Pectinidae

Pectinidae (from the Latin pecten meaning comb), common name scallops, are a family of saltwater clams, marine bivalve mollusks in the superfamily Pectinoidea. Scallops are a cosmopolitan family of bivalves, found in all of the world's oceans, though never in freshwater. (Other families within the same superfamily, Pectinoidea, share a somewhat similar overall shell shape, and species within some of those families are also sometimes referred to as scallops.)

Pectinidae are one of very few groups of bivalves to be primarily free-living; many species are capable of rapidly swimming short distances and even of migrating some distance across the ocean floor. A small minority of pectinid species live cemented to rocky substrates as adults. Some others species are more simply attached, by means of a filament they secrete. The majority of species, however, live recumbent on sandy substrates, but when they sense the presence of a predator such as a starfish, they are able to escape by swimming swiftly but erratically through the water using a form of jet propulsion created by repeatedly clapping the valves of their shells together.

Unlike most other bivalves, pectinids have numerous simple eyes situated around the edges of their mantles. Scallops have a well-developed nervous system.

Anatomy
There is very little variation in the internal arrangement of organs and systems within the scallops, and what follows can be taken to apply to the anatomy of any given scallop species.

Orientation
The shell of a scallop consists of two sides or valves, a left valve and a right one, divided by a plane of symmetry. The animal normally rests on its right valve, and consequently this valve is often shaped differently than the left (i.e., upper) valve. With the hinge of the two valves oriented as shown in the diagram at right, the left side of the image corresponds to the animal's morphological anterior or front, the right 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. However, as many scallop shells are more or less bilaterally symmetrical as well as symmetrical front/back, determining which way a given animal is "facing" requires detailed information about its valves.

Valves
The model scallop shell consists of two similarly shaped valves with a straight hinge line along the top devoid of teeth and which produces a pair of flat wings or "ears" on either side of its center. These ears may be of similar size and shape, or the anterior ear may be somewhat larger. As is the case in almost all bivalves, a series of lines and/ or growth rings originate 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 shell of most scallops is streamlined to facilitate ease of movement during swimming at some point in the life cycle, 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 terms of weight and mass. A feature that is is unique to the members of the scallop family is the presence, at some point during the animal's life cycle, of a distinctive shell feature, a comb-like structure called a ctenolium located on the anterior edge of the right valve next to the 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; the ancestors of modern scallops, the entoliids, did not possess it.

Muscular system
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.

Eyes


Scallops have up to 100 simple, usually brilliantly blue eyes arranged around the edges of each of their two mantles like strings of beads. These are reflector eyes, about one millimeter in diameter, that contain no actual blue pigment but with a retina that is more complex than those of other bivalves. Their eyes contain two retina types, one responding to light and the other to abrupt darkness, such as the shadow of a nearby predator. These eyes cannot resolve shapes, but they can detect changing patterns of light and motion. These reflector eyes are an alternative to those with a lens, where the inside of the eye is lined with a mirrored surface which reflect the image to focus at a central point. The scallop Pecten has up to 100 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive eyes.

Digestive system
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", but which envelops part of the esophagus, intestine, and the entire stomach. Waste is passed on through the intestine (the terminus of which, like that of many mollusks, enters and leaves the animal's heart) and exits via the anus.

Nervous system
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 in color. 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 set of ganglia of any modern bivalve. From these 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. 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 anterior 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 also 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 movement of and sensation in its muscular foot.

Reproduction
The scallop family is unusual in that some members of the family are dioecious (males and females are separate), while other are simultaneous hermaphrodites (both sexes in the same individual), and a few are protoandrous hermaphrodites (males when young then switching to female). Red roe is that of a female, and white, that of a male. Spermatozoa and ova 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.

Locomotion
Scallops are mostly free-living and active, unlike the vast majority of bivalves, which are mostly slow-moving and infaunal. It is believed that all scallops start out with a byssus, which attaches them to some form of substrate such as eel grass 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).

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 will usually land on the sea floor between each contraction of its valves, whereas a swimming scallop will stay in the water column for most or all of its contractions and will travel a much greater distance (though seldom at a height of more than one meter off the sea bed and seldom for a distance of greater than five meters ). Both jumping to 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 can make an audible, soft popping sound as they flap their shells underwater. Other scallops can extend their foot from between their valves, and by contracting the muscles in their foot, they can burrow into sand.

