When attacked by predators, flatfishes perform fast-starts that result
in a rapid take-off from the ocean bottom on which they lie. High-spe
ed video recordings of the blind side of flatfishes indicate that they
expel a coherent jet of water from the blind-side opercular valve dur
ing take-off. Buccal pressure recordings in winter flounder (Pseudople
uronectes americanus) show that a buccal pressure pulse begins 0-20 ms
before the beginning of the fast-start and has a range of mean magnit
udes for three individuals of 1.6-10.7 kPa. We hypothesize that one fu
nction of the opercular jet in flatfishes may be to reduce the effects
of Stefan adhesion. Stefan adhesion occurs as the fish lifts its head
up rapidly from the ocean bottom, when water must flow into the space
forming beneath the fish. Water viscosity opposes this rapid shear, a
nd a suction pressure develops under the fish, making it more difficul
t for the fish to escape from the bottom. To estimate the magnitude of
Stefan adhesion, we simulated fast-starts using a physical model in w
hich a dead flounder was pulled upwards with an acceleration of 95 m s
(-2). Results from the physical model indicate that up to 35% of the t
otal force required to lift the head at 20 ms into the start can be at
tributed to Stefan adhesion. Despite this large adhesion force, previo
us work has shown that live flatfish do not show improved fast-start p
erformance when Stefan adhesion has been eliminated by starting the fi
sh from an open wire grid. Thus, live fishes are likely to be using be
havioral mechanisms to reduce the adhesion force. Both the timing and
location along the body of the opercular jet indicate that it is ideal
ly suited to attenuate the effects of Stefan adhesion. Propping the bo
dy up on the median fins may also reduce adhesion by increasing the in
itial distance between the fish and the ocean floor.