NANFA-L-- What Happens When Fish Suck?

Denkhaus, Robert (
Thu, 17 Aug 2006 14:49:23 -0500

I thought that this might interest some on the list. At least it's an
intriguing title...

Go to to read
in original form and to access the actual article by Higham et al.


Kathryn Phillips

While slurping one's food is frowned upon in some societies, fish do it
all the time; they simply throw their jaws wide, decrease the pressure
within, and suck. In 1982, Mees Muller and his colleagues derived a
sophisticated mathematical model that allowed scientists to calculate
both the pressure in the mouth and speed of fluid drawn in by simply
measuring the mouth's dimensions and its rate of expansion. However, no
one had ever directly measured the fluid flows generated by a gulping
fish while simultaneously recording the mouth pressure until Timothy
Higham, Steven Day and Peter Wainwright put bluegill sunfish and
largemouth bass, two members of the centrarchid family, into a DPIV tank
(p. 3281).

Digital particle image velocimetry (DPIV) is mainly used to calculate
the hydrodynamic forces acting on an animal as it scythes through the
water; a fish swims through a suspension of microscopic reflective beads
illuminated by a thin plane of laser light, allowing scientists to track
the eddies generated by the fish. By swimming the fish in a flow tank
that matches the water's flow to the speed of the swimming fish, it's
possible to hold the fish in one position within the laser plane and
record the glittering flow patters. However, this isn't an option when
you're measuring fluid flows near to a lunging fish's mouth. Fortunately
Higham realised that he could entice the fish to lunge reproducibly in
the laser plane by tempting them with a tasty morsel suspended in the
laser light; they always open their mouths at the same point when
approaching a meal. But even then, the majority of the fish weren't
correctly positioned in the plane of light and Higham had to discard the

After months of patiently filming fish feeding while simultaneously
recording the pressures in their mouths, Higham was able to calculate
the fluid speed as each fish sucked. Comparing the sunfish with the
bass, Higham could see that the fluid speeds were strongly correlated
with the time it took the sunfish to open its mouth fully, but the
relationship was weaker for the bass. The sunfish's powerful suck was
generated simply by opening its mouth, but Higham explains that the
weaker relationship between the time the bass took to open its mouth
fully and fluid flow suggests that the animal modulates its slurp with
other mouth structures.

Next, Higham compared his measurements with fluid speed predictions from
Muller's model and found that the model consistently overestimated the
fluid speed over a range of distances in front of the fish's mouth. Ever
since the model's development, fish kinematicists have used mouth
pressure measurements to predict fluid flow speeds, but Higham's
simultaneous recordings suggest that simply knowing the pressure in a
fish's mouth isn't enough to accurately estimate the flow from Muller's
model. However, he suspects that it is now feasible to increase the
model's complexity to better reflect the true movements in a gaping
fish's mouth, given the massive increase in computing power since the
early 1980s.


Higham, T. E., Day, S. W. and Wainwright, P. C. (2006). The pressures of
suction feeding: the relation between buccal pressure and induced fluid
speed in centrarchid fishes. J. Exp. Biol. 209,3281 -3287.

Rob Denkhaus
Natural Resource Manager
Fort Worth Nature Center & Refuge
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