A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most

jawed fish Gnathostomata (; from Greek language, Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposi ...

es is covered with these protective

scale Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number ...

s, which can also provide effective camouflage through the use of reflection and colouration, as well as possible hydrodynamic advantages. The term ''scale'' derives from the Old French , meaning a shell pod or husk. Scales vary enormously in size, shape, structure, and extent, ranging from strong and rigid armour plates in fishes such as shrimpfishes and boxfishes, to microscopic or absent in fishes such as eels and

anglerfish The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...

es. The morphology of a scale can be used to identify the species of fish it came from. Scales originated within the jawless ostracoderms, ancestors to all jawed fishes today. Most

bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...

are covered with the cycloid scales of salmon and

carp Carp are various species of oily freshwater fish from the family Cyprinidae, a very large group of fish native to Europe and Asia. While carp is consumed in many parts of the world, they are generally considered an invasive species in parts of ...

, or the ctenoid scales of perch, or the ganoid scales of

sturgeon Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretace ...

s and gars.

Cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue ...

es ( sharks and rays) are covered with placoid scales. Some species are covered instead by scutes, and others have no outer covering on part or all of the skin. Fish scales are part of the fish's

integumentary system The integumentary system is the set of organs forming the outermost layer of an animal's body. It comprises the skin and its appendages, which act as a physical barrier between the external environment and the internal environment that it serves ...

, and are produced from the

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...

layer of the

dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...

, which distinguishes them from reptile scales. The same genes involved in tooth and hair development in

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s are also involved in scale development. The placoid scales of cartilaginous fishes are also called dermal denticles and are structurally

homologous Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...

with vertebrate teeth. Most fish are also covered in a layer of mucus or slime which can protect against pathogens such as bacteria, fungi, and viruses, and reduce surface resistance when the fish swims.

Thelodont scales

The bony scales of

thelodont Thelodonti (from Greek: "feeble teeth")Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996. is a class (biology), class of extinct jawless fishes with distinctive Fish scale, scales instead of large pl ...

s, the most abundant form of fossil fish, are well understood. The scales were formed and shed throughout the organisms' lifetimes, and quickly separated after their death. Bone, a tissue that is both resistant to mechanical damage and relatively prone to fossilization, often preserves internal detail, which allows the histology and growth of the scales to be studied in detail. The scales comprise a non-growing "crown" composed of dentine, with a sometimes-ornamented enameloid upper surface and an aspidine base. Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish. Beyond that, there appear to be five types of bone-growth, which may represent five natural groupings within the thelodonts—or a spectrum ranging between the end members meta- (or ortho-) dentine and mesodentine tissues. Each of the five scale morphs appears to resemble the scales of more derived groupings of fish, suggesting that thelodont groups may have been stem groups to succeeding clades of fish. However, using scale morphology alone to distinguish species has some pitfalls. Within each organism, scale shape varies hugely according to body area, with intermediate forms appearing between different areas—and to make matters worse, scale morphology may not even be constant within one area. To confuse things further, scale morphologies are not unique to taxa, and may be indistinguishable on the same area of two different species. The morphology and histology of thelodonts provides the main tool for quantifying their diversity and distinguishing between species, although ultimately using such convergent traits is prone to errors. Nonetheless, a framework comprising three groups has been proposed based upon scale morphology and histology. Comparisons to modern shark species have shown that thelodont scales were functionally similar to those of modern cartilaginous fish, and likewise has allowed an extensive comparison between ecological niches.

Cosmoid scales

Cosmoid scales are found only on ancient lobe-finned fishes, including some of the earliest lungfishes (subclass

Dipnoi Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...

), and in

Crossopterygii Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includin ...

, including the living coelacanth in a modified form (see elasmoid scales, below). They were probably derived from a fusion of placoid-ganoid scales. The inner part of the scales is made of dense lamellar bone called isopedine. On top of this lies a layer of spongy or vascular bone supplied with blood vessels, followed by a complex dentine-like layer called cosmine with a superficial outer coating of

vitrodentine Enameloid, also known as durodentine or vitrodentine, is an enamel-like tissue found in many fish. It is the primary outer component of shark odontodes (teeth and dermal denticles). Although the origin of enameloid is debated, it is probably ho ...

. The upper surface is keratin. Cosmoid scales increase in size through the growth of the lamellar bone layer.

