User:My Unique Deoxyribonucleic Acid/Fish anatomy

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Skin

The skin of the fish are a part of the integumentary system, which contains two layers: the epidermis and the dermis layer. The epidermis is derived from the ectoderm and becomes the most superficial layer that consists entirely of live cells, with only minimal quantities of keratin. It is generally permeable. The dermis is derived from the mesoderm and resembles the little connective tissue which are composed of mostly collagen fibers found in bony fish. Some fish species have scales that emerge from the dermis, penetrate the thin layer of the basement membrane that lies between the epidermis and dermis, and becomes externally visible and covers the epidermis layer.

Generally, the skin also contains sweat glands and sebaceous glands that are both unique to mammals, but additional types of skin glands are found in fish. Found in the epidermis, fish typically have numerous individual mucus-secreting skin cells called goblet cells that produce a slimy substance to the surface of the skin. This aids in insulation and protection from bacterial infection. The skin colour of many mammals are often due to melanin found in their epidermis. In fish, however, the colour of the skin are largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments. Many species, such as flounders, change the colour of their skin by adjusting the relative size of their chromatophores. Some fishes may also have venom glands, photophores, or cells that produce a more watery serous fluid in the dermis.

Placoid scales from a shark. A: Epidermis; B: Dermis; C: Pulp core; D: Dentine; E: Basal plate; F: Enamel; G: Spine

Scales

Main article: Fish scale

Also part of the fish's integumentary system are the scales that cover the outer body of many jawed fish. The commonly known scales are the ones that originate from the dermis or mesoderm, and may be similar in structure to teeth. Some species are covered by scutes instead. Others may have no scales covering the outer body.

Bowfin cycloid scales.

Bowfin cycloid singular scale.

There are four principal types of fish scales that originate from the dermis.

• Placoid scales, also called dermal denticles, are pointed scales. They are similar to the structure of teeth, in which they are made of dentin and covered by enamel. They are typical of cartilaginous fish (even though chimaeras have it on claspers only).

• Ganoid scales are flat, basal-looking scales. Derived from placoid scales, they have a thick coat of enamel, but without the underlying layer of dentin. These scales cover the fish's body with little overlapping. They are typical of gar and bichirs.

• Cycloid scales are small, oval-shaped scales with growth rings like the rings of a tree. They lack enamel, dentin, and a vascular bone layer. Bowfin and remora have cycloid scales.
• Ctenoid scales are similar to cycloid scales, also having growth rings, lack enamel, dentin, and a vascular bone layer. They are distinguished by spines or projections along one edge. Halibut have this type of scale.

  

  Fish scales. 1: Cycloid scale; 2: Ctenoid scale; 3: Placcoid scale; 4: Ganoid scale

Cycloid scale

Fish scales. A: Ganoid; B: Cycloid; C: Ctenoid

Another less common type of scale is the scute, which may be an external, shield-like bony plate; a modified, thickened scale that is often keeled or spiny; or a projecting, modified (rough and strongly ridged) scale. Scutes are usually associated with the lateral line, but may be found on the caudal peduncle (where they form caudal keels) or along the ventral profile. Some fish, such as pineconefish, are completely or partially covered in scutes. Their function are intended for protection as a body armor for fish against environmental abrasions and predations from other species.