Interactive effects of fish kairomone and light on Daphnia escape behavior

We investigated the effects of fish kairomone and light intensity on the in ducibility and effectiveness of escape behavior in four clones of the water flea Daphnia from different habitats. To characterize and determine the ef fectiveness of their escape responses, individuals were observed: (i) escap ing from the hydrodynamic disturbances of a simulated predator (a small sph ere dropped from above); (ii) being preyed upon by small fish (Poecelia ret iculata); (iii) responding to encounters with conspecifics in crowded condi tions. The simulated predation experiments revealed that when exposed to fi sh kairomone for 48 h, two Daphnia pulicaria clones, but not two hybrid clo nes, became about twice as sensitive to fluid disturbances when tested in t he light, relative to no-kairomone and dark treatments. When tested in the dark, kairomone had no effect on sensitivity in any clone. All four clones had an all-or-none escape response, in which the strength of the response, as measured by escape distance and speed, was constant regardless of treatm ent. In the guppy predation trials, kairomone-treated D.pulicaria escaped s ignificantly more often from guppies, in both bright- and dim-light conditi ons. In dim light, similar to natural lighting conditions, regardless of ka iromone, all but the most weakly escaping clone were able to elude attackin g guppies in a significant proportion of attacks. Finally, kairomone had no effect on the number of escapes performed by crowded individuals in respon se to the comparatively weak, non-threatening signals created by other Daph nia, indicating that the kairomone-induced alertness in the D.pulicaria clo nes did not make them 'excessively' sensitive. The results suggest that Dap hnia escape behavior is under complex and efficient environmental regulatio n, and may play a significant role in aquatic trophic relationships.