Fish Scales

Scales and denticles are considered the most complex derivatives of the integument in various fish species.
Scales function as a critical exoskeleton for most fish species.
Exceptions to this rule include catfishes and organisms like Acipenser, which possess scales in only a few regions of the body.
Scales originate from mesenchymal cells located in the dermis, which is the layer situated beneath the epidermis.

In cyclostomes, dermal bones are absent; instead, collagen fibers increase in volume and become densely packed.
In ostracoderms, dermal bones were significantly developed and organized into broad plates that covered the organism's entire body.
Throughout evolution from primitive to modern species, various types of scales emerged, including cosmoid, ganoid, placoid, cycloid, and ctenoid scales.

Scales are primarily categorized into two types based on their developmental origin:
* Placoid Scales: These develop from both the epidermis and the dermis. They are the characteristic scale type found in elasmobranchs.
* Non-Placoid Scales: These develop exclusively from the dermis. They are typical of teleostean fishes and include sub-types such as cosmoid, ganoid, cycloid, and ctenoid scales.

Distribution: Found specifically in elasmobranchs (sharks and rays).
Composition:
* They consist of dentine, which is deposited by osteocyte cells.
* They contain a pulp cavity that is ramified in a manner very similar to the structure of vertebrate teeth.
Structure:
    * Each scale is composed of two main parts: a basal plate and a projecting spine.
    * The basal plate is made of a substance similar to tooth cement, which is secreted by the dermis.
    * The spine projects outward, creating a rough texture on the skin, and develops from the malpighian layer of the epidermis.
Arrangement:
* Placoid scales do not overlap; they are set closely together within the skin.
* Scale size varies depending on the body region; they are typically larger on the snout and along the mid-dorsal line.
Modifications: In the sawfish (Pristis), the large spines or teeth found on the "saw" are actually modified placoid scales.

Physical Characteristics: These scales are thin, transparent, and roughly rounded (circular) in shape.
Surface Features: They exhibit alternating ridges and grooves.
Occurrence and Arrangement:
* Commonly found in major carps, such as Labeo rohita, Cirrhinus mrigala, and Catla catla.
* They project diagonally in an imbricating (overlapping like tiles) pattern to form a protective body covering.

Origin: These are derived from cycloid scales.
Distribution: They are confined to spiny-rayed teleosts and are found in a large number of perciform fishes.
Structure: Like cycloid scales, they are thin and circular. However, they possess a distinct margin and several spines on the posterior part of the body.

Historical Context: Found in extinct Crossopterygii and Dipnoi.
Current Status: They are not present in living fish in their original form.
Modern Variations: In living Crossopterygii (Latimeria) and living Dipnoi (lungfish), these scales have become thin and modified to the point that they resemble cycloid scales.

Characteristics: These are heavy scales that come in various forms and shapes.
Distribution: Found in primitive actinopterygian fishes, often called "ganoid fishes," such as Acipenser, Lepidosteus, and Amia.
Specific Variations:
* Lepidosteus: Scales are hard, rhombic, and plate-like, fitting together edge-to-edge.
* Amia: Scales are soft and possess a thin ganoid layer.

Protection: The primary function of scales is to provide a protective layer for the fish.
Predation: In sharks, scales are modified into teeth within the jaws to assist in catching prey.
Biological Aging: Scales are utilized by scientists to calculate the age of a fish and determine its rate of growth.
Scientific Research:
* Scales provide essential information regarding extinct fish species.
* They are used to identify the food habits of piscivorous (fish-eating) animals by analyzing scale remains.