Characteristics of fish.
Their body is wider in the middle than at the ends. This helps fish move through water better because
there is less resistance.
They have fins (dorsal, pectoral, ventral and caudal).
Their body temperature changes with the environment they live in (that is why we say they are
They breathe through gills: they take in water through their mouths and pump it through their gills.
The gills have a lot of blood vessels that allow gas exchange (oxygen and CO2) between water and
They lay eggs with no shell.
The main types of fish
Fish are classified into bony fish and cartilaginous fish:
Bony fish. They have a bony skeleton. Their gills are protected by a hard cover, called the
operculum. Their mouth is at the end of their head and their caudal fin is divided into two equal
halves. Examples: trout, salmon and hake. These fish can surface and dive easily.
Cartilaginous fish. They have a cartilaginous skeleton. Their gills haven´t an operculum. Their mouth
is found on the underside of the head and their caudal fin is divided into two halves of different sizes.
Examples: sharks, rays and manta rays.