Elasmoid scales

Coelacanth-PaleozoologicalMuseumOfChina-May23-08

Elasmoid scales are thin, imbricated scales composed of a layer of dense, lamellar collagen bone called isopedine, above which is a layer of tubercles usually composed of bone, as in ''

Eusthenopteron ''Eusthenopteron'' (from el, εὖ , 'good', el, σθένος , 'strength', and el, πτερόν 'wing' or 'fin') is a genus of prehistoric sarcopterygian (often called lobe-finned fishes) which has attained an iconic status from its close ...

''. The layer of dentine that was present in the first lobe-finned fish is usually reduced, as in the extant coelacanth, or entirely absent, as in extant lungfish and in the Devonian ''

''. Elasmoid scales have appeared several times over the course of fish evolution. They are present in some lobe-finned fishes, such as all extant and some extinct lungfishes, as well as the coelacanths which have modified cosmoid scales that lack cosmine and are thinner than true cosmoid scales. They are also present in some tetrapodomorphs like ''

'', amiids, and teleosts, whose cycloid and ctenoid scales represent the least mineralized elasmoid scales. The zebrafish elasmoid scales are used in the lab to study bone mineralization process, and can be cultured (kept) outside of the organism.

Ganoid scales

Spotted Gar (Lepisosteus oculatus) (3149758934)

Ganoid scales are found in the

s, paddlefishes, gars,

bowfin The bowfin (''Amia calva'') is a bony fish, native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being the sole surviving species of the Halecomorphi ...

, and bichirs. They are derived from cosmoid scales and often have serrated edges. They are covered with a layer of hard enamel-like dentine in the place of cosmine, and a layer of inorganic bone salt called '' ganoine'' in place of

. Ganoine is a characteristic component of ganoid scales. It is a glassy, often multi-layered mineralized tissue that covers the scales, as well as the

cranial Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

bones and fin rays in some non-teleost ray-finned fishes, such as gars, bichirs, and coelacanths. It is composed of rod-like

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

crystallites. Ganoine is an ancient feature of ray-finned fishes, being found for example on the scales of stem group actinopteryigian '' Cheirolepis''. While often considered a synapomorphic character of ray-finned fishes, ganoine or ganoine-like tissues are also found on the extinct acanthodii. It has been suggested ganoine is

to tooth enamel in vertebrates or even considered a type of enamel. Most ganoid scales are rhomboidal (diamond-shaped) and connected by peg-and-socket joints. They are usually thick and fit together more like a jigsaw rather than overlapping like other scales. In this way, ganoid scales are nearly impenetrable and are excellent protection against predation. File:Alligator gar fish.jpg, The alligator gar has a tough armouring of rhomboidal-shaped ganoid scales. File:Acipenser oxyrhynchus (recropped).png, The

has rows of ganoid scales enlarged into scute-like armour plates. File:Bowfin fish image.jpg, The ganoid scales on a

are reduced in size and resemble

cycloid scales A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as we ...

. In sturgeons, the scales are greatly enlarged into armour plates along the sides and back, while in the bowfin the scales are greatly reduced in thickness to resemble

. File:Earrings from the ganoid scales of alligator gar.jpg, Earrings made from the ganoid scales of an alligator gar File:Schmelzschuppenfischkonkretion.jpg, Fossil of a primitive rayfin with ganoid scales File:Fish Fossil (FindID 64765).jpg, Ganoid scales on a fossilised '' Lepidotes'', circa. 130

mya Mya may refer to: Brands and product names * Mya (program), an intelligent personal assistant created by Motorola * Mya (TV channel), an Italian Television channel * Midwest Young Artists, a comprehensive youth music program Codes * Burmese ...

Native Americans and people of the Caribbean used the tough ganoid scales of the alligator gar for arrow heads, breastplates, and as shielding to cover plows. In current times jewellery is made from these scales.

Leptoid scales

Leptoid (bony-ridge) scales are found on higher-order bony fish, the teleosts (the more

derived Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

of ray-finned fishes). The outer part of these scales fan out with bony ridges while the inner part is criss-crossed with fibrous connective tissue. Leptoid scales are thinner and more translucent than other types of scales, and lack the hardened enamel-like or dentine layers. Unlike ganoid scales, further scales are added in concentric layers as the fish grows. Leptoid scales overlap in a head-to-tail configuration, like roof tiles, making them more flexible than cosmoid and ganoid scales. This arrangement allows a smoother flow of water over the body, and reduces drag. The scales of some species exhibit bands of uneven seasonal growth called ''annuli'' (singular ''annulus''). These bands can be used to age the fish. Leptoid scales come in two forms: cycloid (smooth) and ctenoid (comb-like).

