Placoid scale
D: dentin , E: basal plate, F: enameline layer , G: spine
Placoid scales are scale- like skin teeth that appear in cartilaginous fish and usually cover their entire bodies - if you run your hand over the skin of a shark , it feels like sandpaper . At the edge of the jaw they are transformed into teeth that are homologous to the teeth of the other vertebrates . In the sharks , the placoid scales form a closed exoskeleton , which gives the skin extreme firmness. In the rays of the species, the scale surfaces are reduced to specific regions and individual structures, and in the sea cats there is only a single, partially interrupted, row of placoid scales on both sides of the middle of the body.
Construction and education
A placoid scale consists of a basal plate made of cell-free bone material, which is anchored in the dermis by bone fibers , the Sharpey fibers . The basal plate merges into the actual, tooth-shaped scale. This consists entirely of dentin in the neck area and is coated on its surface, the crown, with a substance similar to tooth enamel , fish enamel . Inside the tooth there is a pulp cavity with connective tissue and blood vessels .
In most types of cartilaginous fish, the tips of the teeth point backwards, i.e. towards the caudal end. As a result, they do not create any flow resistance when the animal is moving in the water - if you run your hand over the shark skin in this direction, it will feel smooth. In the opposite direction, the skin is rough and very sharp like sandpaper. Depending on the species, the size of the crown is between 0.12 and 1.32 millimeters, on average around 0.4 millimeters.
Ontogenetic education
The placoid scale is formed by individual cells of the dermis (corium), which protrude into the epidermis and deposit the dentin required for the structure. These cells are called odontoblasts , and the resulting structure of the scale is called the odontode . The odontoblasts arrange themselves as a so-called dentin organ and release the dentin in a semicircle into the tooth cavity formed from the epidermal cells above. The fish enamel, on the other hand, is released from the surrounding epithelial cells and combines with the crystallizing dentine. The lower tooth neck grows out as a basal plate, which solidifies as a cell-free and fiber-free bone matrix.
Forms of placoid scales
The shape of the scales in the sharks is very diverse and depends on the species considered. The scales can be pavement-like, blunt, roof-tile-like or tapering to a point. In sharks, they cover the entire body and, as an exoskeleton, give the skin a strong firmness.
The design depends primarily on the ecological requirements of the species. Many small species such as the cat (Scyliorhinidae) and dogfish (Squalidae) have scales with ridges and lateral tips that protect them from predators and also from ectoparasites , these scales are very wide at the base. The Schwellhaie ( Cephaloscyllium ) have long, spine-like scales. They can inflate themselves by swallowing water, whereby the thorns stick out from the surface of the body and are also used to spread themselves in rock caves.
Plaster-like or broadly cross-shaped scales have mainly coral reef and shallow water species. The scale armor protects them from abrasion on the sharp-edged reef rocks and corals. You can find this, for example, with the bull head sharks or the noose sharks ( Centrophorus ). Slow swimmers of the benthos can develop long, nail-like scales with a broad base, such as the cuticle sharks ( Echinorhinus ). Angel sharks ( Squatina ) show a hybrid form between protection against enemies and against abrasion: With them, the back scales are thorn-like and the belly scales are rounded.
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. The tightly closed scales show a typical micro- relief of a few millimeter high strips parallel to the swimming direction. These influence the water flow and reduce the water resistance by up to 80%; they also have a complementary effect on stabilization in the water.
The special forms of scales include the teeth of the shark's bite , which are in several rows one behind the other in the shark's bite. The fin spines of spiny dogfish and the saw the sawfish and sawfish are special forms of Placoidschuppe.
Saw rays (Pristidae) and fiddle rays (Rhinobatidae) are also completely covered, they are rounded scales. The real rays (Rajidae) have large nail-like or thorn-like scales on the back, especially near the middle of the body and along the tail. In the stingrays and other species, these also form the tail sting, which in some species is connected to epidermal venom glands at the base of the sting. Electric rays (Torpedininae), eagle rays ( Myliobatidae), cow rays (Rhinopteridae) and devil rays (Mobulidae) do not have placoid scales, but their skin is equipped with significantly more epidemic mucous glands than in species with scales.
In the sea cats there is only a single, partially interrupted, row of placoid scales on both sides of the middle of the body, the mucous glands are also more common in them. Like dogfish, they also have a fin spike. In addition, the males wear a median on the underside of the head and a pair of so-called tenaculum below the pelvic fins . These consist of claw-like scales and are used by the animals to keep claws when mating .
Tribal history development
Tribal History, the placoid scales developed from the already named odontodes that fossil taxa of Placodermi and cartilaginous fish than small zähnchenartige hard tissues of the body covering lamellar bone on eating. In recent forms, the lamellar bones are reduced to the basal plate of the scales.
use
Due to the toughness of the skin and the rough and at the same time solid surface, shark skin is used in many regions of the world for grinding and polishing various materials, especially wood and stone. The very flexible shark leather , on the other hand, does not contain any placoid scales, so these are removed during the manufacturing process.
Due to their positive surface properties for reducing water resistance, similar structures were developed and successfully used primarily by NASA as surfaces for aerospace.
Special swimsuits have been developed for swimming , the surface of which mimics the skin of a shark and therefore has very low flow resistance. In addition, the suits increase the swimmer's buoyancy in the water and keep the athlete in an aerodynamically favorable position thanks to integrated elastic bands.
See also
literature
- Harald Schliemann: integument and appendage organs. and Alfred Goldschmid: Chondrichthyes, cartilaginous fish. In: W. Westheide, R. Rieger: Special Zoology. Part 2: vertebrates or skulls . Spektrum Akademischer Verlag, Munich 2004, ISBN 3-8274-0307-3 , pp. 21f and 200f.
Web links
- Cross-sectional images of placoid scales ( Memento from May 16, 2008 in the Internet Archive )
- Scheme of a placoid scale