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Scallop
Scallop
from Wikipedia

Scallop
Temporal range: Middle Triassic-present
Argopecten irradians, the Atlantic bay scallop
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Pectinida
Superfamily: Pectinoidea
Family: Pectinidae
Wilkes, 1810
Genera

See text

Synonyms[citation needed]

Pectenidae

Scallop (/ˈskɒləp, ˈskæl-/)[a] is a common name that encompasses various species of marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. However, this common name is also sometimes applied to species in other closely related families within the superfamily Pectinoidea, which also includes the thorny oysters.

Scallops are a cosmopolitan family of bivalves found in all of the world's oceans, although never in fresh water. They are one of the very few groups of bivalves to be primarily "free-living", with many species capable of rapidly swimming short distances and even migrating some distance across the ocean floor. A small minority of scallop species live cemented to rocky substrates as adults, while others attach themselves to stationary or rooted objects such as seagrass at some point in their lives by means of a filament they secrete called a byssal thread.

The majority of species, however, live recumbent on sandy substrates, and when they sense the presence of a predator such as a starfish, they may attempt to escape by swimming swiftly but erratically through the water using jet propulsion created by repeatedly clapping their shells together. Scallops have a well-developed nervous system, and unlike most other bivalves all scallops have a ring of numerous simple eyes situated around the edge of their mantles.

Many species of scallops are highly prized as a food source, and some are farmed as aquaculture. The word "scallop" is also applied to the meat of these bivalves, the adductor muscle, that is sold as seafood. The brightly coloured, symmetrical, fan-shaped shells of scallops with their radiating and often fluted ornamentation are valued by shell collectors, and have been used since ancient times as motifs in art, architecture, and design.

Owing to their widespread distribution, scallop shells are a common sight on beaches and are often brightly coloured, making them a popular object to collect among beachcombers and vacationers.[3] The shells also have a significant place in popular culture.

Etymology

[edit]

The family name Pectinidae, which is based on the name of the type genus, Pecten, comes from the Latin pecten meaning comb, in reference to a comb-like structure of the shell which is situated next to the byssal notch.[4]

Description

[edit]

Very little variation occurs in the internal arrangement of organs and systems within the scallop family, and what follows can be taken to apply to the anatomy of any given scallop species.[5][additional citation(s) needed]

Orientation

[edit]
Anatomical diagram of an Atlantic bay scallop with the left (i.e., upper) valve removed; anterior is to the left, posterior to the right

The shell of a scallop consists of two sides or valves, a left valve and a right one, divided by a plane of symmetry.[6] Most species of scallops rest on their right valve, and consequently, this valve is often deeper and more rounded than the left (i.e., upper) valve, which in many species is actually concave. With the hinge of the two valves oriented towards the top, one side corresponds to the animal's morphological anterior or front, the other is the posterior or rear, the hinge is the dorsal or back/top region, and the bottom corresponds to the ventral or (as it were) underside/belly.[7] However, as many scallop shells are more or less bilaterally symmetrical ("equivalved"), as well as symmetrical front/back ("equilateral"), determining which way a given animal is "facing" requires detailed information about its valves.[citation needed]

Valves

[edit]
Diagram of a scallop with two differently sized valves shown positioned in ocean floor sediment: the right valve (shown at the bottom) much deeper than the left, allowing the scallop to appear less visible to predators

The model scallop shell consists of two similarly shaped valves with a straight hinge line along the top, devoid of teeth, and producing a pair of flat wings or "ears" (sometimes called "auricles", though this is also the term for two chambers in its heart) on either side of its midpoint, a feature which is unique to and apparent in all adult scallops.[8] These ears may be of similar size and shape, or the anterior ear may be somewhat larger (the posterior ear is never larger than the anterior one, an important feature for distinguishing which valve is which). As is the case in almost all bivalves, a series of lines and/or growth rings originates at the center of the hinge, at a spot called the "beak" surrounded by a generally raised area called the "umbo". These growth rings increase in size downwards until they reach the curved ventral edge of the shell. The shells of most scallops are streamlined to facilitate ease of movement during swimming at some point in their lifecycles, while also providing protection from predators. Scallops with ridged valves have the advantage of the architectural strength provided by these ridges called "ribs", although the ribs are somewhat costly in weight and mass. A unique feature of the scallop family is the presence, at some point during the animal's lifecycle, of a distinctive and taxonomically important shell feature, a comb-like structure called a ctenolium located on the anterior edge of the right valve next to the valve's byssal notch. Though many scallops lose this feature as they become free-swimming adults, all scallops have a ctenolium at some point during their lives, and no other bivalve has an analogous shell feature. The ctenolium is found in modern scallops only; both putative ancestors of modern scallops, the entoliids and the Aviculopectinidae, did not possess it.[9]

Muscular system

[edit]
A live opened scallop showing the internal anatomy: The pale orange circular part is the adductor muscle; the darker orange curved part is the "coral", a culinary term for the ovary or roe.

Like the true oysters (family Ostreidae), scallops have a single central adductor muscle, thus, the inside of their shells has a characteristic central scar, marking the point of attachment for this muscle. The adductor muscle of scallops is larger and more developed than those of oysters, because scallops are active swimmers; some species of scallops are known to move en masse from one area to another. In scallops, the shell shape tends to be highly regular, and is commonly used as an archetypal form of a seashell.[7]

Adductor muscles

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Adductor muscles of a scallop. a: photo in a live scallop. b: after removal of organs and tissues.

Scallops possess fast (striated) and slow (smooth) adductor muscles, which have different structures and contractile properties. These muscles lie closely apposed to one another but are divided by a connective tissue sheet. The striated adductor muscle contracts very quickly for swimming, whereas the smooth catch adductor muscle lacks striations, and contracts for long periods, keeping shells closed with little expenditure of energy.[10]

Digestive system

[edit]

Scallops are filter feeders, and eat plankton. Unlike many other bivalves, they lack siphons. Water moves over a filtering structure, where food particles become trapped in mucus. Next, the cilia on the structure move the food toward the mouth. Then, the food is digested in the digestive gland, an organ sometimes misleadingly referred to as the "liver, " which envelops part of the oesophagus, intestine, and entire stomach. Waste is passed on through the intestine (the terminus of which, like that of many molluscs, enters and leaves the animal's heart) and exits via the anus.[7]: p.20 

Nervous system

[edit]
Neural map of a giant scallop

Like all bivalves, scallops lack actual brains. Instead, their nervous system is controlled by three paired ganglia located at various points throughout their anatomy, the cerebral or cerebropleural ganglia, the pedal ganglia, and the visceral or parietovisceral ganglia. All are yellowish. The visceral ganglia are by far the largest and most extensive of the three, and occur as an almost-fused mass near the center of the animal – proportionally, these are the largest and most intricate sets of ganglia of any modern bivalve. From this radiate all of the nerves which connect the visceral ganglia to the circumpallial nerve ring which loops around the mantle and connects to all of the scallop's tentacles and eyes. This nerve ring is so well developed that, in some species, it may be legitimately considered an additional ganglion.[7]: p.46  The visceral ganglia are also the origin of the branchial nerves which control the scallop's gills. The cerebral ganglia are the next-largest set of ganglia and lie distinct from each other a significant distance dorsal to the visceral ganglia. They are attached to the visceral ganglia by long cerebral-visceral connectives, and to each other via a cerebral commissure that extends in an arch dorsally around the esophagus. The cerebral ganglia control the scallop's mouth via the palp nerves and connect to statocysts which help the animal sense its position in the surrounding environment. They are connected to the pedal ganglia by short cerebral-pedal connectives. The pedal ganglia, though not fused, are situated very close to each other near the midline. From the pedal ganglia, the scallop puts out pedal nerves which control the movement of, and sensation in, its small muscular foot.[7]: pp. 43–47 

Vision

[edit]
Closeups of the iridescent blue eyes of the bay scallop, Argopecten irradians[11]

Scallops have a large number (up to 200) of small (about 1 mm) eyes arranged along the edge of their mantles. These eyes represent a particular innovation among molluscs, relying on a concave, parabolic mirror of guanine crystals to focus and retro-reflect light instead of a lens as found in many other eye types.[12] Additionally, their eyes possess a double-layered retina, the outer retina responding most strongly to light and the inner to abrupt darkness.[13] While these eyes are unable to resolve shapes with high fidelity, the combined sensitivity of both retinas to light entering the eye and light retro-reflected from the mirror grants scallops exceptional contrast definition, as well as the ability to detect changing patterns of light and motion.[14][15] Scallops primarily rely on their eyes as an 'early-warning' threat detection system, scanning around them for movement and shadows which could potentially indicate predators. Additionally, some scallops alter their swimming or feeding behaviour based on the turbidity or clarity of the water, by detecting the movement of particulate matter in the water column.[16]