Cycloid scales

Cycloid (circular) scales have a smooth texture and are uniform, with a smooth outer edge or margin. They are most common on fish with soft fin rays, such as salmon and

Ctenoid scales

Ctenoid (toothed) scales are like cycloid scales, except they have small teeth or spinules called ctenii along their outer or posterior edges. Because of these teeth, the scales have a rough texture. They are usually found on fishes with spiny fin rays, such as the perch-like fishes. These scales contain almost no bone, being composed of a surface layer containing

hydroxyapatite Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...

and calcium carbonate and a deeper layer composed mostly of

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

. The enamel of the other scale types is reduced to superficial ridges and ctenii. Ctenoid Perch Scales

Ctenoid scales, similar to other epidermal structures, originate from placodes and distinctive cellular differentiation makes them exclusive from other structures that arise from the integument.Kawasaki, Kenta C., "A Genetic Analysis of Cichlid Scale Morphology" (2016). Masters Theses May 2014 - current. 425. http://scholarworks.umass.edu/masters_theses_2/425 Development starts near the caudal fin, along the

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

of the fish. The development process begins with an accumulation of

fibroblast A fibroblast is a type of cell (biology), biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework (Stroma (tissue), stroma) for animal Tissue (biology), tissues, and plays a critical role in wound ...

s between the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...

and

Collagen fibrils Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

begin to organize themselves in the dermal layer, which leads to the initiation of

mineralization Mineralization may refer to: * Mineralization (biology), when an inorganic substance precipitates in an organic matrix ** Biomineralization, a form of mineralization ** Mineralization of bone, an example of mineralization ** Mineralized tissues are ...

. The circumference of the scales grows first, followed by thickness when overlapping layers mineralize together. Ctenoid scales can be further subdivided into three types: * Crenate scales, where the margin of the scale bears indentations and projections. * Spinoid scales, where the scale bears spines that are continuous with the scale itself. * True ctenoid scales, where the spines on the scale are distinct structures. Most ray-finned fishes have ctenoid scales. Some species of

flatfish A flatfish is a member of the Ray-finned fish, ray-finned demersal fish order (biology), order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the ...

es have ctenoid scales on the eyed side and cycloid scales on the blind side, while other species have ctenoid scales in males and cycloid scales in females.

Reflection

Messina Straits Argyropelecus hemigymnus

Many teleost fish are covered with highly reflective scales which function as small mirrors and give the appearance of silvered glass. Reflection through silvering is widespread or dominant in fish of the open sea, especially those that live in the top 100 metres. A transparency effect can be achieved by silvering to make an animal's body highly reflective. At medium depths at sea, light comes from above, so a mirror oriented vertically makes animals such as fish invisible from the side. The marine hatchetfish is extremely flattened laterally (side to side), leaving the body just millimetres thick, and the body is so silvery as to resemble aluminium foil. The mirrors consist of microscopic structures similar to those used to provide structural coloration: stacks of between 5 and 10 crystals of guanine spaced about ¼ of a wavelength apart to interfere constructively and achieve nearly 100 per cent reflection. In the deep waters that the hatchetfish lives in, only blue light with a wavelength of 500 nanometres percolates down and needs to be reflected, so mirrors 125 nanometres apart provide good camouflage. Most fish in the upper ocean are camouflaged by silvering. In fish such as the

herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Oceans, i ...

, which lives in shallower water, the mirrors must reflect a mixture of wavelengths, and the fish accordingly has crystal stacks with a range of different spacings. A further complication for fish with bodies that are rounded in cross-section is that the mirrors would be ineffective if laid flat on the skin, as they would fail to reflect horizontally. The overall mirror effect is achieved with many small reflectors, all oriented vertically. Fish scales with these properties are used in some cosmetics, since they can give a shimmering effect to makeup and lipstick.

Placoid scales

Placoid (pointed, tooth-shaped) scales are found in the

cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue ...

es: sharks, rays. They are also called dermal denticles. Placoid scales are structurally

with vertebrate teeth ("denticle" translates to "small tooth"), having a central

pulp cavity The pulp is the connective tissue, nerves, blood vessels, and odontoblasts that comprise the innermost layer of a tooth. The pulp's activity and signalling processes regulate its behaviour. Anatomy The pulp is the neurovascular bundle cent ...

supplied with blood vessels, surrounded by a conical layer of dentine, all of which sits on top of a rectangular basal plate that rests on the

. The outermost layer is composed of

, a largely inorganic enamel-like substance. Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size. Similar scales can also be found under the head of the

denticle herring The denticle herring (''Denticeps clupeoides'') is a small, up to long, species of ray-finned fish found only in the rivers of Benin, Nigeria, and western Cameroon. It is related to the herrings, but notable for its large anal fin and its arra ...

