Structure of skin
The skin forms the external covering of the body and performs various important functions in fish. The skin of fish is quite firmly attached and is hard and rough, and composed of two layers. The outer layer is called epidermis and the inner layer is called dermis or corium (Figure 4.6). The two layers differ in origin, structure and function. A thick basement membrane is present between the epidermis and dermis. In most fish species, the epidermis is thinner than the dermis.
The epidermis is a thin, multilayered epithelium derived from the ectoderm of the embryo. It does not contain blood vessels. The upper portion of epidermis consists of several layers of flattened epithelial cells, called stratified epithelium. The inner most layer is made up of columnar cells, called stratum germinativum in which cells are always multiplying by mitotic division to replace the outer worn out cells. Different types of unicellular and multicellular glands are formed from the epidermis. Some of these are confined to the epidermis while others grow deep into the underlying tissue of the dermis. These include holocrine mucous cells (produce mucous), chromatophores (impart color), taste buds, sensory cells, ampullary organs and a variety of other secretory cells. The mucous cells are basically of two types, the goblet cells and the club cells. The goblet cells produce mucous, which makes skin slimy and frictionless as well as protects from pathogens. The goblet cells develop from the stratum germinativum and migrate to the surface. The club cells produce specific secretions and are named accordingly as alarm cells, poison cells, photophores, etc. Other secretory cells that the epidermis may include are sacciform cells. Sacciform cells of some fish species may secrete alarm pheromones, whereas sacciform cells of other species may secrete toxic or repellent substances for antipredator defense. The epidermis is a fragile layer which is constantly sloughed off and renewed.
The dermis is mesodermal in origin and is composed mainly of fibrous connective tissues with collagen proteins. It contains blood vessels, nerves, lymph vessels, scales cutaneous sense organs, chromatophores and adipose tissues. The dermis is composed of three layers. The thin upper layer of loose connective tissue, where the scales have their base, is called stratum spongiosum. The thicker dense inner layer is called stratum compactum, which is built out of strong fibrous connective tissue. The middle layer is a subcutaneous layer which contains sense organs. All these three layers are sharply demarked from each other. Immediately below the dermis, and separating the dermis from the underlying skeletal musculature, is a layer of well-vascularized loose connective tissues with chromatophores and lipid cells called the hypodermis or subcutis. Integumentary colors are primarily dependent on the presence of chromatophores, which mostly occur in the stratum spongiosum, in the hypodermis, or both. The scales of teleost fishes arise from the secretary activity of the dermis.
Mucous/Slime
The goblet or mucoid cells are single-celled glands in the epidermis, producing a glycoprotein called mucin. This secretion is fibrous and swells up in water to form a thick layer of viscous mucous. Not all fish species have these cells, and the amount of mucous produced varies from enormous amounts (e.g., a bucketful by the hagfish, Myxine glutinosa) to none. Mucous has a protective function against infections and parasites, reduces the danger of damage to the skin through collisions and helps the fish to escape the grasp of predators. Mucous seals the body and prevents the exchange of ions and water. There are many more special functions described for a variety of species, varying from the protective nightgown of parrotfishes to a foodstuff for young cichlids.
Functions of skin
1. Lubricating function: Mucous secreted by the skin glands lubricates the fish and reduces body friction in water while swimming, thus enabling the fish to move with a greater speed.
2. Protective function: Mucous present in skin protects the body from external injuries, parasites, fungus, bacteria and other microorganisms.
3. Osmoregulatory function: Skin helps in regulating the osmotic exchanges of water and ions between the body fluids and surrounding medium.
4. Excretory function: Excretes metabolic wastes from the body.
5. Repairing and maintenance function: The skin performs an important function in healing the surface wounds.
6. Respiratory function: The skin is an important respiratory organ in certain species like Anguilla spp. (Eel), Anabas spp. (Climbing perch), Monopterus spp. (Mud eel), etc.
7. Offensive or defensive function: The club cells of the epidermis are modified to form poison gland in certain fishes (in many elasmobranches and few teleosts) used for offence as well as defense.
8. Impart color or Tint: The chromatophores of various kinds present in the epidermis and dermis of the fish give beautiful color patterns to the body making it conspicuous or inconspicuous (e.g., carotenoids – yellow-red, melaninsblack, flavins – yellow, erythrophores - red, purines – white, bile pigments- bluish green).
9. Metabolic function: The skin absorbs the dissolved nutrients from the surrounding water.
10. Heat regulation: The skin contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss, while constricted vessels greatly reduce cutaneous blood flow and conserve heat.
