Fish swim with fins and sink and float by regulating gas with a swim bladder.

Fish swim with their torso and tail, but rely on their individual fins for balance, and their swim bladder to regulate gases when surfacing and diving. The fish's tail is responsible for providing forward momentum as well as controlling direction. The pectoral fins, on the other hand, act as oars when the fish is swimming slowly, and are responsible for braking the movement when it is swimming fast.

Fish are variable-temperature aquatic vertebrates that breathe through gills, swim through the oscillations of the tail and trunk and the coordinated action of the fins, and feed by virtue of the upper and lower jaws, belonging to the vertebrate subphylum of the phylum Chordata, and are generally classified into the five major groups of fishes (53 percent), birds (18 percent), reptiles (12 percent), mammals (9 percent), and amphibians (8 percent).

Morphological features of fish

The appendages of fish are fins, which are organs of locomotion for swimming and maintaining body balance. The fins consist of branched fin stretcher bones and fin rays, which are divided into two types, an angular fin that is not segmented or branched and occurs from the epidermis, as seen in cartilaginous fishes, and a squamous or bony fin that is derived from the scales and is segmented, branched, or unbranched, as seen in sclerodactylous fishes, and is connected to each other by a thin flipper.

The bony fins are divided into two types, fin spines and soft strips. Fin spines are formed by deformation of one type of fin and are hard spines that are neither branched nor segmented, and are found in higher fishes. Soft strips are soft and articulated, with their distal ends branching (called branched fins) or not (called unbranched fins), and are formed by merging the left and right halves. The fins are divided into two categories: odd and even fins.

Even fins are paired fins, including one pair each of pectoral and ventral fins, which are equivalent to the anterior and posterior limbs of terrestrial vertebrates; odd fins are unpaired fins, including the dorsal, caudal, and anal (anal) fins. The basic function of the dorsal and anal fins is to keep the body balanced, prevent tilting and swaying, and help swimming, while the caudal fin, like a rudder, controls the direction and propels the fish forward.

All common fish have the above five types of fins: pectoral, ventral, dorsal, anal, and caudal. But there are a few exceptions, such as eels without even fins, odd fins also degraded; eels without ventral fins; electric eels without dorsal fins and so on.