Placoid scales are characteristic of fossil fish that died out tens, and some hundreds of thousands of years ago. However, in our time there are representatives of the underwater world, who still have a similar skin. You can learn about which fish still have placoid scales, about its structure, as well as other interesting facts from this article.
General information
The scales of fish is an outer cover formed by bone plates, composed of several layers, including specialized substances and tissues. Their structure determines the classification that exists in our time. There are four main types of scales - cosmoid, elasmoid, ganoid and placoid. It is about the last of them that we will talk in more detail.
Shapes and sizes of scales
Placoid denticles are isolated cone-like formations with an expanded base or the so-called basal plate, which is immersed in the dermis. A hard spike begins to grow from it at a certain angle. Byas it develops, it breaks through the epidermis and protrudes outward. Teeth with their tops are always directed from head to tail.
Usually, the average size of such a flake is no more than 0.3 mm. In some species of sharks and rays, it can grow up to 4 mm. In this case, the scales will have a more complex structure, since it is already a multi-vertex formation - the result of the fusion of several teeth at once. It was this structure that was inherent in the bone plates of most fossil fish.
In appearance, this type of scale in different fish is very different from each other. Dissimilarity can be both in the form of a spike and in its base. In some species of cartilaginous fish, the scales do not have a pointed tip. It looks like a fairly wide plate with several incisors along the edge and three or five longitudinal ridges. The structure of the base of the placoid scale is quite diverse; besides, its shape is also inherent in other classifications. Its edge is sometimes smooth or with processes, it can be elongated or rounded.
Internal structure
The appearance of bone plates with spikes can vary greatly from each other. As for the internal structure of the placoid scale, it is practically the same in all species. The outer coating can be formed by a durable substance durodentin or vitrodentin, as well as real enamel.
The base of the clove is a plate formed by cellular bone. His body is made of dentin. Under it is the pulp cavity. From her deep into the dentineleaves a whole network of branched tubules containing nerve fibers and blood capillaries. In the layers of the dermis, each clove holds strands of collagen fibers that originate in bone tissue. Interestingly, in humans, all the teeth in the jaw are held in this way. These strands are called Sharpei fibers (after the scientist who discovered and studied them).
Development
The formation of placoid scales begins with the formation of denticles. It occurs in the process of close interaction of two components - the epidermis and the dermis. First, the precursor of the tooth is born in the soft tissues. It is still impossible to determine where is the enamel and where is the dentin layer. Tissues become hard only when they grow to the size of the future toothed plate.
Such a developmental process as formation and its further hardening means that the scales of this type (and, in particular, its denticles), having fully matured, can no longer increase in size. It is known that the growth of fish continues throughout its life. After a certain period of time, the scales begin to wear out, and a new one appears instead. This process may be repeated several times during a lifetime. With each successive generation, the cloves get larger until they reach their maximum size. If the body still continues to grow, then the laying of additional bone plates begins. It is safe to say that such processes in the skin are characteristic of representatives of all types of cartilagefish.
Location
Placoid scales on the body of fish are unevenly distributed. It is not grouped in any clearly defined rows, but on the contrary - it happens randomly, because the teeth can be placed both on a separate area of \u200b\u200bthe skin, and over its entire surface.
Most often there is a certain distance between them, so a continuous scaly cover is not common. As a rule, such fish look "naked", but at the same time their skin has a special roughness. Sometimes you can observe a completely different picture, when some scales lean on others, thus covering the entire body and reliably protecting their owner.
Functions
Based on this information, scientists concluded that placoid scales do not always perform the tasks that other types of bone plates do. If they play the role of protective structures that form a fairly hard and reliable shell around the soft body of the fish, then the situation is different with placoid denticles. Their main task is to cut through the stream of water flowing along it at the time when the fish swims. In this case, micro-vortices appear, which significantly reduce the friction of the body, thereby simplifying its forward movement.
Shark teeth and bony plates
The placoid scales of these cartilaginous fish are known to have a variety of shapes. These can be spikes or bone plates with an uneven edge and longitudinal ridges. Each species of shark has its own shape of both teeth and scales. bone platesalmost the entire body of a predatory fish is covered. Only the gill slits remain vulnerable. It is noteworthy that in most safety instructions intended for divers, during a shark attack, it is recommended to strike at this unprotected place on the body of the fish. Experts say that such actions often help to scare away an aggressive predator.
It is also interesting that the scales on the body of a shark almost always have the same shape as the teeth. In addition, both of them have an almost identical structure and a special ability to constantly update. Scientists have proven that shark teeth are modified placoid scales. However, due to the fact that they perform different functions, they also have certain differences in both size and structure. Located in the oral cavity of the shark, the scales, increasing in size, become teeth. It can also change, forming other bony outgrowths on the skin, for example, saw blades in pylons or spines in katrans.
The body of a shark, covered with scales, reliably protects it from adverse external influences and from the teeth of other predators. If you run your hand along a rough surface in the direction from the caudal fin to the head, then you can peel off your skin to the point of blood. The scales of the predator are so strong that even a knife blow cannot damage it. There are places on the body of a shark where the skin is very thick. It easily carries loads up to 500 kg per 1 cm².