Distribution and habitat
Pectinidae 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 meters, 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. Pectinidae can be found living within, upon, or under either rocks, coral, rubble, sea grass, kelp, sand, or mud. Most adult specimens are either byssally attached or cemented to a substrate, while others are free swimmers.

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

Other species of Pectinidae 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 the use of an extendable foot located between their valves or burrowing themselves in the sand by extending and retracting their feet.

Pectinidae are highly sensitive to shadows, vibrations, water movement, and chemical stimuli. 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.

Physiological fitness and exercise of Pectinidae decreases with age due to the decline of cellular and especially mitochondrial function, 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. Environmental factors, such as changes in oxidative stress parameters, can inhibit the growth and development of Pectinidae.

Seasonal changes in temperature and food availability have 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 Pectinidae in May have lower maximal oxidative capacities and substrate oxidation than any other times in the year. This phenomenon is due to lower protein levels in adductor muscles.

Mutualism
Some scallops, including Chlamys hastata, frequently carry epibionts such as sponges and barnacles on their shell. 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,   camouflages Chlamys hastata from predators, or forms a physical barrier around byssal openings to prevent sea stars from inserting their digestive membranes. Sponge encrustation protects C. hastata from barnacle larvae settlement, serving as a protection from epibionts that increase susceptibility to predators. Thus, barnacle larvae settlement will occur more frequently on sponge-free shells than sponge-encrusted shells.

In fact, 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.

Lifecycle and growth
Many Pectinidae are hermaphrodites (having female and male organs simultaneously), altering their gender throughout their lives, while others exist as dioecious species, having a definite gender. 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 the age of four. The process of reproduction takes place 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. The females of Pectinidae are highly fecund, capable of producing hundreds of millions of eggs per year.

Once an egg is fertilized, it is then planktonic, which is a collection of microorganisms that drift abundantly in fresh or salt water. Larvae stay in the water column for the next 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 Pectinidae species into free swimmers. There is rapid growth within the first several years, with an increase of 50 to 80% in shell height and quadrupled size in meat weight and reach commercial size at about four to five years of age. The lifespans of some Pectinidae have been known to extend over 20 years.

Fossil record


The fossil history of Pectinidae is rich in species and specimens. The earliest known records of true Pectinidae (those with a ctenolium) can be found from the Triassic period, over 200 million years ago. 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. 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.

Taxonomy and list of genera
More than 30 genera and around 350 species are in the family Pectinidae. Raines and Poppe list nearly 900 species names, but most of these are considered either questionable or invalid. They mention over 50 genera and around 250 species and subspecies. While species are generally well circumscribed, their attribution to subfamilies and genera is sometimes equivocal, and information about phylogeny and relationships of the species is minimal, not the least because most work has been based on adult morphology.

Evolution
''' 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 300 extant species in 60 genera. Its origin dates back to the Middle Triassic Period, approximately 240 million years ago, and has been a thriving family to present day. 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 plays an extremely important role in many benthic communities and exhibits a wide range of shell shape, sizes, sculpture, and culture.

The earliest and most comprehensive taxonomic handlings of the family are based on macroscopic morphological characters of the adult shells and represent broadly divergent classification schemes. 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.

The framework of its phylogeny shows that repeated life habit states derive from evolutionary convergence and parallelism. Studies have determined the Pectinidae family is monophyletic, developing from a single common ancestor. The direct ancestors of Pectinidae were scallop-like bivalves of the family Entoliidae. Entoliids had auricles and 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.

Recently, 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.

There have been a number of efforts to address phylogenetic studies. Only three have assessed more than 10 species  and only one has included multiple outgroups. 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.

In molecular phylogenies of the Bivalvia, both the Spondylidae and the Propeamussiidae have been resolved as sister to the Pectinidae. A useful strategy would be to include outgroup species from two or more closely related families.

