Pisces - classification and structure

Fish are essentially aquatic species, able to breathe in water through their gills. With a total of about 32,000 species, fish represent 50% of vertebrates; together with molluscs and crustaceans, they are included in the "fishery products" of " man, for whom they constitute a "large (but increasingly less available) food resource.
Fish can be classified according to many criteria; among these, the main ones are the "living environment and the structural features.

Classification of fish according to "living environment

The classification based on the living environment distinguishes:

Sea fish, which in the Italian diet consist mainly of: anchovy, herring, sea bass, mullet, gurnard, lemon (like flounder or plaice), cod, hake, sea bream, flounder, St. Peter's fish, swordfish, ray, turbot, sardine, redfish, mackerel, sole, tuna, mullet etc.
Freshwater fish: barbel, carp, chub, whitefish, pike, catfish, perch (perch and yellow perch), char, tench, trout etc.
Mixed water fish: eel, salmon, sturgeon, etc.

Not all the fish listed above are Italian fish; moreover, for reasons of fishing density and productivity, most of the fish on the market come from oceanic seas or in any case from abroad.
NB. Many fish species can be farmed but, curiously, none of the bluefish species fall into this category. Furthermore, distinguishing saltwater fish from freshwater fish is not always easy, since many species considered marine possess the ability to survive and reproduce effectively even within the mouths and marsh valleys with brackish water.

Classification of fish according to structural features

The structural characteristics of fish essentially depend on their habitat of life; the classification differentiates two types of fish:

Flat fish: live lying on the bottom and have a flattened shape: sole, plaice, monkfish (or monkfish), turbot, soaso, dab, etc;
Tapered fish: those with "normal" shape, so to speak, demonstrate great hydrodynamics and propulsion in the fluid as excellent swimmers.

Structure of fish

The body of fish is essentially composed of 3 distinct parts:

The head, which includes the visual, olfactory, and gustatory organs, much of the central nervous system, and the gill slits for breathing
The central body, always covered by skin and often also by scales (or more correctly by scales) internally encloses the viscera and, between the thoracic cavity and the skin, it contains the muscular faci and adipose tissue (the latter especially in correspondence belly)
The tail or caudal fin is the appendix responsible for transforming muscle contraction into propulsion.

Fish are characterized by the presence of fins, essential appendages for propulsion, direction and swim stabilization. In particular:

Dorsal fin: it is in the back of the body, it can be more or less developed, consisting of a single segment or differentiate into 2 or 3 separate lobes
Caudal fin: as anticipated, it is the tail of the fish and is essential for the advancement
Anal fin: sometimes there are 2, it is in the belly of the body and from the center ends at the anus
Two pairs of paired fins: 2 pectoral or thoracic and 2 pelvic; of the two couples, each is positioned on one side of the body as if they were the front and rear "limbs".

The number and arrangement of the fins characterizes the various families of fish and represents a criterion of classification as well as recognition, but it is a rather complex topic of mainly zoological and biological interest.

Classification of fish according to skeleton tissue

Fish are "vertebrate" organisms, therefore they have only (or almost ...) an internal skeleton (the so-called bone or spine); this skeleton, which in fish can be of cartilage or bone tissue, represents a "further classification:

Pisces - classification and structure

Cartilaginous fish or Selaci: generally large, they are almost all predators and have the mouth in the ventral area of the head; they possess gill slits which open outwards and have heterocercial tails. It is a less numerous group than that of bony fishes and includes: sharks, rays, torpedoes, etc. They do not have scales but some have bony plates (such as sturgeon). Few species are used in gastronomy, some are: dogfish, blue shark, cagnolo, dogfish, emery, breed etc.
Bone fish (or Teleostei according to a classification now no longer used): it is an extremely large group and includes almost all the species used in gastronomy; they have the mouth positioned at the end of the head and the gills covered by a gill operculum, while the tail is almost always homocercal. They may have bare skin (such as eel, catfish, mostella, ling, etc.) or covered of scales (even tiny and not very visible, such as those of tuna or amberjack); they all have numerous muciparous glands, which make them slimy and hydrodynamic, and protect them from infections.

Classification of fish according to EEC regulation no. 2455/70, art. 6

Extra Category: the fish must be free of signs of pressure, scratches, dirt, strong discoloration
Category A: slight scratches are allowed
Category B: corresponds to the aforementioned requirements, except for some variations on scoring and deep pressures
Category C: does not meet the requirements indicated for Extra, A and B.

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