Distribution and habitat

[edit]

Scallops inhabit all the oceans of the world, with the largest number of species living in the Indo-Pacific region. Most species live in relatively shallow waters from the low tide line to 100 m, while others prefer much deeper water. Although some species only live in very narrow environments, most are opportunistic and can live under a wide variety of conditions. Scallops can be found living within, upon, or under either rocks, coral, rubble, sea grass, kelp, sand, or mud. Most scallops begin their lives as byssally attached juveniles, an ability that some retain throughout their lives while others grow into free-living adults.[17]

Biology

[edit]

Locomotion

[edit]
Overhead view of a scallop engaged in a zig-zag swimming motion
Overhead view of a scallop engaged in a unidirectional jumping motion

Scallops are mostly free-living and active, unlike the vast majority of bivalves, which are mostly slow-moving and infaunal. All scallops are thought to start out with a byssus, which attaches them to some form of substrate such as eelgrass when they are very young. Most species lose the byssus as they grow larger. A very few species go on to cement themselves to a hard substrate (e.g. Chlamys distorta and Hinnites multirigosus).[18]

However, the majority of scallops are free-living and can swim with brief bursts of speed to escape predators (mostly starfish) by rapidly opening and closing their valves. Indeed, everything about their characteristic shell shape – its symmetry, narrowness, smooth and/ or grooved surface, small flexible hinge, powerful adductor muscle, and continuous and uniformly curved edge – facilitates such activity. They often do this in spurts of several seconds before closing the shell entirely and sinking back to the bottom of their environment. Scallops are able to move through the water column either forward/ventrally (termed swimming) by sucking water in through the space between their valves, an area called the gape, and ejecting it through small holes near the hinge line called exhalant apertures, or backward/dorsally (termed jumping) by ejecting the water out the same way it came in (i.e. ventrally). A jumping scallop usually lands on the sea floor between each contraction of its valves, whereas a swimming scallop stays in the water column for most or all of its contractions and travels a much greater distance (though seldom at a height of more than 1 m off the sea bed and seldom for a distance of greater than 5 m).[18] Both jumping and swimming movements are very energy-intensive, and most scallops cannot perform more than four or five in a row before becoming completely exhausted and requiring several hours of rest. Should a swimming scallop land on its left side, it is capable of flipping itself over to its right side via a similar shell-clapping movement called the righting reflex. So-called singing scallops are rumored to make an audible, soft popping sound as they flap their shells underwater (though whether or not this happens is open to some debate).[19] Other scallops can extend their foot from between their valves, and by contracting the muscles in their foot, they can burrow into sand.[20]

Mobility and behavior

[edit]
A scallop swims by slowly opening its two halves, then quickly closing them. This is a successful swimming method because the inertial force of the surrounding water dominates over the viscous force. As demonstrated by the scallop theorem, in a low Reynolds number environment, a scallop would only oscillate in place.[21]

Most species of the scallop family are free-living, active swimmers, propelling themselves through the water through the adductor muscles to open and close their shells. Swimming occurs through the clapping of valves for water intake. Closing the valves propels water with a strong force near the hinge via the velum, a curtain-like fold of the mantle that directs water expulsion around the hinge. Scallops swim in the direction of the valve opening unless the velum directs an abrupt change in course direction.[22][23]

Other species of scallops can be found on the ocean floor attached to objects by byssal threads. Byssal threads are strong, silky fibers extending from the muscular foot, used to attach to a firm support, such as a rock. Some can also be found on the ocean floor, moving with an extendable foot between their valves or burrowing themselves in the sand by extending and retracting their feet.[7] Scallops are highly sensitive to shadows, vibrations, water movement, and chemical stimuli.[24] All possess a series of 100 blue eyes, embedded on the edge of the mantle of their upper and lower valves that can distinguish between light and darkness. They serve as a vital defense mechanism for avoiding predators. Though rather weak, their series of eyes can detect surrounding movement and alert precaution in the presence of predators, most commonly sea stars, crabs, and snails.[7] Physiological fitness and exercise of scallops decrease with age due to the decline of cellular and especially mitochondrial function,[25] thus increasing the risk of capture and lowering rates of survival. Older individuals show lower mitochondrial volume density and aerobic capacity, as well as decreased anaerobic capacity construed from the amount of glycogen stored in muscle tissue.[25] Environmental factors, such as changes in oxidative stress parameters, can inhibit the growth and development of scallops.[26]

Seasonal changes in temperature and food availability have been shown to affect muscle metabolic capabilities. The properties of mitochondria from the phasic adductor muscle of Euvola ziczac varied significantly during their annual reproductive cycle. Summer scallops in May have lower maximal oxidative capacities and substrate oxidation than any other time in the year. This phenomenon is due to lower protein levels in adductor muscles.[27]

Scallops are known to be infected by viruses, bacteria, microalgae of the heterokonts and dinoflagellates.[28] : 71 

Mutualism

[edit]

Some scallops, including Chlamys hastata, frequently carry epibionts such as sponges and barnacles on their shells. The relationship of the sponge to the scallop is characterized as a form of mutualism, because the sponge provides protection by interfering with adhesion of predatory sea-star tube feet,[29][30][31] camouflages Chlamys hastata from predators,[30] or forms a physical barrier around byssal openings to prevent sea stars from inserting their digestive membranes.[31] Sponge encrustation protects C. hastata from barnacle larvae settlement, serving as a protection from epibionts that increase susceptibility to predators. Thus, barnacle larvae settlement occurs more frequently on sponge-free shells than on sponge-encrusted shells.[29]

Barnacle encrustation negatively influences swimming in C. hastata. Those swimming with barnacle encrustation require more energy and show a detectable difference in anaerobic energy expenditure than those without encrustation. In the absence of barnacle encrustation, individual scallops swim significantly longer, travel further, and attain greater elevation.[32]

Lifecycle and growth

[edit]
Life cycle of the giant Yesso scallop found north of Japan [33]

The scallop family is unusual in that some members of the family are dioecious (males and females are separate), while others are simultaneous hermaphrodites (both sexes in the same individual), and a few are protoandrous hermaphrodites (males when young then switching to female). Female scallops have red roe and male scallops have white roe, corresponding to ova and spermatozoa respectively, which are released freely into the water during mating season and fertilized ova sink to the bottom. After several weeks, the immature scallops hatch and the larvae, miniature transparent versions of the adults called "spat", drift in the plankton until settling to the bottom again (an event called spatfall) to grow, usually attaching by means of byssal threads. Some scallops, such as the Atlantic bay scallop Argopecten irradians, are short-lived, while others can live 20 years or more. Age can often be inferred from annuli, the concentric rings of their shells.[7]

Many scallops are hermaphrodites (having female and male organs simultaneously), altering their sex throughout their lives, while others exist as dioecious species, having a definite sex. In this case, males are distinguished by roe-containing white testes and females with roe-containing orange ovaries. At the age of two, they usually become sexually active, but do not contribute significantly to egg production until four. The reproduction process occurs externally through spawning, in which eggs and sperm are released into the water. Spawning typically occurs in late summer and early autumn; spring spawning may also take place in the Mid-Atlantic Bight.[34] The females of scallops are highly fecund, capable of producing hundreds of millions of eggs per year.[34]

Once an egg is fertilized, it is then planktonic, a collection of microorganisms that drift abundantly in fresh or salt water. Larvae stay in the water column for four to seven weeks before dissipating to the ocean floor, where they attach themselves to objects through byssus threads. Byssus is eventually lost with adulthood, transitioning almost all scallop species into free swimmers. Rapid growth occurs within the first several years, with an increase of 50–80 % in shell height and quadrupled size in meat weight, and reaches a commercial size at about four to five years of age.[34] The lifespans of some scallops have been known to extend over 20 years.[35]

Phylogeny

[edit]
Fossil scallop Chlamys with encrusters; Nicosia Formation (Pliocene) of Cyprus

The fossil record of scallops is rich in both species and specimens. The earliest known records of true scallops (those with a ctenolium) can be found from the Triassic period, over 200 million years ago.[9] The earliest species were divided into two groups, one with a nearly smooth exterior: Pleuronectis von Schlotheim, 1820, while the other had radial ribs or riblets and auricles: Praechlamys Allasinaz, 1972.[36] Fossil records also indicate that the abundance of species within the Pectinidae has varied greatly over time; Pectinidae was the most diverse bivalve family in the Mesozoic era, but the group almost disappeared completely by the end of the Cretaceous period. The survivors speciated rapidly during the Tertiary period. Nearly 7,000 species and subspecies names have been introduced for both fossil and recent Pectinidae.[37]

The cladogram is based on molecular phylogeny using mitochondrial (12S, 16S) and nuclear (18S, 28S, and H3) gene markers by Yaron Malkowsky and Annette Klussmann-Kolb in 2012.[38]

Pteriomorphia
(c. 247 mya) Pectinidae

(70 mya) Pecten

Limidae (file shells)

other Pteriomorphia (oysters, mussels)

Taxonomic structure

[edit]
Bractechlamys vexillum
Pecten tigris Lamarck, 1819, museum specimens

Scallops are the family Pectinidae, marine bivalve molluscs within the superfamily Pectinoidea. Other families within this same superfamily share a somewhat similar overall shell shape, and some species within some of the related families are also commonly referred to as "scallops" (for example, Propeamussiidae, the glass scallops).

