72-525: The eagle rays are a group of cartilaginous fishes in the family Myliobatidae , consisting mostly of large species living in the open ocean rather than on the sea bottom. Eagle rays feed on mollusks and crustaceans , crushing their shells with their flattened teeth. They are excellent swimmers and are able to breach the water up to several metres above the surface. Compared with other rays, they have long tails, and well-defined, rhomboidal bodies. They are ovoviviparous , giving birth to up to six young at
144-506: A complex dentine -like layer called cosmine with a superficial outer coating of vitrodentine . The upper surface is keratin . Cosmoid scales increase in size through the growth of the lamellar bone layer. 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 . The layer of dentine that
216-478: A jigsaw rather than overlapping like other scales. In this way, ganoid scales are nearly impenetrable and are excellent protection against predation. 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 cycloid scales . Native Americans and people of the Caribbean used the tough ganoid scales of
288-506: 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. The bony scales of thelodonts , 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
360-442: A long time ago. However, there is currently no evidence of this. All chondrichthyans breathe through five to seven pairs of gills , depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills, whilst demersal species can actively pump water in through their spiracles and out through their gills. However, this is only a general rule and many species differ. A spiracle
432-429: 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
504-493: A mosaic of chondrichthyan and acanthodian traits. Dating back to the Middle and Late Ordovician Period, many isolated scales, made of dentine and bone, have a structure and growth form that is chondrichthyan-like. They may be the remains of stem -chondrichthyans, but their classification remains uncertain. The earliest unequivocal fossils of acanthodian-grade cartilaginous fishes are Qianodus and Fanjingshania from
576-409: A paraphyletic assemblage leading to Chondrichthyes. Some characteristics previously thought to be exclusive to acanthodians are also present in basal cartilaginous fish. In particular, new phylogenetic studies find cartilaginous fish to be well nested among acanthodians, with Doliodus and Tamiobatis being the closest relatives to Chondrichthyes. Recent studies vindicate this, as Doliodus had
648-402: A recent research experiment biomimetic samples of shark denticles with a crescent like microstructure were 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
720-402: A rectangular basal plate that rests on the dermis . The outermost layer is composed of vitrodentine , 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 amount of scale coverage is much less in rays. Rhomboidal scales with
792-418: 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 dermis 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
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#1732780850846864-406: A row of scutes following the lateral line on either side. 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 occurs, then morphogenesis is induced, and finally
936-492: 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 are covered with the cycloid scales of salmon and carp , or the ctenoid scales of perch , or the ganoid scales of sturgeons and gars . Cartilaginous fishes ( 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
1008-425: 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%. Denticles also provide drag reduction on objects where the main form of drag is caused by turbulent flow at the surface. A large portion of
1080-434: A shimmering effect to makeup and lipstick. Placoid (pointed, tooth-shaped) scales are found in the cartilaginous fishes : sharks , rays . They are also called dermal denticles . Placoid scales are structurally homologous with vertebrate teeth ("denticle" translates to "small tooth"), having a central pulp cavity supplied with blood vessels , surrounded by a conical layer of dentine , all of which sits on top of
1152-440: A spinal cord with spinal nerves. They have several sensory organs which provide information to be processed. Ampullae of Lorenzini are a network of small jelly filled pores called electroreceptors which help the fish sense electric fields in water. This aids in finding prey, navigation, and sensing temperature. The Lateral line system has modified epithelial cells located externally which sense motion, vibration, and pressure in
1224-637: A time. They range from 0.48 to 5.1 m (1.6 to 16.7 ft) in length and 7 m (23 ft) in wingspan. Nelson's book Fishes of the World treats cownose rays, mantas, and devil rays as subfamilies in the Myliobatidae. However, most authors (including William Toby White ) have preferred to leave the Rhinopteridae and Mobulidae outside of the Myliobatidae. White (2014) retained three genera ( Aetobatus , Aetomylaeus , and Myliobatis ) in
1296-412: A turbulent boundary layer forcing the laminar flow farther away from the sharks skin. Unlike bony fish, sharks have a complicated dermal corset made of flexible collagenous 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
1368-417: 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 , which lives in shallower water,
1440-784: 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%. Sharks decrease drag and overall cost of transport (COT) through multiple different avenues. Pressure drag
1512-422: 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 are abruptly liquidated into Strouhal arrays of hairpin vortices lifting off
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#17327808508461584-442: Is a small hole found behind each eye. These can be tiny and circular, such as found on the nurse shark ( Ginglymostoma cirratum ), to extended and slit-like, such as found on the wobbegongs (Orectolobidae). Many larger, pelagic species, such as the mackerel sharks (Lamnidae) and the thresher sharks (Alopiidae), no longer possess them. In chondrichthyans, the nervous system is composed of a small brain, 8–10 pairs of cranial nerves, and
1656-675: 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 well as possible hydrodynamic advantages. The term scale derives from the Old French escale , 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 anglerfishes . The morphology of
