable of swimming by the opening and rapid closing of the valves. The adductor muscle which is used to close and open the valves is very large and powerful. The foot is a finger-like organ, which spins the byssal threads, which pass through the byssal notch on the ears. The margin of the mantle has two layers: the inner layer is finely fringed whilst on the outer is lined with long tentacles with two series totalling 30–36 dark blue or green simple eyes or ocelli in two rows at their base.
File:Pecten maximus 01.jpg, Right valve File:Pecten maximus 02.jpg, Left valve
Forma ''maculata'' File:Pecten maximus 03.jpg, Right valve File:Pecten maximus 04.jpg, Left valve


Distribution

''Pecten maximus'' occurs in the eastern Atlantic along the European coast from northern
Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...
, south to the
Iberian Peninsula The Iberian Peninsula ( ), also known as Iberia, is a peninsula in south-western Europe. Mostly separated from the rest of the European landmass by the Pyrenees, it includes the territories of peninsular Spain and Continental Portugal, comprisin ...
, it has also been reported off West Africa, off the
Macaronesia Macaronesia (; ) is a collection of four volcanic archipelagos in the Atlantic Ocean#North Atlantic, North Atlantic Ocean, off the coast of North Africa and Europe. Each archipelago is made up of a number of list of islands in the Atlantic Oc ...
n Islands. In Great Britain and Ireland it is distributed all round the coast but it is uncommon and localised on the eastern
North Sea The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...
coast. It prefers offshore waters down to depth.


Biology

''Pecten maximus'' frequently creates a slight hollow in the substrate to lie in by opening and closing for its
shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...
s to eject water from the mantle cavity, which raises the shell at an angle to the substrate so that subsequent water jets into the sediment and create a recess. Once settled, sand, mud, gravel or living organisms coat the upper valve and the margin of the shell, with the tentacles and eyes, is all that is visible. They are
filter feeder Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a s ...
s, which extract particles from the surrounding water via a feeding current, which is drawn by
cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
across the gills, where the food particles are trapped then taken to the mouth in a stream of mucous. ''Pecten maximus'' swims but this is generally limited to escape reactions. The main predators which cause this reaction when detected are the mollusc eating
starfish Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...
'' Asterias rubens'' and '' Astropecten irregularis'', although starfish which do not feed on molluscs can cause limited jumping or valve-closing reactions. The swimming action is performed by rapidly clapping the valves and expelling jets of water from each side of the hinge so that it moves with the curved edge of the shell at the front. The scallop jumps forwards by gradually relaxing the adductor muscle and then rapidly opening and closing the valves. ''Pecten maximus'' tends to be more numerous in areas where they are not fully exposed to strong currents. Scallops which live in sheltered habitats grow faster than scallops in areas exposed to wave action, possibly due to the filter feeding apparatus being unable to function because of high concentrations of particulate matter in the water in areas subject to high levels of wave exposure. Another factor that may be significant is that the processes of larval settlement and byssal attachment are rather delicate and would be disturbed in strong currents. Abundance and growth rates are negatively correlated with the amount of mud in the substrate. Scallops use larval motility to distribute themselves, as the adult scallops have limited mobility. The distribution of the larvae is affected by factors such as local hydrographic regimes and their survival, and this results in the scallops having an aggregated distribution within their geographic range. This means that the major fishing grounds are normally widely separated and each fishing ground's environmental conditions mean there are marked differences in structures of the populations, although the genetics of scallops are rather uniform across its range. The reproductive cycle of ''Pecten maximus'' is extremely variable and the spawning may be influenced by both internal and external factors such as age and temperature respectively but is also influenced by genetic adaptation. Generally, mature scallops spawn over the summer months starting in April or May and lasting to September. They are hermaphroditic and have distinct tongue-shaped gonads which are red or orange in colour for the female gonad and white for the male. It is estimated that a three-year-old scallop releases 15–21 million oocytes per emission. There appear to be two spawnings in many parts of the range, normally there is a partial one in the Spring and a full one in late August, however younger scallops have a single spawning event in the late summer. In some areas this pattern is reversed and the major spawning is in the Spring. After spawning the animals undergo period of recovery of the gonad before they spawn again. Fertilization of the gametes is external and either
sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...
or
oocyte An oocyte (, oöcyte, or ovocyte) is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ger ...
s can be released into the water column first. Since the larval stage of ''Pecten maximus'' is relatively long, up to a month, the potential for dispersal is quite high, even smaller adults can use the
byssus A byssus () is a bundle of filaments secreted by many species of bivalve mollusc that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including pen shells ( Pinnidae), true mussels (Mytili ...
to drift too. However, in at least some populations, genetic studies show that there is little contribution from more distant populations and that these populations probably sustain themselves. In waters around the United Kingdom ''Pecten maximus'' become sexually mature at around 2–3 years old and when they reach 80 to 90 mm in shell length. Where they are not exploited, they may live for more than 20 years and reach shell lengths of more than 200mm. Scallops in shallow water grow faster than those in deeper water; the growth halts in winter and starts again in spring, producing concentric growth rings which are used to age the scallops.