The family Pectinidae is the most diversified of the pectinoideans in present-day oceans. It is one of the largest marine bivalve families and contains over 300 extant species in 60 genera. Its origin dates back to the Middle Triassic Period, approximately 240 million years ago;[9] in terms of diversity, it has been a thriving family to the present day.[39]

Evolution from its origin has resulted in a successful and diverse group: pectinids are present in the world's seas, found in environments ranging from the intertidal zone to the hadal depths. The Pectinidae play an extremely important role in many benthic communities and exhibit a wide range of shell shapes, sizes, sculptures, and cultures.[40]

Raines and Poppe[b] listed nearly 900 species names of scallops, but most of these are considered either questionable or invalid. Raines and Poppe mentioned over 50 genera, around 250 species, and subspecies. Although species are generally well-circumscribed, their attribution to subfamilies and genera is sometimes equivocal, and information about phylogeny and relationships of the species are minimal, not the least because most work has been based only on adult morphology.[42]

This family's earliest and most comprehensive taxonomic treatments were based on macroscopic morphological characters of the adult shells and represent broadly divergent classification schemes.[43][28] Some level of taxonomic stability was achieved when Waller's studies in 1986, 1991, and 1993 concluded evolutionary relationships between pectinid taxa based on hypothesized morphological synapomorphies, which previous classification systems of Pectinidae failed to do. He created three Pectinidae subfamilies: Camptonectinidae, Chlamydinae and Pectininae.[44][45][46]

The framework of its phylogeny shows that repeated life habit states derive from evolutionary convergence and parallelism.[47][48] Studies have determined the family Pectinidae is monophyletic, developing from a single common ancestor. The direct ancestors of Pectinidae were scallop-like bivalves of the family Entoliidae.[49] Entoliids had auricles and a byssal notch only at youth, but they did not have a ctenolium, a comb-like arrangement along the margins of the byssal notch in Pectinidae. The ctenolium is the defining feature of the modern family Pectinidae and is a characteristic that has evolved within the lineage.[50]

In a 2008 paper, Puslednik et al. identified considerable convergence of shell morphology in a subset species of gliding Pectinidae, which suggests iterative morphological evolution may be more prevalent in the family than previously believed.[51]

There have been a number of efforts to address phylogenetic studies. Only three have assessed more than ten species[52][53][54] and only one has included multiple outgroups.[53] Nearly all previous molecular analyses of the Pectinidae have only utilized mitochondrial data. Phylogenies based only on mitochondrial sequence data do not always provide an accurate estimation on the species tree. Complicated factors can arise due to the presence of genetic polymorphisms in ancestral species and resultant lineage sorting.[55][56]

In molecular phylogenies of the Bivalvia, both the Spondylidae and the Propeamussiidae have been resolved as sister to the Pectinidae.[53][57]

List of subfamilies and genera

[edit]

The following are recognised in the family Pectinidae:

Relation to humans

[edit]

Scalloped shape

[edit]
Scalloped edges of a porcelain basket

The term "scalloped" is used to designate a decorative pattern, resembling the wavy scallop surface, that is used at the edges of furniture, fabrics, and other items.[59]

Fisheries

[edit]
Pearl nets used to grow spat to juveniles in scallop aquaculture

The largest wild scallop fishery is for the Atlantic sea scallop (Placopecten magellanicus) found off the northeastern United States and eastern Canada. Scallops are harvested using scallop dredges or bottom trawls. Most of the rest of the world's production of scallops is from Japan (wild, enhanced, and aquaculture) and China (mostly cultured Atlantic bay scallops).[60]: p.661 

In the D'Entrecasteaux Channel in the south of Tasmania dredging was banned in 1969, and since then divers have caught them in this area.[61] Attempts to use lighted pots to attract lobster and crab led to the discovery that they were effective in attracting scallops.[62]

Sustainability

[edit]

The scallop fishery in New Zealand declined from a catch of 1246 tonnes in 1975 to 41 tonnes in 1980, at which point the government ordered the fishery closed. Spat seeding in the 1980s helped it recover, and catches in the 1990s were up to 684 tonnes.[63] The Tasman Bay area was closed to commercial scallop harvesting from 2009 to 2011 due to a decline in the numbers. The commercial catch was down to 22 tonnes in 2015, and the fishery was closed again. The main causes for the decline seem to be fishing, climate effects, disease, pollutants, and sediment runoff from farming and forestry.[63] Forest and Bird list scallops as the "Worst Choice" in their Best Fish Guide for sustainable seafood species.[64][better source needed]

On the east coast of the United States, over the last 100 years, the populations of bay scallops have greatly diminished due to several factors but probably mostly due to a reduction in seagrasses (to which bay scallop spat attach) caused by increased coastal development and concomitant nutrient runoff. Another possible factor is the reduction of sharks from overfishing. A variety of sharks used to feed on rays, which are the main predator of bay scallops. With the shark population reduced – this apex predator in some places almost eliminated – the rays have been free to feed on scallops to greatly decrease their numbers.[65] By contrast, the Atlantic sea scallop (Placopecten magellanicus) is at historically high levels of abundance after recovery from overfishing.[66]

As food

[edit]
Scallops browning in clarified butter in a very hot pan

Scallops are characterized by offering two flavors and textures in one shell: the meat, called "scallop", which is firm and white, and the roe, called "coral", which is soft and often brightly coloured reddish-orange. Sometimes, markets sell scallops already prepared in the shell, with only the meat remaining. Outside the U.S., the scallop is often sold whole. They are available both with and without coral in the UK and Australia.[67]

Scallops without any additives are called "dry-packed", while scallops that are treated with sodium tripolyphosphate (STPP) are called "wet-packed". STPP causes the scallops to absorb moisture prior to the freezing process, thereby increasing their weight. The freezing process takes about two days.[68][better source needed]

In French cuisine, scallops are often quickly cooked in a hot buttered pan, sometimes with calvados and served with creamed leeks, or prepared in a white wine sauce. In Galician cuisine, scallops are baked with breadcrumbs, ham, and onions.[citation needed]

Scallops are sometimes breaded, deep-fried, and served with coleslaw and french fries in the northeastern United States (either on their own or as part of a fisherman's platter).[69] In New England, some seafood restaurants offer scallop rolls, consisting of breaded scallops on a grilled, split-top hot dog bun.[70]

In Japanese cuisine, scallops may be served in soup or prepared as sashimi or sushi.[citation needed] In a sushi bar, hotategai (帆立貝, 海扇) is the traditional scallop on rice and, while kaibashira (貝柱) is more loosely used to include other shellfish species with round-shaped flesh (the adductor muscle), such as Atrina pectinata (帶子).

Dried scallop is known in Cantonese Chinese cuisine as conpoy (乾瑤柱, 乾貝, 干貝).[citation needed] Smoked scallops are sometimes served as appetizers or used as an ingredient in the preparation of various dishes and appetizers.[71]

Scallops have lent their name to the culinary term "scalloped", which originally referred to seafood creamed and served hot in the shell.[72] Today, it means a creamed casserole dish such as scalloped potatoes, which contains no seafood at all.

Pearls

[edit]
A scallop pearl

Scallops do occasionally produce pearls, though scallop pearls do not have the buildup of translucent layers or "nacre" which give desirability to the pearls of the feather oysters, and usually lack both lustre and iridescence. They can be dull, small, and of varying colour, but exceptions occur that are appreciated for their aesthetic qualities.[73]

Symbolism

[edit]

The Shell company, one of the world's biggest companies, is represented by a scallop.

