Box jellyfish have four morphologically different types of eyes. Two of these eye types called the upper and lower lens eyes, are camera type eyes with spherical fish-like lenses.

Dr Anders Garm of Lund University, Sweden, has now demonstrated that obstacle avoidance was visually guided and likely mediated by the lower lens eye, as it was found that the jellyfish did not respond to objects above the surface of the water, which are detected by the upper lens eye.
 
More importantly, the strength of response correlated with the intensity contrast between the obstacle and its surroundings. “Contrast is important because, without contrast, the object cannot be detected by any eye,” says Dr Garm. However, there are two kinds of visual contrast: colour contrast and intensity contrast.

“Obstacle avoidance is governed by intensity contrast, which fits with our other data, which strongly suggest that the jellyfish are, in fact, colour blind.”

Contrast is important, because without contrast, the object cannot be detected by any eye