Genera
Family Pectinidae 
 * Subfamily Camptonectinae Habe, 1977
 * Delectopecten Stewart, 1920
 * Ciclopecten Seguenza, 1877
 * Lyropecten Conrad, 1862
 * Pseudohinnites Dijkstra, 1989
 * Subfamily Hemipectinae Habe, 1977 (disputed, often in Chlamydinae: Chlamydini)
 * Hemipecten Adams & Reeve, 1849
 * Subfamily Palliolinae Korbkov in Eberzin, 1960
 * Tribe Palliolini Waller, 1993
 * Palliolum Monterosato, 1884
 * Lissochlamys Sacco, 1897
 * Placopecten Verrill, 1897
 * Pseudamussium Mörch, 1853
 * Mesopeplum Iredale, 1929
 * Subfamily Pectininae
 * Tribe Decatopectinini Waller, 1986
 * Anguipecten Dall, Bartsch & Rehder, 1938
 * Annachlamys Iredale, 1939
 * Bractechlamys Iredale, 1939
 * Decatopecten Rüppell in G. B. Sowerby II, 1839
 * Excellichlamys Iredale, 1939
 * Flexopecten Sacco, 1897
 * Glorichlamys Dijkstra, 1991
 * Gloripallium Iredale, 1939
 * Juxtamusium Iredale, 1939
 * Mirapecten Dall, Bartsch & Rehder, 1938
 * Tribe Pectinini Wilkes, 1810
 * Dentamussium Dijkstra, 1990
 * Pecten Müller, 1776 (includes the Great or King scallop, Pecten maximus, Japanese (sea) scallop, Pecten yessoensis, the New Zealand scallop, Pecten novaezealandiae, and the Ravenel or Round-rib scallop, Pecten raveneli)
 * Euvola Dall, 1898 (includes the Zigzag scallop, Euvola ziczac)
 * Minnivola Iredale, 1939
 * Serratovola Habe, 1951
 * Subfamily Chlamydinae von Teppner, 1922
 * Tribe Clamydini von Teppner, 1922 Semipallium fulvicostatum 001.jpg'']]
 * Chlamys Röding, 1798
 * Complichlamys Iredale, 1939
 * Coralichlamys Iredale, 1939
 * Equichlamys Iredale, 1929
 * Hinnites Deference, 1821
 * Laevichlamys Waller, 1993
 * Manupecten Monterosato, 1872
 * Nodipecten Dall, 1898
 * Notochlamys Cotton, 1930
 * Pascahinnites Dijkstra & Raines, 1999
 * Pedum Bruguière, 1791
 * Psychrochlamys Jonkers, 2003
 * Scaeochlamys Iredale, 1929
 * Semipallium Jousseaume in Lamy, 1928
 * Swiftopecten Hertlein, 1936
 * Veprichlamys Iredale, 1929
 * Tribe Austrochlamydini Jonkers, 2003
 * Austrochlamys Jonkers, 2003
 * Tribe Adamussiini Habe, 1977
 * Adamussium Thiele, 1934
 * Tribe Fortipectinini Masuda, 1963
 * Mizuhopecten Masuda, 1963
 * Patinopecten Dall, 1898
 * Tribe Crassadomini Waller, 1993
 * Crassadoma Bernard, 1986
 * Caribachlamys Waller, 1993
 * Tribe Mimachlamydini Waller, 1993
 * Mimachlamys Iredale, 1929
 * Spathochlamys Waller, 1993
 * Talochlamys Iredale, 1935 includes Talochlamys pusio (Linnaeus, 1758) == Chlamys distorta (da Costa, 1778)
 * Tribe Aequipectinini F. Nordsieck, 1969
 * Aequipecten Fischer, 1886 (includes Rough scallop Aequipecten muscosus)
 * Argopecten Monterosato, 1889 (includes bay scallop, Argopecten irradians, Atlantic calico scallop Argopecten gibbus and Pacific calico scallop, Argopecten ventricosus)
 * Cryptopecten Dall, Bartsch & Rehder, 1938
 * Haumea Dall, Bartsch & Rehder, 1938
 * Leptopecten Verrill, 1897
 * Leptopecten latiauratus Conrad, 1837
 * Volachlamys Iredale, 1939
 * Subfamily incertae sedis
 * Hyalopecten Verrill, 1897

Gallery
 Image:Pecten maximus(02).jpg|The great scallop, Pecten maximus, on the right, next to the native European oyster Ostrea edulis Image:Placopecten magellanicus.jpg|A live individual of Placopecten magellanicus Image:SerpulidsPecten.JPG|A shell of a Pecten species with serpulid worm encrustation; Duck Harbor Beach on Cape Cod Bay, Wellfleet, Massachusetts Image:Aviculopecten subcardiformis01.JPG|External mold of a scallop shell in the fossil genus Aviculopecten, from the Logan Formation, Lower Carboniferous, Ohio Image:Pedum_spondyloideum.jpg| Pedum spondyloideum from the North coast of East Timor 