A scallop shell in a German coat of arms

The scallop shell symbol found its way into heraldry as a badge of those who had been on the pilgrimage to Compostela, although later, it became a symbol of pilgrimage in general. Sir Winston Churchill and Lady Diana's family, the Spencer family coat of arms includes a scallop, as well as both of Diana's sons Prince William and Prince Harry's personal coats of arms; also Pope Benedict XVI's personal coat of arms includes a scallop; another example is the surname Wilmot and also John Wesley's (which as a result the scallop shell is used as an emblem of Methodism). However, charges in heraldry do not always have an unvarying symbolic meaning, and there are cases of arms in which no family member went on a pilgrimage, and the occurrence of the scallop is simply a pun on the name of the armiger (as in the case of Jacques Coeur), or for other reasons.[74] In 1988, the State of New York in the US chose the bay scallop (Argopecten irradians) as its state shell.[75]

Shell of Saint James

[edit]
Left: Saint James by Carlo Crivelli, c. 1480

 

Right: Shield with symbol of St. James the Great, Church of the Good Shepherd (Rosemont, Pennsylvania)

The scallop shell is the traditional emblem of St James the Great and is popular with pilgrims travelling the Way of St James (Camino de Santiago).[76] Medieval Christians would collect a scallop shell while at Compostela as evidence of having made the journey. The association of Saint James with the scallop can most likely be traced to the legend that the apostle once rescued a knight covered in scallops.[77] An alternative version of the legend holds that while St. James' remains were being transported to Galicia (Spain) from Jerusalem. As the ship approached land, the wedding of the daughter of Queen Lupa was taking place on shore.[78] The young groom was on horseback, and, upon seeing the ship's approach, his horse got spooked, and horse and rider plunged into the sea.[79] Through miraculous intervention, the horse and rider emerged from the water alive, covered in seashells.[80]

Indeed, in French, the mollusc itself – as well as a popular preparation of it in cream sauce – is called coquille St. Jacques. In German they are Jakobsmuscheln – literally "James's shellfish". Curiously the Linnaean name Pecten jacobeus is given to the Mediterranean scallop, while the scallop endemic to Galicia is called Pecten maximus due to its bigger size.[81] The scallop shell is represented in the decoration of churches named after St. James, such as in St James' Church, Sydney, where it appears in a number of places, including in the mosaics on the floor of the chancel.[82]

When referring to St James, a scallop shell valve is displayed with its convex outer surface showing. In contrast, when the shell refers to the goddess Venus (see below), it is displayed with its concave interior surface showing.[81]

Fertility symbol

[edit]
Aphrodite Anadyomene, from Amisos, 1st century BC – 1st century AD

Throughout antiquity, scallops and other hinged shells have symbolized the feminine principle.[83] Outwardly, the shell can symbolize the protective and nurturing principle, and inwardly, the "life-force slumbering within the Earth",[84] an emblem of the vulva.[85][86]

Many paintings of Venus, the Roman goddess of love and fertility, included a scallop shell in the painting to identify her. This is evident in Botticelli's classically inspired 15th century painting The Birth of Venus.[87]

One legend of the Way of St. James holds that the route was seen as a fertility pilgrimage, undertaken when a young couple desired to bear offspring. The scallop shell is believed to have originally been carried by pagans as a symbol of fertility.[88][89]

Other interpretations

[edit]

Alternatively, the scallop resembles the setting sun, which was the focus of the pre-Christian Celtic rituals of the area.[citation needed] To wit, the pre-Christian roots of the Way of St. James was a Celtic death journey westwards towards the setting sun, terminating at the End of the World (Finisterra) on the "Coast of Death" (Costa da Morte) and the "Sea of Darkness" (i.e., the Abyss of Death, the Mare Tenebrosum, Latin for the Atlantic Ocean, itself named after the Dying Civilization of Atlantis).[90][better source needed]

Contemporary art

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Large sculpture by Maggi Hambling titled The Scallop erected in 2003 on the beach at Aldeburgh, England

The beach at Aldeburgh, Suffolk, England, features Maggi Hambling's steel sculpture, The Scallop, erected in 2003 as a memorial to the composer Benjamin Britten, who had a long association with the town.[91]

See also

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Explanatory notes

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Citations

[edit]
  1. ^ Whitney, D. W. (1890) "Scallop". The Century Dictionary: An Encyclopedic Lexicon of the English Language p.5371, The Century Company.
  2. ^ "Scallop" (2009). The Oxford English Dictionary (2nd ed.), Oxford University Press.
  3. ^ Robinson & Robinson 2000, p. 65.
  4. ^ Rice 2012, p. 47.
  5. ^ Shumway & Parsons 2011, p. 124.
  6. ^ Milsom & Rigby 2009, p. 62.
  7. ^ a b c d e f g h Drew 1906, pp. 5–6.
  8. ^ Shumway & Parsons 2011, p. 59.
  9. ^ a b c Hautmann, Michael (2010). "The first scallop" (PDF). Paläontologische Zeitschrift. 84 (2): 317–322. Bibcode:2010PalZ...84..317H. doi:10.1007/s12542-009-0041-5. S2CID 84457522.
  10. ^ Sun, Xiujun; Liu, Zhihong; Wu, Biao; Zhou, Liqing; Wang, Qi; Wu, Wei; Yang, Aiguo (2018). "Differences between fast and slow muscles in scallops revealed through proteomics and transcriptomics". BMC Genomics. 19 (1): 377. doi:10.1186/s12864-018-4770-2. PMC 5963113. PMID 29783952. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
  11. ^ Harris, Olivia K.; Kingston, Alexandra C. N.; Wolfe, Caitlin S.; Ghoshroy, Soumitra; Johnsen, Sönke; Speiser, Daniel I. (2019). "Core–shell nanospheres behind the blue eyes of the bay scallop Argopecten irradians". Journal of the Royal Society Interface. 16 (159). doi:10.1098/rsif.2019.0383. PMC 6833330. PMID 31640501.
  12. ^ Speiser, Daniel I.; Johnsen, Sönke (29 December 2008). "Comparative Morphology of the Concave Mirror Eyes of Scallops (Pectinoidea)". American Malacological Bulletin. 26 (1–2): 27–33. Bibcode:2008AMalB..26...27S. doi:10.4003/006.026.0204. S2CID 11584708.
  13. ^ Speiser, Daniel I.; Loew, Ellis R.; Johnsen, Sönke (1 February 2011). "Spectral sensitivity of the concave mirror eyes of scallops: potential influences of habitat, self-screening and longitudinal chromatic aberration". Journal of Experimental Biology. 214 (3): 422–431. Bibcode:2011JExpB.214..422S. doi:10.1242/jeb.048108. PMID 21228201.
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  41. ^ Raines, Poppe & Groh 2006.
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  44. ^ Waller, Thomas R. (1986). "A new genus and species of scallop (Bivalvia: Pectinidae) from off Somalia, and the definition of a new tribe Decatopectinin i". Nautilus. 100 (2): 39–46. Bibcode:1986Nauti.100...39.. doi:10.5962/bhl.part.26491.
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General bibliography

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia
from Grokipedia

Scallops are marine bivalve mollusks belonging to the family Pectinidae, comprising approximately 264 distributed globally in diverse benthic habitats from shallow coastal waters to deep seas. These organisms are characterized by two asymmetrical, fan-shaped shells with radial ribs, hinged together and capable of rapid opening and closing via a powerful adductor muscle that enables jet-propelled swimming—a rare mobility among bivalves achieved by expelling water from cavity. Unlike sedentary bivalves such as clams, many scallops are free-living, resting on the seabed or attaching temporarily via a thread in juveniles, and they filter-feed on using a siphoning mechanism. A defining feature is their mantle margin lined with numerous simple eyes—up to 200 in some —equipped with corneas, lenses, and retinas that detect light, motion, and shadows for predator evasion, representing an advanced sensory adaptation in the phylum . Commercially, scallops support lucrative fisheries, particularly the Atlantic sea scallop (), which drives significant economic value in regions like through practices.