. The amount of scale coverage is much less in rays. Rhomboidal scales with the properties of both placoid and ganoid scales are suspected to exist in modern jawed fish ancestors: jawless

ostracoderms Ostracoderms () are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes) (may also be polyphyletic if anaspids are closer to cyclostomes) and ...

and then jawed

placoderms Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally 'Plate (animal anatomy), plate-skinned') is a Class (biology), class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devoni ...

Shark skin

Shark skin is almost entirely covered by small placoid scales. The scales are supported by spines, which feel rough when stroked in a backward direction, but when flattened by the forward movement of water, create tiny vortices that reduce hydrodynamic drag and reduce turbulence, making swimming both more efficient and quieter compared to that of bony fishes. It also serves a role in anti-fouling by exhibiting the lotus effect. All denticles are composed of an interior pulp cavity with a nervous and arterial supply rooted in the

to supply the denticle with mucus. Denticles contain riblet structures that protrude from the surface of the scale; under a microscope this riblet can look like a hook or ridges coming out of the scale. The overall shape of the protrusion from the denticle is dependent on the type of shark and can be generally described with two appearances. The first is a scale in which ridges are placed laterally down the shark and parallel with the flow of the water. The second form is a smooth scale with what looks like a hooked riblet curling out of the surface aiming towards the posterior side of the shark. Both riblet shapes assist in creating a turbulent boundary layer forcing the

laminar flow In fluid dynamics, laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral mi ...

farther away from the sharks skin. Unlike bony fish, sharks have a complicated dermal corset made of flexible

ous fibers arranged as a helical network surrounding their body. The corset works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy. Depending on the position of these placoid scales on the body, they can be flexible and can be passively erected, allowing them to change their angle of attack. These scales also have riblets which are aligned in the direction of flow, these riblets reduce the drag force acting on the shark skin by pushing the vortex further away from the skin surface, inhibiting any high-velocity cross-stream flow.

Scale morphology

The general anatomy of the scales varies, but all of them can be divided into three parts: the crown, the neck and the base. The scale pliability is related to the size of the base of the scale. The scales with higher flexibility have a smaller base, and thus are less rigidly attached to the ''stratum laxum.'' On the crown of the fast-swimming sharks there are a series of parallel riblets or ridges which run from an anterior to posterior direction. Analyzing the three components of the scale it can be concluded that the base of the denticle does not come into contact with any portion of the fluid flow. The crown and the neck of the denticles however play a key role and are responsible for creating the turbulent vortices and

eddies In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid beh ...

found near the skin's surface. Because denticles come in so many different shapes and sizes, it can be expected that not all shapes will produce the same type of turbulent flow. During a recent research experiment biomimetic samples of shark denticles with a crescent like microstructure and tested in a water tank using a traction table as a slide. The experiment showed that the surface with denticles experienced a 10% drag reduction overall versus the smooth sample. The reason for this drag reduction was that the turbulent vortices became trapped between the denticles, creating a ‘cushion like’ barrier against the laminar flow. This same type of experiment was performed by another research group which implemented more variation in their biomimetic sample. The second group arrived at the same conclusion as the first, however, because their experiment contained more variation within the samples they were able to achieve a high degree of experimental accuracy. In conclusion they stated that more practical shapes were more durable than ones with intricate ridge-lines. The practical shapes were low profile and contained trapezoidal or semi-circular trough-like cross sections, and were less effective but nonetheless reduced drag by 6 or 7%.

Drag reduction

Sharks decrease drag and overall cost of transport (COT) through multiple different avenues.

Pressure drag Parasitic drag, also known as profile drag, is a type of aerodynamic drag that acts on any object when the object is moving through a fluid. Parasitic drag is a combination of form drag and skin friction drag. It affects all objects regardless of ...

is created from the pressure difference between the anterior and posterior side of the shark due to the amount of volume that is pushed past the shark to propel itself forward. This type of drag is also directly proportional to the

