While placoid scales are similar in some ways to the scales of bony fish, they are more like teeth covered with hard enamel. Unlike the scales of other fish, these do not grow after an organism has fully matured. Placoid scales are often called dermal denticles because they grow out of the dermis layer. Placoid scales are packed tightly together, are supported by spines, and grow with their tips facing backward and laying flat.
Placoid scales are rough to the touch and the structure they form is nearly impossible to penetrate. These scales function to protect a fish from predators and can even be used to injure or kill prey. The v-shape of a placoid scale reduces drag and increases turbulence as a fish moves through the water so that they may swim more quickly and quietly, all while expending less energy.
Placoid scales form a matrix that is so dynamic and fluid that swimsuits have been designed to mimic their composition. The flat rectangular base plate of a placoid scale is embedded in a fish's skin. Like teeth, placoid scales have an inner core of pulp composed of connective tissues, blood vessels, and nerves.
They are a part of the fish. The pulp cavity is nursed by a layer of odontoblast cells that secrete dentine. This hard, calcified material forms the next layer of scales, which fits tightly between the old layers.
The dentine is coated in vitrodentine, which is an enamel-like substance that is produced by the ectoderm and is even harder than dentine. Once the scale erupts through the epidermis, it cannot be coated in any more enamel. Different species of cartilaginous fish support their scales with unique spines based on the shape and role of the fish.
A species can be identified by the shape of its scales. Because rays are flat and sharks are more angular, the spines of their placoid scales are slightly different to allow both fish to swim quickly. Probably that they live in the water, breathe using gills and swim with their fins. You might also say that fish bodies are covered in scales. Did you know that sharks, just like other fish, have scales, too?
The crowns generally end in a pointy tip that faces towards the tail, which is why shark skin is smooth when you rub it from head to tail, but rough when you rub from tail to head. In fact, these scales are so rough that the ancient Greeks used shark skin as sandpaper to smooth wooden objects.
So why are they shaped like that? These special scales have several functions that help sharks go about their lives in the ocean. Sharks can encounter things in the ocean that can scrape them like coral reefs, rocky areas and marine litter like leftover fishing gear. Sharks also have a thick fibrous dermis that part of the skin which is below the epidermis which supports the scales and helps protect the animal.
Other cartilaginous fish that swim like sharks, such as the Guitar-fish Rhinobatidae and the Saw-fish Pristidae also have a complete covering of denticles.
However, those species that are dorso-ventrally flattened — such as the rays — tend to have many fewer denticles. In the Skates Rajidae they are scattered in patches across the pectorals and on the head. In the Eagle Rays Myliobatidae they are very few in number and in the Electric Rays Torpedinidae they are absent, except in the modified form of the tail spines.
The Devil-fishes Mobulidae have none at all. In their ancient form, they had four layers. One of dense bone, one of spongy bone, one of dentine and one of enamel. The scales of the remaining bony fishes have only two layers, a calcified one and a fibrous one. Ganoid scales are derived from Cosmoid scales and are the evolutionary older style. They are found on Bichirs, Gar-fish, Sturgeons and Reedfish. The most common form of fish scale is the elasmoid scale.
It is the thin plate that you find on most fishes. However there are intermediate forms and these two terms are really just adjectives that represent the extremes of a continuum. The scales of a fish may be all of one kind perch have ctenoid scales while herrings, minnows and trout all have cycloid scales or both type of scales may be found on the same fish. Sea Perches, Epinephelus sp. Not all fish have scales, some species such the Sun-fish Mola mola and the Siluroidei Naked Catfish have none at all.
Other species, like the Common Eel appear to have no scales, but they really have microscopic scales deeply embedded in their dermis. Even in those fish that have scales, they do not always cover the whole body. Thus fish that swim quickly, or that live in fast flowing waters Trout, Tuna etc , tend to have small scales. While fish that swim slowly in slow moving waters, tend to have larger scales, i.
In some S. American Catfishes, the scales have become modified into bony plates to make armour. This is also the case in Sea horses and Pipe fish. In these cases the added protection is paid for by reduced flexibility and speed.
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