Swimming movements
Each side of the vertebral column (flexible backbone) of a fish is made of muscle blocks or myotomes. The muscle blocks are arranged in a zig-zag fashion from head to tail. Each muscle block starts from the transverse process of one vertebra and ends on the transverse process of next vertebra. Each muscle block is separated from another by a connective sheet.
Swimming is brought about when the muscle blocks of one side contract bending the flexible vertebral column and at the same time the muscle blocks of the other side relax resulting in the bending of the body. The alternate contractions and relaxations of the muscle blocks and the bending of the vertebral column create wave-like movements that run from head to the caudal fin causing the side to side (lateral) movements of the caudal fin.
The bending of the body which pushes the tail against water provides forward movements. Also the lateral movements (side to side lashings) of the caudal fins push water backward and propel the fish forward. When the fish is swimming, the head and the tail turn to opposite direction, thus enabling the fish to maintain a straight course.
In a compact fish like tilapia, it is the penducle (tail stalk) that lashes from side to side providing most of the forward movement. In slender fish such as African spiny eel and Erpetoichithys calabaricus (sail fish) the whole body bends from side to side creating two or more curved areas that help to push water backward, thus providing forward movement.
Fin movements (Fins)
There are two kinds of fins: the paired fins which are the pectoral and pelvic fins and the unpaired fins which are the dorsal, anal and tail (caudal) fins.
1) Paired fins
The pectoral and pelvic fins are used for slow movement. Their movements when the fish is not in motion help to bring in fresh oxygenated water. They are used for balancing and steering. They also help to act as brakes.
2) Unpaired fins
a) Dorsal fin: Is used as a stabilizer that keeps the fish steady.
b) Anal fin: Also act as a stabilizer and helps the fish to maintain a straight course.
c) Tall fin: Is used for steering during locomotion. The streamlined shape of the body also helps for easy movement through the water.
Swim-bladder (Air bladder)
A swim-bladder is a sac-like organ in which gas accumulates in many fish. It functions as a hydrostatic organ which helps to maintain the specific gravity of the fish. By filling the swimming bladder with gas, the fish becomes lighter than water and thus rises up while the removal of the gas from the swim-bladder, the fish becomes heavier than water and therefore sinks.