. When the laminar flow increases around the fish the pressure drag does as well. Frictional drag is a result of the interaction between the fluid against the shark's skin and can vary depending on how the boundary layer changes against the surface of the fish. The riblets impede the cross-stream translation of the streamwise vortices in the viscous sublayer. The mechanism is complex and not yet understood fully. Basically, the riblets inhibit the vortex formation near the surface because the vortex cannot fit in the valleys formed by the riblets. This pushes the vortex further up from the surface, interacting only with the riblet tips, not causing any high-velocity flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. Recent research has shown that there is a pre and post breakdown regime in the near-wall boundary layer where the sublayer thickens at a declining rate and then abruptly undergoes a breakdown into turbulent vortices before finally collapsing. This system is completely self regulating and mediates the growth and decay cycle; the vortices accumulate during the growth period and abruptly liquidated in into Strouhal arrays of hairpin vortices lifting off the wall. Lifting vortices are what push the boundary layer out and away from the surface of the shark which results in reducing the overall drag experienced by the fish. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called

shagreen Shagreen is a type of rawhide consisting of rough untanned skin, historically from a horse's or onager's back, or from shark or ray. Etymology The word derives from the French ''chagrin'' and is related to Italian ''zigrino'' and Venetian '' ...

. One of the many historical applications of shark shagreen was in making hand-grips for

sword A sword is an edged, bladed weapon intended for manual cutting or thrusting. Its blade, longer than a knife or dagger, is attached to a hilt and can be straight or curved. A thrusting sword tends to have a straighter blade with a pointed ti ...

s. The rough texture of the skin is also used in Japanese cuisine to make graters called '' oroshiki'', by attaching pieces of shark skin to wooden boards. The small size of the scales grates the food very finely.

Technical application

In the marine industry, there is an extremely large market and need for anti-fouling surfaces. In laymen's terms, fouling is known as the process by which something becomes encrusted with material from the surrounding environment such as

barnacle A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in eros ...

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

, and green sludge. Dermal denticles are an extremely promising area of research for this type of application due to the fact that sharks are among the only fish without build up or growth on their scales. Studies by the U.S. Navy have shown that if a biomimetic material can be engineered, it could potentially lead to fuel cost savings for military vessels of up to 45%. There are many examples of

biomimetic material Biomimetic materials are materials developed using inspiration from nature. This may be useful in the design of composite materials. Natural structures have inspired and innovated human creations. Notable examples of these natural structures includ ...

s and surfaces based on the structure of aquatic organisms, including sharks. Such applications intend to enable more efficient movement through fluid mediums such as air, water, and oil. Surfaces that mimic the skin of sharks have also been used in order to keep microorganisms and

from coating the hulls of submarines and ships. One variety is traded as " sharklet". A lot of the new methods for replicating shark skin involve the use of polydimethylsiloxane (PDMS) for creating a mold. Usually the process involves taking a flat piece of shark skin, covering it with the PDMS to form a mold and pouring PDMS into that mold again to get a shark skin replica. This method has been used to create a biomimetic surface which has superhydrophobic properties, exhibiting the lotus effect. One study found that these biomimetic surfaces reduced drag by up to 9%, while with flapping motion drag reduction reached 12.3%. Along with marine applications, the aerospace industry can also benefit from these biomimetic designs.

Parametric model In statistics, a parametric model or parametric family or finite-dimensional model is a particular class of statistical models. Specifically, a parametric model is a family of probability distributions that has a finite number of parameters. Def ...

ing has been done on shark denticles with a wide range of design variations such as low and high-profile vortex generators. These biomimetic models were designed and analyzed to see the effects of applying the denticle-like structures to the wings of various airplanes. During the simulation, it was noted that the sample altered how the low and high angles of attack reacted. Out of both the low and high-profile samples tested, the low profile vortex generators outperformed the current smooth wing structures by 323%. This increase in performance is due to a separation bubble in the denticles wake and stream-wise vortices that replenish momentum lost in the boundary layer due to skin friction.

Scutes

FMIB 51542 Pine-cone fish, Monocentris japonicus (Houttnyn) Waka, Japan

Scutes are similar to scales and serve the same function. Unlike the scales of fish, which are formed from the epidermis, scutes are formed in the lower vascular layer of the skin and the epidermal element is only the top surface. Forming in the living dermis, the scutes produce a horny outer layer, that is superficially similar to that of scales. Scute comes from Latin for ''shield'', and can take the form of: * an external shield-like bony plate, or * a modified, thickened scale that often is keeled or spiny, or * a projecting, modified (rough and strongly ridged) scale, usually associated with the lateral line, or on the caudal peduncle forming caudal keels, or along the ventral profile. Some fish, such as

pineconefish Pinecone fishes are small and unusual marine fish of the family ''Monocentridae''. The family contains just four species in two genera, one of which is monotypic. Their distribution is limited to tropical and subtropical waters of the Indo-Pac ...

