Behavioral Response to Light in Hydromedusae: Effects of Photoreceptor Complexity Across Three Hydrozoan Species

Abstract

Cnidarians possess a simple and diffuse nervous system, yet they are capable of exhibiting behavioral responses to light stimuli. Examining the relationship between photoreceptor complexity and the extent to which hydromedusae translate light cues into behavioral responses can provide critical insights into the evolutionary development of nervous systems. We tested the behavioral responses of three marine hydrozoan species in their hydromedusa stage: Sarsia tubulosa, Clytia gregaria, and Mitrocoma cellularia. We exposed ten individuals from each species first to different constant light intensities to observe behavioral differences, then to a half-illuminated environment to assess phototactic preferences. Consistent with previous studies, none of the three species exhibited significant differences in swimming behavior across light intensities. Under the half-illuminated condition, only S. tubulosa displayed positive phototaxis, whereas C. gregaria and M. cellularia showed no significant preference for either illuminated or dark regions. These findings suggest that greater photoreceptor complexity does not necessarily correspond to different behavioral responses, despite enabling the detection of additional visual cues, including the presence of a shadow in a light environment.