1728-534: Is also thought to play a role in the immune system). They are also produced in the Leydig's organ , which is only found in certain cartilaginous fishes. The subclass Holocephali , which is a very specialized group, lacks both the Leydig's and epigonal organs. Apart from electric rays , which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth (again, Holocephali
1800-545: 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 homologous 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
1872-553: Is an exception, as the teeth are lost in adults, only kept on the clasping organ seen on the caudal ventral surface of the male), also called placoid scales (or dermal denticles ), making it feel like sandpaper. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other. Originally, the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in
1944-402: Is assumed that their oral teeth evolved from dermal denticles that migrated into the mouth, but it could be the other way around, as the teleost bony fish Denticeps clupeoides has most of its head covered by dermal teeth (as does, probably, Atherion elymus , another bony fish). This is most likely a secondary evolved characteristic, which means there is not necessarily a connection between
2016-408: Is claimed with competitive swimwear. Parametric modeling has been done on shark denticles with a wide range of design variations such as low and high-profile vortex generators. Through this method, the most thorough characterization has been completed for symmetrical two-dimensional riblets with sawtooth, scalloped and blade cross sections. These biomimetic models were designed and analyzed to see
2088-403: Is created from the pressure difference between the anterior and posterior sides 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 laminar flow . 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
2160-409: Is internal. Development is usually live birth ( ovoviviparous species) but can be through eggs ( oviparous ). Some rare species are viviparous . There is no parental care after birth; however, some chondrichthyans do guard their eggs. Capture-induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks/rays when fished. Capture-induced parturition
2232-407: Is often mistaken for natural birth by recreational fishers and is rarely considered in commercial fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date). The class Chondrichthyes has two subclasses: the subclass Elasmobranchii ( sharks , rays, skates, and sawfish ) and the subclass Holocephali ( chimaeras ). To see the full list of
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2304-482: The alligator gar for arrow heads, breastplates, and as shielding to cover plows. In current times jewellery is made from these scales. Leptoid (bony-ridge) scales are found on higher-order bony fish, the teleosts (the more derived clade 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
2376-665: The bony fishes , and the clade that includes spiny sharks and early cartilaginous fish . The modern bony fishes, class Osteichthyes , appeared in the late Silurian or early Devonian, about 416 million years ago. The first abundant genus of shark, Cladoselache , appeared in the oceans during the Devonian Period. The first Cartilaginous fishes evolved from Doliodus -like spiny shark ancestors. Zangerl, 1981 [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] Placoid scale A fish scale
2448-474: The cartilaginous fish or chondrichthyans , which all have skeletons primarily composed of cartilage . They can be contrasted with the Osteichthyes or bony fish , which have skeletons primarily composed of bone tissue . Chondrichthyes are aquatic vertebrates with paired fins , paired nares , placoid scales , conus arteriosus in the heart , and a lack of opercula and swim bladders . Within
2520-443: 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
2592-600: The integument . Development starts near the caudal fin , along the lateral line of the fish. The development process begins with an accumulation of fibroblasts between the epidermis and dermis . Collagen fibrils begin to organize themselves in the dermal layer, which leads to the initiation of mineralization . The circumference of the scales grows first, followed by thickness when overlapping layers mineralize together. Ctenoid scales can be further subdivided into three types: Most ray-finned fishes have ctenoid scales. Some species of flatfishes have ctenoid scales on
2664-486: The Myliobatidae, while a fourth ( Pteromylaeus ) was synoymized with Aetomylaeus . A 2016 paper placed Aetobatus in its own family, the Aetobatidae . Cartilaginous fish Chondrichthyes ( / k ɒ n ˈ d r ɪ k θ i iː z / ; from Ancient Greek χόνδρος ( khóndros ) 'cartilage' and ἰχθύς ( ikhthús ) 'fish') is a class of jawed fish that contains
2736-409: 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 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
2808-510: 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. The general anatomy of the scales varies, but all calcium composites hydrolize scales out side of main skeleton of them it's can be divided into three parts:
2880-431: 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
2952-399: The current smooth wing structures by 323%. This increase in performance is due to a separation bubble in the denticle's wake and stream-wise vortices that replenish momentum lost in the boundary layer due to skin friction. 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
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3024-401: 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
3096-462: The earliest lungfishes (subclass Dipnoi ), and in Crossopterygii , 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
3168-544: The early Silurian ( Aeronian ) of Guizhou , China around 439 million years ago, which are also the oldest unambiguous remains of any jawed vertebrates. Shenacanthus vermiformis , which lived 436 million years ago, had thoracic armour plates resembling those of placoderms. By the start of the Early Devonian, 419 million years ago, jawed fishes had divided into three distinct groups: the now extinct placoderms (a paraphyletic assemblage of ancient armoured fishes),
3240-422: 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. Both the geometry of the denticles and their arrangement have a profound effect on the aerodynamic response of the aerofoils. Out of both the low and high-profile samples tested, the low-profile vortex generators outperformed
3312-547: The eyed side and cycloid scales on the blind side, while other species have ctenoid scales in males and cycloid scales in females. 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