, are completely or partially covered in scutes. River herrings and threadfins have an abdominal row of scutes, which are scales with raised, sharp points that are used for protection. Some jacks have a row of scutes following the

on either side.

Scale development

Scales typically appear late in the development of fish. In the case of zebrafish, it takes 30 days after fertilization before the different layers needed to start forming the scales have differentiated and become organized. For this it is necessary that consolidation of the

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every o ...

occurs, then morphogenesis is induced, and finally the process of differentiation or late

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

occurs. * Mesenchyme consolidation: The consolidation or structuring of the mesenchyme originates during the development of the

. This process depends on whether the fish is cartilaginous or bony. For cartilaginous fish the structuring originates through the formation of two layers. The first is superficial and wide and the second is thin and compact. These two layers are separated by

mesenchymal cell Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage cel ...

s. Bony fish generate an

acellular Non-cellular life, or acellular life is life that exists without a cellular structure for at least part of its life cycle. Historically, most (descriptive) definitions of life postulated that an organism must be composed of one or more cells, ...

substrate organized by perpendicularly by collagen fibers. Subsequently, for both fish the

s elongate. These penetrate the compact layer of the mesenchyme, which consolidates prior to the formation of the scale, in order to initiate the dermal plate. * Morphogenesis induction: The morphogenesis is due to the formation of the epidermal papilla, which is generated by joining the

and

through a process of invagination. Morphogenesis begins at the time when fibroblasts are relocated to the upper part of the compact mesenchyme. Throughout this process, the basal cells of the

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

form a delimiting layer, which is located in the upper part of the mesenchyme. Subsequently, these cells will differentiate in the area where the scale primordium will arise. * Differentiation or late metamorphosis: This differentiation is generated by two different forms according to the type of scale being formed. The formation of elasmoid scales (cycloids and ctenoids) occurs through the formation of a space between the matrix of the epidermal papilla. This space contains collagen fibers. Around this space elasmoblasts differentiate and are responsible for generating the necessary material for the formation of the scale. Subsequently, matrix mineralization occurs, allowing the scale to acquire the rigid characteristic that identifies them. Unlike elasmoid scales, ganoid scales are composed of mineralized and non-mineralized collagen in different regions. The formation of these occurs through the entry of the surface cells of the mesenchyme into the matrix, the latter is composed of collagen fibers and is located around the vascular capillaries, thus giving rise to vascular cavities. At this point, elasmoblasts are replaced by

osteoblast Osteoblasts (from the Greek language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the p ...

s, thus forming bone. The patches of the matrix of the scale that are not ossified are composed of compacted collagen that allow it to maintain the union with the mesenchyme. This are known as

Sharpey fibers Sharpey's fibres (bone fibres, or perforating fibres) are a matrix of connective tissue consisting of bundles of strong predominantly type I collagen fibres connecting periosteum to bone. They are part of the outer fibrous layer of periosteum, ...

. One of the genes that regulate the development of scale formation in fish is the

sonic hedgehog Sonic hedgehog protein (SHH) is encoded for by the ''SHH'' gene. The protein is named after the character ''Sonic the Hedgehog''. This signaling molecule is key in regulating embryonic morphogenesis in all animals. SHH controls organogenesis and ...

(shh) gene, which by means of the (shh) protein, involved in organogenesis and in the process of

cellular communication A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...

, enable the formation of the scales. The

apolipoprotein E Apolipoprotein E (APOE) is a protein involved in the metabolism of fats in the body of mammals. A subtype is implicated in Alzheimer's disease and cardiovascular disease. APOE belongs to a family of fat-binding proteins called apolipoproteins. ...

(ApoE), that allows the transport and metabolism of triglycerides and cholesterol, has an interaction with shh, because ApoE provides cholesterol to the shh signaling pathway. It has been shown that during the process of

cell differentiation Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellula ...

and interaction, the level of ApoE transcription is high, which has led to the conclusion that this protein is important for the late development of scales.

