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Fish Classification


There are two classes of fish: the class of chondrocytes (from Greek khondros: 'cartilage'; and ichthyes: 'fish'), or cartilaginous fish, and the class of osteitisers (from Greek osteon: 'bone'), or bony fish.

Bone fish are the most abundant in number of known species, representing about 95% of the total of these species. There are several differences between cartilaginous and bony fish.

The cartilaginous fish

Cartilaginous fish such as shark and ray live mainly in salt water. But some ray species live in freshwater.

Among the characteristics of cartilage fish we can consider:

  • cartilaginous skeleton and relatively light;
  • presence of five pairs of gill slits and a hole called spiraclewhere water enters and bathes the gills;
  • ventrally located mouth and gut ending in a kind of bag called cloaca - In it, the final ducts of the digestive, urinary and genital systems converge.


Basking Shark (Çetorhinus maximus)

The bony fish

Bone fish are abundant both in saltwater (mullets, sea bass, seahorses, hake, etc.) and in freshwater (lambaris, dorado, spotted, pacus, kingfisher, etc.).

Let's consider some characteristics of bone fish:

  • predominantly bone skeleton;
  • presence of four pairs of gill slits and absence of spirulum, gills protected by a structure called operculum;
  • mouth located anteriorly and intestine terminated in the anus, no cloaca;
  • presence, in many species, of a gas storage vesicle called swimming bladder or gas bladder.

Absent in cartilage fish, the swimming bladder - which can increase and decrease in volume according to the depth of the fish - favors flotation and thus allows the animal to save energy as it can remain more or less stable at a certain depth without requires a lot of muscle effort for swimming.

Fish breeding

Most bony fish have external fertilization: the female and the male release their gametes in the water. After fertilization of the egg by a sperm, the zygote forms. In many species of bony fish, the development is indirectwith larvae called fingerlings.

In cartilage fish, the fertilization is usually internal: The male introduces his sperm into the female's body, where the eggs are fertilized. O development is straightforward: Eggs give birth to puppies that are born with the general appearance of an adult, only smaller.


Paulistinha fish in the larval phase and in the adult phase; species is widely used in biomedical research