Taxonomy

Etymology and Nomenclature

The common English name "scallop" derives from the mid-14th century scalop, borrowed from escalope, denoting a shell or , in reference to the mollusk's fan-shaped, ribbed . The term likely stems from a Germanic root akin to skol or Proto-Germanic skalōną, implying "to cut" or "split," evoking the shell's serrated, scalloped margins or the act of prying it open. Variant spellings like scollop or escallop persisted into , with the latter often denoting the shell itself in culinary or heraldic contexts, as in dishes served in emptied shells. In taxonomic nomenclature, "scallop" encompasses marine bivalve mollusks of the family Pectinidae Rafinesque, 1815, within the order Pectinida and superfamily Pectinoidea. The family name Pectinidae originates from the Pecten Oken, 1815 (originally established by Scopoli in 1777 under binomial conventions), derived from Latin pecten ("" or "rake"), alluding to the comb-like serial arrangement of radial costae or ribs on the exterior of the valves. This includes like Pecten maximus (great scallop), while over 60 other genera in the family, such as Argopecten, Chlamys, and Placopecten, bear -specific binomials reflecting morphological traits, geographic distribution, or historical descriptors, per the . Common names vary regionally—e.g., "bay scallop" for Argopecten irradians in North American fisheries—but uniformly apply to Pectinidae members distinguished from superficially similar bivalves like oysters by their free-swimming ability and auricles.

Phylogenetic Classification

Scallops, comprising the family Pectinidae, are classified within the superfamily Pectinoidea, order Pectinida, subclass (infraclass Autobranchia), class , phylum , kingdom Animalia. This hierarchical placement reflects both morphological traits, such as the inequivalved, auriculate shells and byssal notch, and molecular data supporting of the family based on mitochondrial and nuclear genes. Within , Pectinidae nest within the clade , characterized by aragonitic crossed-lamellar shell microstructure and cementation or byssal attachment in ancestral forms, diverging from protobranch and lineages around 500 million years ago based on fossil-calibrated phylogenies. Molecular phylogenies, incorporating genes like 16S rRNA, COI, and 28S rRNA, resolve Pectinidae as a robust monophyletic group sister to certain clades within the paraphyletic Propeamussiidae, challenging earlier superfamily-level classifications that treated Propeamussiidae as a distinct family. These analyses, sampling over 60 taxa across Pectinoidea, highlight in shell ornamentation and swimming capabilities, with Pectinidae exhibiting high-performance escape responses distinct from slower Propeamussiid relatives. Ingroup within Pectinidae often separates into major clades corresponding to subfamilies such as Pectininae (including Pecten species with radial ribs) and Chlamydinae (e.g., with smoother or scaly valves), though outgroup selection influences resolution, and some studies recover only two primary clades from sampled species. Five subfamilies are currently recognized—Pectininae, Chlamydinae, Palliolinae, Camptonectinae, and Pedinae—with the latter two underrepresented in molecular datasets due to rarity or deep-water habits. Morphological classifications, relying on hinge dentition, auricle proportions, and byssal features, conflict with molecular trees in subfamilial boundaries; for instance, shell-based systems proposed by Waller (2006) emphasize auricle , but genetic data suggest in these traits driven by predation pressures and habitat shifts. Fossil records, extending to the (~240 million years ago), support Pectinidae diversification post-Permian , with molecular clock estimates aligning crown-group radiation around 70-100 million years ago in the , coinciding with angiosperm-driven productivity increases in shelf seas. Ongoing and multi-gene approaches continue to refine this framework, addressing long-standing uncertainties in basal pteriomorph relationships by incorporating transcriptomic data from understudied taxa.

Species Diversity and Recent Discoveries

The family Pectinidae, commonly known as scallops, exhibits substantial , with taxonomic databases recognizing between 250 and 309 extant across approximately 68 genera. This diversity spans subfamilies such as Chlamydinae, Palliolinae, and Pectininae, reflecting adaptations to varied marine habitats from shallow coastal waters to deep-sea environments. Morphological variations include differences in shell sculpture, hinge structure, and byssal adaptations, though in shell shape has historically obscured phylogenetic distinctions among gliding . Recent discoveries have expanded understanding of this diversity through integrative approaches combining morphology, mitogenomics, and . In February 2025, Delectopecten thermus was formally described as a new vent-dwelling from hydrothermal sites in Japan's Okinawa Trough, notable for its translucent, glass-like shell measuring up to 20 mm, asymmetrical hinge, and lack of chemosynthetic in tissues, as confirmed by 16S rRNA . This finding underscores the untapped in extreme deep-sea habitats and highlights the Delectopecten's previously understudied phylogenetic position within Pectinidae. Additional genomic studies since 2020 have revealed heterogeneous divergence patterns between closely related species, such as and Pecten jacobeus, informing conservation amid environmental pressures, while parasite surveys have identified novel trematodes in bay scallops () since 2012, potentially influencing without constituting new host species. These advancements emphasize the role of molecular tools in resolving taxonomic ambiguities and documenting scallop evolutionary history.

Morphology and Anatomy

Shell and Valves

The shell of scallops in the family Pectinidae comprises two —a left and a right—joined dorsally by a resilifer-embedded elastic and, in some taxa, cardinal teeth. These exhibit a generally circular to fan-shaped outline, with the left often more convex than the flatter right, conferring an inequivalved morphology that facilitates adduction for . Auricles, wing-like projections flanking the umbo, extend anteriorly and posteriorly from the margin; the anterior auricles predominate in size and asymmetry, aiding in shell balance during swimming. The right bears a distinctive byssal notch ventral to the anterior auricle, containing the ctenolium—a series of comb-like denticles serving as a synapomorphy of Pectinidae and anchoring byssal threads in early post-larval stages before detachment in mobile adults. Externally, display radial costae radiating from the umbo, formed through accretionary growth increments marked by concentric varices or lines, which enhance structural rigidity and hydrodynamics. Internally, a pallial line delineates mantle attachment, with the adductor muscle scar prominently central and striated. Shell microstructure features stratified layers: an outer prismatic or homogeneous calcitic zone, a middle columnar or foliated layer, and an inner nacreous or crossed-lamellar layer, optimizing fracture resistance and lightness. Compositionally, the shell is 95-99% by weight, polymorphs of and embedded in an organic conchiolin matrix (∼1-5%), secreted sequentially by the outer and inner mantle epithelium for continuous . This biomineralsation yields shells up to 20 cm in height, with thickness varying by and age to balance protection against predation and energy costs of burrowing or escape responses.

Muscular and Locomotory Systems

Scallops possess a bipartite adductor muscle comprising a striated phasic portion for fast contractions and a smooth catch portion for prolonged tension with minimal energy expenditure. The striated muscle features ordered sarcomeres with and filaments, enabling quick calcium-triggered contractions via proteins like , ideal for escape responses. In contrast, the catch muscle relies on paramyosin and twitchin proteins for a low-energy "catch" state, maintaining shell closure through rather than continuous calcium binding. This muscular system powers the scallop's primary locomotion: jet-propelled . To initiate movement, the adductor relaxes, allowing the elastic hinge ligament to open the valves and draw water into cavity. Rapid contraction of the striated adductor then claps the valves shut, expelling water through directed mantle openings for , with a typical cycle duration of 0.28 seconds divided into closing, gliding, and opening phases. The muscular mantle margins steer the exhalant jet, enabling directional control during bursts of speed up to accelerations of 1370° s⁻². Unlike sedentary bivalves, adult scallops are free-living, relying on this system for predator evasion rather than permanent attachment, though juveniles may use temporary byssal threads. Peak adductor stress during reaches 1.06×10⁵ N m⁻², generating power outputs of 185 kg⁻¹.

Internal Organs and Sensory Capabilities

Scallops feature a suite of internal organs adapted for filter-feeding, , and within their bivalve . The digestive system includes a surrounded by labial palps that direct filtered particles from the gills via a short to the , which is embedded in the digestive and employs a crystalline style to release . The intestine loops through the before exiting via the anus in the excurrent chamber. Circulation occurs in an open system, with a heart consisting of two auricles and one ventricle located in the near the adductor muscle; is distributed through anterior and posterior aortae to tissue sinuses and returns to the gills and mantle for reoxygenation. Respiration relies on ctenidial gills for both feeding and oxygen uptake, augmented by the extensively vascularized mantle, which serves as a primary respiratory surface. Excretion is handled by paired kidneys that filter waste from the , while the gonads produce gametes released through renal ducts into the mantle cavity for broadcast spawning. Sensory functions are mediated by a well-developed nervous system comprising cerebral, pedal, and paired visceral ganglia that innervate internal organs such as the gills, heart, kidneys, digestive tract, and gonads. The most distinctive sensory structures are the numerous image-forming eyes—ranging from dozens to over 100—arrayed along the mantle edge, each utilizing a concave spherical mirror for focusing light onto dual retinas: a proximal rhabdomeric retina that depolarizes to light and a distal ciliary retina that hyperpolarizes. Visual pigments vary by retina and species, with peak sensitivities (λ_max) of approximately 490 nm (proximal) and 520 nm (distal) in Placopecten magellanicus, and 504 nm (proximal) and 549 nm (distal) in Argopecten irradians, tuned to ambient light spectra in their habitats. These eyes enable panoramic spatial vision across a 270° field without body rotation, achieving resolutions as fine as 2° to detect moving shadows or predators at distance. Adjacent chemotactile tentacles extend toward visual stimuli for close-range verification via touch and chemical sensing, integrating with eye inputs for threat assessment. This multimodal system supports rapid escape responses, distinguishing scallops from less visually oriented bivalves.

