Anatomy of finfish and shellfish

UNIT 1 - Anatomy of Fin Fish

1.6. Scalation
Outstanding among the special feature of the skin are the very prominent scales that most fishes have. However some do not have scales. Examples include the lampreys (petromyzonidae) and a number of cat fishes North America (F.W) (Ietaluridae). An intermediate structural category have seals only on a few palces of the body for example: Paddle fish (polydon) three spines stickle bask Gasterosteus aculeatus and mirror carp. Then there are a few fishes that have very small deeply imbedded scales that they look scale less for example Fresh Water Eels (Anguilla) Brook trauts (Salvelius fontinalis) and the barbot (lota) of Cod family (F.W) (Gadidae)
In arrangements scales are most often “imbricated” and thus overlap like roof tiles with free margin directed towards the tail in a manner that minimize friction with water. In some fishes such as barbot (lota) and F.W eels (Anguilla) the pattern is mosaic rather than overlapping one another, the scales are minutely separated or meet their neighbors only at their margins

Scales shapes

Although they comprise only a few basic structural types, Scales exhibit many modifications that are often characteristic of groups or species on the basis of shape one type is plate like (Placoid), with each plate carrying a small cusp, as common among the sharks (Elasmobranchil) a Second type is diamond shaped (rhombic) and found in gars (Lepirosteidae) and the bichirs (pdypterus). A third type of scale is called Cycloid because it is typically smooth disc like and more or less circular mostly found in soft rayed fishes. (Malacopterygi). In fourth type is ctenoid the posterior surface or margin is toothed or comb like and a characteristic of spiny rayed bony fishes (Acunthopterygil)
However, some of the soft rayed fishes have ctenoid like contact organs on their scaes, examples characins (characid) Killi fishes (cyprinodontidac). Some spiny rayed fishes have cepeloid scales exclusively. For example the brooke silverside labidestus. Many spiny rayed species exhibit both cycloid and ctenoid scales.
Example: Common basses (Micropterus).

Structural types

Structurally, there are two types of fish scales, placoid and Non-placoid. Non-placoid scales are basically of the three kinds – cosmoid, Ganoid and Bony ridged scales.
I) Placoid scale
Placoid scales also called dermal denticles have an ectodermal cap or covering of enamel like substances (as an human teeth) termed “Vitrodentine” beneath this is a thicker layer of entine with a pulp cavity and dentinal tubules emanating form it. Each scale has a disc like basal plate in the dermis with a cusp projecting outward from it though the epidermis. Placoid scales occur among the sharks and their relatives (chandrichthyes).
II) Non – Placoid scales
1) Cosmoid scales: The cosmoid scales have a thinner, harder outer layer than the placoid ones. It is also termed as vitrodentine. The layer below this hard and non cellular and is called cosmine. The layer beneath is vascularised mid layer of perforated bony substances called “Iropedine” The growth of the scale is at the edge and beneath no ligivng cell layer covers the scale. The cosmoid scales found in (latimeria).
2) Ganoid Scale: In ganoid scale the outer layer is a hard inorganic substances called ganoine which is different from Vitrodentine. The layer below the ganoine is a cosmine like layer. The innermost layer is a bony layer called isopedine. The growth takes place at the edge, beneath and on the surface. Three types of scales are found in bichirs (pdypterus) and gars (Lepisosteidae).
3) Bony Ridges Scales: Bony –ridged scales are typically thin and transclucent lacking both dense enameled and dentinal layers of the three other kinds. Bony ridged scales characterizes the many living species of bony fishes (Osteichthyes) that have either cycloid or ctenoid scales. The outer layer of there scales is marked with bony ridges alternate with depression. The inner part or plate of the scale is made up of layers of criss – crossing fibrous connective tissues. The growth of the scales is both on the outer surface and from beneath.
For ridges on the scale two terms are used ridges and circuli changes in the growth pattern of the individuals fish is reflected in the character and distribution of ridges. “Breeding” and “Yearmarks” can be identified on this basis in many species. In both cycloid and ctenoid scales, a nuclear central zone can be recognized. This zone may be called as “focus” the scale. However this central position shifts due to differential growth of fore or after part of the scale, shifting the focus to more posterior and anterior margin. In many species grooves or radii radiate from or near focus towards one or more margins of the scale. The ctenoid scales have teeth on its posterior margins.