Visual cues eliciting the feeding reaction of a planktivorous fish swimming in a current
2005
SUMMARY The visual plankivorous feeding behaviour of the shiner perch
( Cymatogaster aggregata ) was investigated by means of a flow tank
operated at various current speeds. Artemia salina was used as prey.
In a second set of experiments, Artemia was darkened with black ink,
to compare the visually mediated behaviour of C. aggregata while
feeding on dark prey vs feeding on natural (i.e. semi-transparent)
prey. The positions of the fish and its prey at the time of the feeding
reaction of C. aggregata were measured in three dimensions. Prey were
on average closer and more in line with the fish9s axis when feeding reactions
to darkened Artemia were considered, in comparison with natural
Artemia. Three potential mechanisms triggering the feeding reaction
of C. aggregata were explored: the prey may trigger a reaction in
C. aggregata when it reaches a threshold (1) angular size, (2)
angular velocity, or (3) rate of change of the angular size (i.e. loom) of the
prey as it is carried passively by the current towards the fish. Our results
show that angular velocity may trigger the fish9s reaction when using
semi-transparent prey, while loom may trigger the reaction to darkened prey.
This suggests that feeding behaviour of planktivorous fish is flexible and can
use different cues to trigger a motor reaction to prey with different visual
characteristics. The feeding reaction appeared to occur at longer distances
for semi-transparent rather than darkened Artemia . We suggest that
semi-transparent Artemia were visible at greater distances because of
their higher scattering (i.e. diffuse reflectance) that made them appear
brighter when viewed against a dark background.
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