Data for: Experimental test of the influence of light availability on the evolution of eye size and behavior in Daphnia

<p>There exists extensive variation in eye size. Much work has provided a connection between light availability and differences in eye size across taxa. Experimental tests of the role of the light environment on the evolution of eye size are lacking. Here we performed a selection experiment that examined the influence of light availability on shifts in eye size and the connection between eye size and phototactic (anti-predator) behavior in <em>Daphnia</em>. We set up replicate experimental populations of <em>Daphnia</em>, repeatedly evaluated phenotypic shifts in eye size during the ~50-day experiment and performed a common garden experiment at the end of the experiment to test for evolutionary shifts in eye size and behavior. Our phenotypic analyses showed that eye size rapidly diverged between the light treatments; relative eye size was consistently larger in the low versus high light treatments. Selection on eye size was also modified by variation in density as increases in <em>Daphnia</em> density favored a larger eye. However, we did not observe differences in eye size between the light treatments following two generations of common garden rearing at the end of the experiment. We instead observed strong shifts in anti-predator behavior. <em>Daphnia</em> from the low light treatment exhibited decreased phototactic responses to light. Our results show that decreased light relaxes selection on anti-predator behavior. Such trends provide new insights into selection on eye size and behavior.     </p>