Modified scales

Different groups of fish have evolved a number of modified scales to serve various functions. * Almost all fishes have a

, a system of

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...

s that detect water movements. In bony fishes, the scales along the lateral line have central pores that allow water to contact the sensory cells. * The dorsal fin spines of

dogfish shark Squalidae, more commonly known as dogfish, dog sharks, or spiny dogfish, are one of several families of sharks categorized under Squaliformes, making it the second largest order of sharks, numbering 119 species across 7 families. Having earned t ...

s and chimaeras, the stinging tail spines of stingrays, and the "saw" teeth of sawfishes and sawsharks are fused and modified placoid scales. *

Surgeonfish Acanthuridae are the family of surgeonfishes, tangs, and unicornfishes. The family includes about 86 extant species of marine fish living in tropical seas, usually around coral reefs. Many of the species are brightly colored and popular in ...

have a scalpel-like blade, which is a modified scale, on either side of the caudal peduncle. * Some

s, anchovies, and

halfbeak Hemiramphidae is a family of fishes that are commonly called halfbeaks, spipe fish or spipefish. They are a geographically widespread and numerically abundant family of epipelagic fish inhabiting warm waters around the world. The halfbeaks are ...

s have deciduous scales, which are easily shed and aid in escaping predators. * Male ''

Percina ''Percina'' is a genus of small freshwater ray-finned fish, classified within the subfamily Etheostomatinae, part of the family Percidae, which also contains the perches, ruffes and pikeperches from North America. Like the similar fishes in the ...

'' darters have a row of enlarged caducous scales between the

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

s and the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

. *

Porcupine fish Porcupinefish are fish belonging to the family Diodontidae (order Tetraodontiformes), also commonly called blowfish and, sometimes, balloonfish and globefish. They are sometimes collectively called pufferfish, not to be confused with the morpholo ...

es have scales modified into large external

spine Spine or spinal may refer to: Science Biology * Vertebral column, also known as the backbone * Dendritic spine, a small membranous protrusion from a neuron's dendrite * Thorns, spines, and prickles, needle-like structures in plants * Spine (zoolog ...

s. * By contrast,

pufferfish Tetraodontidae is a family of primarily marine and estuarine fish of the order Tetraodontiformes. The family includes many familiar species variously called pufferfish, puffers, balloonfish, blowfish, blowies, bubblefish, globefish, swellfis ...

have thinner, more hidden spines than porcupine fish, which become visible only when the fish puffs up. Unlike the porcupine fish, these spines are not modified scales, but develop under the control of the same network of genes that produce feathers and hairs in other vertebrates. File:FMIB 45807 Atopomycterus nichthemerus.jpeg,

have scales modified into

s. File:Tetrodon patoca Achilles 182.jpg,

Pufferfish Tetraodontidae is a family of primarily marine and estuarine fish of the order Tetraodontiformes. The family includes many familiar species variously called pufferfish, puffers, balloonfish, blowfish, blowies, bubblefish, globefish, swellfis ...

spines are not modified scales but are developed by an independent gene network.

Fish without scales

File:Synchiropus splendidus 2 Luc Viatour cropped.png, Mandarinfish lack scales and protect themselves with a layer of smelly and bitter slime. Fish without scales usually evolve alternatives to the protection scales can provide, such as tough leathery skin or bony plates. * Jawless fish ( lampreys and hagfishes) have smooth skin without scales and without dermal bone. Lampreys get some protection from a tough leathery skin. Hagfish exude copious quantities slime or mucus if they are threatened. They can tie themselves in an overhand knot, scraping off the slime as they go and freeing themselves from a predator. * Most eels are scaleless, though some species are covered with tiny smooth cycloid scales. * Most catfish lack scales, though several families have body armour in the form of dermal plates or some sort of scute. * Mandarinfish lack scales and have a layer of smelly and bitter slime which blocks out disease and probably discourages predators, implying their bright coloration is aposematic. *

Anglerfish The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...

have loose, thin skin often covered with fine forked dermal prickles or tubercles, but they do not have regular scales. They rely on camouflage to avoid the attention of predators, while their loose skin makes it difficult for predators to grab them. Many groups of bony fishes, including pipefish, seahorses, boxfish, poachers, and several families of

stickleback The sticklebacks are a family of ray-finned fishes, the Gasterosteidae which have a Holarctic distribution in fresh, brackish and marine waters. They were thought to be related to the pipefish and seahorses but are now thought to be more closel ...

