2.2 Fish fins are flexible
Fish fins are typically divided into two major
classes: median, which are along the body center
line, and paired (see Figs. 1 and 2)[12], and both
types of fins play an important and active role during
locomotion[3335]. Although many models of fish fin
function treat the fins as rigid flat plates with constant
area, fish fins are anything but rigid and their flexibility
is important for vectoring forces and for thrust production. Fig. 3 illustrates a variety of pectoral fin conformations in rainbow trout[34]. During steady swimming,
the fins are held near the body, but during hovering,
turning, or braking the fin is twisted into a variety of
conformations. In addition, the base of the fin can be
rotated by musculature in the body wall which reorients the entire fin surface. Fig. 4 shows deformation in
the sunfish pectoral fin during steady locomotion: sun-
fish pectoral fins exhibit considerable change in area
and shape as the fins beat.