Genomics

A draft
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
is presented by Kenny et al. 2020. Their assembly is 918Mb, and they estimate a
genome size Genome size is the total amount of DNA contained within one copy of a single complete genome. It is typically measured in terms of mass in picograms (trillionths or 10−12 of a gram, abbreviated pg) or less frequently in daltons, or as the tot ...
of 1,150Mb, 1.7%
heterozygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...
, with 27.0% being repeats, in total coding 67,741 genes. Recent () improvements in read length helped Kenny to resolve questions of
copy number variation Copy number variation (CNV) is a phenomenon in which sections of the genome are repeated and the number of repeats in the genome varies between individuals. Copy number variation is a type of structural variation: specifically, it is a type of ...
in ''P. maximus'' which were previously indecipherable.


Predators and diseases

As well as the
starfish Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...
species '' Asterias rubens'' and '' Astropecten irregularis'', major predators of ''Pecten maximus'' are crabs such as '' Cancer pagurus'', ''
Carcinus maenas ''Carcinus maenas'' is a common littoral crab. It is known by different names around the world. In the British Isles, it is generally referred to as the shore crab or green shore crab. In North America and South Africa, it bears the name Europe ...
'', '' Liocarcinus depurator'' and '' Necora puber'', which will prey on the scallops as they grow. The anemone '' Anthopleura ballii'' was found preying on young specimens of ''P.maximus'' in south-western Ireland. The larvae of ''Pecten maximus'' are attacked by the bacterium '' Vibrio pectenicida'', which was described in 1998 as a new species after incidents of mortality among cultured scallops in France in the early 1990s. Other strains of pathogenic bacteria were detected in Norway following mass mortality of larvae in culture.


Fisheries and aquaculture

In 1999 the total catch reported by the United Nations Food and Agriculture Organisation was 35,411 tonnes with the two biggest catches being reported from the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...
and
France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...
which landed 19,108 tonnes and 12,745 tonnes respectively. It is believed that some natural stocks are showing indications of over exploitation resulting in strict enforcement of fisheries legislation and by the development of stock enhancement practices. Great scallops are fished for using Newhaven scallop dredges, and less than 5% is gathered by hand by divers. In total the scallop fisheries for ''P. maximus'' and for the Queen scallop '' Aequipecten opercularis'' are one of the top five fisheries by value in United Kingdom waters. However, the use of towed great to harvest scallops causes damage to the wider ecosystem. ''Pecten maximus'' can be cultivated in
aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. Nelu ...
and this is reasonably advanced in France and Norway. Spain, France, Ireland, the United Kingdom and Norway have been involved in the aquaculture of scallops; production peaked in 1998 when 512 tonnes were landed but production later decreased, with only 213 tonnes landed in 2004, having an estimated value of €852 000, equivalent to €4 per kilogramme. ''Pecten maximus'' has been found to contain
domoic acid Domoic acid (DA) is a kainic acid-type neurotoxin that causes amnesic shellfish poisoning (ASP). It is produced by algae and accumulates in shellfish, sardines, and anchovies. When sea lions, otters, cetaceans, humans, and other predators eat cont ...
, a toxin that can cause Amnesic Shellfish Poisoning. The risk associated with scallop consumption is regarded as a significant threat to both public health and the shellfish industry.


Cultural significance

The oil company
Shell plc Shell plc is a British Multinational corporation, multinational petroleum, oil and natural gas, gas company, headquartered in London, England. Shell is a public limited company with a primary listing on the London Stock Exchange (LSE) and second ...
derives its highly recognizable logo from this species. Pilgrims travelling to the town of
Santiago de Compostella Santiago de Compostela, simply Santiago, or Compostela, in the province of A Coruña, is the capital of the autonomous community of Galicia, in northwestern Spain. The city has its origin in the shrine of Saint James the Great, now the Cathedra ...
in Galicia took the shells of scallop with them, in honor of Saint James. This gave rise to the alternative name St James shell. Michel Callon, a sociologist, used a case study of scallop fishing in St Brieuc Bay in France to illustrate how the sociology of translation can be applied to understand the dynamics of science and technology. This study then became a seminal work of actor-network theory (ANT).


See also

* Symbolism of scallop shells


References


External links


A live individual, upper shell covered in sediment


{{Authority control Pectinidae Molluscs of the Atlantic Ocean Molluscs described in 1758 Taxa named by Carl Linnaeus