Distribution and Habitat

Global Range

Scallops of the family Pectinidae exhibit a cosmopolitan distribution across all major ocean basins, including the Atlantic, Pacific, Indian, Arctic, and Southern Oceans. Species occur from polar latitudes, such as northern Norway and the Antarctic periphery, to equatorial tropics. This broad latitudinal span reflects adaptations to varied thermal regimes, with polar species like Chlamys islandica documented in sub-Arctic waters and tropical forms prevalent in Indo-West Pacific coral reef-associated habitats. Species richness is highest in the region, where environmental heterogeneity supports elevated compared to Atlantic or other basins. Approximately 250 to 400 extant are recognized globally, with Indo-Pacific endemics comprising a significant proportion of this total. In contrast, the Atlantic hosts fewer , such as Placopecten magellanicus ranging from to , while Pacific distributions include clusters off to and in the . Habitat depth varies widely, from intertidal and shallow subtidal zones to bathyal and abyssal depths exceeding 2,000 meters in some cases. Epifaunal attachment to substrates predominates in shallower waters, transitioning to free-lying or semi-infaunal lifestyles in deeper, soft-sediment environments. Regional abundances fluctuate due to oceanographic factors, with dense aggregations reported in temperate shelf seas like the Northwest Atlantic, where billions of individuals of commercial occupy discrete beds.

Environmental Preferences and Adaptations

Scallops of the family Pectinidae primarily inhabit subtidal marine environments on substrates consisting of clean sand, fine , or beds, where juveniles often settle and adults rest partially buried or attached by a byssus thread. These preferences facilitate filter feeding by maintaining access to suspended particulates while providing stability against currents. Species distribution correlates with depths ranging from the low to approximately 100 meters, though some extend to greater depths in colder waters. Temperature tolerances vary among species, with optimal growth for many occurring between 10°C and 20°C; for example, shell growth in the Yesso scallop (Patinopecten yessoensis) slows significantly above 20°C due to metabolic stress. preferences align with full marine conditions around 30-35 ppt, as deviations—particularly reductions—can inhibit growth and , as observed in trials where low periods coincided with minimal somatic increases. Moderate currents enhance suitability by delivering food and oxygen while preventing accumulation, with inshore populations favoring areas of stronger flow. Key adaptations include valvular snapping for jet-propelled swimming, which allows scallops to evade adverse conditions such as hypoxia or temperature extremes and actively select preferable microhabitats. This mobility, powered by a hypertrophied adductor muscle, contrasts with the sessile of most bivalves and supports exploitation of patchy resources. Additionally, behavioral responses to environmental cues, such as burrowing into during low oxygen events, aid survival in fluctuating coastal regimes. Genomic analyses reveal molecular underpinnings for these traits, including expanded families for and sensory that facilitate rapid environmental responsiveness.

Physiology and Life Cycle

Feeding and Digestion

Scallops are suspension feeders that capture particulate organic matter, primarily and , from ambient using specialized known as ctenidia. Unlike infaunal bivalves, scallops lack siphons, allowing water to enter cavity through an open gape and exit via a lateral pore, with flow generated by ciliary beating on the gills and supplemented by valve adductions during locomotion. The gills feature heterorhabdic plicate structures comprising principal filaments for ingestion and ordinary filaments for rejection; particles larger than approximately 5 μm are retained on gill filaments via nets formed by latero-frontal cirri, with retention exceeding 50% for particles over 4 μm in species like . Captured particles are transported dorsally in low-viscosity streams toward the labial palps, a secondary selection site where ridges and ciliated troughs direct suitable to the via an oral groove, while rejects are expelled ventrally in high-viscosity mucus as pseudofeces. Ingestion occurs through a simple ciliated leading to the , with no salivary glands present, distinguishing scallops from some other bivalves like mussels. In the , classified as type IV, commences as food particles are triturated against a chitinous gastric shield by the rotating crystalline style—a translucent rod composed of mucin-type glycoproteins that secretes enzymes such as α-amylase and laminarinase while stirring gastric contents. The style's rotation facilitates mechanical breakdown and enzymatic , with its size varying inversely with fullness; in Pecten maximus larvae, initial begins about 6 hours post-ingestion at 17°C. Partially digested material passes to the digestive , a hepatopancreas-like organ of blind-ending tubules drained by principal and secondary ducts into the , where secretory cells produce enzymes including chitinase and absorptive digestive cells perform via and , achieving absorption within acini. The intestine, featuring descending and ascending loops that traverse the digestive and , supports further enzymatic action by proteases and chitinase in ciliated epithelial cells with microvilli, enabling metabolite transfer to reproductive tissues; complete digestion in continuously feeding P. maximus larvae occurs in about 10 hours at 17°C. Undigested wastes form fecal pellets expelled through the into the exhalant stream, with digestive rhythms often synchronized to tidal cycles influencing feeding rates and activity. rates, such as 4 L h⁻¹ g⁻¹ dry tissue in P. magellanicus for algal suspensions, vary with seston concentration and , optimizing particle clearance up to a threshold beyond which rates decline.

Reproduction and Development

Scallops in the Pectinidae reproduce sexually through broadcast spawning, releasing eggs and into the surrounding for . Most species are gonochoristic, with distinct males and females, though some, such as Argopecten nucleus and Pecten fumatus, are simultaneous hermaphrodites that typically prioritize cross-fertilization by timing release to minimize self-fertilization. Spawning is often induced by environmental cues, including rising water temperatures and blooms, with gonadal development progressing through stages of inactivity, gamete growth, maturation, and release, sometimes followed by resorption of unspawned gametes. varies by species and size; for instance, a single P. fumatus can produce up to 1 million eggs per spawning event. Fertilized eggs develop rapidly into free-swimming trochophore larvae within approximately 24 hours, followed by the emergence of D-shaped veliger larvae after about three days, which begin planktotrophic feeding on . Veliger larvae progress through straight-hinge and umbo stages, developing sensory organs and growing via and feeding, before reaching the pediveliger stage, characterized by a functional foot and byssal organ for substrate attachment. The planktonic larval duration spans 2 to 6 weeks, influenced by temperature and species; for example, giant scallop () veligers reach pediveliger at 15°C in 28 days, while Atlantic sea scallops () may require up to 45 days. Pediveliger larvae exhibit vertical migration behaviors, descending to the at rates of about 1.7 mm/s to select settlement substrates such as shells, rocks, or , where they metamorphose by resorbing the velum and developing juvenile features like the adductor muscle and auricles. Settlement success depends on , with optimal pediveliger stocking enhancing post-larval production and early growth, as demonstrated in controlled studies optimizing densities for species like Argopecten ventricosus. Juveniles, or spat, initially remain attached via byssal threads before adopting a free-living, epibenthic , with rates influenced by predation, substrate quality, and water flow. Sexual maturity is typically reached by age 2 in many , enabling annual or semi-annual reproductive cycles thereafter.

Growth Patterns and Longevity

Scallops display , incrementally depositing layers along the shell margins, which form visible annual growth rings used for age estimation via sclerochronology. Growth trajectories are typically modeled with the , L(t) = L_∞ (1 - e^{-k(t - t_0)}), where L_∞ represents asymptotic shell height, k the growth coefficient reflecting rate to maximum size, and t_0 the theoretical age at zero length; this model captures decelerating growth after initial rapid juvenile phases, with parameters varying by and environment. For instance, in the great scallop (), maximum annual shell height increments reach 40-50 mm in early years, declining with age and , as northern populations exhibit slower but larger asymptotic sizes compared to southern ones. Growth rates are modulated by environmental factors, including temperature, where sea scallops (Placopecten magellanicus) achieve peak somatic growth at 12-13°C, with above 15°C reducing clearance rates and energy allocation to maintenance over shell deposition. density drives feeding efficiency and scope for growth, with bay scallops () exhibiting higher rates (0.031 mg dry weight per day) in bed edges versus open sediments due to enhanced food access and reduced predation. Spatial heterogeneity further influences outcomes; in show elevated growth compared to other northwest Atlantic areas, attributed to optimal and prey abundance. Longevity spans 2-35 years across Pectinidae species, correlating inversely with metabolic rates and growth velocity per predictions. Short-lived taxa like bay scallops reach maturity in months and maximum ages of 2 years, prioritizing rapid over somatic maintenance. In contrast, great scallops attain 22 years, scallops (Adamussium colbecki) median 14 years with maxima to 19, and scallops (Chlamys islandica) up to 35 years, reflecting adaptations to colder, stable habitats that minimize oxidative damage and support extended . Peruvian scallops (Argopecten purpuratus) live 7-10 years, with longevity linked to antioxidative efficacy under varying latitudes.

