* Water has a higher relative density (800 times higher than air) and is a viscous medium to move through. * The body of a fish is of a streamlined shape to move quickly in water. * The streamlined shape of a fish:
* a) is long ellipse tapering to a point (shaped like a torpedo) in the direction of the flow of water. * b) reduces the drag (a drag is water resistance caused by friction between moving water and the body of the fish).
* The general adaptive features in a fish for locomotion in water: * a) the anterior of a fish is smooth and rounded.
* b) there is no neck and the head merges with the body * c) its body is broad at the interior and tapers to the tail. * d) the body is covered with overlapping scales and a slimy coating to reduce friction. * e) the presence of fins to stabilise the fish and help it move efficiently through water. * A fish has different types of fins with different functions for locomotion: * a) Dorsal Fin– increases the vertical surface of the fish and keeps it upright by preventing it from rolling sideways. * b) Caudal Fin– generates the forward force (thrust) which propels a fish forward and aids in steering the fish * c) Anal Fin– provides the same function as the dorsal fin. * d) Pelvic Fin– a pair of pelvic fins which is used to provide stability and steering while swimming. * e) Pectoral Fin– a pair of fins located on each side of the fish that allow the fish to swim backwards, stay in one spot, and move up and down or from side to side.
* A fish swims by wavelike movement, moving from head to tail. * These movement:
* a) push the water backwards and the fish
* b) cause the fish to become unstable in water
when they are swimming, leading to
i. Yawing (sideway movement),
ii. Pitching (vertical plane movement) and
iii. Rolling (transverse plane movement).
* c) are overcome by the fins on the fish’s body.
* Propulsion is generated by myotomes – the antagonistic muscles:
* a) The wavelike movement are generated by antagonistic muscle blocks called myotomes on both sides of its vertebral column.
* b) Dorsally, each myotome has a zig-zag shape.
* c) When myotomes on either side contacts, the backbone (vertebral column curves) towards it.
* d) Starting from the anterior towards the tail, myotomes on both sides contract and relax alternately producing a series of waves which propel the fish through the water.