page_title: Fish locomotion
section_title: Swimming
hierarchical_section_title: Fish locomotion / Larval fish / Locomotion / Swimming
caption_attribution_description: English: Salmon egg hatching (Salmo salar) - the Alevin (larva) has grown around the remains of the yolk sac - visible are the arteries spinning around the yolk and little oildrops, also the gut, the spine, the main caudal blood vessel, the bladder and the arcs of the gills. In about 24hrs it will be a fry without yolk sac. Français: Eclosion d'un œuf de saumon (Salmo salar). La larve (ou alevin) a grandi autour des restes du sac vitellin. On peut voir les artères qui entourent ce sac, des goutelettes de graisse, ainsi que les intestins, la colonne vertébrale, les principaux vaissaux sangin, la vessie et les arcs branchiaux. Dans 24 heures l'alevin n'aura plus ce sac vitellin.
context_page_description: Fish locomotion is the variety of types of animal locomotion used by fish, principally by swimming. This however is achieved in different groups of fish by a variety of mechanisms of propulsion in water, most often by wavelike movements of the fish's body and tail, and in various specialised fish by movements of the fins. The major forms of locomotion in fish are anguilliform, in which a wave passes evenly along a long slender body; sub-carangiform, in which the wave increases quickly in amplitude towards the tail; carangiform, in which the wave is concentrated near the tail, which oscillates rapidly; thunniform, rapid swimming with a large powerful crescent-shaped tail; and ostraciiform, with almost no oscillation except of the tail fin. More specialised fish include movement by pectoral fins with a mainly stiff body, as in the sunfish; and movement by propagating a wave along the long fins with a motionless body in fish with electric organs such as the knifefish.
  In addition, some fish can variously "walk", i.e., move over land, burrow in mud, and glide through the air.
context_section_description: 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 is particularly true for the larval fishes with higher metabolic rate and smaller size which makes them more susceptible to predators. The swimming ability of a reef fish larva helps it to settle at a suitable reef and for locating its home as it is often isolated from its home reef in search of food. Hence the swimming speed of reef fish larvae are quite high (~12 cm/s - 100 cm/s) compared to other larvae. The swimming speeds of larvae from the same families at the two locations are relatively similar. However, the variation among individuals is quite large. At the species level, length is significantly related to swimming ability. However, at the family level, only 16% of variation in swimming ability can be explained by length. There is also a negative correlation between the fineness ratio and the swimming ability of reef fish larvae. This suggests a minimization of overall drag and maximization of volume. Reef fish larvae differ significantly in their critical swimming speed abilities among taxa which leads to high variability in sustainable swimming speed. This again leads to sustainable variability in their ability to alter dispersal patterns, overall dispersal distances and control their temporal and spatial patterns of settlement.
caption: Salmon larva emerging from its egg
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