either forwards or backwards. Larval fish Locomotion Swimming Fish larvae, like many adult fishes, swim by undulating their body. The swimming speed varies proportionally with the size of the animals, in that smaller animals tend to swim at lower speeds than larger animals. The swimming mechanism is controlled by the flow regime of the larvae. Reynolds number (Re) is defined as the ratio of inertial force to viscous force. Smaller organisms are affected more by viscous forces, like friction, and swim at a smaller Reynolds number. Larger organisms use a larger proportion of inertial forces, like pressure, to swim, at a higher Reynolds number. The larvae of ray finned fishes, the Actinopterygii, swim at a quite large range of Reynolds number (Re ≈10 to 900). This puts them in an intermediate flow regime where both inertial and viscous forces play an important role. As the size of the larvae increases, the use of pressure forces to swim at higher Reynolds number increases. Undulatory swimmers generally shed at least two types of wake: Carangiform swimmers shed connected vortex loops and Anguilliform swimmers shed individual vortex rings. These vortex rings depend upon the shape and arrangement of the trailing edge from which the vortices are shed. These patterns depend upon the swimming speed, ratio of swimming speed to body wave speed and the shape of body wave. A spontaneous bout of swimming has three phases. The first phase is the start or acceleration phase: In this phase the larva tends to rotate its body to make a 'C' shape which is termed the preparatory stroke. It then pushes in the opposite direction to straighten its body, which is called a propulsive stroke, or a power stroke, which powers the larva to move forward. The second phase is cyclic swimming. In this phase, the larva swims with an approximately constant speed. The last phase is deceleration. In this phase, the swimming speed of the larva gradually slows down to a complete stop. In the preparatory stroke, due to the bending of the body, the larva creates 4 vortices around its body, and 2 of those are shed in the propulsive stroke. Similar phenomena can be seen in the deceleration phase. However, in the vortices of the deceleration phase, a large area of elevated vorticity can be seen compared to the starting phase. The swimming abilities of larval fishes are important for survival. This