Cubomedusae possess a total of 24 eyes, some of which are structurally similar to vertebrate eyes. Accordingly, the medusae also display a range of light-guided behaviours including obstacle avoidance, diurnal activity patterns and navigation. Navigation is supported by spatial resolution and image formation in the so-called upper lens eye. Further, there are indications that obstacle avoidance requires image information from the lower lens eye. Here we use a behavioural assay to examine the obstacle avoidance behaviour of the Caribbean cubomedusa Tripedalia cystophora and test whether it requires spatial resolution. The possible influence of the contrast and orientation of the obstacles is also examined. We show that the medusae can only perform the behaviour when spatial information is present, and fail to avoid a uniformly dark wall, directly proving the use of spatial vision. We also show that the medusae respond stronger to high contrast lines than to low contrast lines in a graded fashion, and propose that the medusae use contrast as a semi-reliable measure of distance to the obstacle.
Keywords: behaviour; cubomedusae; eyes; spatial resolution; vision.