State-dependent, visually guided behaviors in the nudibranch Berghia stephanieae.

Nudibranch molluscs have structurally simple eyes whose behavioral roles have not been established. We tested the effects of visual stimuli on the behavior of the nudibranch Berghia stephanieae under different food and hunger conditions. In an arena that was half shaded, animals spent most of their time in the dark, where they also decreased their speed and made more changes in heading. These behavioral differences between the light and dark were less evident in uniformly illuminated or darkened arenas, suggesting that they were not caused by the level of illumination. Berghia responded to distant visual targets; animals approached a black stripe that was at least 15° wide on a white background. They did not approach a stripe that was lighter than the background but approached a stripe that was isoluminant with the background, suggesting the detection of spatial information. Animals travelled in convoluted paths in a featureless arena but straightened their paths when a visual target was present even if they did not approach it, suggesting that visual cues were used for navigation. Individuals were less responsive to visual stimuli when food-deprived or in the presence of food odor. Thus, Berghia exhibits visually guided behaviors that are influenced by odors and hunger state.