11. In some species, special structures like the electric organs, and phosphorescent organs are also integumentary derivatives.
Scales
Types and Structure
The body of all fishes (except the members of the Order- Siluriformes and some bottom-dwellers) is covered by an exoskeleton in the form of scales. In a few teleosts, scales are entirely absent (e.g., Silurids); or they exist only as microscopic vestiges hidden in the skin (e.g., Eels) and in some fishes, they become reduced to mare papillae of the dermis (e.g., Silurids). The body of some fishes is also covered by scutes, a bony plate (e.g., Sturgeons). The number of scales covering the body remains constant throughout life, and in general, scale growth is proportional to the fish growth. According to the mode of origin, there are two types of scales, placoid and non-placoid. The placoid scale is formed due to the secretary activities of both epidermis and dermis, while the non-placoid scale is derived from the dermis only. The scale of teleosts is of non-placoid type. Structurally, the non-placoid scales are classified as cosmoid, ganoid or rhomboid, cycloid, and ctenoid. Cycloid and ctenoid scales are also called bony ridge scales.
a. Placoid scale
The placoid scales are found in the Elasmobranchii and Holocephali. It consists of two parts: a flattened, disk like basal plate (embedded in the dermis) and a spine or cusp (projecting out through epidermis) (Figure 4.7). In structure, a placoid scale resembles that of a tooth. The spine has an external covering of enamel-like, hard, transparent material called vitrodentine. This is followed by a layer of dentine enclosing a pulp cavity from which several branching dental tubules radiate in different directions. The centre of the basal plate is perforated by an aperature known as opening of pulp cavity through which blood vessels and nerve from dermis supplied to the scale. These scales are closely set but do not overlap each other except where they protect the lateral line canal. The protruding spines of placoid scales give skin its characteristic rough texture. Placoid scales do not exhibit contineous, indefinite growth; they may be replaced when old, worn out, or lost.
b. Cosmoid scale
The cosmoid scales were found in the extinct Crossopterygii and Dipnoi. They are not present in the living fish. In the living Crossopterygii (Latimeria) and Dipnoi, they have become thin and modified to look like cycloid scales. The external layer of the cosmoid scale is thin and enamel-like and is called the vitrodentine (Figure 4.8). The middle layer is made up of hard, non-cellular, dentine-like material called the cosmine, and contains a large number of branching tubules and chambers. The inner layer is made up of vascularised bony substance, isopedine. These scales grew by the addition of new isopedine material from below, along the edges.
c. Ganoid scale
The ganoid scales are found in the primitive Actinopterygii and are of various forms and structures. Ganoid scales are usually rhomboidal or diamond-shaped. These scales are heavy and have an outer layer of hard, inorganic, enamel-like material called ganoine. The middle layer is cosmine containing numerous branching tubules. The innermost layer is thickest and is made up of lamellar bone, isopedine. These scales grow by the addition of new layers to lower as well as upper surface. They may slightly overlap and articulate one another by peg-and-sucket joints at the margin.
Ctenoid scales
• The ctenoid scales are also roughly rounded in shape as in cycloid scales (Figure 4.10-B).
• The anterior margin has more or less serrated edges.
• Several spines (ctenii) are present on the surface of the posterior area of the scale which give a rough texture to spiny-rayed fish. The word 'ctenoid' comes from the Greek word cteno, meaning comb or spine.
• Found in teleosts having spiny-rayed fins. E.g., Channa spp., Anabas spp., and other fishes of Perciformes order.
Both cycloid and ctenoid scales may be present on the same individual of certain fish species, such as Glossogobius giuris. In G. giuris, cycloid and cteoid scales are found on the body and head regions, respectively. Cyloid and ctenoid scales are of considerable help in calculating the age and growth rate of fishes. Many species undergo seasonal growth, which is apparent form, the lines of growth on the scale. In some species such as Salmo, spawning marks can be seen on the scales, so that it is possible to find how many times a fish has spawned.
Functions of scale
1. The scales of fishes are in the form of exoskeleton, which provides protection to the body.
2. It protects the body from external injury, parasites, fungus, bacteria and other microorganisms.
3. In some species (e.g. Puffers), the scales are elongated to form spines for protection.
4. In most teleosts, the scales along the lateral line become perforated to communicate sensory canal with the exterior.
5. The scales are of considerable help in calculating the age and growth rate of fishes.
6. In some fishes, scales exhibit remarkable coloration, which helps the fish to match with the surroundings.
7. The scales of many fishes show spawning rings and marks (e.g. Salmo spp.), which are the result of the cessation of feeding and exhaustion during the spawning period.
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