Ecology and Behavior

Locomotion and Escape Responses

Scallops propel themselves through water via , achieved by cyclic opening and closing of their valves using the adductor muscle and elastic hinge ligament. Water is drawn into the cavity as the valves gape, then expelled forcefully through lateral vents during rapid adduction, generating thrust. This mechanism relies on the phasic portion of the adductor muscle for quick contractions, distinct from the tonic portion used for sustained closure. The primary function of this is predator evasion, where scallops exhibit an escape response involving bursts of clapping to produce directed jets. In species like the Atlantic sea scallop (), swimming occurs to flee threats or reposition, with sequences of at least four sequential adductions qualifying as true swimming rather than mere jumping. Escape performance varies with body size; larger individuals in Aequipecten opercularis show enhanced response efficacy due to greater muscle mass and propulsive force. Swimming trajectories can be unidirectional or zig-zag, influenced by mantle edge contact points with predators and environmental factors such as substratum type. In bay scallops (), the probability of initiating swimming increases on sandy substrates compared to beds, aiding rapid evasion. Observed speeds reach up to 73 cm/s in species like Amusium pleuronectes, though distances are typically short, reflecting energy costs of repeated adductions. musculature directs jet orientation, enabling backward propulsion away from stimuli while the auricles facilitate realignment post-swim.

Predation, Defense, and Symbioses

Scallops are preyed upon by a variety of marine predators, including sea stars such as Asterias rubens and Astropecten irregularis, crabs like the rock crab Cancer irroratus and blue crabs, American lobsters (Homarus americanus), and certain fish and rays. Predation intensity varies by habitat; for instance, bay scallops (Argopecten irradians) along edges of Thalassia testudinum seagrass beds suffer over 20% daily loss to predators, compared to lower rates within dense beds. Post-settlement juveniles experience significant mortality from sea stars and crabs, influencing recruitment dynamics. In marine protected areas, elevated scallop densities can attract higher predator numbers, such as sea stars, increasing natural mortality. To counter predation, scallops utilize behavioral defenses centered on rapid escape responses, primarily through sequential valve adductions or "claps" of the shell that expel water for propulsion, enabling or away from threats. These responses are elicited by contact or visual cues, with violent reactions to predatory sea stars versus milder ones to non-predators. Sensory capabilities include rows of simple eyes along edge, numbering up to 200 in some , which detect light, shadows, and movement to initiate flight-or-freeze decisions. Additional tentacles serve as chemosensory and mechanosensory organs for predator detection. Scallops also employ tonic adductor muscles for prolonged shell closure and can bury in sediment for , though remains the primary anti-predator strategy. Escape performance varies with size, age, and conditioning; larger individuals show reduced reactivity, while predator-exposed juveniles exhibit faster responses. Symbiotic associations enhance scallop defenses and survival. Encrusting sponges, such as Myxilla species on the spiny scallop Chlamys hastata, form mutualisms where the sponge camouflages the host and disrupts predator tube-foot adhesion, deterring attacks, while the mobile scallop transports the sponge away from threats like dorid nudibranchs. In deep-sea species, gill-surface ectosymbionts like sulfur-oxidizing provide nutritional benefits via , supporting host metabolism in low-food environments. Bay scallops host diverse symbionts, including prokaryotes like a novel species and eukaryotes, with population density influencing composition; these may modulate immunity but can include pathogenic viruses linked to moribund states. Epibionts such as oysters attach commensally without harming the scallop, potentially aiding . Scallops rely on innate immune effectors from hemocytes for humoral and cellular defense against parasites and , coordinating with symbiotic microbiomes.

Ecosystem Roles

Scallops function as benthic , drawing in water through their gills to capture , , and , thereby clarifying water and facilitating nutrient transfer from pelagic to benthic zones. This process supports nutrient cycling by converting suspended into and pseudofeces, which deposit on the seafloor and influence microbial decomposition and sediment chemistry. In coastal systems prone to , dense scallop populations can mitigate excess algal blooms by reducing standing stocks, though farming densities may alter dissolved dynamics through enhanced microbial activity. As intermediate consumers in marine food webs, scallops serve as prey for a range of predators, including sea stars, , whelks (such as Busycon spp.), and , channeling energy upward to higher trophic levels. Predation rates vary by size and habitat; for instance, juvenile bay scallops () face high mortality from epifaunal and bottom-feeders, while adults employ valve-clapping propulsion to evade threats, potentially resuspending sediments and affecting local prey availability. Declines in scallop abundance due to or predation can disrupt these dynamics, altering predator populations and cascading to other benthic species. Scallops contribute to habitat complexity as ecosystem engineers, with their shells providing attachment sites for epifauna such as sponges, bryozoans, and , thereby enhancing local and three-dimensional structure in otherwise sandy or gravelly seabeds. Live and dead shells foster microhabitats that support juvenile settlement of conspecifics and other , while scallop mobility—via swimming or burial—can redistribute sediments, promoting heterogeneity in soft-bottom communities. In managed fisheries, for scallops often damages this emergent epifauna, underscoring their role in maintaining resilient benthic assemblages.

Human Interactions

Commercial Fisheries

Commercial scallop fisheries target wild populations of various Pectinidae species using primarily bottom-towed dredge gear, which consists of rigid frames with a toothed bar or teeth that scrape the seabed to dislodge scallops into a trailing chain-mesh bag. The Atlantic sea scallop () in the Northwest Atlantic represents the world's largest wild scallop , with U.S. commercial landings reaching 27.4 million pounds of adductor muscle meats valued at $360 million in 2023. Operations occur from southward to the Mid-Atlantic Bight, at depths typically ranging from 40 to 100 meters, under strict management by the Fishery Management Council, including dredge ring sizes of at least 4 inches to enable of undersized scallops and rotational closures to safeguard recruitment areas. Smaller-scale fisheries exploit bay scallops () in shallow coastal waters along the U.S. East Coast, particularly in , New York, and , where state-managed harvests occur seasonally using similar dredges or hand-gathering in permitted areas. Calico scallops (Argopecten gibbus) are dredged from deeper waters (30-240 feet) off Florida's east coast and , though yields fluctuate due to variable recruitment. On the U.S. West Coast, weathervane scallops (Patinopecten caurinus) are harvested via New Bedford-style dredges averaging 15 feet wide and weighing 2,600 pounds, primarily in and the . These fisheries hold substantial economic weight, particularly in , where Atlantic sea scallop revenues have comprised 75-80% of fishery values over the past five years, supporting thousands of jobs in harvesting, processing, and related sectors. Pound-for-pound, sea scallops rank among the highest-value U.S. products, driven by demand for the large adductor muscle, with management practices emphasizing quota allocations and minimization to sustain yields.

Aquaculture Practices

Scallop primarily involves the production of juvenile (spat) followed by grow-out to marketable size, with global production exceeding 2 million metric tons in 2018, dominated by at over 90% of output. Key cultured species include the Yesso scallop (Patinopecten yessoensis) in and , the northern bay scallop (Argopecten purpuratus) in and , and Zhikong scallop (Chlamys farreri) in . Operations rely on either natural spat collection from wild settlement or controlled rearing to ensure supply reliability, particularly in regions with variable natural . Hatchery practices begin with conditioning under controlled temperatures and fed diets to induce spawning, typically via thermal or serotonin stimulation, yielding veliger larvae that are reared in tanks at optimal salinities (around 25-30 ppt) and temperatures (13-25°C depending on species) while fed like Isochrysis and Chaetoceros. Larval development spans 2-4 weeks until settlement onto collectors, after which spat are transferred to nursery systems for further growth before on-growing. This closed-cycle approach mitigates risks from wild seed variability but requires precise management to prevent bacterial overgrowth and high mortality rates, which can exceed 50% in early stages without optimized protocols. Grow-out methods emphasize suspended using rafts, longlines, or poles with multi-tiered pearl lanterns or mesh bags to hold juveniles, allowing natural filtration of without supplemental feed, which minimizes organic pollution compared to fed . Bottom , seeding juveniles directly onto seabeds, is less common due to predation risks but practiced in some areas like parts of for sea scallops. Harvest occurs after 1-3 years when adductor muscles reach commercial size, with producing about 11,000 metric tons annually in recent years through such systems. Site selection prioritizes high flows and low to support growth rates of 0.5-1 mm per day in optimal conditions. Challenges include disease outbreaks from pathogens like Vibrio species, exacerbated by dense stocking, and environmental stressors such as temperature fluctuations linked to climate variability, which contributed to production collapses in regions like . Overcrowding in suspended systems can lead to and reduced water flow, impacting feeding efficiency, while broader concerns involve potential genetic dilution of wild stocks from escapes, though evidence of significant impacts remains limited for scallops due to their low and extractive nature. Sustainable practices incorporate fallowing, spat size grading to reduce uniformity in vulnerability, and monitoring for algal toxins that scallops may bioaccumulate.

