Probably the most common tactic for overcoming gape
limitation is nibbling. Many small-mouthed shallow water
marine and freshwater fishes can nibble small pieces from
large prey, including centrarchid sunfishes, cichlids, damselfishes,
wrasses, and surgeonfishes. But the great majority
of fishes, those that utilize suction regularly during
feeding, lack both the dentition and the jaw strength to
nibble effectively. However, one body form among otherwise
suction feeders has allowed the development of an
alternative solution to gape limitation. Eel-like fishes and
other elongate, aquatic vertebrates can spin rapidly around
their long body axis while holding on to food and thus tear
chunks from the larger mass of a prey item.
Eels as a group are predators and scavengers. The
American Eel, Anguilla rostrata, and other members of the
family Anguillidae feed by cruising close to the bottom and
poking their snouts into sediment and crevices. Typical
stomach contents after a night’s foraging include a variety
of small invertebrates and fishes, most of which are inhaled
via the standard teleostean process of inertial suction.
Occasionally, an eel will be fortunate enough to encounter
a dead or dying fish, or a crab that has just molted and
has a soft carapace, or a clam with its siphon sticking
above the surface. Anguillids lack both the dentition and
jaw musculature necessary for chopping up or nibbling at
such items. What they do instead is grasp the item in their
mouth and try to suction it. If this does not work, they give
it a few shakes and tugs. If the item still does not yield, the
eel will hold onto the food and rotate rapidly, up to 14
rotations per second (Fig. 19.6). This action twists the food
and shears off a smaller piece. If the piece is small enough
to swallow, it is suctioned. Otherwise the eel will shake the
food some more, or wedge it against the bottom or in a
crevice and start spinning again until a small enough piece
is removed.
Rotational feeding has