3384-518: 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 materials 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
3456-447: The fish . Leptoid scales come in two forms: cycloid (smooth) and ctenoid (comb-like). 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 carp . 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,
3528-401: The forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development. Like all other jawed vertebrates, members of Chondrichthyes have an adaptive immune system . Fertilization
3600-486: 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
3672-417: The infraphylum Gnathostomata , cartilaginous fishes are distinct from all other jawed vertebrates. The class is divided into two subclasses: Elasmobranchii ( sharks , rays , skates and sawfish ) and Holocephali ( chimaeras , sometimes called ghost sharks, which are sometimes separated into their own class). Extant chondrichthyans range in size from the 10 cm (3.9 in) finless sleeper ray to
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#17327808508463744-410: 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 are found in the sturgeons , paddlefishes , gars , bowfin , 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
3816-462: The middle when scapulocoracoid and puboischiadic bars evolved. In rays , the pectoral fins are connected to the head and are very flexible. One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion. Chondrichthyans have tooth-like scales called dermal denticles or placoid scales. Denticles usually provide protection, and in most cases, streamlining. Mucous glands exist in some species, as well. It
3888-495: 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
3960-477: The over 10 m (33 ft) whale shark . The skeleton is cartilaginous. The notochord is gradually replaced by a vertebral column during development, except in Holocephali , where the notochord stays intact. In some deepwater sharks, the column is reduced. As they do not have bone marrow , red blood cells are produced in the spleen and the epigonal organ (special tissue around the gonads , which
4032-440: The place of cosmine , and a layer of inorganic bone salt called ganoine in place of vitrodentine . 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 bones and fin rays in some non-teleost ray-finned fishes , such as gars , bichirs , and coelacanths . It is composed of rod-like apatite crystallites. Ganoine
4104-543: The properties of both placoid and ganoid scales are suspected to exist in modern jawed fish ancestors: jawless ostracoderms and then jawed placoderms . 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
4176-652: 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 are found only on ancient lobe-finned fishes , including some of
4248-556: 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 and calcium carbonate and a deeper layer composed mostly of collagen . The enamel of the other scale types is reduced to superficial ridges and ctenii. Ctenoid scales, similar to other epidermal structures, originate from placodes and distinctive cellular differentiation makes them exclusive from other structures that arise from
4320-413: 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
4392-485: 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: Some fish, such as pineconefish , 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
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#17327808508464464-628: 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. 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 barnacles , algae , and green sludge . Dermal denticles are an extremely promising area of research for this type of application due to
4536-496: The skin of sharks have also been used in order to keep microorganisms and algae 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
4608-452: The skin. Fish scales are part of the fish's integumentary system , and are produced from the mesoderm layer of the dermis , which distinguishes them from reptile scales . The same genes involved in tooth and hair development in mammals are also involved in scale development. The placoid scales of cartilaginous fishes are also called dermal denticles and are structurally homologous with vertebrate teeth. Most fish are also covered in
4680-704: The species , click here . The fossil record of the Holocephali starts in the Devonian period. The record is extensive, but most fossils are teeth, and the body forms of numerous species are not known, or at best poorly understood. * position uncertain Cartilaginous fish are considered to have evolved from acanthodians . The discovery of Entelognathus and several examinations of acanthodian characteristics indicate that bony fish evolved directly from placoderm like ancestors, while acanthodians represent
4752-404: The teeth and the original dermal scales. The old placoderms did not have teeth at all, but had sharp bony plates in their mouth. Thus, it is unknown whether the dermal or oral teeth evolved first. It has even been suggested that the original bony plates of all vertebrates are now gone and that the present scales are just modified teeth, even if both the teeth and body armor had a common origin
4824-699: 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
4896-409: The total drag on long objects with relatively flat sides usually comes from turbulence at the wall, so riblets will have an appreciable effect. Along with marine applications, the aerospace industry can benefit greatly from these biomimetic designs. Other applications include pipes, where they score the insides to a riblet-like roughness and have discovered a 5% drag reduction, and a few percent reduction
4968-477: 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
5040-455: 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 . One of the many historical applications of shark shagreen was in making hand-grips for swords . The rough texture of
5112-450: The water around them. Most species have large well-developed eyes. Also, they have very powerful nostrils and olfactory organs. Their inner ears consist of 3 large semicircular canals which aid in balance and orientation. Their sound detecting apparatus has limited range and is typically more powerful at lower frequencies. Some species have electric organs which can be used for defense and predation. They have relatively simple brains with
5184-689: 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 Eusthenopteron . 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 Eusthenopteron , amiids, and teleosts, whose cycloid and ctenoid scales represent
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