Fish scale#Placoid scales

Placoid scales are imbricated Hautzähnchen that occur in the cartilaginous fish and usually cover their entire body - it takes about a hand over the skin of a shark, this will feel them like sandpaper. At the jaw edges are formed into teeth the teeth of the other vertebrates homologous. In the sharks the placoid scales form a closed exoskeleton through which the skin takes on an extreme strength. In the rays the shed surfaces reduce Type specific to certain regions and individual structures and the chimaeras only a single, partially broken, number of placoid scales exist on both sides of the midsection.

Design and Education

A Placoidschuppe consists of a basal plate from cell-free bone material, which is anchored by bone fibers, the Sharpey fibers in the dermis. The basal plate goes into the actual, tooth-shaped scale. This is entirely in the neck area of dentin and is on its surface, the Crown, with a enamel -like substance, the melting fish, covered. Inside the Zähnchens is a pulp cavity with connective tissue and blood vessels.

The tips of the teeth are in most species of cartilaginous fish backwards, to the caudal end. Thus they do not form a resistance to flow when the animal moves away in the water - can be spread by hand in this direction over the shark's skin feels this smooth. In the opposite direction the skin is rough and very sharp like sandpaper. The size of the crown is species-specific, 0.12 to 1.32 millimeters, the average is around 0.4 millimeters.

Ontogenetic Education

Is formed by the Placoidschuppe individual cells of the dermis ( dermis ), which protrude into the epidermis and deposit the dentin required for the construction. These cells are called odontoblasts, the resulting preliminary structure of the scale as Odontode. The odontoblasts arrange themselves as so-called Dentinorgan and enter the dentin in the semi-circular, formed from the overlying epidermal cells, tooth cavity from. The fish melting is given, however, from the surrounding epithelial cells and connects to the crystallizing dentin. The lower tooth neck grows from a basal plate which solidifies as cell-free and fiber-free bone matrix.

Forms of placoid scales

The shape of the scales is very diverse in sharks and depending on the considered Art The scales can both plaster -like blunt, imbricate or pointed his tapering. When the sharks they cover the entire body and give the skin according exoskeleton as a strong resistance.

The design is mainly dependent on the ecological requirements of the species. So have many small species such as the cat ( Scyliorhinidae ) and spiny dogfish ( Squalidae ) scales with ridges and lateral tips to protect them from predators and also from ectoparasites, these scales are very wide at the base. The Schwellhaie ( Cephaloscyllium ) have long, thorn-like scales. You could blow through water swallowing, so that the spines protruding from the body surface and are also used to splay themselves in caves.

Plaster -like or broad cross-shaped scales have mainly species of coral reefs or shallow water species. For them, the scale armor protects against abrasion on the sharp-edged Riffgesteinen and coral. One finds this example, with the bull's head sharks or the Schlinghaien ( Centrophorus ). Slowly float the benthos may develop long, nail-like scales, broad-based, such as the nail sharks ( Echinorhinus ). Angel sharks ( Squatina ) show a mixed form of protection against enemies and against abrasion: For them the back shed the belly scales are formed like a mandrel and roundish.

Deep-sea forms, such as the blue shark ( Prionace glauca ), the hammerhead sharks ( Sphyrnidae ), mackerel sharks ( Lamnidae ) and the thresher shark ( Alopias vulpinus ) are very fast swimmers. For them, the tight closed horny dress, a typical micro-relief of a few millimeters high strips parallel to the swimming direction. These affect the flow of water and put the water capacity is reduced by up to 80%, they also act in addition to the stabilization in water.

Among the special forms of dandruff include primarily the teeth of Haigebisses that after the other in several rows in the teeth of the shark. The fin spines of the spiny dogfish and the saw of the sawfish and sawfish are special forms of Placoidschuppe.

Sawfish ( Pristidae ) and violin (Rhinobatidae ) are also completely covered in them is roundish scales. The Real skates ( Rajidae ) have on the dorsal side, especially near the center of the body and along the tail large nail or thorn-like scales. In the stingrays and other species these also form the tail sting, which is in some species associated with epidermal poison glands at the base of the spine. Electric Rays ( Torpedininae ), eagle rays ( Myliobatidae ) Kuhrochen ( Rhinopteridae ) and Devil Rays ( Mobulidae ) do not have placoid scales, because their skin is, however, equipped with significantly more epidemialen mucous glands than in species with scales.

In the chimaeras only a single, partially broken, number of placoid scales exist on both sides of the middle of the body, even in them the mucous glands are increasingly ago. Like the spiny dogfish also they have a fin spine. In addition, the males carry on the head underside a median and a paired so-called Tenaculum below the pelvic fins. These consist of claw -shaped flakes and serve the animals at the clawing during copulation.

Phylogenetic development

Tribal History developed the placoid scales of the odontodes already named that rise ate in fossil taxa of placoderms and the cartilaginous fish as small zähnchenartige hard tissues of the body covering lamellar bone. In recent forms the lamellar bone are reduced to the basal plate of the scales

Use

Due to the toughness of the skin and harsh, while solid surface sharkskin is used in many regions of the world for grinding and polishing of different materials, especially wood and stone. In contrast, the very flexible sharkskin contains no placoid scales, these are therefore removed in the manufacturing process.

Due to their positive surface properties to reduce water resistance -like structures were mainly developed by NASA as a surface for the aerospace and used successfully.

For the sport of swimming special swimsuits were developed whose surface imitates the skin of a shark and thus has a very low flow resistance. In addition, the suits increase the buoyancy of the float in the water and keep the athlete by incorporated elastic bands in the aero position.