s, have developed external bony plates, structurally resembling placoid scales, as protective armour against predators. * Seahorses lack scales but have thin skin stretched over a bony plate armour arranged in rings through the length of their bodies. * In boxfish, the plates fuse together to form a rigid shell or exoskeleton enclosing the entire body. These bony plates are not modified scales but skin that has been ossified. Because of this heavy armour boxfish are limited to slow movements, but few other fish are able to eat the adults. File:Lactoria fornasini1.jpg, Boxfish have plates of ossified skin fused together to form a rigid shell. File:Hippocampus guttulatus Achilles 174.jpg, Seahorses have thin skin stretched over bony plates arranged in rings. Some fish, such as hoki and swordfish, are born with scales but shed them as they grow. Filefish have rough non-overlapping scales with small spikes, which is why they are called filefish. Some filefish appear scaleless because their scales are so small. Prominent scaling appears on tuna only along the lateral line and in the corselet, a protective band of thickened and enlarged scales in the shoulder region. Over most of their body tuna have scales so small that to casual inspection they seems scaleless. File:Cantherhines fronticinctus.jpg, Some

filefish The filefish (Monacanthidae) are a diverse family of tropical to subtropical tetraodontiform marine fish, which are also known as foolfish, leatherjackets or shingles. They live in the Atlantic, Pacific and Indian Oceans. Filefish are closely rel ...

appear scaleless because their scales are so small. File:Bluefin-big.jpg, To casual examination tuna seem largely free of scales, but they are not.

Leviticus

A passage in Leviticus declares that, "of all that are in the waters... in the seas, and in the rivers" those that do not have both fins and scales "shall be an abomination unto you" and may not be eaten. This eliminates all aquatic invertebrates as

abominations ''Abominations'' is a three-issue Marvel Comics limited series created by Ivan Velez Jr Ivan () is a Slavic male given name, connected with the variant of the Greek name (English: John) from Hebrew meaning 'God is gracious'. It is associ ...

and

unclean Unclean may refer to: * Unclean animals, those whose consumption or handling is labeled a taboo * Unclean food, in Jewish dietary custom * Ritually impure, in various religions * Unclean hands, a term in contract law * ''Unclean'' (album), a 19 ...

, as well as any fish that lack scales (there do not seem to be fish that lack fins). According to the ''chok'' or divine decrees of the Torah and the Talmud, for a fish to be declared

kosher (also or , ) is a set of dietary laws dealing with the foods that Jewish people are permitted to eat and how those foods must be prepared according to Jewish law. Food that may be consumed is deemed kosher ( in English, yi, כּשר), fro ...

, it must have scales and fins.Aryeh Citron
"All About Kosher Fish"
/ref> The definition of "scale" differs from the definitions presented in biology, in that the scales of a kosher fish must be visible to the eye, and can be easily removed from the skin either by hand or scaling knife. According to the kosher certification agency of the

Orthodox Union The Orthodox Union (abbreviated OU) is one of the largest Orthodox Jewish organizations in the United States. Founded in 1898, the OU supports a network of synagogues, youth programs, Jewish and Religious Zionist advocacy programs, programs for ...

, a fish is kosher if the scales can be removed without tearing its skin. Thus carp and salmon are kosher, whereas a shark, whose scales are microscopic, a sturgeon, whose scutes can not be easily removed without cutting them out of the body, are all not kosher. Other non-kosher fish include catfish, eels, freshwater cod, snake mackerels, and puffer fish.

Lepidophagy

Dorsal view of right-bending and left-bending mouth morphs of the cichlid Perissodus microlepis - journal

Lepidophagy (Ancient Greek for ''scale-eating'') is a specialised feeding behaviour in fish that involves eating the scales of other fish. Lepidophagy has independently evolved in at least five freshwater families and seven marine families. Fish scales can be nutritious, containing a dermal portion and a layer of protein-rich mucus apart from the layers of keratin and enamel. They are a rich source of calcium phosphate. However, the energy expended to make a strike versus the amount of scales consumed per strike puts a limit on the size of lepidophagous fish, and they are usually much smaller than their prey. Scale eating behaviour usually evolves because of lack of food and extreme environmental conditions. The eating of scales and the skin surrounding the scales provides protein rich nutrients that may not be available elsewhere in the niche.

Fish jaw Most bony fishes have two sets of jaws made mainly of bone. The primary oral jaws open and close the mouth, and a second set of pharyngeal jaws are positioned at the back of the throat. The oral jaws are used to capture and manipulate prey by b ...

s normally show

bilateral symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...

. An exception occurs with the scale-eating

cichlid Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted this ...

'' Perissodus microlepis''. The jaws of this fish occur in two distinct morphological forms. One morph has its jaw twisted to the left, allowing it to eat scales more readily on its victim's right flank. The other morph has its jaw twisted to the right, which makes it easier to eat scales on its victim's left flank. The relative abundance of the two morphs in populations is regulated by frequency-dependent selection.

References

External links

* Hydrodynamic aspects of shark scale

*Fish scales and flow manipulatio

{{Authority control Fish anatomy