Culinary and Nutritional Value

Scallops are harvested primarily for their adductor muscle, the firm white tissue responsible for valve closure, which constitutes the main edible portion in most cuisines due to its tender texture and subtly sweet, briny flavor. , consumption typically involves only this muscle, and separated from the viscera and roe, whereas in regions like and parts of , the coral-like roe (gonad) is also eaten for its richer taste and nutritional content. The muscle's composition, high in free such as , , , and , contributes to its profile, enhancing palatability in various preparations. Common cooking methods emphasize brevity to avoid toughness: pan-searing over high heat in butter or oil for 1-2 minutes per side yields a golden crust via the while preserving moisture, as overcooking causes protein denaturation and rubbery consistency. or broiling imparts smokiness, suitable for skewers or appetizers, while raw applications like seviche rely on acid marination to denature proteins without heat. Nutritionally, raw scallop adductor muscle offers a low-calorie, high-protein profile: per 100 grams, it contains approximately 69 calories, 12.1 grams of protein, 0.8 grams of fat (predominantly polyunsaturated), and 5.4 grams of carbohydrates, primarily . This yields a favorable , supporting muscle and with minimal caloric intake. Key micronutrients include at 334 mg (48% of daily value), providing skeletal support, and sodium at 392 mg, though levels vary by and habitat . Scallops are also notable for content, aiding defense, and like B12 for neurological function, though omega-3 fatty acids are present in modest amounts compared to finfish. Cooking methods like or minimally alter macronutrients but can concentrate minerals by reducing ; for instance, cooked scallops average 111 calories per 100 grams with elevated protein density. Potential concerns include moderate (around 37 mg per 100 grams raw) and allergenicity for shellfish-sensitive individuals, but no significant heavy metal in monitored fisheries.

Cultural Symbolism and Other Uses

The scallop shell has served as a prominent emblem of since the , particularly for journeys to the shrine of Saint James at in , where pilgrims collected shells from Galician beaches as souvenirs and badges of completion. Pilgrims affixed these shells to their clothing or hats for identification and practical use as scoops for from streams. The shell's radiating ridges symbolize converging paths leading to a single destination, representing spiritual unity and the multifaceted routes to enlightenment. In heraldry, the escallop—often depicted as an oriented scallop shell—functions as a charge denoting or maritime themes, linked to Saint James the Great and adopted in coats of arms, including those of families with historical ties to the . It also appears in ecclesiastical contexts as a symbol of , with scallop-shaped fonts evoking the shell's capacity to hold and signifying rebirth through immersion. Beyond symbolism, scallop shells have found utilitarian applications across eras; in prehistoric contexts, they were employed as cutting, scraping, and serving tools due to their durable, concave form. Roman artifacts reveal shells repurposed as cosmetic containers, a practice extending from earlier Mesopotamian uses around 2500 B.C. In , scallop motifs adorn facades and interiors from medieval cathedrals to designs, evoking pilgrimage without exclusive religious connotation.

Conservation and Environmental Considerations

Threats from Climate and Human Activities

, driven by increased atmospheric CO2 absorption since the , reduces the pH of surface ocean waters and impairs in bivalves like scallops, leading to thinner shells and higher mortality in larval stages. For Atlantic sea scallops (Placopecten magellanicus), a NOAA study modeling future scenarios under RCP8.5 projects a potential biomass decline exceeding 50% by 2100, threatening the $500 million annual Northeast U.S. . Empirical experiments confirm that elevated levels hinder juvenile growth and survival, with post-larval scallops showing reduced development under acidification conditions simulating projected 21st-century levels. Rising sea temperatures exacerbate these risks, causing that disrupts reproduction and increases susceptibility to hypoxia. In northern bay scallops ( irradians), summer heatwaves combined with low dissolved oxygen have triggered mass die-offs, as documented in New York waters from 2019–2021, crippling local fisheries through repeated recruitment failures. For Atlantic sea scallops, warming contributes to smaller adult sizes by altering growth rates, with statistical models attributing up to 30% of size variance to thermal effects interacting with fishing pressure. Human activities pose direct threats through and disruption, primarily via scallop and , which scrape seafloors and damage sedimentary habitats essential for juvenile settlement. causes immediate mortality by crushing undersized or buried scallops and long-term declines in bed density, as evidenced by observational data from fisheries showing unsustainable additional mortality beyond harvest quotas. exacerbates this by resuspending sediments, reducing water clarity, and destroying biogenic structures that stabilize scallop habitats, leading to ecosystem-wide and indirect predation increases. depletes stocks beyond recovery thresholds, with historical examples like Atlantic removals allowing ray populations to surge and consume juvenile scallops, further collapsing fisheries. from nutrient runoff contributes to localized hypoxia events, compounding climate-driven oxygen deficits in scallop grounds.

Sustainability Management and Restoration Efforts

The Atlantic sea scallop (Placopecten magellanicus) fishery in the United States is managed under the Atlantic Sea Scallop Fishery Management Plan, implemented in 1982 by the New England Fishery Management Council and NOAA Fisheries to rebuild depleted stocks and stabilize abundance through rotational area closures and controlled harvesting. This approach divides fishing grounds into rotational areas, where portions are closed to commercial dredging for 2-3 years to allow biomass recovery, followed by sequential openings based on survey data from NOAA's Northeast Fisheries Science Center, which has supported stock rebuilding from near-collapse in the 1990s to record highs by the 2010s. Annual specifications, such as those in Framework Adjustment 39 for fishing years 2025-2026, set catch limits, access area quotas, and effort controls, including a 2025 open area days-at-sea allocation of approximately 30,000 days and incidental catch total allowable catch of 50,000 pounds, to prevent overexploitation while accommodating industry needs. The Scallop Research Set-Aside Program deducts 1-2% of projected catch to fund surveys and gear research, with NOAA approving eight projects in 2025 to refine stock assessments and reduce bycatch. For bay scallops (Argopecten irradians), sustainability management emphasizes quota systems and habitat-linked controls, as populations have declined due to loss and episodic events like red tides; in the Northern , state-specific seasons and possession limits, such as Florida's zone-based openings from July to September with 2-gallon whole scallop limits per person, aim to curb recreational overharvest. contributes to sustainability, with off-bottom culture methods—where scallops are grown on substrates mimicking natural habitats—rated as low-impact by Monterey Bay Aquarium's program due to minimal ecosystem disruption and disease risk compared to intensive pond systems. In Canadian Maritime waters, annual surveys on fishing vessels count scallop densities to inform total allowable catches, supporting stable yields since the early 2000s. Restoration efforts target bay scallop declines, often integrating propagation with enhancement. In 's Panhandle, the Florida Fish and Wildlife Conservation Commission's 10-year project, launched in 2016, involves caging wild adults for larval release, direct juvenile stocking, and monitoring in bays like St. Joseph and St. Andrew, aiming for self-sustaining recruitment amid historical crashes from 50 tons annual meat landings in the 1980s to near-zero by the 2010s. Institute of Marine Science efforts since 2001 restored eelgrass beds—critical nursery —leading to bay scallop reintroduction after 70 years of on the Eastern Shore, with 2025 surveys showing population surges tied to improved cover and reduced predation. In New York's Peconic Bay, Cornell Cooperative Extension and partners have released -reared juveniles since 2006, using remote setting techniques to boost settlement on substrates, which increased wild spatfall and supported limited fisheries by 2024. Similar -based releases by the since 2005 in waters vary juvenile sizes to enhance survival against predators, contributing to localized recovery. These initiatives underscore that restoration success depends on addressing causal factors like and algal blooms, rather than release volume alone, with adaptive monitoring essential for long-term viability.

References

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